Patent Publication Number: US-11653981-B2

Title: Hip replacement navigation system and method

Description:
INCORPORATION BY REFERENCE TO ANY PRIORITY APPLICATIONS 
     The present application is a continuation of U.S. patent application Ser. No. 15/474,037, filed Mar. 30, 2017, which is a continuation of U.S. patent application Ser. No. 13/800,620, filed Mar. 13, 2013, which claims priority under 35 U.S.C. § 119(e) to U.S. Provisional Application No. 61/683,167 filed on Aug. 14, 2012 and U.S. Provisional Application No. 61/761,617 filed on Feb. 6, 2013. Any and all applications for which a foreign or domestic priority claim is identified in the Application Data Sheet as filed with the present application including U.S. patent application Ser. No. 13/800,620 filed Mar. 13, 2013, U.S. provisional application No. 61/683,167, filed Aug. 14, 2012, and U.S. provisional application No. 61/761,617, filed Feb. 6, 2013, are hereby incorporated by reference under 37 CFR 1.57. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This application is directed to the field of hip replacement, and particularly to surgical tools and methods for guiding the preparation of the bones in connection therewith. 
     Description of the Related Art 
     Hip replacement surgery is common and getting more common by the year. One persistent issue with hip replacement is the relatively high incidence of poor placement of the cup and ball components of the prosthetic hip joint. For example, the cup is optimally placed in a specific alignment with a plane including a rim of the acetabulum of the pelvis. For several reasons an unacceptably high percentage of patients have the cup of the artificial hip joint out of alignment with this plane. 
     Unfortunately, misalignment can lead to dislocation of the hip as soon as within one year of the implantation procedure. This is particularly problematic because recovery from a hip procedure can take many months. Patients undergoing a revision so soon after the initial implantation will certainly be dissatisfied with their care, being subject to addition redundant surgery. Of course, all surgery carries some degree of risk. These poor outcomes are unsatisfactory for patients and surgeons and are inefficient for the healthcare system as a whole. 
     Also, in cup placement in total hip arthroplasty, the inclination and anteversion angles are with respect to the Anterior Pelvic Plane (defined as a plane created by the two anterior superior iliac spines (ASIS) and the pubic symphysis). While these anatomical features are visible/palpable while the patient is in a supine position, the majority of total hip replacements are accomplished via a posterolateral approach with the patient in some variation of a lateral position, in which most of these landmarks are not accessible or visible. Historically, navigation for posterior approach hip replacement has been accomplished by registering the anatomical features of the Anterior Pelvic Plane with the patient first in a supine position and, once this plane is recorded by the navigation computer, moving the patient to a lateral position in order to perform hip surgery—with navigation performed with respect to the directly registered Anterior Pelvic Plane. This approach to hip navigation is sub-optimal for surgical workflow because the extra movement of the patient from supine to lateral position takes more surgeon and staff time and requires breaking sterility and re-draping. This is one of the key reasons why hip navigation has failed to be adopted by most of the market. 
     Additionally, altered leg length is a common patient complaint arising from hip replacement surgery and has been a common cause of medical malpractice lawsuits that arise from hip replacement. Because part of the hip replacement procedure requires precise measurements of patient leg length and joint off-set that are frequently difficult to visualize utilizing conventional instrumentation, there are opportunities to improve the surgeon&#39;s performance of these measurements using computer technology. 
     SUMMARY OF THE INVENTION 
     There is a need for improved systems and methods for providing for proper alignment of hip components with a patient&#39;s anatomy during a hip replacement procedure. This can involve techniques for locating one or more anatomical landmarks, e.g., discrete anatomy and/or planes including multiple points. This can involve techniques for confirming alignment of a prosthetic component with an anatomical landmark. 
     In one embodiment, a method is provided for navigating a hip joint replacement procedure. The method includes advancing a first portion of a jig into a portion of the pelvis. The portion of the pelvis is an anatomical landmark in some techniques. In others it is not. At least one inertial sensor is coupled to the jig. The second portion of the jig is moved relative to the first portion to touch, e.g., sequentially, a plurality of anatomical landmarks. This can include touching two or three landmarks, for example. A cup portion of a replacement joint is placed in the acetabulum by reference to a plane calculated based on data from the at least one of a plurality of inertial sensors. 
     In another embodiment, a hip joint navigation system is provided. The system includes a jig, a first inertial navigation device and a second inertial navigation device. The jig has an anchor portion adapted to be placed on the hip, e.g., at an anatomical landmark. The jig also has a landmark acquisition probe coupled with the anchor portion. The probe is moveable in at least three degrees of freedom. The first inertial navigation device is configured to be fixed to a pelvis of a patient to track movements of the pelvis. The first inertial navigation device can be immovably connected to the pelvis. The second inertial navigation device is coupled with the landmark acquisition probe. The landmark acquisition probe can be moved to touch a plurality of landmarks. The inertial navigation devices determine the orientation of a plane of the acetabulum based at least in part on the position of the anatomical landmarks. 
     In another embodiment, a method of navigating a hip replacement procedure is provided. A first hip of a patient is positioned on a surgical table and a second hip is positioned off of the table such that the anterior pelvic plane is disposed upright (e.g., vertically). A jig is coupled with a bone adjacent to a second hip joint. The jig has a moveable orientation guide. An inertial sensor is coupled with the orientation guide. The orientation guide can be an arm of a registration probe in some embodiments. The orientation guide is oriented in a plane substantially parallel to the plane of the table. The orientation of the inertial sensor is recorded as an indication of the orientation of the anterior pelvic plane. If the anterior pelvic plane is vertical the inertial sensor can indicate the plane of the table, which is perpendicular to the anterior pelvic plane. A cup of an artificial hip joint is placed in the acetabulum with reference to the orientation of the anterior pelvic plane based on the orientation of the inertial sensor. 
     In another embodiment, a system for determining orientation data in connection with a hip joint procedure is provided. The system includes a data capture module, a computational module, and a user interface module. The data capture module is configured to receive inertial data from an inertial sensor. The computational module is configured to provide, based on the inertial data, one or more angles of a proxy acetabular plane relative to an anterior pelvic plane. The user interface module is configured to output a user interface configured to communicate orientation data to a user. One or more of these modules is implemented by one or more processors. 
     In another embodiment, a method of navigating a hip replacement procedure is provided. A patient is positioned for posterior or posterolateral approach. A jig is coupled with a bone adjacent to a hip joint. The jig comprising a landmark acquisition probe having an inertial sensor coupled therewith. Patient condition can be assessed, and based on the patient condition, a selection can be made between a first plurality of landmarks and a second set of landmarks. The first set of landmarks can be disposed on an acetabular rim. The plurality of landmarks can be disposed off of an acetabular rim. The orientation of the inertial sensor can be recorded when the landmark acquisition probe is in contact with each of the points of the selected plurality. A cup of an artificial hip joint is positioned in the acetabulum with reference to the recorded orientation to the selected plurality of points. 
     In another approach, a method of navigating a procedure on a hip joint is provided. At least one aspect of the hip joint is characterized pre-operatively. A patient is positioned for posterior or posterolateral approach. A jig is coupled with a bone adjacent to the hip joint (e.g., part of the pelvis). The jig has a landmark acquisition probe having an inertial sensor coupled therewith. The orientation of the inertial sensor is recorded when the landmark acquisition probe is in contact with each of a plurality of landmarks. A cup of an artificial hip joint is positioned in the acetabulum with reference to recorded orientation and to estimations of at least one of anteversion and inclination angles. Estimations of these angles can be based upon the pre-operatively recorded characterization of the hip joint. The recorded orientation can be that of the inertial sensor when the probe is in contact with each of the landmarks. 
     In another embodiment, a method of navigating a hip replacement procedure is provided. The method includes positioning a patient for posterior or posterolateral approach. A jig is coupled with an acetabular socket of the patient, the jig having an engagement surface formed to closely mate to acetabular bone contours of the specific patient. The jig comprising a registration feature configured to be in a pre-determined orientation relative to the anterior pelvic plane of the patient when the jig is so-coupled. An inertial sensor is coupled with the registration feature such that the inertial sensor generates a signal indicating at least one angle relative to the anterior pelvic plane. A prosthetic cup is placed based on the signal. 
     In another embodiment, a patient specific jig system for hip replacement is provided. The jig system includes an engagement surface and a registration feature. The engagement surface is formed to closely mate to acetabular bone contours of a specific patient. The registration feature is configured to be in a pre-determined orientation relative to the anterior pelvic plane of the patient when the jig is coupled to closely mate to acetabular bone contours of the specific patient. 
     In another embodiment, a method of replacing a hip joint is provided. The method includes coupling a trackable member with a limb forming a part of a hip joint of the patient. The limb is moved to at least four points disposed away from a neutral position of the hip. The four points include at least one medial extent, at least one lateral extent, at least one anterior extent, and at least one posterior extent of a patient&#39;s range of motion. During the moving step, data is collected from the trackable member indicating the displacement from the neutral position to each of the extents. A socked component of the prosthetic hip joint is placed within the acetabulum. A stem of a femoral component of a prosthetic hip joint is placed into a proximal femur. A ball of the femoral component is placed in the socket component. In the method, when the prosthetic hip joint is in the neutral position, a stem axis connecting the center of rotation of the ball and a centroid of the stem at the mouth of the socket component is in a central zone between the at least four extents. At least one of the steps of placing is performed with the aid of a display of the position and/or orientation of the stem axis relative to the central zone. 
     In the system described above, the inertial navigation devices can be replaced with or supplemented by one or more cameras for monitoring distance, linear position, or angular position. 
     In the system described above, the inertial navigation devices can be replaced with or supplemented by one or more cameras for determining the spatial position of trackers coupled with instruments, such as a stylus. In such a system, the jig can be simplified without requiring moveable portions for example. 
     In some variations of the methods discussed herein, patient data can be used to enhance the accuracy of orientation of a component, such as the plane of the acetabulum. Patient data can include CT, MRI, X-Ray or other pre-operative planning data. 
     In another embodiment, a hip joint navigation jig is provided that includes a platform, a cannula coupling device, and a registration jig mounting feature. The cannula coupling device is disposed on the platform and is configured to enable a cannula to be detachably coupled with a bottom surface of the platform. The cannula is configured for detachably coupling the platform with a bone adjacent to a hip joint. The registration jig mounting feature is disposed on the platform. The hip navigation jig also includes registration jig. The registration jig includes an upright member, a rotatable member, and a probe. The upright member is configured to be detachably coupled to the platform at the registration jig mounting feature. The rotatable member is coupled with the upright member for rotation about an axis that is not vertical when the jig is mounted to the bone adjacent to a hip joint and the upright member is disposed generally vertically. The probe had a tip for engaging anatomy. The anatomy engaging tip is disposed at a distal end of an elongate body coupled with the rotatable member for rotation about the axis. The orientation and position of the elongate body of the probe can be adjusted to bring the anatomy engaging tip into contact with a plurality of anatomical landmarks during a landmark acquisition maneuver. 
     Although the platform can have any shape or configuration, it is elongate in some implementations, for example, including a first end and a second end. The first end can be configured to be oriented inferiorly and the second end to be oriented superiorly when the navigation jig is applied to the patient. If the platform is elongate, the cannula coupling device can be disposed adjacent to the first end, which may be located inferior of the second end when placed on the patient. The cannula coupling device can be detachably coupled with a bottom surface of the platform in some embodiments. The registration jig mounting feature is disposed on a top surface of the platform and can be positioned adjacent to the first end. Again, the first end may be inferior end, just superior to or at the superior portion of the surgical field. 
     In another embodiment, a hip joint navigation jig is provided that includes an anatomical interface comprising a bone engagement portion. A registration jig is also provided that is coupled, e.g., removeably, with the anatomical interface. A rotatable member is provided for rotation about an axis that is not vertical when the jig is mounted to the bone adjacent to a hip joint and the registration jig is coupled with the anatomical interface. An anatomy engaging probe is coupled with the rotatable member for rotation about the axis and is translatable to enable the probe to be brought into contact with a plurality of anatomical landmarks during a procedure. An inertial sensor is coupled with the probe to indicate orientation related to the landmarks, the sensor being disposed in a different orientation relative to horizontal when the probe is in contact with the landmarks. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features, aspects and advantages are described below with reference to the drawings, which are intended to illustrate but not to limit the inventions. In the drawings, like reference characters denote corresponding features consistently throughout similar embodiments. 
         FIG.  1    is a perspective view of a hip navigation system applied to a patient illustrating a measurement of leg length and/or joint offset after implantation of the prosthetic hip joint. 
         FIG.  2    is an image of hip anatomy illustrating some examples of anatomical landmarks that can be used in a method of navigating a hip prosthesis with the navigation system of  FIG.  1   . 
         FIG.  3    shows a navigation base assembly coupled with a first anatomical landmark, in this case the ilium on the pelvis of the patient. 
         FIG.  4    is a perspective view illustrating first and second orientation detecting devices coupled with the base of  FIG.  3   . 
         FIG.  5    is a perspective view of the navigating system, illustrating one technique for synchronizing a plurality of orientation and/or position detecting devices of the navigating system of  FIG.  1   . 
         FIG.  6    is a perspective view of the navigation system of  FIG.  1    coupled with the pelvis and illustrating a step of registering a landmark of a femur prior to resecting the femur. 
         FIG.  7    shows the anatomy after the femoral head has been resected and an optional step of synchronizing a plurality of inertial sensors of the navigation system. 
         FIG.  8    illustrates a step of registering an anatomical landmark disposed about the acetabular rim on the pelvis. 
         FIG.  9    illustrates a step of registering another anatomical landmark disposed about the acetabular rim of the pelvis. 
         FIG.  10    illustrates initial placement of an impactor in the acetabulum. 
         FIG.  11    illustrates a hip prosthesis placement system, including an inertial sensing device. 
         FIGS.  11 A- 11 C  illustrate an embodiment of an impactor assembly. 
         FIG.  12    illustrates a step of navigating placement of a cup portion of an artificial hip joint. 
         FIG.  13    is a perspective view of another embodiment of a hip navigation system. 
         FIG.  14    is a detail view of portion of the system of  FIG.  13   , with a camera recording linear position of a registration arm. 
         FIG.  15    shows a variation of the embodiment of  FIGS.  13  and  14    in which rotational orientation and linear position can be acquired by a camera viewing a radial scale. 
         FIG.  16    is an exploded view of an assembly showing a tilt/rotation mechanism adapted to enable a camera to track at least one rotational position. 
         FIGS.  17 - 17 C- 2    illustrate modified systems configured for navigating a posterior approach hip replacement procedure. 
         FIGS.  18 - 21 B  illustrate a hip navigation system configured for an anterior approach hip replacement procedures, and various aspects of such procedures. 
