Patent Description:
Navigation systems assist users in locating objects. For instance, navigation systems are used in industrial, aerospace, defense, and medical applications. In the medical field, navigation systems assist surgeons in placing surgical instruments relative to a patient's anatomy. Surgeries in which navigation systems are used include neurosurgery and orthopedic surgery. Typically, the instrument and the anatomy are tracked together with their relative movement shown on a display.

Navigation systems may employ light signals, sound waves, magnetic fields, radio frequency signals, etc. in order to track the position and/or orientation of objects. Often the navigation system includes tracking devices attached to the object being tracked. A localizer cooperates with tracking elements on the tracking devices to determine a position of the tracking devices, and ultimately to determine a position and/or orientation of the object. The navigation system monitors movement of the object via the tracking devices.

Many navigation systems rely on an unobstructed line-of-sight between the tracking elements and sensors that receive tracking signals from the tracking elements. When the line-of-sight is obstructed, tracking signals being transmitted from the tracking elements are not received by the sensors. As a result, errors can occur. Typically, in this situation, navigation is discontinued and error messages are conveyed to the user until the line-of-sight returns or the system is reset. In the medical field, in many instances, the error messages are displayed on a monitor remote from the surgeon making it difficult for the surgeon to notice and remedy the error in a timely manner. This can cause delays to surgical procedures.

As a result, there is a need in the art for navigation systems that quickly identify line-of-sight issues so that they can be resolved without significant delay. There is also a need in the art for navigation systems and methods that help to improve the line-of-sight and reduce possible errors associated with obstructions to the line-of-sight between tracking elements and sensors.

Document <CIT> may be construed to disclose a disposable tool suitable for use in orthopedic alignment comprises a sensor communicatively coupled to the wand to register points of interest on a first and second bone and transmit location data related to the points of interest to the sensor to assess orthopedic alignment with the points of interest. A display via a wireless connection to the tool reports and visually displays alignment information in real-time. The wand and the sensor each have at least two ultrasonic transducers. The wand has a housing fits in a hand and includes a tip for identifying and registering a location. The wand can be attached to a mount in a predetermined position within the surgical field during a portion of the alignment procedure. Sensor and wand remain within the surgical field throughout the surgery and are disposed of after use in surgery.

According to the present disclosure, there are provided navigation systems according to the independent claims. Further developments are set forth in the dependent claims.

In one example, a navigation system is provided for tracking an object. The navigation system comprises a tracking element for transmitting a tracking signal. A sensor is provided to receive the tracking signal from the tracking element to determine a position of the object. One of the tracking element and the sensor is attachable to the object. An error indicator is also attachable to the object. The error indicator indicates an error in the sensor receiving the tracking signal from the tracking element. The error indicator includes a first light emitter and a second light emitter. A computing system determines if the sensor receives the tracking signal from the tracking element. The computing system also generates an error signal if the sensor did not receive the tracking signal and activates the first light emitter in response to generating the error signal. The computing system further deactivates the second light emitter in response to generating the error signal.

In another example, which is not part of the invention, a method is also provided for tracking an object. The method includes attaching one of a tracking element and a sensor to the object. The method also includes transmitting a tracking signal from the tracking element and determining if the sensor received the tracking signal from the tracking element. An error signal is generated if the sensor did not receive the tracking signal. A first light emitter is activated in response to the error signal and a second light emitter is deactivated in response to the error signal.

One advantage of this navigation system and method is that the error is identified directly on the object being tracked. As a result, the user can better isolate signal transmission issues and remedy the issues without significant delay. Also, in some instances, by placing the indicator on the object being tracked, there is a greater likelihood that the user will notice the error as compared to an error message displayed on a remote monitor.

In another example, a navigation system is provided to reduce line-of-sight issues while tracking an object. The navigation system comprises a tracking device. The tracking device includes a base attachable to the object, a tracking element, and a connector coupling the tracking element to the base to allow movement of the tracking element in at least one degree of freedom relative to the base to place the tracking element in a desired orientation. A computing system determines if the tracking element is in the desired orientation and instructs a user to move the tracking element relative to the base in the at least one degree of freedom if the tracking element is not in the desired orientation.

