Patent Application: US-74290908-A

Abstract:
a computer assisted surgical navigation system and method is disclosed for registering the position of prosthetic hip joint components . elements are applied to the pelvis and femur , generating a three - dimensional array . these two arrays combine to derive a reference point representing the native joint . a tracking device with a pre - determined shape and dimensions that precisely articulate with an acetabular cup component generates a third three - dimensional array . the device has a further shape and dimensions that independently articulate precisely with a neck portion of a femoral component , which represents a prosthetic joint center that is independently registered in the system . the tracking device concurrently registers the three dimensional positions of the femoral and acetabular prosthetic components , along with the prosthetic joint center and the native joint center , respectively enabling alterations in three dimensional location of the leg length and offset prior to reduction of the prosthetic joint .

Description:
the present invention is used in combination with a computer assisted surgical ( cas ) system . the localization principle of the cas system may be based on optic , magnetic , radio - frequency , ultrasound measurement technologies or articulated passive or robotic arms . referring to fig1 - 12 , a tracker 1 is a device that is localized by the localizer system 3 of the cas system . for each tracker is defined a unique coordinate system ( o , x , y , z ) 2 , also called a reference . one tracker is fixed on the femur 13 ( fig4 ), one on the pelvis 14 ( fig5 ), and one on the device 5 ( fig2 ). by measuring those trackers in real - time , the localizer system provides the relative position of the femoral reference , the pelvis reference , and the device reference during the surgical procedure . for convenience purpose , the tracker is represented in all figures by 3 branches device with 3 spherical balls , as it looks like for an optical passive wireless tracker like standard spectra camera from northern digital inc . ( ontario , canada ), even if the tracker technology may be not optical , but magnetic , ultrasounds , radio - frequency , or robotics . a reference 4 is also attached to the localizer system 3 . for each tracker 1 detected by the localizer system 3 , the localizer system 3 gives the transformation matrix , composed of 3 translations and 3 rotations , between the localizer reference 4 and the tracker reference 2 . as an example , if mf i is the matrix between the localizer reference and the femoral reference , and if md i is the matrix between the localizer reference and the device reference , then the relative position m i between the device reference and the femoral reference is m i = md i − 1 . mf i . with this method one can express every geometrical data ( point , line , shapes ) that are known in a given reference into another reference . as an example , if c is the 3d point corresponding to the ball center 9 expressed by its coordinates ( x , y , z ) in the device reference , and if c ′ is the 3d point corresponding to the ball center 9 in the femoral reference , expressed by ( x ′, y ′, z ′), then c ′= mj · c . this typical workflow for transferring geometrical from one reference to another is described in fig1 . the device , described in fig2 , is composed of a tracker 5 to allow the cas system to track the whole device . the tracker is mounted on a handle 6 . the handle helps the operator manipulating the device . a reproducible fixation system 7 allows the operator to assemble and disassemble easily the ball 8 during the surgical procedure . the ball contains a tapered trunion hole 10 that fits with femoral necks . the geometry of the device is perfectly known , meaning that the coordinate of the trunion hole axis 11 , and the ball center 10 are perfectly known in the tracker reference 5 . preferably , the tracker fixation 7 allows the operator to assemble and disassemble the tracker easily from the ball . one may provide the operator with several ball types , such as different combinations of diameters and trunion hole shapes , which implies that the operator changes the ball before or during the surgery . the fixation is reproducible and the components geometry is perfectly known , so that once the ball is attached to the tracker , the ball position is known in the tracker reference . preferably , the fixation is manipulated without any tool such as screwdriver . optionally , the tracker fixation can be fixed and rigid , with no possibility to disassemble the ball . for a large majority of hip implants , the top part of the prosthetic femoral neck ( fig7 ) 22 can be modeled by a cone , with a revolution axis , called the neck axis 21 . in addition , there are few cone types , in order to allow a cup and head manufacturer to be compatible with other manufacturer &# 39 ; s stems . the invention is best used with the cones that are standardized , which represents a very large proportion of the market , or it is necessary to know the specific geometry of cones for other cases . the trunion hole 10 is designed to fit with a total hip prosthetic femoral neck so that there is a unique reproducible position of the ball with respect to the femoral neck , except around the neck axis . once in position ( fig8 ), the trunion hole axis 11 and the neck axis 21 are coincident 19 , and the ball center 9 corresponds to the future prosthetic head center 18 , 20 . the inner trunion hole is positioned with respect to the ball center such that once it is placed on the neck , the ball center corresponds to a “ head neck 0 ” of the most common implants . the “ head neck 0 ” is always defined by implant manufacturers as the default neck . other heads are proposed by implant manufacturers with an offset along the neck axis , such as “ head neck + 2mm ”, “ head neck − 2mm ”, etc . optionally , if the ball center does not correspond to the “ neck 0 ” of a given implant , one may enter in the cas system the offset value between the ball center and the “ head neck 0 ” of the implant . by default , this offset value is 0 . thanks to the trackers that give the position of the device reference with respect to the femoral reference , one may deduce the prosthetic neck axis 21 and the prosthetic head center 18 in the femoral reference . the trunion hole authorizes the ball to be assembled and disassembled easily from the femoral neck , allowing the operator to remove the ball , change the prosthetic parameters such as the neck axis or neck length , and redo the measurement with the ball . the software workflow that computes the femoral head center 18 position with the help of the device is described in fig1 . the software workflow that computes the femoral neck axis 21 position with the help of the device is similar to the one presented fig1 by changing the geometrical data , i . e . replacing the head center 18 of the neck axis 21 . the external shape of the ball 8 is designed so that it precisely articulates with the desired dimension of the acetabular component 24 . the external shape is partially spherical , at least one fourth of a sphere , but preferably one half sphere , in order to fit well with the acetabular component 24 ( fig9 ). there is a unique and reproducible position in translation of the ball with respect to the acetabular component . that means that once the ball is placed inside the acetabular component ( fig1 ), the acetabular center 23 and the ball center 9 are coincident 25 . thanks to the trackers that give the position of the device reference with respect to the pelvic reference , one may deduce the acetabular center in the pelvic reference . the external shape authorizes the ball to be assembled and disassembled easily from the acetabular component , allowing the operator to remove the ball , change the prosthetic parameters , and redo the measurement with the ball . the software workflow that computes the acetabular center 23 position with the help of the device is similar to the one presented fig1 , except the device is positioned in the acetabular component instead of the femoral neck , and the femoral reference is replaced by the pelvic reference . there are several solutions to perform the anatomical registration of the hip joint parameters at the beginning of surgery . preferably , the anatomical center of the hip joint is determined with a kinematic reference method ( solution 1 ) where the center can be defined by movement of the femoral reference in a circular fashion also known as pivoting . as the hip joint is fixed in the pelvic reference , movement of the femoral reference describes a cone of movement , the center of which is the center of the joint 17 ( fig6 ). in this case , the device helps comparing the prosthetic hip center to the anatomical hip center . optionally , the anatomical center of the hip is computed separately on the femur and the pelvis ( solution 2 ). on the femoral side , one may acquire the femoral head surface its most spherical part with a dedicated pointer reference , in order to reconstruct the sphere passing best by the points collected with the pointer using a least - squares method , and compute the anatomical femoral head center of the center of the reconstructed sphere 15 ( fig4 ). on the acetabular side , one may acquire the acetabular surface with a dedicated pointer reference in a similar way , by calculating the least - squares sphere that fits with the points collected in non worn parts of the anatomical acetabulum , in order to compute the anatomical acetabular center 16 ( fig5 ). in this case , the device helps comparing the prosthetic acetabular and femoral centers respectively to the acetabular and femoral hip centers . optionally , one may simply compute the femoral position with respect to the pelvis position ( solution 3 ). in this case , the device helps comparing the prosthetic position with respect to the anatomical position