         FIGS.  22 - 31    illustrate various aspects of methods involving custom patient-specific positioning jigs. 
         FIG.  32    illustrates methods for defining a patient-specific safe zone in a hip placement procedure. 
         FIG.  33    is an embodiment of a system for close range optical tracking. 
         FIGS.  34 - 35    illustrate various anatomical landmarks that can be used in various methods involving navigating with landmarks. 
         FIG.  36    is a pre-operative image that can be used to enhance alignment in a hip procedure by providing patient specific data. 
         FIGS.  37  and  38    are views of a hip procedure navigation system applied to a pelvis in a posterior approach. 
         FIGS.  39  and  40    are view of the hip procedure navigation system of  FIG.  37 - 38    modified and applied to a pelvis in an anterior approach. 
         FIGS.  41 - 41 A  illustrate a first embodiment of pin securement devices. 
         FIGS.  42 - 42 B  illustrate a second embodiment of pin securement devices. 
         FIGS.  43 - 43 B  illustrate a third embodiment of pin securement devices. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A variety of systems and methods are discussed below that can be used to improve outcomes for patients by increasing the likelihood of proper placement of a hip joint. These systems can be focused on inertial navigation techniques, close range optical navigation, or a combination of inertial and optical navigation. 
     I. Hip Navigation Using Inertial Sensors 
     Systems and methods described below can improve prosthetic hip joint placement using navigation in connection with referencing anatomical landmarks, incorporating preoperative custom fit jigs based on imaging, and a combination of pre-operative imaging and landmark referencing. These hip procedures generally guide a prosthetic hip to an orientation within the acetabulum that minimizes the chance of dislocation due to impingement of the femoral neck on the cup or on bones around the acetabulum or other reasons related to suboptimal orientation of the prosthetic. Various techniques leverage population averages of proper placement while others are amenable to patient specific refinements. Also various techniques for registering and confirming the position and/or orientation of the femur pre- and post-implantation are discussed herein, which are useful to control leg length and joint offset at the end of the procedure. 
     A. Navigation Using Inertial Sensors and Jigs for Referencing Anatomical Landmarks with Posterior Approach 
     Most hip replacement procedures presently are performed from a posterior approach. In this approach, the patient is positioned on his/her side and the anterior pelvic plane is oriented vertically, e.g., perpendicular to the plane of the table on which the patient is positioned. Most surgeons performing hip replacement are very familiar with this approach and will immediately recognize the benefit of enhanced certainty about the orientation of the relevant anatomy when the patient is in this position. 
     1. Apparatuses and Methods for Posterior Approach Hip Navigation 
       FIGS.  1  and  4    show a hip navigation system  100  adapted to navigate a hip joint procedure with reference to anatomical landmarks without requiring, but not necessarily excluding, pre-operative imaging or other inputs apart from those discussed below. The system  100  is shown mounted on a pelvis in a posterior approach in  FIG.  1   .  FIG.  4    shows an early phase of a procedure prior to the joint being dislocated but after the system  10  is mounted to the pelvis.  FIG.  1    shows a late phase of some variations of techniques for which the system  100  is adapted. As discussed further below, such variations involve registering the femur prior to and after the joint is replaced to confirm an aspect of the relative position and/or orientation of the femur, e.g., leg length, joint offset, and rotational orientation of the femoral neck. 
     The system  100  includes a registration jig  104 , an alignment assembly  108  and a landmark acquisition assembly  112 . The alignment assembly  108  is rigidly connected to the hip in the illustrated configuration so that motion of the hip cause corresponding motion of sensor(s) in the assembly  108  as discussed below. Sensing this motion enables the system  100  to eliminate movement of the patient as a source of error in the navigation. The landmark acquisition assembly  112  provides a full range of controlled motion and sensor(s) that are able to track the motion, in concert with sensor(s) in the assembly  108 . Additional details of systems, devices, sensors, and methods are set forth in U.S. Pat. No. 8,118,815; US US2010/0076505; and U.S. Pat. No. 8,057,479 which are all incorporated by reference herein in their entireties for all purposes. The sensors in assemblies  108 ,  112  preferably transfer data among themselves and in some cases with external devices and monitors wirelessly, using Bluetooth, Wifi® or other standard wireless telemetry protocol. 
     The registration jig  104  includes a fixation cannula  124  that has a distal end that can be advanced to a pelvic bone at an anatomical location or landmark or other selected location. In the illustrated technique, the cannula  124  is secured by a pin  132  (see  FIG.  3   ) that is driven into the ilium on the pelvis through the cannula  124 . A distal end  128  of the pin  132  is shown in  FIG.  1   . 
     As discussed further below, the cannula  124  can be coupled with other bones in other techniques with a posterior approach. For example, the cannula  124  can be coupled with the ischium or the pubis in other techniques. In some techniques, the cannula  124  is mounted to a pelvic bone but not at a landmark. The hip navigation system  450  discussed below in connection with  FIG.  17 - 17 B  can be used such that the fixation member  466  is coupled at a point superior to the superior-most point on the acetabular rim. In a specific technique, the member  466  is about 10 mm above the superior-most point on the acetabular rim. In such techniques, three or more anatomical landmarks disposed about the acetabulum can be acquired, as discussed below. When the cannula  124  is coupled with a landmark, only two additional landmarks are acquired in some embodiments as discussed below. In another variation, a clamp can be used to couple with a bone without requiring that the pin  132  be driven through the cannula  124  into the bone. For example, if the bone is thinner in the region where the system  100  is to be anchored, placing the pin may be disadvantageous.  FIG.  2    shows a region where a clamp may be used beneath the point “A” on the ischium. One reason for mounting or clamping the cannula  124  away from the landmarks is that the landmarks may not be visible or accessible before dislocating the hip joint. If the clinician wishes to use the system  100  to reference the femur (as discussed below), it may be required to mount or clamp the cannula  124  away from the landmarks. 
       FIG.  1    illustrates a step toward the end of a navigated hip joint implant procedure discussed in detail below. Some of the preceding steps involve removing the to-be-replaced joint, navigating the hip joint, preparing the implant location for the artificial joint, and placing the joint, as elaborated below. As discussed further below,  FIG.  1    illustrates a technique for confirming that these steps were properly performed. 
       FIG.  2    shows some of the anatomy that is relevant to various methods and systems herein. In some embodiments, the navigation system  100  is configured to locate a relevant anatomical feature to aid in proper placement of a prosthetic hip joint. For example, a plane can be located using the system  100  that includes at least a portion of a patient&#39;s acetabular rim. In practice, the acetabular rim may be uneven due to development of ostephytes. So, in the context of this application locating the anatomical plane can be an approximation of the actual topography, for example an estimate of the plane, a plane including a substantial fraction, e.g., a majority of the surface of the acetabular rim, or some other manner of estimating a relevant anatomical feature. Preferably the anatomical landmark being located is used to confirm accurate placement of at least the cup and preferably the complete artificial hip joint. 
       FIG.  2    also shows an example of anatomical landmarks that can be used to approximate the acetabular rim or another plane relevant anatomical landmark. In many patients the acetabular rim is not well defined, due to injury, advanced stages of arthritis or other conditions. Accordingly, approximating the acetabular rim for these patients includes calculating in the system  100  a plane that references but may not include most or any of the actual acetabular rim. The plane that is defined is located near the rim but more importantly has a known anteversion and abduction angle relative to the anterior pelvic plane. For example, three points can be used to estimate the plane of the acetabular rim. In one technique, some or all of the points illustrated in  FIG.  2    are used. 
     As illustrated by  FIG.  2   , three landmarks are defined at “A”, “B”, and “H”. The landmark “H” is located on the ilium at a location that is spaced away from the rim by an amount sufficient to avoid irregular bony growth due to injury, advanced stages of arthritis or other conditions, for example 1 cm superior to the most superior point on the acetabular rim. The landmarks “A” and “B” can be located on the ischium and pubis respectively and can be similarly spaced from the rim to avoid damaged/diseased areas. Each of these landmarks preferably is close enough to the rim, however, to be within the standard open area, e.g., the area exposed by the surgical cut down. Other landmarks that could be used include: anterior insertion point of trans-acetabular ligament to the ischium, mid-point of the inferior aspect of the acetabular notch, the anterior superior iliac spine, anterior inferior iliac spine, convergence of the acetabulum and anterior inferior iliac spine, as well as the other landmarks illustrate on  FIGS.  34  and  35   . In the techniques discussed below all of the ilium, the pubis, and the ischium are used to locate the acetabular rim. The navigation system  100  has one or more processors that receive(s) data and determines the relative position of these (or other) anatomical landmarks from these points. The data can be generated by inertial sensors, as discussed elsewhere herein, or other types of sensors. Preferably the sensors are small enough to be mounted on or in handheld housings or embedded in the instruments. The navigation system  100  preferably also has a memory device to at least temporarily store the position of these points or relevant orientation data. 
       FIG.  3    shows further details of the registration jig  104  and further aspects of methods of navigating an artificial hip joint. A proximal end of the pin  132  is coupled with or disposed above a platform  136  that is configured to couple with the alignment assembly  108  and/or the landmark acquisition assembly  112 . As shown in  FIG.  1   , the platform  136  can be connected to both of the alignment assembly  108  and the landmark acquisition assembly  112  at the same time. The platform  136  comprises a rigid bar fixed to the proximal end of the pin  132  and/or the cannula  124  in the illustrated embodiment. The platform  136  includes a plurality of mount features  140 A,  140 B, e.g., a mount feature on each of two lateral ends  144 A,  144 B of the platform. The mount feature  140 A is configured to permit non-rotational attachment to the alignment assembly  108 . 
       FIG.  3    illustrates that the registration jig  104  is configured to be used in left and right hip procedures, for example having a dedicated mount feature  140 A for each hip. Preferably the mount feature  140 A provides a post spaced away from the joint being treated so that the alignment assembly  108  can be mounted as far away from the hip joint as possible.  FIG.  5    shows the alignment assembly  108  on this post and another post exposed. The exposed post is not used during the procedure on the hip joint illustrated in  FIG.  5   . However, if the other hip of the patient is being treated, the platform  136  is in the opposite orientation and the posted exposed in  FIG.  5    will be coupled with the alignment assembly  108 . Stated another way, a longitudinal axis of the platform  136  extends between two mount posts, each of which can be dedicated to a hip on one side of the medial-lateral mid-plane of the patient. 
     The mount feature  140 B enables rotational mounting of the landmark acquisition assembly  112 . For example, the mount feature  140 B can include a pivotally mounted jig  148  that projects upward to a free end that is adapted to mate with an orientation sensing device as discussed below. The joint  148  permits a registration arm, such as the elongate member  224  discussed below to be tilted downward to touch landmarks at different elevations. 
     In one technique, the registration jig  104  is preassembled and is driven into a suitable anatomical landmark, such as the ilium. In other techniques, an anchor jig can be mounted off-set from a landmark to be acquired. The ilium will have been previously identified by conventional means, such as by X-ray examination, palpation, or by making an incision and visually inspecting the pelvis. In one technique, the cannula  124 , the pin  132 , and the platform  136  are separable so that the pin can be placed and the platform  136  coupled to the pin at a later time. The cannula  124  can be coupled with other landmarks in some variations. 
       FIG.  4    illustrates further steps of various techniques. For example, the alignment assembly  108  can be coupled with the mount feature  140 A. In one embodiment, the alignment assembly  108  includes a rigid extension  160  that is adapted to be mounted detachably to the mount feature  140 A. The extension  160  has a first end  164  and a second end  168 . The second end  168  is detachably mountable to a surgical orientation device  172  that detects orientation and rotation of the device  172  relative to a reference frame. The orientation device  172  preferably comprises at least one sourceless sensor, such as an accelerometer, a gyroscope, or a combination of these sensors and other sensors. In one preferred embodiment, the orientation device includes a three axis accelerometer to detect orientation relative to gravity and a plurality of gyroscopes to detect rotation. Other sensors could be used in various modifications. Examples of specific sensor combinations include Analog Devices ADIS 16445 and Invensense MPU-6050 or MPU-9150 among others. In some approaches, the orientation device  172  can be disposable and so the sensors preferably are less expensive sensors. Sensors on the landmark acquisition assembly  112  may be reusable in some configurations and thus may incorporate more expensive, more rugged or more accurate sensors. 
     The first end  164  of the detachable extension provides several functions. The first end  164  has a device to engage the mount  140 A in a secure but releasable manner. The engagement between the extension  160  and the platform  136  minimizes or prevents relative movement therebetween to avoid any mechanical relative movement during navigation procedures so that movement of the orientation device  172  corresponds to movement of the hip. The first end  164  also has a docking device that, as discussed further below, provides a stable and controlled manner to position the landmark acquisition assembly  112  relative to the orientation device  172 . 
       FIG.  4    also illustrates that the landmark acquisition assembly  112  can be securely coupled to the platform  136 , e.g., at the mount  140 B. In one embodiment, the landmark acquisition assembly  112  includes a gimbaled jig  200  and an orientation sensing device  204 . The jig  200  includes a coupler  208  for detachably coupling with the mount feature  140 B of the platform  136 . The coupler  208  is pivotally connected to a sliding support  212 . The sliding support  212  includes a slot that permits slideable extension of an elongate member  224 . The slideable extension permits a range of motion of a distal end  228  of the elongate member to facilitate acquiring a plurality of landmarks that are different distances from the attachment location of the cannula  124 , as discussed further below. In other words, the distal end  228  can be extended away from the axis of the sliding support  212  or can be retracted to a position closer to the axis of the sliding support  212 . 
       FIGS.  4  and  6    illustrate the moveability of the landmark acquisition assembly  112  relative to the platform  136  between two positions. In  FIG.  4   , the elongate member  224  is swung about an axis that may be parallel to a longitudinal axis of the cannula  124  to move the distal end  228  away from the first end  164  of the extension  160 . This is a moving configuration of the gimbaled jig  200 . In addition to rotation enabled by the pivotal coupling between the coupler  208  and sliding support  212 , the pivotally mounted joint  148  can enable the elongate member  224  to pivot about an axis that is not parallel to the axis of the cannula  124 . The axis of rotation of the joint  148  can be perpendicular to the axis of rotation of the sliding support  212 . This rotatability enables the distal end  228  of the elongate member  224  to pivot down to contact anatomical landmarks, as discussed above. Additionally, the slideability of the elongate member  224  within the sliding support  212 , discussed above, enables the distal end  228  to move to reach anatomical landmarks in the same plane but closer to or farther from the distal end of the cannula  124  or pin  132 .  FIG.  6    shows the distal end  228  of the elongate member  224  positioned closer to the platform  136  for referencing landmarks at higher elevation or closer positions, e.g. on the lateral side of the femur. 