In another example, which is not part of the invention, a method is also provided for setting up a navigation system to track an object wherein the navigation system comprises a computing system and a tracking device including a base, a tracking element, and a connector. The method comprises attaching the base to the object and determining if the tracking element is in a desired orientation. A user is instructed to move the tracking element relative to the base in at least one degree of freedom if the tracking element is not in the desired orientation.

One advantage of this navigation system and method is to guide the user during setup of the tracking devices so that the tracking devices are arranged in desired orientations. For instance, the desired orientation may provide the tracking device with the necessary line-of-sight to other devices such as sensors of the navigation system.

Referring to <FIG> a navigation system <NUM> is illustrated. The navigation system <NUM> is shown in a surgical setting such as an operating room of a medical facility. The navigation system <NUM> is set up to track movement of various objects in the operating room. Such objects include, for example, a surgical instrument <NUM>, a femur F of a patient, and a tibia T of the patient. The navigation system <NUM> tracks these objects for purposes of displaying their relative positions and orientations to the surgeon and, in some cases, for purposes of controlling or constraining movement of the surgical instrument <NUM> relative to a predefined path or anatomical boundary.

The navigation system <NUM> includes a computer cart assembly <NUM> that houses a navigation computer <NUM>. A navigation interface is in operative communication with the navigation computer <NUM>. The navigation interface includes a first display <NUM> adapted to be situated outside of a sterile field and a second display <NUM> adapted to be situated inside the sterile field. The displays <NUM>, <NUM> are adjustably mounted to the computer cart assembly <NUM>. First and second input devices <NUM>, <NUM> such as a mouse and keyboard can be used to input information into the navigation computer <NUM> or otherwise select/control certain aspects of the navigation computer <NUM>. Other input devices are contemplated including a touch screen (not shown) on displays <NUM>, <NUM> or voice-activation.

A localizer <NUM> communicates with the navigation computer <NUM>. In the embodiment shown, the localizer <NUM> is an optical localizer and includes a camera unit <NUM> (also referred to as a sensing device). The camera unit <NUM> has an outer casing <NUM> that houses one or more optical position sensors <NUM>. In some embodiments at least two optical sensors <NUM> are employed, preferably three. The optical sensors <NUM> may be three separate charge-coupled devices (CCD). In one embodiment three, one-dimensional CCDs are employed. It should be appreciated that in other embodiments, separate camera units, each with a separate CCD, or two or more CCDs, could also be arranged around the operating room. The CCDs detect infrared (IR) signals.

Camera unit <NUM> is mounted on an adjustable arm to position the optical sensors <NUM> with a field of view of the below discussed trackers that, ideally, is free from obstructions.

The camera unit <NUM> includes a camera controller <NUM> in communication with the optical sensors <NUM> to receive signals from the optical sensors <NUM>. The camera controller <NUM> communicates with the navigation computer <NUM> through either a wired or wireless connection (not shown). One such connection may be an IEEE <NUM> interface, which is a serial bus interface standard for high-speed communications and isochronous real-time data transfer. The connection could also use a company specific protocol. In other embodiments, the optical sensors <NUM> communicate directly with the navigation computer <NUM>.

Position and orientation signals and/or data are transmitted to the navigation computer <NUM> for purposes of tracking the objects. The computer cart assembly <NUM>, display <NUM>, and camera unit <NUM> may be like those described in <CIT>, entitled "Surgery System".

The navigation computer <NUM> can be a personal computer or laptop computer. Navigation computer <NUM> has the displays <NUM>, <NUM>, central processing unit (CPU) and/or other processors, memory (not shown), and storage (not shown). The navigation computer <NUM> is loaded with software as described below. The software converts the signals received from the camera unit <NUM> into data representative of the position and orientation of the objects being tracked.

Navigation system <NUM> includes a plurality of tracking devices <NUM>, <NUM>, <NUM>, also referred to herein as trackers. In the illustrated embodiment, one tracker <NUM> is firmly affixed to the femur F of the patient and another tracker <NUM> is firmly affixed to the tibia T of the patient. Trackers <NUM>, <NUM> are firmly affixed to sections of bone. Trackers <NUM>, <NUM> may be attached to the femur F and tibia T in the manner shown in <CIT>. Other methods of attachment are described further below. In additional embodiments, a tracker (not shown) is attached to the patella to track a position and orientation of the patella. In yet further embodiments, the trackers <NUM>, <NUM> could be mounted to other tissue types or parts of the anatomy.