of the lower limb , without any indication on the anatomical and prosthetic hip centers . preferably , the surgical protocols 1 and 2 using the device are described fig1 . the protocol 1 particularly fits with the non - modular neck implants , where the implants are not in the cas database , because this protocol is universal and highly independent of the implant . in this protocol , the operator registers the acetabular component ( d ) position and the femoral neck ( e 1 ) position . then , the cas system displays on the screen the virtual prosthetic hip ( f ) in relation with the anatomical measurements . this step allows the surgeon to visualize the simulation of the hip parameters without reduction of the hip joint , and change accordingly the neck length in order to restore a correct leg length and lateral offset for best hip stability . if the surgeon is still not satisfied of the prosthesis positions ( g ), he may change the implant stem parameters ( h ) such as position , orientation , or chose a stem with different characteristics such as lateralization or inclination , and then redo the neck registration with the device ( e 1 ). the protocol 2 particularly fits with modular neck implants where the implant database is known by the cas system . in this protocol , the operator registers the acetabular component position ( d ) and the stem ( e 2 ) position . then , the cas system displays the virtual prosthetic hip like in the previous protocol ( f ). as the cas system knows the implant database , it may allow the surgeon to change virtually not only the neck length like in the protocol 1 , but also the neck type . it is a significant advantage if one considers that a modular neck implant may offer more than one hundred of possibilities . by simulating virtually the neck type using the touch screen of the cas system , the operator does not need to change actually the implants and redo the measurement like in the protocol 1 . for both protocols , if the anatomical registration has recorded a hip center , by solution 1 or 2 described before , then the cas system displays the offset between the prosthetic hip center and the anatomical hip center in the 3 projection plans ( frontal , sagittal , axial ), as well as the resulting position in terms of lengthening , lateral offset , and sagittal offset of the prosthetic lower limb compared to the anatomical situation . for both protocols , once the virtual proposal is accepted , the operator selects and inserts final implants ( i ), reduces the hip ( j ) and makes a final check ( k ). final check consists in measuring the final changes of leg length and offset of the hip joint in comparison with pre - operative measurements . final check may also include registering the final prosthetic hip center , by a kinematic movement for instance , and display the comparison with the anatomical situation . optionally , the acetabular registration ( d ) is done just after the cup insertion ( b ) and the femoral neck registration ( e 1 ) or the stem registration ( e 2 ) is done just after the stem insertion ( c ). in some cases it is preferable to digitize the stem without any neck . this situation occurs with modular neck implants in particular . in those cases , we use an adaptor to the stem 12 ( fig3 ), instead of a trunion hole adaptor to the neck . the fixation system to the stem is the one used by the standard stem impactor , usually dependent of the implant type and brand . most of stem fixation systems are female on the stem part , in order to avoid conflicting soft tissues once the stem impactor is removed , which leads to design in most cases a male fixation part on the device 12 . if the fixation part of the device is female , there is a possibility to place the female part inside the ball ( fig1 ). wherever the female part 10 is on the ball , except on the tracker fixation system 7 , it does not conflict with the acetabular component 24 registration functionality . however , if the fixation part of the device is male , one need to make sure that the male part does not conflict with the acetabular registration . if placing the male part on the ball surface is not convenient , one may optionally design a second tip 12 that is fixed to the handle 6 , instead of the ball 8 . the device 12 may be manufactured from any suitable medical material , which would include stainless steel or plastic . the device 12 may be embodied in other specific forms without departing from its spirit or essential characteristics . the described embodiment is to be considered in all respects only , as illustrative and not restrictive and the scope of the device is , therefore , indicated by the appended claims rather than by the foregoing description . all changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope . nogler m . navigated minimally invasive total hip arthroplasty . surg tech . int . 2004 ; 12 : 259 - 262 , lewinnek g e , lewis j l , tarr r , compere 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