       FIG.  4    also shows that the distal end  228  can include an angled length that enables the elongate member  224  to avoid minor irregularities in height adjacent to the anatomical landmarks being registered. Such irregularities may be normal anatomy, osteophytes or irregular bone growth of various types. 
       FIG.  5    illustrates a parked configuration  260  of the landmark acquisition assembly  112 . In particular, a portion of the elongate member  224  is moved into a latch  262  disposed at the first end  164  of the upright extension  160 . The parked configuration  260  enables the navigation system  100  to manage errors that can compound in some inertial sensors. For example, in one embodiment, gyroscopic sensors in the orientation device  172  and in the orientation sensing device  204  can be synchronized when a stable and known orientation is detected and one or more of the gyroscopes, e.g., the gyroscope in the device  204 , can be zeroed after that condition is met. Further techniques employing the parked configuration  260  will be discussed further below. As discussed below in connection with the system  450 , some jigs have a registration point adjacent to the distal end of the anchor jig or bone connection site. The system  450  is capable of accurately acquiring landmarks based on only accelerometers operating in the device  204  in one mode. In such a mode, a registration feature can provide an analogous function to the parked configuration, e.g., to enhance accuracy of the sensing devices in the system. 
     Another example of a parked configuration of the system  100  can be provided. For example, the parked configuration advantageously includes the ability to stably position and hold the devices  172 ,  204  for substantially no relative movement. In one approach, the orientation sensing device  204  is mounted on the rigid extension  160 . Other arrangements could include a mounting post on the platform  136  adjacent to the rigid extension  160 . 
     Where error management is less an issue, the parked configuration  260  can still be useful in that it prevents unwanted swinging or other movement in the surgical field. 
     In one basic method, the jigs discussed above are connected to the pelvic bone, the system  100  is put into the parked configuration  260 , and the sensors are initialized. The initializing can include synchronizing at least two sensors. In some cases, the initializing can include zeroing one or more sensors. In this context, “zeroing” is a broad term that includes any method of eliminating accumulated error in the system, including any form of resetting of the sensors, and/or confirming in one device that the data from the other device is reliable for at least a fixed period. 
       FIG.  6    illustrates an optional step of acquiring a landmark of a femur in connection with a hip replacement procedure. The hip is positioned in a neutral flexion/abduction position. The landmark acquisition assembly  112  is in a withdrawn configuration  266  with the elongate member  224  moved, such as by sliding in the sliding support  212 , to accommodate the relatively short distance from the platform  136  and a landmark of the proximal femur. In one technique the tip of the distal end  228  is brought into contact with a part of the greater trochanter or elsewhere on the proximal femur. After the landmark is found and/or contacted, the clinician can make a mark on the femur Fm, such as a bovie mark, a pen mark, a stitch or other durable indication. Once the tip of the distal end  228  is in contact with the desired landmark, the navigation system  100  processes data from and stores the orientation of one or more sensor(s) in the orientation sensing device  204 . Additionally, in some embodiments, the elongate member  224  is provided with a scale  226  indicating position of the tip of the elongate member  224 , e.g., relative to the cannula  124  or some other relevant fixed feature of the patient or the system  100 . By providing the scale  226  to be read by the clinician, the system is made simple and cost effective. 
     After the optional step illustrated in  FIG.  6   , the proximal femur can be resected to remove the natural ball thereon. 
       FIG.  7    illustrates that in one advantageous technique, the user returns the system  100  to the parked configuration  260 . This step may be optional depending on the sensor(s) and the timing of the resecting of the femur. In this position, the sensor(s) in the orientation devices  172 ,  204  can be initialized again, e.g., zeroed. As discussed above, this is one technique for minimizing accumulation of error in some inertial sensors. By providing this optional step, less costly sensors can be used enabling the system  100  to deliver highly accurate hip replacement while helping to manage cost for the patient, medical provider and healthcare system generally. 
       FIG.  8    illustrates a first extended configuration  264  provided in a step after the resecting of the proximal femur in which a second anatomical landmark is acquired or referenced. In particular, the elongate member  224  can be extended and can be rotated by the jigs  148 ,  200  to be in contact with any suitable landmark. In one technique, contact is made between the distal end  228  and the ischium. To provide maximum accuracy, this contact may be provided within a short period, e.g., within about 20 seconds of being disengaged from the parked configuration  260 . Once contact is made, the system  100  is configured to store the orientation of the sensing device  204 . In one configuration, the orientation is stored after a button or other indirect means is pressed on the orientation device  172 . In addition to acquiring the orientation, a position value is input to the system. For example, the scale  226  on the elongate member  224  can be read by the user and the value of the scale input into the system. In one technique where the scale  226  is read and input by the user, the orientation device  172  has a user interface with an input device for inputting such variables. As can be seen in the drawings, the scale  226  can in fact be two different scales, one for each of the retracted configuration  266  and the extended configuration  264 . Alternatively, the scale  226  can extend the entire length of the elongate member  224  to provide a greater range of positions that can be read by the clinician or by the system as in  FIGS.  13  and  14   . 
     The extended configuration  264  is one in which the distal end  228  of the elongate member  224  is adapted to touch an anatomical landmark located between the medial cephalad-caudal plane of the patient and the acetabulum of the pelvis. 
     Depending on the sensors used and the timing of landmark acquiring step of  FIG.  8   , the user may return the system  100  to the parked configuration  260  and also may initialize, e.g., zero, the system  100  again. 
       FIG.  9    illustrates a second extended configuration  272  provided in a step after the resecting of the proximal femur in which a third anatomical landmark is acquired or referenced. The third anatomical landmark can be acquired before the second anatomical landmark in some techniques. In the second extended configuration the distal end  228  of the elongate member  224  moved to contact a landmark, such as the pubis. To provide maximum accuracy, this contact may be provided within a short period, e.g., within about 20 seconds of being disengaged from the parked configuration  260 . Once contact is made, the system  100  can store the orientation of the sensing device  204 . The orientation can be stored by pushing a button or other user interface device. In some techniques orientation and position are input into the system. For example, the scale  226  on the elongate member  224  can be read by the user and the value of the scale input into the system. In one technique where the scale is read and input by the user, the orientation device  172  has an input device, such as a user interface for inputting such variables. 
     The extended configuration  272  is one in which the distal end  228  of the elongate member  224  is adapted to touch an anatomical landmark located anteriorly of the acetabulum. 
     Once landmarks have been acquired, the system  100  can determine the bearing of three landmarks including that of the attachment location of the cannula  124 , if the pin is attached to a relevant landmark. The system can calculate the orientation of the orientation device  172  relative the plane containing these three (or in other methods another group of three or more) landmarks. From this, a variety of post processing can be performed. For example, the orientation (anteversion and/or abduction) can be adjusted based on the known mean orientation of the plane containing these three (or another three or more, if used) landmarks from the pelvic anatomic reference planes. 
     One variant of the system  100  enables a user to select between multiple sets of landmarks for use in the above calculations. The method discussed above exploits the use of three points that are off of the acetabular rim. These points are less impacted by local prominences at the rim that may be due to disease or deformity. Thus, they have a lower likelihood of requiring intra-operative improvisation. On the other hand, another set of landmarks can be selected where the rim is free of deformities, which might be confirmed pre-operatively. For example, two or three points can be selected on the acetabular rim for landmark acquisition. The on-rim landmarks are advantageous in that they are easier to access through a smaller incision. For example, on-rim points can include the center of the posterior insertion of the transacetabular ligament, the center of the anterior insertion of the transacetabular ligament and the most superior point on the rim. A group of anatomical landmarks including one or more extra-acetabular landmarks can include the ilium (where the registration jig  104  or other anchor member can be inserted), the lowest point of the acetabular sulcus of the ischium, and the prominence of the superior pelvis ramus. 
     Some techniques involve referencing a fourth point. The fourth point can be used in connection with some forms of patient specific registration. The fourth point can be extra-acetabular or can be disposed on the acetabular rim. An example of an acetabular landmark is the acetabular notch. Other landmarks are discussed herein, for example in connection with  FIGS.  34  and  35   . 
     The posterior approach systems are advantageously configured to allow intra-operative selection between on-rim and off-rim points. For example, if the rim looks free of deformities pre-operatively but when exposed presents differently, the surgeon can select an off-rim landmark set. 
     Several techniques for enhancing the accuracy of the relationship between the sensed landmarks and the location of calculated anatomical features, such as the anterior pelvic plane or angle of the acetabulum can be employed. For example, user input can be collected indicating whether the hip being treated is on the left or the right side of the patient and whether the patient is male or female. A more refined estimation of the model can be provided based on a characterization of a study group. For example, hip joints of a group of 30 or more patients can be studied to identify the correspondence between a feature that can be accessed in one approach and an anatomical feature of more surgical relevance that cannot. A group of subjects can be studied for any number of demographic characteristics such as gender, age, weight, height or any other variable in a relevant population. For those sub-groups, a correlation or transformation between a measured parameter and a parameter that cannot be measured but is desirable to know can be generated. Once such a correlation or transformation is established, transforming a measured feature into the unmeasurable but useful to have feature can be achieved by operating software on a processor. The software can be programmed to calculate one or two angles, e.g., inclination and anteversion based on a registered pelvic plane, such as a proxy acetabular plane. Such a system can be used to generate in real time the angles of a free hand instrument relative to the anatomy, e.g., relative to an acetabulum in placing a hip socket component. 
     Additionally, data from the use of pre-operative imaging or positioning (discussed below) can be used to enhance the accuracy of these calculations. Thus, the posterior approach systems preferably are configured to take user input directly by actuating buttons on the orientation device  172  or by connecting an auxiliary data storage device, such as a flash memory device, to the system or by any means of other communication with the system, including wifi connection, Bluetooth, Internet connection among others. 
     In some techniques, the posterior approach systems described herein are adapted to determine, monitor, and confirm proper leg length and joint offset outcome in a hip replacement. For example, the system  100  can calculate and store components of a leg length metric, e.g., a vector along the superior-inferior axis (leg length) and/or along the medial-lateral axis (offset). In one approach, the device  172  has a display that indicates when the femur is in the same position pre- and post-operatively. For example, it can indicate “0” meaning no displacement causing a leg length change and “0” indicating no movement of the femur farther away from the cephalad-caudal mid-plane of the patient pre- and post-operatively. For enhanced accuracy, a plurality of points, e.g., three points, can be marked acquired and/or marked on the femur. The points can be spaced apart by an amount sufficient to provide increased accuracy. These three points can be used to confirm proper placement of the femur in abduction, rotation, and flexion. 
     One enhancement involves referencing the femoral neck to assure that after implanting the hip joint, the femur is positioned properly rotationally. For example, it may be desired to make sure that a feature of the femur like the greater trochanter resides in the same rotational orientation relative to an axis extending through the center of rotation of the femoral head and perpendicular to the plane of the acetabulum. To assure a substantially unchanged rotation orientation post-implantation, the system  100  can record one or more, e.g., three points on the femoral neck pre- and post-implantation. Three points that would be convenient from either the posterior approach or the anterior approach (discussed below in connection with  FIGS.  18 - 21   ) are the greater trochanter, lesser trochanter and the insertion of the obdurator externus. 
     The foregoing are some steps that can be used to determine and store a variety of parameters useful in a navigated hip procedures. After some or all of these steps have been performed, in one embodiment, the acetabulum can be prepared for receiving a cup. For example, the acetabulum can be reamed in a conventional manner. In some embodiments, the reamer can be coupled with an orientation device containing an inertial sensor to guide the reaming process. This is discussed in some detail in US2010/0076505, published Mar. 25, 2010 which is incorporated by reference herein in its entirety for this purpose and for all disclosure therein generally. 
       FIG.  10    shows that after reaming, an impactor  300  may be used to place a cup of an artificial hip joint. The impactor handle  304  may be positioned in the approximate correct orientation, e.g., with a longitudinal axis of the impactor being disposed perpendicular to the plane navigated above or a plane determined based on the navigated plane.  FIG.  10    shows that this initial placement can be done while the system  100  is in the park configuration  260 . The impactor  300  can be substantially aligned at this time, based on visual inspection. As part of the step illustrated in  FIG.  10    or shortly thereafter, the sensors can be initialized, e.g., zeroed as discussed above. 
       FIG.  11    shows that in a subsequent step the orientation sensing device  204  can be undocked from the proximal end  230  of the elongate member  224  and thereafter docked to the impactor  300 . Preferably this step is performed while the impactor  300  is in place on the hip, close the proper alignment. In another embodiment, a third sensing device similar to the sensing device  204  be coupled with, e.g., pre-attached to, the impactor and the data collected above transferred to the third device. The impactor  300  and sensing device  204  comprise a cup orientation navigation assembly. Preferably the impactor  300  has a cylindrical shell  312  that is moveable relative to an inner shaft  316  of the handle  304 . The shell has a docking device  320  that can receive the docking device of the sending device  204 . The moveability of the shell  312  helps to isolate the sensing device  204  from the forces that are transmitted through the impactor  300 . These forces are applied by a mallet or other device for forcibly moving the cup into position. By providing at least some force isolation between the shell  312  and the sensing device, impact on the sensors in the sensing device  204  can be reduced. Excessive force being applied to the sensing device  204  can put the device  204  out of service, for example until synched with the device  172 . 
       FIG.  11 A  illustrates a further embodiment of an impactor  300 A in which the movement of a shell  312 A is cushioned by a plurality of spring members  340 ,  344  which are configured to absorb at lease some of the shock of the impact on the impactor  300 A. The impactor  300 A also is configured to be modified to suit any of a plurality of hip prostheses. For example, a plurality of tip components  348  can be provided in a kit where each tip component is attachable to and detachable from a distal end of the shaft  316 A of the impactor  300 A. 
       FIGS.  11 B-C  show more detail of distal features of the impactor  300 A. In particular, the tip component  348  is removable from a shaft  316 A of the impactor  300 A.  FIG.  11 C  shows that the tip component  348  can have a recess  352  formed on the proximal side thereof and an engagement device  356  formed on the distal side thereof. The recess  352  can comprises a plurality of flats  350 A corresponding to a plurality of flats  350 B on the distal end of the shaft  316 A. The flats permit proximal-distal sliding of the recess  352  over the distal end of the shaft  316 A. Preferably a detent device or other mechanism is provided between the tip component  348  and the shaft  316 A so that the component does not fall off the shaft. The flats prevent the tip components  348  from rotating relative to the shaft  316 A. The engagement device  356  comprises threads in one embodiment so that the cup  360  of the prosthetic hip can be screwed onto the distal end of the tip component  348 . The sliding engagement of the tip component  348  on the shaft  316 A is important because the impactor  300 A is intended to be used with hip prostheses of a variety of manufacturers. Often the cup  360  will have a hole pattern for securing the cup to the prepared acetabulum that is unique to the manufacturer and that is dictated by the anatomy. The flats enable many discrete alternate relative angular positions of the tip component  348  (and hence the cup  360 ) to the shaft  316 A. A plurality of flutes or elongate axial ridges  364  on the outer surface of the tip component  348  enable the user to securely grasp the tip component for mounting and dismounting the tip component on the shaft  316 A. 