An instrument tracker <NUM> is firmly attached to the surgical instrument <NUM>. The instrument tracker <NUM> may be integrated into the surgical instrument <NUM> during manufacture or may be separately mounted to the surgical instrument <NUM> in preparation for the surgical procedures. The working end of the surgical instrument <NUM>, which is being tracked, may be a rotating bur, electrical ablation device, or the like.

The trackers <NUM>, <NUM>, <NUM> can be battery powered with an internal battery or may have leads to receive power through the navigation computer <NUM>, which, like the camera unit <NUM>, preferably receives external power.

In the embodiment shown, the surgical instrument <NUM> is an end effector of a surgical manipulator. Such an arrangement is shown in <CIT>, entitled, "Surgical Manipulator Capable of Controlling a Surgical Instrument in Multiple Modes".

In other embodiments, the surgical instrument <NUM> may be manually positioned by only the hand of the user, without the aid of any cutting guide, jib, or other constraining mechanism such as a manipulator or robot. Such a surgical instrument is described in <CIT>, entitled, "Surgical Instrument Including Housing, a Cutting Accessory that Extends from the Housing and Actuators that Establish the Position of the Cutting Accessory Relative to the Housing".

The optical sensors <NUM> of the localizer <NUM> receive light signals from the trackers <NUM>, <NUM>, <NUM>. In the illustrated embodiment, the trackers <NUM>, <NUM>, <NUM> are active trackers. In this embodiment, each tracker <NUM>, <NUM>, <NUM> has at least three active tracking elements or markers <NUM> for transmitting light signals to the optical sensors <NUM>. The active markers <NUM> can be, for example, light emitting diodes (LEDs) <NUM> transmitting light, such as infrared light. The optical sensors <NUM> preferably have sampling rates of <NUM> or more, more preferably <NUM> or more, and most preferably <NUM> or more. In some embodiments, the optical sensors <NUM> have sampling rates of <NUM>. The sampling rate is the rate at which the optical sensors <NUM> receive light signals from sequentially fired LEDs <NUM>. In some embodiments, the light signals from the LEDs <NUM> are fired at different rates for each tracker <NUM>, <NUM>, <NUM>.

Referring to <FIG>, each of the LEDs <NUM> are connected to a tracker controller <NUM> located in a housing (not shown) of the associated tracker <NUM>, <NUM>, <NUM> that transmits/receives data to/from the navigation computer <NUM>. In one embodiment, the tracker controllers <NUM> transmit data on the order of several Megabytes/second through wired connections with the navigation computer <NUM>. In other embodiments, a wireless connection may be used. In these embodiments, the navigation computer <NUM> has a transceiver (not shown) to receive the data from the tracker controller <NUM>.

In other embodiments, the trackers <NUM>, <NUM>, <NUM> may have passive markers (not shown), such as reflectors that reflect light emitted from the camera unit <NUM>. The reflected light is then received by the optical sensors <NUM>. Active and passive marker arrangements are well known in the art.

Each of the trackers <NUM>, <NUM>, <NUM> also includes a <NUM>-dimensional gyroscope sensor <NUM> that measures angular velocities of the trackers <NUM>, <NUM>, <NUM>. As is well known to those skilled in the art, the gyroscope sensors <NUM> output readings indicative of the angular velocities relative to x, y, and z axes of a gyroscope coordinate system. These readings are multiplied by a conversion constant defined by the manufacturer to obtain measurements in degrees/second with respect to each of the x, y, and z axes of the gyroscope coordinate system. These measurements can then be converted to an angular velocity vector defined in radians/second.

The angular velocities measured by the gyroscope sensors <NUM> provide additional non-optically based kinematic data for the navigation system <NUM> with which to track the trackers <NUM>, <NUM>, <NUM>. The gyroscope sensors <NUM> may be oriented along the axis of each coordinate system of the trackers <NUM>, <NUM>, <NUM>. In other embodiments, each gyroscope coordinate system is transformed to its tracker coordinate system such that the gyroscope data reflects the angular velocities with respect to the x, y, and z axes of the coordinate systems of the trackers <NUM>, <NUM>, <NUM>.