       FIG.  12    shows the cup orientation placement navigation assembly of  FIGS.  11 A-C  adjacent to the anatomy. This figure also illustrates a free-hand navigation configuration  274 , in which at least the orientation devices  172 ,  204  are capable of six degrees of motion relative to each other. Any of the variations of  FIG.  11 A- 11 C  could be substituted in the illustration. In particular, the handle  304  is oriented as desired. In one embodiment, the system  100  displays in real time the angle of the cup relative to the navigated plane, which was acquired as discussed above. Angles that can be displayed include any one or more of anteversion and abduction for example. Preferably the clinician can confirm the position of the cup within a short fixed time, such as within about 20 seconds. In one embodiment, the angles displayed can be adjusted by about 40 degrees abduction and 20 degrees anteversion. These angles are not critical, but they relate to the range of motion of the leg. It is preferred to be close to these angles because motion in abduction and anteversion extends on either side of these angles. It is believed that the systems discussed herein can increase the percentage of patients in a “safe zone” close to these angles, typically described as within 10 degrees of these angles. In contrast, studies show that conventional techniques yield close to 50% of patients outside the “safe zone.” 
     Depending on the sensors and the timing of cup placement step of  FIG.  12   , the user may mount the sensing device  204  on the elongate member  224  again and may return the system  100  to the parked configuration  260  and also may initialize or zero the system  100 . 
     The system  100  can be configured to provide a pre- and/or post-operative estimation of an angle relative to the angle of the table. In the posterior approach, the patient is placed on his/her side. In this approach, there is more chance for the patient&#39;s position to shift intra-operatively. In one embodiment, an alignment rod can be coupled with the sensing device  204  and aligned with the plane of the table. The orientation of the sensing device  204  when so aligned is recorded in the system. Later in the procedure, one or more angles is calculated and displayed to the user based on the assumption that the pelvis has not moved. At such later stages, the orientation of the sensing device  204  can be confirmed again relative to the table to provide information about whether the patient has moved. If significant movement has occurred, such that any assumptions of no movement are violated, some or all of the landmark acquisition steps can be repeated. Alternatively, the movement of the pelvis can be tracked by the sensing device and corrected for. The manner of incorporating the table orientation with landmark acquisition is discussed in greater detail below. 
     The user will have placed the artificial ball of the replacement hip join in the proximal femur and thereafter can place the ball in the cup, which was properly oriented using the techniques discussed above. 
       FIG.  1    shows that thereafter, the user can optionally confirm orientation and/or leg length using the system  100 . The leg with the artificial hip joint assembled is placed in a neutral flexion and/or abduction and/or rotation position. The acquisition assembly  112  can be placed in the retracted configuration  266 . The distal end  228  of the elongate member  224  can be brought into contact with a landmark, which may be the same landmark acquired in  FIG.  6   . Once contact is made with this landmark (e.g., the bovie mark), the orientation of the sensing device  204  is determined by the system  100 . Also, the distance indicated on the scale  226  of the elongate member  224  is input into the system in any of the manners discussed above (e.g., manual or sensed). The system  100  can thereafter calculate components of vectors along the S-I axis (leg length) or M-L axis (offset). 
     Once leg length and offset are determined post-operatively, they can be compared the pre-operative measurements ( FIG.  6   ) to let the surgeon know if any adjustments should be made before completing the hip replacement surgery. 
       FIGS.  13  and  14    show other embodiments of a hip navigation system  400  that can include any of the features discussed above. In addition, the system  400  includes a free-hand sensor mount  404  that can be used to mount a freehand orientation device  204 A in one configuration. The freehand orientation device  204 A preferably includes inertial sensors, similar to those hereinbefore described. The device  204 A preferably also includes a camera  412 . The field of view is illustrated by the cone projecting downwardly from the base of the freehand orientation device  204 A.  FIG.  14    shows that the field of view includes a window  418  in a sliding support  420 . The window  418  enables the scale  226  to be viewed therethrough. 
     Because hip replacement procedures involve an open surgical field with a substantial amount of exposed tissue and blood the line of sight the camera  412  to the scales can become obstructed. In one embodiment, a hood is provided above the window  418 . The hood keeps most of the blood and tissue out of the space where the camera views the scales. Additionally, a scrubber component, e.g., a thin rubber member, can be provided above the scales  226 ,  226 A (discussed below) to prevent this tissue or fluids from entering into the field of view laterally. 
     One advantage of the system  400  is that the camera  412  can automatically process the image captured through the window  418  and thereby determine the position of the elongate member  224  relative to the sliding support  420 . A further advantage of this is to eliminate one step from the navigation process, e.g., to eliminate the need to enter the linear dimension into the system  400 . Eliminating the step can reduce time and/or personnel in the operating room. Also, the camera  412  can be configured to read a much higher resolution than can be read by a clinician. This can provide greater accuracy in the system overall. Not only that, but he camera can be configured to make fewer or no errors in reading the position, which can improve outcomes overall. For example, miniature cameras can produce data in JPEG or other image format that a processor in one or both the orientation devices  172 ,  204 A can process to extract the linear position of the elongate member  224 . 
     A further modified embodiment is described in  FIG.  15   , which shows an arcuate scale  226 A that can be positioned on a structure beneath the elongate member  224 , e.g., on a structure beneath the orientation device  204 A that is rotationally fixed relative to an axis extending out of the page.  FIG.  16    shows one configuration with this arrangement. A pivot  440  enables the sliding support  420  to rotate about an axis extending upward on the page. Although the pivot  440  is fixed about this upward extending axis, it can rotate about a pivot  444 . A window  418 A in the elongate member  224  enables the camera to see through the support to view the scale  226 A disposed on an arcuate or disk shaped feature of the pivot  440 . The scale  226 A can be read by the camera  412  or a second camera to provide accurate determination of the rotational position of the elongate member  224 . This can enable one of the sensors in the orientation device  204 A to be eliminated or inactivated. In another embodiment, camera date derived from the scale  226 A can be used to confirm the data from sensors in the orientation device  204 A. Preferably the scale  226 A has markings over a range of from about 15 to about 90 degrees, for example, between about 30 and about 60 degrees, e.g., at least between about 40 and about 50 degrees. 
     2. Posterior Approach Systems Adapted for Accelerometer Sensitivity 
       FIGS.  17 - 17 B  illustrate another embodiment of a system  450  for navigating a hip procedure from a posterior approach. The system  450  includes an anchor jig  454 , an alignment system  458 , and a landmark acquisition assembly  462 . The components may be similar in some respects to those discussed above, and such descriptions are incorporated with this embodiment where consistent. 
     The jig  454  includes a hollow fixation member  466  and a platform  468  for coupling a plurality of devices to the pelvis. The platform has a generally T-shaped configuration including a first portion  468 A coupled with the proximal end of the fixation member  466  and a second portion  468 B disposed transversely to the first portion  468 A. The first portion  468 A provides a support for a cradle  476  discussed further below. The second portion  468 B can include a plurality of docking devices  469  for coupling directly or indirectly with the orientation device  172 . The T-shaped configuration provides the advantage that the docking devices  469  can be disposed father away from the surgical site than is the case with the system  100 . This reduces any intrusion of the orientation device  172  into the working field. 
     In some cases, the fixation member  466  provides adequate stability in anchoring the system  450  to the pelvis. In other situations, the jig  454  can be coupled with the pelvis from the second portion  468 B. For example, a slot  470  can be formed in the second portion  468 B on one or both sides of location where the first portion  468 A extends from the second portion  468 B. The slots  470  can extend from a lateral edge of the second portion  468 B toward location where the first portion  468 A extends from the second portion  468 B. The slots  470  can include a plurality of channels  471  configured to receive fixation pins (e.g., Steinmann pins) that can be advanced into the pelvis. The channels  471  extend generally parallel to the fixation member  466 . The fixation pins can be securely connected to the second portion  468 B in the channels  471  by a clamp device  472 . The clamp device can include a screw configured to draw the portions of the second portion  468 B on either sides of the slot  470  toward each other and thus to create large frictional forces on the pins in the slots  471 . 
     The slots  470  preferably are aligned such that a plane extends along both of the slots  470  along their length. Because the slots  470  are long and slender this plane can be readily visualized in an X-ray image. It is preferred that the jig  454  be aligned to the pelvis such that the plane extending along the slots  470  is perpendicular to an axis of the patient (e.g., the intersection of the medial lateral plane and the transverse mid-plane of the patient). This feature provides a convenient way to visually confirm proper positioning of the jig  454  in one embodiment. 
     The fixation member  466  includes a registration feature  473  and a foot  474  adjacent to a distal end thereof and a coupling  475  adjacent to the proximal end thereof for connecting to the platform  468 . The foot  474  includes a plurality of spaced apart spikes extending from a distal end thereof capable of preventing or limiting rotation of the jig  454  when the fixation member  466  is connected to the pelvis.  FIG.  17    shows that securing the jig  462  to the pelvis can include positioning a pin or other bone engaging device through the fixation member  466 . The pin and spikes extending from the foot  474  can provide three or more points of contact with the pelvis providing secure mounting of the jig  462 . 
     The coupling  475  generally secures the platform  468  to the fixation member  466 . In some embodiment, the coupling  475  has a rotational capability that enables the platform to be positioned at selective locations about the longitudinal axis of the pin  466 , for example to enable the platform  468  to be initially positioned in the correct orientation or to be moved during or after the procedure to make space for other surgical devices. One arrangement provides matching splines that extend parallel to the longitudinal axis of the fixation member  466 . This arrangement would permit splines on an upper portion of the coupling  475  to be disengaged from splines on a lower portion of the coupling  475 . When disengaged, the platform  468  and the upper portion of the coupling  475  can be rotated relative to the lower portion of the coupling  475 . The splines can thereafter be re-engaged. 
     The jig  454  also preferably includes a cradle  476  that can be used to hold a probe arm  477 . The cradle  476  includes a U-shaped recess having a width between two upright members that is about equal to the width of an arm  477  of the landmark acquisition system  462 .  FIG.  17    shows the probe arm  477  in a parked configuration as discussed above. If the sensor  204  operates with components that are prone to accumulated error sources, the parked configuration can be used to eliminate such error. As discussed above, the system  450  can be configured such that the position and/or orientation of the sensor  204  relative to the orientation device  172  is known. Thus, when the arm  477  is in the cradle  476  any accumulated error of components of the sensor  204  can be eliminated. 
     The cradle  476  can provide other convenient functions even if the sensing devices in the sensor  204  are not subject to sources of accumulated error. As discussed elsewhere herein, for confirmation of accuracy of the system or to provide a simplified reference frame not requiring landmark acquisition, it may be desirable at some point of the procedure to use the probe arm  477  and the sensor  204  to estimate the plane of the surgical table upon which the patient is resting. If, as discussed above, the plane intersecting the slots  470  is oriented perpendicular to the axis of the patient when the jig  454  is mounted to the pelvis, the cradle will be parallel to the axis of the patient. If the fixation member  466  is oriented vertically, the arm  477  will be parallel to the plane of the table when in the cradle  476 . The system  450  can thus use the plane of the table as a reference frame for guiding the placement of the cup without registering landmarks. Or, the plane of the table can be used in combination with registering the anatomy about the acetabular rim, as discussed above, to increase the accuracy of navigating the cup. 
     The cradle  476  also provides a convenient home position that keeps the arm  477  stationary and out of the way of other surgical instruments.  FIG.  17 A  illustrates the probe arm  477  withdrawn from the cradle  476  and free to move into contact with landmarks. 
     The jig  454  also includes a pivot feature  478  that is disposed horizontally.  FIG.  17 B  shows that the pivot feature  478  includes two horizontal apertures  480 . One of the apertures  480  is formed in the same structure forming the cradle  476  but at an elevation below the cradle  476 . The other aperture  480  formed between the cradle  476  and a projection of the fixation member  466 .  FIG.  17 A  shows that the probe arm  477  is connected to the pivot feature  478  by a shaft  482  that extends through the apertures. A movement device is provided between the shaft  482  and the arm  477  to enable a distal tip of the arm to be rotated about perpendicular axes and to be advanced linearly relative to the stationary jig  454 . One axis of rotation A of the movement device is disposed parallel to and at an elevation above the platform  468 . Another axis of rotation B is disposed generally perpendicular to the axis A. Sliding of the arm  477  is enabled by a snug but sliding fit of the arm in a housing C. By orienting the axis A in this manner, the sensitivity of accelerometers in the sensor  204  to small angular motions references points about the acetabulum is heighted or maximized. This can enable landmark acquisition with the system  450  based solely on accelerometers, which advantageously are not subject to accumulated error, which can simplify the landmark acquisition process. Further variations of systems that are configured to allow landmark acquisition based solely on accelerometers are discussed below in connection with Figures*. 
     The registration feature  473  is a convenient way to enhance the accuracy of the sensor  204 . In particular, in one variation of the method discussed above, a distal tip of the probe arm  477  is brought into contact with the registration feature  473 . In one embodiment, the registration feature  473  is a notch configured to receive and temporarily retain the tip. Thereafter, the user can interact with the orientation device  172  to initialize accelerometers within the sensor  204 . Thereafter the points to be acquired can be sequentially contacted and the orientation and position of the sensor  204  can be sequentially recorded in the system  450 . Because the accelerometers are initialized close to the points to be acquired, accuracy of the reading is enhanced as the angular error resulting from an error in the scale factor of the accelerometers is minimized due to the small arc from the registration feature. For example, the jig  454  is configured to enable the landmark acquisition assembly  458  to reach all points to be registered by moving less than about 45 degrees from an initial or home position in some embodiments. In other embodiments, the jig  454  is configured to enable the landmark acquisition assembly  458  to reach all points to be registered by moving less than about 25 degrees from the initial position. In other embodiments, the jig  454  is configured to enable the landmark acquisition assembly  458  to reach all points to be registered by moving less than about 15 degrees from the initial position. 
     The jig  454  also is configured to interact well with the soft tissue that is disposed around the surgical site in the posterior approach. In this approach, an incision is made in soft tissue that is kept as small as possible. In one approach, the fixation member  466  is positioned at the end of the incision. Where the incision is made as minimal as possible, the jig  454  can also function as a retractor. The T-shaped configuration is particularly well suited for this function because the first portion  468 A of the platform  468  can be received between the middle and ring fingers of the user with the second portion  468 B in the palm of the hand. With the foot  474  gripping the pelvis, the jig  454  can be tilted from the platform  468  away from the hip joint to retract the tissue away. 
       FIGS.  17 C- 1  and  17 C- 2    illustrate further embodiments of a posterior approach jig  454 A having a mounting device  488  disposed adjacent to the distal end of a fixation member  466 A, which is otherwise similar to the fixation member  466 . The fixation member  466 A includes a tubular body  490  coupled with the fixation member  466 A, which in this embodiment acts as a primary fixation member. The tubular body  490  extends along a lumen that is angled relative to the lumen of the fixation member  466 A. The lumen in the tubular body  490  is configured to accept a fixation pin  492  that can be driven into the bone, as illustrated in  FIG.  17 C- 2    at an oblique angle. The fixation pin  492  supplements the fixation provided by the fixation member  466 A. The fixation pin  492  can be used in conjunction with the optional long pin(s) extending through the channels  471 , e.g., into the ilium or as a substitute for that option. The fixation pin  492  has the advantage of not requiring any additional holes in the skin because it is located within the primary incision made to access the joint in the procedure. The fixation pin  492  can be threaded to engage the bone in one embodiment. In some embodiments, a locking device  494  can be provided to secure the pin  492  in the lumen of the tubular body  490 . A set screw is one example of a locking device  494  that can be used. The locking device  494  enables the fixation pin  492  to be headless, which avoids issues with screw threads stripping the hole in the bone into which the pin  492  is inserted. 
     3. Workflow Considerations for Posterior Approach 
     As noted above, a workflow problem arises in typical hip replacement procedures in that anatomical features that can be more easily references are unavailable in the traditional posterior approach for operating on the joint. 
     By performing a CT-based study of a large number of human pelvises, the assignee of this application has been able to calculate a population-based average relationship between multiple planes created by various points in, on or around the acetabulum that are accessible during posterior approach hip replacement (each plane, an “Acetabular Plane”), and the Anterior Pelvic Plane. One of the key features of posterior hip navigation for some embodiments disclosed herein is the ability of a module, e.g., software incorporated into a processor, which may be on a computer, or one or both of the orientation device  172  and sensor  204 , to calculate a transformation from one reference frame to another. As described in more detail elsewhere herein, several points are referenced in, on or around the acetabulum and from these points a proxy Acetabular Plane is calculated. 
     Next, in certain embodiments described herein a module operable to process an algorithm, e.g., by executing software in one or both of the orientation device  172  and sensor  204  alone or with a separate computer, is able to calculate a transformation from the proxy Acetabular Plane to Anterior Pelvic Plane. The approach indirectly registers the Anterior Pelvic Plane without requiring a direct supine registration and subsequent patient movement and re-draping necessary in standard navigation. A module in certain embodiments described herein is then able to provide the user real time navigation data of the orientation of a hip instrument (e.g., the impactors  300 ,  300 A) with respect to the Anterior Pelvic Plane. 
     In certain systems described herein, a further advantage is that the systems are able to implement the plane transformation algorithm to calculate an Anterior Pelvic Plane from one of any number of proxy Acetabular Planes that the surgeon chooses to register. This enables the surgeon to have greater flexibility in Acetabular Plane landmark selection to take into account the quality or accessibility of certain landmarks. For example, in cases of minimal deformity around the acetabular rim, the surgeon may choose to register landmarks around the rim, which are easily accessible. In cases where there is great deformity or high presence of osteophytes on the acetabular rim, the surgeon may instead choose to register an Acetabular Plane based on extra-acetabular landmarks (or described as “off-rim” elsewhere herein) outside of the rim that are unaffected by disease or prior hip replacement surgery. 
     Examples of anatomical landmarks that may be used to create a proxy Acetabular Plane and that are shown in  FIG.  2    include but are not limited to: 
     Extra-acetabular landmarks (Ischium/Ilium/Pubis)
         (A) The lowest point of the acetabular sulcus of the ischium   (B) The prominence of the superior pubic ramus   (G) The confluence of the anterior inferior iliac spine (AIIS) and the outer border of the acetabular rim       

     Acetabular Rim Landmarks
         (E) The center of the anterior insertion of the trans-acetabular ligament   (F) The center of the posterior insertion of the trans-acetabular ligament   (H) The most superior point of the acetabular rim.       

     Additional points can be combined with either of the groups of points listed above. For example, in one embodiment, point “D” is used. Point D is the midpoint of the inferior border of the acetabular notch. As discussed in connection with  FIG.  36    below, point D corresponds to the bottom landmark  380 B used to form the line  382 . Point D is used in that approach to provide patient specific refinements to the positioning. 
     A further key benefit of certain embodiment discussed herein is that the foregoing plane transformation capabilities increase the accuracy of the transformation between the proxy Acetabular Plane registered and the Anterior Pelvic Plane above the general population average data by the user inputting certain patient-specific information, such as gender. 
     Additionally, certain embodiments of systems including one or more of the orientation device  172 , sensor  204 , or a separate computer may have modules that are operable, e.g., by processing software, to allow the user to input an angular or plane relationship between an proxy Acetabular Plane and Anterior Pelvic Plane that the surgeon measured based on pre-operative imaging, allowing for a partial or whole plane transformation based on patient-specific data rather than population data. By way of example, the surgeon may choose to pre-operatively measure an angle created by (a) landmarks that are both visible on an A/P pelvis x-ray and that can be referenced during posterior hip replacement, and (b) landmarks that are both visible on the pelvis x-ray and that are directly associated with inclination measurement in the Anterior Pelvic Plane. If this angular relationship is inputted into a module of a system including one or more of the orientation device  172 , the sensor  204 , or a separate computer, which module is capable of making calculations processing software and the surgeon registers the landmarks described in (a), inclination navigation will be based specifically on that patient rather than a population average. Landmarks (D) and (H) listed above are examples of landmarks that are both visible on an A/P pelvis x-ray and that can be referenced to create a proxy Acetabular Plane in posterior hip replacement. 
     These aspects of the systems adapted for posterior approach hip joint replacement can greatly enhance both workflow and accuracy in such procedures. 
     B. Navigation Using Inertial Sensors and Jigs for Referencing Anatomical Landmarks with Anterior Approach 
     1. Apparatuses for Anterior Approach Hip Navigation 
       FIGS.  18 - 21    illustrate a hip navigation system  500  adapted to navigate a hip joint procedure from an anterior approach. Anterior approach to hip replacement advantageously can be less invasive than posterior approach. In particular, the anterior approach can enable smaller incisions, less soft tissue dissection, and shorten recovery time for patients. The system  500  includes an anchor system  504 , an alignment assembly  508  and a landmark acquisition assembly  512 . 
       FIG.  18    shows the anchor system  504  in more detail. The system  504  is configured to securely couple the navigation system  500  to the hip, such that movement between the system and the hip are minimized or eliminated. The anchor system  504  includes a cannula  516  having a distal end  520  and a proximal end  524  with a lumen  532  extending between the distal and proximal ends. The proximal end  524  of the cannula  516  is coupled with a platform  536 , for example adjacent to one lateral end of the platform. The platform  536  is similar to those hereinbefore described having a plurality of docking device  538 ,  538 A disposed away from the location where the proximal end  524  and the platform  536  are connected. 
     The docking devices  538  are configured to couple with detachable mounting devices that securely but temporarily couple sensor to the anchor system  504 . The two docking device  538  on the top surface of the platform  536  enable the anchor system  504  to be used for either left or right hip procedures. As shown in  FIG.  18   , the docking device  538  on the side of the platform  536  closest to the medial plane of the patient is preferably used for docking. The top side docking feature not in use in  FIG.  18    would in fact be used in performing a procedure from the other side of the patient. The docking device  538 A on the side surface of the platform  536  is provided for a temporary intra-procedure mounting of a sensor to the platform  536 . As discussed further below, this temporary mounting provides a known orientation and/or location of two sensors relative to each other during a procedure, which enables the system  500  to control sources of error with certain types of sensors. 
     The platform  536  also can have a channel  540  disposed away from the cannula  516 . The channel  540  can have a lumen disposed along an axis substantially parallel to the lumen  532  of the cannula  516 . In one embodiment, the anchor system  504  is configured to securely couple the platform  536  to the hip by placement of two spaced apart pins  544 A,  544 B.  FIG.  18    shows that the pin  544 A can be advanced through the cannula  516  such that a distal end of the pin  544 A contacts and penetrates a bony prominence of the pelvis. In one technique the pin  544 A is positioned at or as close as possible to the anterior superior iliac spine (ASIS) of the pelvis. The pin  544 B is advanced through the channel  540  and into the pelvis at a location offset form the ASIS. The distance between the pins  544 A,  544 B and the precise positioning of the pin  544 B are not critical, but are determined by the locations of the connection of the cannula  520  to the platform  536  and of the channel  540 . 
     The pins  544 A,  544 B can take any suitable form but preferably have the same cross-sectional profile as the lumens in the cannula  520  and in the channel  540 , e.g., they can be circular in cross-section. The pins  544 A,  544 B can be modified Stienmann pins, e.g., configured to extend at least about 5 cm above the platform  536  and having a diameter of about 4 mm. 
     The anchor system  504  also has a locking device  556  for securing the platform  536  to the pins  544 A,  544 B. In one embodiment, the portion of the platform disposed around the pins comprises medial and lateral portions  560 M,  560 L that can move away from each other to release the pins  544 A,  544 B or toward each other to frictionally engage the pins. For example, a pair of hex-driven screws can engage the medial and lateral portion  560 M,  560 L to translate them toward and away from each other respectively. The locking device  556  preferably is quickly and easily removed from the pins such that other instrument, such as X-Ray or other diagnostic devices can be brought into the vicinity of the surgical field during the procedure. Preferably the pins  544 A,  544 B have markings along their length such that if the platform  536  is removed for imaging or other reasons it can be quickly re-positioned at the same elevation. 
     The cannula  520  also has a foot  568  adjacent to or at the distal end  528  to minimize or eliminate error that could arise due to uneven penetration depth of the anchor system  504  when compared to the position of a distal probe of the landmark acquisition system  512  when landmarks are being acquired. The foot  568  can include an annular projection disposed outward of the cannula  520 . Preferably the foot  568  extends laterally from the outer surface of the cannula  520  by a distance equal to or greater than the wall thickness of the cannula  520 . In some embodiment, the surface area beneath the foot is equal to or grater than the surface area of the cannula when viewed in cross-section at a location where the foot  568  is not located, e.g., at an elevation about the foot  568 . 
     The alignment assembly  508  is similar to those hereinbefore described. It can have a rigid extension  570  configured to detachably secure a orientation device  172  to the docking device  538 . 
     The landmark acquisition assembly  512  is similar to those hereinbefore described, but is configured to be unobstructed in use by soft tissue anterior to the pelvis of the patient. In one embodiment, an extension  578  is provided to elevate a pivoting and sliding mechanism  582 . The pivoting and sliding mechanism enables a probe arm  584  to slide away from the extension  578  toward the location of landmarks to be acquired. The pivoting and sliding mechanism  582  can be similar to any of those discussed above. The distal (lower) end of the extension  578  can be coupled to the platform  536  in any suitable way. For example, the distal end can include a pin-like projection that is received in, e.g., friction fit in, an aperture  578 A having the same shape. Detents or other locking features can be provided to securely connect the extension to the platform  536  in the aperture  578 A.  FIG.  19    shows that the aperture  578 A can be formed in a portion of the platform  536  that is elevated compared to the portions of the platform through which the pin  544 A extends. This portion is elevated to provide sufficient bearing engagement to minimize play. It also has a slot generally parallel to the top surface of the platform  536  which serve the function of engaging a ball detent on the lower end of the extension  578 . 
     The probe arm  584  can be configured as an elongate member with a plurality of markings, discussed below. A distal end of the probe arm  584  can include an angled tip  586  that assists in probing anatomy in some techniques, e.g., portions of the femur for leg length and femoral head positioning confirmation. In the posterior approach, the angled tip  586  is used to directly contact anatomy. 
     In the anterior approach, the angled tip  586  is coupled with a probe extension  590  configured to contact selected anatomy. The probe extension  590  has an upright member  592  that is configured to extend, in the anterior approach, between the elevation of the probe  584  down toward the elevation of the tissue to be probed. A foot  594  on the distal (lower) end of the upright member  592  is configured to engage the tissue in a way that minimizes error due to uneven tissue compression between the point of mounting of the pin  544 A and the foot  594 . For example, the foot  594  can have a cross configuration that spreads out the force or pressure applied by the landmark acquisition system  512  in use. The proximal end of the extension  590  includes a coupler  596  that connects a distal end of the probe arm  584  with the upright member  592 . Preferably the coupler  596  is easily manipulable by the user to modify connect to the probe arm  584 . The coupler can include an L-shaped member with an aperture configured to receive the tip  586  of the probe arm  584 . A set screw can be advanced through the L-shaped portion to lock the arm  584  in place. The L-shaped portion is configured to couple to the arm  584  such that the tip of the angled tip  586  rests on a projection of the longitudinal axis of the upright member  592 . 
     2. Example Methods for Navigating Using the Anterior Approach 
     The system  500  can be used to navigate from an anterior approach in the following ways. The orientation device  172  and the sensor  204  can be paired such that they are in wireless communication with each other. This permits one or other of the device  172  and sensor  204  to control the other, store data from the other, and/or display information based on signals from the other. In one method, the orientation device  172  has a display that confirms to the surgeons certain angles based on the data sensed by the sensor  204 . The pairing the device and sensor  172 ,  204  can involve coupling them together and comparing sensor output between the two devices at a plurality of orientations, e.g., horizontal, vertical, and angled at 30 degrees. Some of these positions may be repeated with a plurality of attitudes, e.g., vertical with left side up, vertical with right side up, and vertical with top side up. 
     As noted above, the components discussed herein can be provided as a kit that enables the surgeon to select among different surgical approaches, e.g., posterior and anterior approaches. The orientation device  172  and sensor  204  may operate differently in these different approaches. Thus, in one method the user will enter into one or both of the orientation device and sensor  172 ,  204  which approach is being used. This will implement a software module in the orientation device  172  (or in the sensor  204  is the processor running the software is located there) corresponding to the selected approach. 
     In various embodiments suitable for the anterior approach, the orientation device  172  and the sensor  204  can both have a plurality of sourceless sensors. These components can have both accelerometers and gyroscopes in some embodiments. Some gyroscopes are subject to accumulated error that can be significant in the time frames relevant to these methods. Accordingly, various methods are provided to prevent such errors from affecting the accuracy and reliability of the angles displayed to the surgeon by the system  500 . Some approaches can be performed with accelerometers only. For example, variations of the anterior approach can be performed with accelerometers with somewhat less but still acceptable accuracy using accelerometers only. The reduction in accuracy of the accelerometers is balanced against the benefit of eliminating the accumulated error that arises with some gyroscopes. The resolution of accelerometers is sufficient because the points navigated are relatively far apart. 
     The calculations performed by the system  500  are unique to the hip being treated in some embodiment, so the system receives input of the hip being treated. 
     The foot  568  is placed on a selected anatomical location, e.g., on the ASIS as discussed above. With the cannula  520  in an approximately vertical orientation the platform  536  is secured to the hip. Securing the platform  536  to the hip can be done in any suitable way, such as with two spaced apart Stienmann pins. Thereafter, the orientation device  172  and the sensor  204  are attached to the platform  536  in the manner shown in  FIG.  20 A . Depending on the nature of the sensing devices deployed in the sensor  204  it may be advantageous to initialize the sensor at this point of the procedure. As discussed above, certain inertial sensors (e.g., some gyroscopes) are subject to accumulated error. One technique for managing this error source is to periodically initialize or zero out this error. Some techniques involve initialing at this point. 
     In some embodiment, a frame of reference based on the plane of the table can be input into the system  500 . The table reference frame can be a secondary reference frame. In one technique, the sensor  204  is moved from the platform dock position of  FIG.  20 A  to the navigating position on the probe arm  584  as shown in  FIG.  18   . The probe arm  584  is then pivoted by the mechanism  582  such that the arm points in a direction that is parallel to the patient&#39;s medial-lateral mid-plane and the angled tip  586  superiorly (generally toward the patient&#39;s head). The probe arm  584  is also held substantially parallel to the plane of the table. With this heading and orientation the user interacts with a user interface on the orientation device  172  to signal to the orientation system  508  to capture the orientation of the sensor  204 . This orientation provides an estimation of the orientation of the anterior pelvic plane. This estimation may be tracked in the system  500  and may alone provide an improvement over the state of the art in un-navigated hip replacement, which involves discrete maneuvers guided by the unaided eye. 
     At the surgeon&#39;s discretion the system  500  can be used to navigate a condition of the femur prior to hip replacement. A mark Fm may be made on the proximal femur. Thereafter the sensor  204  can be initialized or zeroed such as by placing it back in the dock position on the platform (as in  FIG.  20 A ). Thereafter, the probe tip  586  can be brought into contact with the femur mark Fm and locked in place in such contact. See  FIG.  21 A . The sensor  204  can be transferred to the proximal end of the probe  584  and the orientation device  172  can be signaled to record the orientation of the sensor  204 . A distance from the point of attachment of the cannula  520  to the ASIS to the marked position on the femur can then be recorded in the orientation device  172 . The position can be based on reading graduated marks on the probe  584  or can be captured automatically by a camera system or a sensor built into the system  500 . In one embodiment, graduated marks are read at an upright edge  598  within a bight of a sliding portion of the pivoting and sliding mechanism  582 . 
       FIG.  20 A  illustrates a further step of navigating the anterior pelvic plane. As shown, the sensor  204  is docked on the platform  536 , in which position any accumulated error associated with some sensors can be eliminated. In a preceding step, the extension  590  is coupled with the distal portion of the probe  584 . The foot  594  is brought into contact with the contralateral ASIS. Thereafter, the sensor  204  can be attached to the proximal end of the probe  284  as shown in  FIG.  18   . The landmark acquisition system  512  can be immobilized and the orientation of the sensor  204  can be recorded in memory in the orientation device  172 . Additionally, the distance that the probe  584  is extended to contact the contralateral ASIS can be recorded in the orientation device  172 . As noted above, that distance can be read from the scale on the probe  584  at the upright edge  598 . 
     The process to record the contralateral ASIS can be repeated for one or more additional points. The sensor  204  can be docked to the platform as in  FIG.  20 A  to eliminate sources of accumulated error. The probe  584  can then be moved to cause the foot  594  to be in contact with a pubic tubercle. The probe  584  can be immobilized and the sensor coupled with the proximal end as shown in  FIG.  20 B . Thereafter data indicative of the orientation of the sensor  204  and the distance to the pubic tubercle are recorded in the orientation device  172  in any of the manners discussed above. 
     Once the foregoing points of the pelvis have been navigated and the data recorded into the orientation device  172  the anterior pelvic plane can be calculated from data indicating the navigated points. The orientation of the anterior pelvic plane is a baseline for placement of the cup portion of a hip prosthesis. 
     The sensor  204  and the orientation device  172  can at this point be used to guide placement of the cup  360  in the prescribed orientation. Prior to placement the impactor  300 ,  300 A is provided. For example, the impactor  300 A can be provided by selecting the appropriate tip component  348  onto the distal end of the shaft  316 A. The tip component  348  is coupled with the cup  360 , e.g., by threads. The rotational orientation of the cup  360  to the shaft  316 A that is most convenient given hole patterns and position of the sensor  204  is selected by matching up the flats  350 A,  350 B as appropriate. During the process of providing the impactor  300  the sensor  204  can be docked to the platform  536  and source of accumulated error can be eliminated just prior to navigating the cup  360  into place in the acetabulum. 
     In one technique, the cup  360  is inserted into the acetabulum and placed to approximately the correct orientation. Thereafter the sensor  204  is connected to a docking device  338  on the impactor as shown in  FIG.  11 A . The orientation device  172  is the activated to display angles indicative of the orientation of the cup, e.g., degrees of inclination and anteversion with respect to the anterior pelvic plane. The angle displayed can directly reflect the table reference frame discussed above. The angle displayed can directly reflect the frame of reference from the acquisition of landmarks. In some cases, angles can be displayed that directly reflect both table reference frame and landmark reference frame. In other embodiments, the table reference frame is not displayed but rather causes a user instruction to be displayed on the orientation device  172 , such as a direction to re-acquire landmarks due to disagreement between the angles generated by the two reference frames. 
     Any of the foregoing combinations of table and landmark reference frames provides redundancy that ensures that the angle information provided to the user is accurate and reliable such that the procedures performed will be better contained within the “safe zone”. 
     When the correct angles are achieved, a tool is used to strike the proximal end of the impactor  300  to lodge the cup  360  in place at the desired angle. In some techniques, the sensor  204  is removed prior to striking the proximal end of the impactor  300 . The system  500  includes a module that monitors signals from the sensor  204  and if a large deviation in the readings occurs, the module prevents the angles on the display of the orientation device from changing. This “freezing” of the display is both a safety and an accuracy precaution because a large force due to impact can affect the accuracy of the sensor  204 . 
     If femoral landmarks are acquired in the procedure prior to separating the natural joint, the same landmarks can be acquired after the prosthetic joint is placed to confirm that the replacement of the joint has not changed either the length of the leg, the off-set of the leg from the trunk of the patient or both. For example, the sensor  204  can be docked to the docking device  538 A as shown in  FIG.  20 A . Sources of accumulated error can be eliminated by initializing the sensor  204 . Thereafter, the probe arm  538  can be brought into contact with the same landmark (e.g., Fm) acquired early in the procedure. See  FIG.  21 B . The probe arm  538  can be locked into place and thereafter the sensor  204  can be coupled with the proximal end of the probe arm  538 . The orientation of the sensor and the distance to the probe arm  538  can be input into the orientation device  172 . These data enable the orientation device  172  to output amounts of change in leg length and leg offset. 
     In one variation a plurality of points, e.g., three points, on the femur are acquired before and after the joint is replaced. This approach enables a further confirmation that the rotation orientation of the neck of the femur relative to an axis extending through the center of the cup  360  perpendicular to the plane of the acetabulum is unchanged after the procedure. 
     Of course, the femur registration procedures enable correction of diagnosed deformities including excessive leg length offset and joint offset, as well as mal-orientation of the femoral neck in the natural joint. In other words, the surgeon can begin the procedure with the intent of adding some offset or changing rotational orientation to improve the patient&#39;s bone positions and/or orientations post-operatively. 
     C. Navigation Using Pre-Operative Imaging or Characterization 
     Although the foregoing approaches can improve the standard of care currently in place, further increases in accuracy and even better outcomes and streamlining of the procedure can be provided if the system is configured to account for patient specific anatomical variability. 
     1. Navigation Using Inertial Sensors and a Custom Jig 
       FIG.  22    shows the placement of a hip movement tracking sensor  204  on a pin  732  adjacent to the acetabulum. This position is not limiting, in that the hip movement tracking sensor  204  can be mounted anywhere on the pelvis, but adjacent to the acetabulum is convenient. The pin  732  has been placed with the aid of a pre-operative characterization of the hip of the specific patient. In these methods the pin  732  is placed without the need for intra-operative landmark acquisition. 
     In one approach, a pre-operative three-dimensional characterization of the acetabulum is performed using any suitable technology, such as CT scan or MRI. This pre-operative procedure can be performed to fully characterize the pelvis and, in some cases, the proximal femur. Thereafter, the shape, location and orientation of the acetabulum are known. Also, the bony features around the acetabulum are known. From this data, a custom jig  700  can be fabricated specific to the patient. The custom jig  700  not only has features that are specific to the individual patient&#39;s anatomy but also a registration feature  702  that will be at a known orientation to the plane of the acetabulum and to the anterior pelvic plane. 
       FIG.  23    shows an example of the custom jig  700 . The jig  700  has an anterior side  704  and a posterior side  708 . The posterior side  708  is formed with an acetabular portion  712  configured to mate with at least one feature of the acetabulum in a secure manner. For example, the acetabular portion  712  can fit snugly over the acetabular rim with a central portion of the posterior side  708  positioned in the acetabulum. The jig  700  preferably has only one pre-defined orientation. A surface on a posterior portion of the jig can define a plane that corresponds to a preferred orientation angle of the cup post-implantation. One or more channels  716  can be formed on the posterior side  708  that receive the local bony prominences of the acetabular rim only when the jig  700  is in the proper position and orientation. In another approach, the registration feature  702  of the jig  700  has a face or a hole that is oriented in the desired orientation for the shell or cup of the implant. Thus, once the jig  700  is placed, the sensing device  204  can be positioned against the face or surface or, if coupled with a pin  732 , the pin can be inserted into the hole. From the orientation of the device when so placed, the orientation of the acetabular rim or a proxy thereof can be recorded in one or both of the devices  172 ,  204 . The hole  702  preferably extends from the anterior side  704  to the posterior side  708  of the jig  700 . The distance between the anterior and posterior surfaces  704 ,  708  provides the depth of the hole  702  being sufficient to guide a pin to specific anatomy along a specific direction. 
       FIG.  24    shows initial placement of the jig  700  in the acetabulum in an orientation dictated by the fit of the jig  700  over the anatomy. The profile of the posterior side  708  including the channel(s)  716  receives the specific patient&#39;s acetabular rim including local prominences and recesses of the bone at and around the acetabulum. The hole  702  is located on a peripheral projection  720  of the jig  700 . The configuration of the projection  720  is such that the hole  702  is disposed over a specific bone or bone region of the hip. In this example, the projection  720  is configured to be disposed over the bone superior to the acetabulum. Other regions of bone around the acetabulum can be used if sufficiently thick or strong and in a convenient position to not block actions of the surgeon during the procedure. The precise location of the projection  720  chosen can be determined by the pre-operative imaging and factored into the forming the custom jig  700 . 
       FIG.  25    shows that after the jig is placed the pin  732  can be placed through the hole  702 . The pin  732  has a length that extends above the anterior surface  704  of the jig  700  such that the sensor  204  can be mounted thereto. Once the sensor  204  is mounted to the pin, the sensor can track any movement of the pelvis during the procedure. There is no need for registration of landmarks in this technique because the position and orientation of the pin relative to the acetabulum and/or to the anterior pelvic plane are known from the pre-operative imaging. 
       FIG.  26    shows that the plug  700  advantageously can include an alignment guide  736  to control rotational orientation of the sensor  204  on the pin  732 . The alignment guide  736  can be a line extending along a specific direction relative to the registration feature  702 . As noted above, the sensing devices inside the sensor  204  can be sensitive to the direction of gravity. Tilting of the sensor about the pin  732  can change the readings of these sensing devices. To eliminate sources of error associated with this sensitivity, the navigation system incorporating the sensor  204  can be programmed to assume that the sensor will be at a specific rotation position about the longitudinal axis of the pin  732 . The sensor  204  may be mechanically or visually aligned with the guiding mark  738  to assure that this assumption is met in use. In one variation, the sensor  204  has a laser that projects onto the jig  700  and can be aliened with the mark  736  to facilitate alignment. Alternatively, the pin  732  may be configured to only enter the hole in a unique orientation (for example, with an asymmetric non-circular cross-section), and to allow the sensor to mount to the pin in a unique orientation (by including asymmetric coupling features). 
     Once the sensor  204  is mounted to the pin  732 , the jig  700  can be removed from the surgical area. For example, the jig  700  can be made of material can be cut along a line  742  in a lateral edge of the jig. A saw or rongeur can be used to cut through the jig  700 . Thereafter, the majority of the body of the jig  700  can be removed from the surgical area.  FIG.  28    shows that in some methods, the projection  720  is left in place so that the position and orientation of the sensor  204  is not disrupted. 
     A second sensor  204  is attached to a cup impactor, which may be the same as in  FIGS.  11 A- 11 C . The impactor guides the placement of the cup with reference to the signals from the sensor  204  mounted on the pin  732  on the pelvis. Signals from the sensor on the impactor can be corrected if movement of the hip is detected by the sensor on the pin  732 . 
     2. Navigation Using Inertial Sensors and a Cannulated Guide 
       FIGS.  29 - 31    illustrate one way of implementing cannulated guide delivery methods. Cannulated methods are advantageous in that once a guide member is mounted, the tracking of orientation is simplified and may no longer be necessary in some cases, which can eliminate accumulated errors, sensor drift, or erroneous readings of other sorts as a concern. 
     A custom jig  750  is formed by the process discussed above in connection with the jig  700 . The jig  750  has many of the same components as those of the jig  700 , including a registration feature  752  extending between the anterior and posterior surfaces  754 ,  758 . A guiding mark  738  can be provided on the anterior surface  754  to align the sensor  204  rotationally about the pin  732 . The jig  750  also has a guide channel  762  located generally centrally in the jig  750 . The guide channel  762  has an anterior opening on the anterior surface  754 , a posterior opening on the posterior surface  758 , and a wall extending between these openings. The wall is disposed about a central axis A. The position and orientation of the axis A can be determined based on the pre-operative characterization of the acetabulum. In one embodiment, an MRI or CT scan reveals an optimal axis for delivering a prosthetic cup along. The wall forming the guide channel  762  is formed about the axis A which coincides with this optimal axis when the jig  750  is placed on the specific patient&#39;s acetabulum. 
       FIG.  30    shows that the impactor  300 A can then be advanced along the axis A into the guide channel  762 . A distally facing shoulder  766  on the impactor  300 A can mate in a pre-defined way with the anterior surface  754  and the entrance to the channel  762  and when so mated the orientation of the sensor  204  on the impactor  300 A can be recorded. In this technique, the jig  750  is a cannula with the channel  762  configured to receive the impactor  300 A. If patient movement is possible, the sensor  204  on the pin  732  can be retained in place to track such movement. If not, the sensor  204  on the pin  732  can be removed. The sensor  204  on the impactor  300 A will have stored the orientation of the axis A in memory and will be able to inform the user of any variance of the impactor from this axis. It is preferred to retain the sensor  204  on the pin  732 , as the orientation can only be accurate stored by the sensor  204  on the impactor  300 A for a short time due to accumulated error (e.g., drift) of some sensors, e.g., some lower cost gyroscopes. 
     In one variation, the impactor  300 A has a central channel that coincides with the axis A when the impactor is placed into the guide channel  762  and the shoulder  766  abutted with the surface  754 . A guide pin can be advanced through this channel and into the acetabulum. The guide pin can be lodged in the base of the acetabulum. The sensor  204  coupled with the pelvis by the pin  732  can be removed because the guide pin placed through the channel of the impactor  300 A provides a mechanical way of tracking movement of the hip. Thereafter the impactor  300 A with the cup mounted thereon can be slide over the guide pin and into place in the acetabulum. 
     In a further variation, the sensor  204  coupled with the impactor  300 A can also be removed. In this further variation, the guide pin is configured along with the cup to prevent tilting of the prosthetic cup relative to the axis A. In particular, an interface between the guide member and the cup of the hip prosthesis could be made to have sufficient length along the axis A that tilting is prevented by this interface. In some cases, the cup  360  is coupled to the impactor  300 ,  300 A. A variation of the impactor  300 ,  300 A can be tubular or have another feature for interfacing with, e.g., tracking along the guide pin in the pelvis. 
     3. Navigation Using Inertial Sensors and Pre-Operative Imaging 
     In another technique using less comprehensive imaging, a correspondence between one or more linear dimensions and an angle can be exploited to enhance accuracy. For example, a clinician can use an X-ray or other standard radiographic imaging device to provide an anterior pelvic bone image. This image can be read to derive the location of the anterior pelvic plane and a dimension on the anatomy. For example, an angle between top and bottom landmarks around the acetabulum (as further describe below) and a trans-ischial line or other anatomic medial-lateral reference line can be a useful patient specific variable to minimize patient-to-patient variation in at least one relevant angle, e.g., the abduction angle. 
     Patient specific data can be provided for use by the surgeon based on best medical judgment. For example, any of the systems herein can be used in a mode that is based on broad population studies. Such studies can define a distribution of patients with sufficient clarity and detail to enable significant improvement over the current standard of care. In one mode, the dimensions taken from radiograph or CT can be used to inform the surgeon whether some patient specific adjustments should be considered. Alternatively, patient specific adjustments can be coded into the system  100  so that they are transparent to the doctor. Such adjustments can be downloaded to either or both of the devices  172 ,  204  or into a separate monitor or control device that communicates wirelessly with the devices  172 ,  204 . Thus, the system  100  can either fully implement patient specific adjustment, e.g., for anteversion, abduction, leg length, joint offset, or other parameter or can enable the surgeon to make a judgment as to whether to do so. 
       FIG.  36    illustrates an example of a pre-operative image that can be used in one technique. The lines  380  point to landmarks which are used intraoperatively and are also visible in an anterior pelvic radiograph. The top landmark  380 A is about 1 cm superior to most superior point of acetabulum. In another approach, the top landmark  380 A can be the most superior point on the acetabular rim. The bottom landmark  380 B is adjacent to or at the acetabular notch (tear drop). An angle between line  382  and the line  384  is a patient-specific abduction of line formed by landmarks, which can be entered into an interface of the system  100  (or the other systems herein) at time of surgery to provide patient specific reference frame. Line  384  may be any anatomic medial-lateral reference line. Examples include trans-ischial line and line across the inferior borders of the obturator foramina (shown in  FIG.  36   ). 
     4. Navigation Using Drift Insensitive Inertial Sensors 
     In one variation, one or both of the devices  172 ,  204  can comprise only accelerometers and can be configured as tilt meters, or the devices could be put into a mode that relies mostly on the accelerometer data or otherwise be configured to be insensitive to accumulated errors that arise from integration of data. If the patient is set in a reproducible and stable position, patient movement and mis-orientation can be eliminated. This enables some methods to be performed without using rate sensor data. In one variation of this tilt-meter approach, one or both of the sensors  172 ,  204  can be configured to inform the surgeon if a condition is sensed that suggests a landmark acquisition approach would yield a superior alignment outcome. This method can advantageously be used for procedures that do not require complex movements, like freehand motions. Where freehand motion is involved, incorporating some indication of heading (gyroscopes, magnetometer, or other indication of heading) would be useful. 
     5. Navigation Using Inertial Sensors to Track Motion to Define a Patient-Specific Safe Zone 
     In another technique illustrated by  FIG.  32   , a patient-specific “safe zone” is defined by recording the patient&#39;s natural range of motion of one more of the patient&#39;s joints. For example, if a hip procedure is to be performed, the patient&#39;s range of motion can be recorded pre-operatively. If the hip to be replaced is not overly arthritic, the range of motion can be determined on the hip to be replaced. If the range of motion of the hip to be replaced is unnatural due to disease state, the contralateral hip can be characterized. 
     In one hip replacement technique a sensor S is coupled with the femur. The sensor can be coupled above the knee to prevent movements at the knee from affecting the measurements made. The sensor S can be connected below knee if the knee is immobilized. The sensor S can be initialized and otherwise prepared to record accurate readings. Thereafter one or more movements of the hip joint can be performed with the output of the sensor recorded and processed. The movements can include, for example, movement in anterior and posterior (A-P) directions to the full extent of the range of motion and movement in medial and lateral (M-L) directions to the full extent of the range of motion. These motions define the patient&#39;s natural range of motions in these planes. 
     Based on the extents of motion in the A-P and M-L directions, a cone of motion CM can be defined. The cone of motion CM can be defined as originating at a point defined as the center of rotation of the femoral head and extending out from the acetabulum to a circular base located a distance from the center of rotation equal to the distance to the mount point of the sensor. The circular base can be defined as having a radius equal to the average extent of motion in the A-P and M-L directions. In  FIG.  32   , the cone of motion is shown on the contralateral side for clarity. As noted above, the data collected to estimate the cone of motion can be based on the leg to be treated or the contralateral leg. 
     Placement of the cup of the hip prosthesis is dictated by some metric of centering within the cone of motion. For example, the cup can be centered such that an axis extending perpendicular to the plane of the entrance to the cup crosses the circular base of the cone of motion precise in the center of the cone. In some systems, the orientation of the cup is controlled such that the crossing point of the axis so projecting is closer to the center of the circular base than it is to the periphery of the circular base. In other systems, the orientation of the cup is controlled such that the crossing point of the axis so projecting is within a distance from the center point that is less than 25% of the radius of the circular base. 
     In a class of patients, the movement of the hip is not symmetrical in each of the A-P and M-L directions. As such, the cone of motion can have a more complex geometry. For example, the cone of motion can originate at the center of rotation of the femoral head and extend to a base having an oblong shape, for example shortened in the medial direction, but longer in the lateral, anterior, and/or posterior directions. Various metrics of “within the safe zone” can be defined based on these irregular shaped cones. For example the geometric center of a complex base shape can be calculated and the cup of the prosthetic joint can be centered such that an axis extending perpendicular to the plane of the entrance to the cup crosses the irregular shaped base of the cone of motion at or within some maximum distance of the centroid of the cone. 
     Any suitable set of motions can be used to obtain the center of rotation of the femoral head and/or the boundaries of the base of the cone of motion. Examples of methods for determining the center of rotation of a femoral head using inertial sensors are discussed in U.S. Pat. No. 8,118,815, which is hereby incorporated by reference for this and all other purposes. A more complete perimeter of the base of the cone of motion can be directly recorded using sensors that are capable of tracking both position and orientation. For example, several other points between the A-P and M-L direction can be taken so that six, eight, ten, twelve or more extents are recorded. In other embodiments, arcuate motions of along all or portions of the perimeter of the base of the cone of motion can be traced and recorded. Because several degrees of freedom of the sensor S are constrained, the sensor can operate based on accelerometers only in some approaches, which simplifies sensor S and enables it to be disposable and/or less expensive to make. Such approaches may be most accurate if rotations about a vertical axis are minimized or eliminated. 
     In one embodiment, the procedure illustrated in  FIG.  32    generates an origin and a direction that can be input to a cup placement system. The origin can be the center of rotation of the femoral head and the corresponding center of rotation of a prosthetic socket. The direction can be a line connecting the origin and the point of intersection with the base of the cone of motion. This data is transferred to a cup placement system, such as any of those discussed above. For example, the impactor  300 A can include the sensor  204  to which this data has been saved. Thereafter movements of the impactor  300 A can be tracked with reference to this origin and direction to assure proper placement of the cup. Such placement can be with the aid of a patient movement tracking sensor pinned to the pelvis for example. 
     In other embodiments, cannulated systems can be used to minimize the number of steps during which inertial sensors are used. For example, once the origin and direction of the axis connecting the center of rotation and the intersection with the base of the cone of motion are determined, a guide member can be placed via a cannulated impactor (or other cannula). The guide member can dock with an impactor-mounted cup. The cup can be slid over the guide member into place in the acetabulum. The direction and origin information collected in the steps illustrated by  FIG.  32    are preserved by the guide member and by the tilt preventing features on the guide member and/or prosthetic cup. 
     If the patient&#39;s joint is subject to extensive disease, a cone of motion can be established by a combination of data collected in motions similar to those discussed above in connection with  FIG.  32    and pre-operative imaging. For example, X-rays can be taken when the femoral neck is moved close to the acetabular rim to supplement some of the data points defining the cone of motion. Thus, the cone of motion can be in part established by inertial sensing and in part by imaging to characterize the native anatomy. 
     D. Modular System for Anterior or Posterior Approach to Navigation Using Inertial Sensors and Anatomical Landmark Acquisition Jigs 
       FIGS.  37 - 40    illustrate a system  900  for navigating a hip procedure. The system  900  can be similar to some of those discussed above. But, while some of the foregoing systems are specialized for a particular approach, the system  900  includes a first sub-system  900 A adapted for a posterior approach and a second sub-system  900 B adapted for an anterior approach. As discussed more below, both systems  900 A,  900 B are configured to enable navigation to be conducted without requiring gyroscopic or other sensors that are subject to accumulated error (drift). This refinement makes the system simpler to implement and to use in a wider variety of settings and with more patients. 
     The system  900 A includes a jig  904 A that is adapted for hip joint navigation from a posterior approach. The jig  904 A is similar in some respects to the jig  454 , and any consistent description thereof is incorporated herein. The jig  904 A includes a platform  908 , a cannula coupling device  912 , and a registration jig mounting feature  914 . The platform  908  can have any shape, but in some implementations can be elongate, e.g., having a first end  916  and a second end  920 . The elongate shape enables at least a portion of the jig  904 A to be low profile in one direction and to provide a plurality of positions along a length for coupling devices to the jig. The first end  916  is configured to be oriented inferiorly and the second end  920  to be oriented superiorly when the navigation jig is applied to the patient. The medial-lateral dimensions or extent can be minimized to not obstruct the surgical field or the surgeon. 
     The cannula coupling device  912  is disposed adjacent to the first end  916  and is configured to enable a cannula  924  to be held adjacent to a bottom surface of the platform  908 . The cannula  924  can have a top surface connected to a bottom surface of the platform  908 . A connection between these components can be secured by a device disposed above within or below the platform  908 . In one form, a proximal structure of the cannula  924  can be received within a bottom recess of the platform  908  and can be held within the recess by a compression device, such as a set screw S. Details of several variants of cannula coupling devices  912  are discussed below in connection with  FIGS.  41 - 43 B . A connection to a bone adjacent to a hip joint is made through the cannula  924 . For example, a pin  928  can be placed through the platform  908  and the cannula  924  into the bone. 
     An anterior approach cannula  926  is shown in  FIGS.  39  and  40    and is similar to the cannula  516 , the description of which is incorporated herein. The description of the cannula coupling device  912  applies equally to the cannula  924  for posterior approach and to the cannula  926  for anterior approach. 
     The registration jig mounting feature  914  is disposed on a top surface  932  of the platform  908  adjacent to the first end  916 . In one form, the mounting feature  914  includes an elevated portion of the platform. The mounting feature can include one or more, e.g., two recesses into which pins can be received. In one embodiment, the elevated portion includes a window, e.g., a through hole, for viewing such a pin to confirm correct placement. As illustrated in  FIG.  41   , in one variant, a circular recess can be provided for a first pin and an U-shaped slot can be provided for another pin or member. 
     The hip navigation jig  904 A also includes registration jig  940 . The registration jig  940  can have some features similar to those discussed above. The registration jig  940  includes an upright member  942 , a rotatable member  948 , and a probe  952 . The upright member  942  is configured to be detachably coupled to the platform  908  at the registration jig mounting feature  914 . For example, a plurality of (e.g., two) pins can project from a lower surface of the upright member  942 , the pins being configured to be received in corresponding recesses in the registration jig mounting feature  914 . One of such pins is visible through the window in the registration jig mounting feature  914  seen in  FIG.  38   . The upright member  942  includes a first portion  944  and a second portion  946  disposed above the first portion  944 . The first portion  944  is substantially vertical and increases the elevation of the second portion  946  when the registration jig  940  is mounted to the registration jig mounting feature  914 . The second portion  946  is inclined away from a vertical longitudinal axis of the first portion  944 . The incline of the second portion  946  provides several advantages. It enables the upright member  942  to be out of the way of the range of motion of the probe  952 , as discussed below. This is important because the probe  952  has to be able to easily and quickly reach a plurality of anatomical features. 
     The incline of the second portion  946  also provides a simple way to incline an angle of rotation of the rotatable member  948  relative to a vertical axis. The rotatable member  948  is coupled with the upright member  942  for rotation about an axis A that is not vertical when the jig is mounted to the bone adjacent to a hip joint and the upright member is disposed generally vertically. This arrangement is one way to enable a navigation system employing inertial sensors to eliminate the need to manage sensor drift. As discussed above, certain sensors, such as gyroscopes, are more subject to accumulated errors (drift). The orientation of the axis A enables the jig  904  to be used in a system that includes accelerometers and other sensors that are sufficiently sensitive if activated and moved about axes that are not vertical. 
     As in the registration devices discussed above, other degrees of freedom of rotation and position can be provided in the registration jig  940  and such description is incorporated here. 
     The probe  952  had a tip  956  for engaging anatomy. The anatomy engaging tip  956  is disposed at a distal end of an elongate body  960  coupled with the rotatable member for rotation about the axis. The orientation and position of the elongate body  960  of the probe can be adjusted to bring the anatomy engaging tip into contact with a plurality of anatomical landmarks during a landmark acquisition maneuver. Such adjustments can be by sliding through a sliding support, similar to those hereinbefore described. 
     The upright member  942  can include a cradle  954  that allows the elongate body  960  of the probe  952  to be held in place when not in use during a procedure. The cradle  954  can be used to latch the sensor  204 , as discussed above. In various implementations, the system  900  does not require any steps of zeroing, however, since the sensors are configured to be generally drift insensitive. Eliminating sensitivity to drift can be achieved by configuring the sensor  204  as a tilt meter, and/or by using any sort of inertial sensor that will not introduce excessive error due to drift during the procedure time. As such, even sensors that have some drift can be used, so long as their accumulation of error does not reach a significant level until during the procedure. The cradle  954  could be used to zero error if a procedure was unexpectedly long and the sensor were subject to some drift. In one advantageous embodiment, the sensor  204  can operate solely with signals from accelerometers, which are insensitive to drift. 
       FIGS.  37 - 40    show that the systems  900 A,  900 B can include one or more sensors for detecting orientation of the probe  952 . The sensors can take any form, e.g., can include the surgical orientation device  172  and the sensor  204  discussed above. Accordingly, the jig  904  can include a sensor mounting feature  962  disposed on the platform  908 . Where the platform is elongate, the sensor mounting feature  962  can be disposed at the second end  920 . Another advantage of the jig  904 A,  904 B is that it is symmetrical and can be used on both hips. The jig  904 A,  904 B thus can have a single sensor mounting feature disposed on a plane of symmetry. If the platform  908  is elongate, the sensor mounting feature  962  can be located on a vertical mid-plane of the platform. Vertical here refers to the orientation of the jig  904 A,  904 B when applied to the hip in a posterior or anterior approach. 
     The registration jig  940  can include a sensor mounting feature  964  disposed thereon for movement with the probe  952 . For example, the sensor mounting feature  964  can be located at a proximal end of the elongate body  960 . This location is one of convenience, placing the sensor  204  at the proximal end. However, the sensor mounting feature  964  and the sensor  204  could be located on a side surface of the elongate body  960 . 
     As discussed herein, the orientation of the axis of rotation A of the rotatable member  948  enables the change of orientation of the sensor  204  to be other than in the horizontal plane. This is accomplished by orienting the axis A other than in the vertical direction. With this arrangement, it is possible to configure at least the sensor  204  as a tilt meter, e.g., using primarily or only accelerometers to output a signal indicative of orientation of a component, such as of the prove  952 . Example of angles or ranges of angles of the axis A that can be provided include about 20 degrees from horizontal, about 30 degrees from horizontal, about 45 degrees from horizontal, at less than about 60 degrees from horizontal. 
       FIG.  38    shows a further feature of the system  900 A, which includes the jig  904 A and the cannula  924 . The cannula  924  is adapted for posterior approach is similar to or the same as the hollow fixation member  466 . An upper or first end of the cannula  924  is configured to couple with the cannula coupling device  912 , such as by a set screw as discussed above. A second end of the cannula  924  is configured to couple with a bone adjacent to the hip joint. The bone can be any of those discussed above for coupling the fixation member  466  or other analogous structures discussed in any embodiments above. A home point feature  968  is disposed adjacent to the second (lower) end of the cannula  924 . The home point feature  968  is in a predefined, known position and can receive the anatomy engaging tip  956  of the probe  952 . When these structures contact, they are in a predefined position and orientation. The home point feature  968  can be similar to the registration feature  473  discussed above. 
     Because the system  900  can be adapted for posterior approach or for anterior approach (discussed below), the cannula  924  should be made removable from the platform  908  in the operating room or at a back table in preparation for surgery. As such, the connection between the cannula  924  and the platform  908  can be made orientation specific. This reduces a potential source of operator error, i.e., the home point feature  968  always faces toward the surgical field from the hip bone attachment location, e.g., faces inferiorly if the jig  904  is mounted to a superior location of the surgical field. For example, a projection on a proximal portion of the cannula  924  and a corresponding projection in a recess on the lower side of the platform  908  can define only one rotational orientation of the cannula relative to the platform in which these components can be coupled. 
     As discussed above, the cannula  926  is provided in the system  900  to enable a surgeon to switch to an anterior approach. Anterior approach is discussed in great detail above, e.g., in connection with  FIGS.  18 - 21 B , which description are incorporated here as well. The system  900 B differs from the system  500  in that the orientation of the axis A of rotation in the system  900 B is not vertical, as discussed above. As such, the sensors can be greatly simplified compared to the system  500 . The cannula  926  has a home point feature  968 B. The home point feature  968 B is in a predefined, known position and can receive the anatomy engaging tip  956  of the probe  952 . When these structures contact, they are in a predefined position and orientation. The home point feature  968 B can be similar to the registration feature  473  discussed above. The cannula  926  and the platform  908  can be configured for limited, e.g., only one, rotational position of attachment. This assures that when the jig  904 B is assembled in the operating room or back table that the jig  904 B will be properly set up. 
     In one method to maximize the accuracy of the landmark acquisition, jig  904 B is coupled with the patient in an anterior approach. The tip  956  is put into contact with the home point feature  968 B. Thereafter, user input can be applied to the surgical orientation device  172 A to indicate that the tip  956  is in the home point feature  968 B. Thereafter, the system registers movements and landmark acquisition in the manner discussed above. These data provide a basis to guide the placement of the acetabular cup, as discussed above. 
     The placement of the acetabular cup using a device such as the impactor  300 A can be an operation that benefits from inertial sensors that may include one or more drift-sensitive sensors, e.g., gyroscopes. The system  900  provides a calibration mount  998  for coupling a sensor  204  in a known, fixed position and orientation relative to the surgical orientation device  172 A. The calibration mount  998  is a docking device that positions the sensor  204  just prior to a step of eliminating any potential source of accumulated error, e.g., zeroing a drift-sensitive sensor.  FIG.  37 - 38    show that they system  900 A can include two sensors  204 , one mounted to the registration jig  940  and one to the calibration mount  998 . These two sensors  204  can be identical or can be dedicated for their specific function.  FIGS.  39  and  40    shows only one sensor  204 . In this system, a single sensor  204  is used to gather landmark data and to work in combination with the impactor  300 A to place an acetabular implant. 
       FIGS.  41 - 43 B  illustrate various features for clamping structures to the platform  908 . In particular, these figures show fixation pin securement devices  970  that are incorporated into the platform  908 . The fixation pin securement devices  970  can have low profile to be out of the way of other tools in the surgical field.  FIG.  41 - 41 A  show one embodiment of a pin securement device  970  that includes a compression member  972 . The cannula coupling device  912  can include a similar mechanism to clamp a pin disposed through the cannula  924 . The platform  908  includes a slot or plurality of slots formed on a surface thereof, e.g., on the top surface. The slots  974  are larger in at least one direction than the compression member  972  such that the compression member can fit in the slot and move to some extent therein. The compression member  972  has a tapered channel  976 . Movement of a tapered member  978  vertically in the tapered channel  976  shifts the compression member  972  to narrow a gap G between the compression member  972  and a rigid feature of the platform. The gap G can be between a curved lateral surface of the compression member  972  and a curved surface of the platform  908 . 
     In one method, a pin or other fixation member is advanced through the gap G and into the bone. The platform  908  is positioned on the fixation member at an appropriate height and the pin securement device  970  is affixed to the fixation member. The fixation member can be a Steinmann pin or other similar device. In one technique, the tapered member  978  is a threaded elongate body that is advanced along internal threads formed in the platform  908  until the tapered surface thereof acts on the tapered surface  976  to shift the compression member  972  laterally to narrow the gap G. Further advancement of the tapered member  978  further shifts the compression member  972  to enhanced securement of the fixation member. The method can be repeated for a second pin, where one pin extends through the cannula  924  and one extends parallel to the cannula  924 , but off-set superiorly therefrom on the patient. 
       FIGS.  42 - 42 B  illustrate another approach to a fixation pin securement devices  970 A in which the fixation pin securement device comprises a compression member  972 A pivotally mounted to the platform  908 .  FIG.  42 A  shows two compression members  972 A, each of which is mounted to pivot about a pin or shaft  980 . The securement device  970 A on the left in  FIG.  42 A  corresponds to a configuration in which a fixation member can freely pass through a gap G in the mechanism. The securement device  970 A on the right in  FIG.  42 A  corresponds to a configuration in which the gap G is narrowed and a fixation member disposed in the gap G will be securely clamped and unable to move relative to the platform  908 . A rigid surface of the platform  908  opposite the pivoting compression member  972  along with the compression member holds the fixation member in place. 
     In one method, pins or other fixation members are placed in the fixation pin securement device  970 A and the cannula coupling device  912 . In the illustrated embodiment, these devices can employ similar clamping mechanisms. Thereafter, screws  982  are advanced to cause the compression member  972  to pivot about the pin or shaft  980  from a first position in which the gap G provided between a clamping surface of the compression member  972 A and a rigid surface of the platform  908  is larger to a second position in which the gap G is smaller. The second position is a clamped position for the fixation member and will retain the platform in position until the screw  982  is withdrawn enlarging the gap G. 
       FIGS.  43 - 43 B  illustrate another approach to a fixation pin securement devices  970 B in which the fixation pin securement device comprises a compression member  972 B configured to clamp a plurality of segments of an outside surface of a fixation member. The platform  908  includes a plurality of projections  984  extending upward from an upper surface of the platform. The projections preferably are threaded. Each projection includes a collet  986  or similar device disposed therein having an inner lumen sized to receive a fixation member. A plurality of slots extends downward from an upper surface of the collet  986  and an angled surface  988  is disposed between top ends of each member defined between a pair of such slots. A corresponding angled surface  990  is provided on an inside of a cap  992 . The cap  992  has internal threads that act on the threads of the projection  984  to advance the angled surfaces  990  onto the angled surfaces  988 . Further advancement collapses the slots of the collet  986  causing compression about the outer surface of the fixation member.  FIG.  43    shows that this approach can be used for the fixation pin securement devices  970 B and/or for the cannula coupling device  912 . 
     While the systems discussed above are well suited for specific approaches, the system  900  can be adapted for a posterior approach or for an anterior approach. This provides a great deal of flexibility to the surgeon and only adds minimal additional components to a universal kit. The orientation of the axis of rotation A (see  FIGS.  38  and  40   ) enhances the sensitivity of a system that incorporates accelerometers and other sensor drift insensitive components. The home point features  968 A,  968 B enable the surgeon to obtain maximal accuracy by allowing the acquisition of position and orientation data for a number of anatomical landmarks at close range to the home point position. This allows the system to initialize the sensors near the points to be acquired to enhance accuracy. 
     II. Hip Navigation Using Camera Tracking 
       FIG.  33    illustrates one embodiment of a system  800  that includes close-range optical tracking capabilities. In this context “close range” is a broad term that means near the patient, such as any of in the surgical field, directly above the pelvis but below the surgeon&#39;s head, within the boundaries of the surgical table, etc. This term is intended to exclude systems where cameras are outside the surgical fields. Close range greatly reduces or eliminates “line of sight” problems that plague traditional optical navigation. 
     In the illustrated embodiment, a jig system  804  is provided for connecting to patient bone. The jig system  804  can include any of the features of any of the jig systems discussed herein. For simplicity, the jig system is illustrated with that of  FIG.  1   , e.g., including the cannula  124  and the platform  136 . A surgical orientation device  172 A is mounted to the platform  136 . The orientation device  172 A can be similar to those hereinbefore described, but also includes one or more cameras  812 . Preferably the orientation device  172 A includes two or more cameras  812  to enable capture of binocular data. The cameras preferably are small cameras, for example the Aptina MT9T111, which is discussed at http://www.aptina.com/products/soc/mt9t111d00stc/. The cones projecting from the lower side of the device  172 A schematically represent the direction of the field of view of the cameras  812 . 
     This data can at least be used to determine the heading of and in some cases six degrees of freedom of a stylus  816 . The stylus has a distal end  828  configured to touch landmarks as part of a landmark acquisition maneuver, as discussed above. A proximal (or other) portion  832  of the stylus  816  has an array of trackers  836  that can be tracked by the cameras  812  to provide orientation, position, heading, attitude, or other combinations of spatial characteristics of portions of the stylus  816  or anatomy with which it is coupled. 
     The cameras  812  can operate without any additional sensor, such as inertial sensors. In some embodiments, the cameras  812  are used in concert with inertial sensors to confirm or to improve accuracy of the sensors. For example, drift in a rate sensor, e.g., accumulated errors, can be monitored by comparing the output of the rate sensor with the viewed position from the cameras. The system can intervene if the sensor output drifts too much, for example, telling the user to reset the rate sensors. 
     Another optical device such as a laser or an IR emitter  814  can be provided in the orientation device  172 A. An IR emitter can be useful to illuminate the fiduciaries to make them more readily detectable by the cameras under the intense lighting in the surgical field. 
     Although these inventions have been disclosed in the context of certain preferred embodiments and examples, it will be understood by those skilled in the art that this application extends beyond the specifically disclosed embodiments to other alternative embodiments and/or uses of the invention and obvious modifications and equivalents thereof. In addition, while a number of variations of the inventions have been shown and described in detail, other modifications, which are within the scope of the inventions, will be readily apparent to those of skill in the art based upon this disclosure. It is also contemplated that various combinations or sub-combinations of the specific features and aspects of the embodiments may be made and still fall within the scope of the application. For example, the application contemplates the connection hub alone or in combination with any of the other modules could comprise a separate aspect. Or, any one or a combination of the modules could be directly connected to an umbrella hub or overhead support to form another separate aspect. Accordingly, it should be understood that various features and aspects of the disclosed embodiments can be combined with or substituted for one another in order to form varying modes of the disclosed embodiments. Thus, it is intended that the scope of the present invention herein disclosed should not be limited by the particular disclosed embodiments described above, but should be determined only by a fair reading of the claims that follow. 
     Similarly, this method of disclosure, is not to be interpreted as reflecting an intention that any claim require more features than are expressly recited in that claim. Rather, as the following claims reflect, inventive aspects lie in a combination of fewer than all features of any single foregoing disclosed embodiment. Thus, the claims following the Detailed Description are hereby expressly incorporated into this Detailed Description, with each claim standing on its own as a separate embodiment.