Each of the trackers <NUM>, <NUM>, <NUM> also includes a <NUM>-axis accelerometer <NUM> that measures acceleration along each of x, y, and z axes of an accelerometer coordinate system. The accelerometers <NUM> provide additional non-optically based data for the navigation system <NUM> with which to track the trackers <NUM>, <NUM>, <NUM>.

The accelerometers <NUM> may be oriented along the axis of each coordinate system of the trackers <NUM>, <NUM>, <NUM>. In other embodiments, each accelerometer coordinate system is transformed to its tracker coordinate system such that the accelerometer data reflects the accelerations with respect to the x, y, and z axes of the coordinate systems of the trackers <NUM>, <NUM>, <NUM>.

Each of the gyroscope sensors <NUM> and accelerometers <NUM> communicate with the tracker controller <NUM> located in the housing of the associated tracker that transmits/receives data to/from the navigation computer <NUM>. The data can be received either through a wired or wireless connection.

The navigation computer <NUM> includes a navigation processor <NUM>. The camera unit <NUM> receives optical signals from the LEDs <NUM> of the trackers <NUM>, <NUM>, <NUM> and outputs to the processor <NUM> signals and/or data relating to the position of the LEDs <NUM> of the trackers <NUM>, <NUM>, <NUM> relative to the localizer <NUM>. The gyroscope sensors <NUM> transmit non-optical signals to the processor <NUM> relating to the <NUM>-dimensional angular velocities measured by the gyroscope sensors <NUM>. Based on the received optical and non-optical signals, navigation processor <NUM> generates data indicating the relative positions and orientations of the trackers <NUM>, <NUM>, <NUM> relative to the localizer <NUM>.

It should be understood that the navigation processor <NUM> could include one or more processors to control operation of the navigation computer <NUM>. The processors can be any type of microprocessor or multiprocessor system. The term processor is not intended to be limited to a single processor.

Prior to the start of the surgical procedure, additional data are loaded into the navigation processor <NUM>. Based on the position and orientation of the trackers <NUM>, <NUM>, <NUM> and the previously loaded data, navigation processor <NUM> determines the position of the working end of the surgical instrument <NUM> and the orientation of the surgical instrument <NUM> relative to the tissue against which the working end is to be applied. In some embodiments, navigation processor <NUM> forwards these data to a manipulator controller <NUM>. The manipulator controller <NUM> can then use the data to control a robotic manipulator <NUM> as described in <CIT>, entitled, "Surgical Manipulator Capable of Controlling a Surgical Instrument in Multiple Modes".

The navigation processor <NUM> also generates image signals that indicate the relative position of the surgical instrument working end to the surgical site. These image signals are applied to the displays <NUM>, <NUM>. Displays <NUM>, <NUM>, based on these signals, generate images that allow the surgeon and staff to view the relative position of the surgical instrument working end to the surgical site. The displays, <NUM>, <NUM>, as discussed above, may include a touch screen or other input/output device that allows entry of commands.

Referring to <FIG>, tracking of objects is generally conducted with reference to a localizer coordinate system LCLZ. The localizer coordinate system has an origin and an orientation (a set of x, y, and z axes).

Each tracker <NUM>, <NUM>, <NUM> and object being tracked also has its own coordinate system separate from localizer coordinate system LCLZ. Components of the navigation system <NUM> that have their own coordinate systems are the bone trackers <NUM>, <NUM> and the instrument tracker <NUM>. These coordinate systems are represented as, respectively, bone tracker coordinate systems BTRK1, BTRK2, and instrument tracker coordinate system TLTR.

Navigation system <NUM> monitors the positions of the femur F and tibia T of the patient by monitoring the position of bone trackers <NUM>, <NUM> firmly attached to bone. Femur coordinate system is FBONE and tibia coordinate system is TBONE, which are the coordinate systems of the bones to which the bone trackers <NUM>, <NUM> are firmly attached.

Prior to the start of the procedure, pre-operative images of the femur F and tibia T are generated (or of other tissues in other embodiments). These images may be based on magnetic resonance imaging (MRI) scans, radiological scans or computed tomography (CT) scans of the patient's anatomy. These images are mapped to the femur coordinate system FBONE and tibia coordinate system TBONE using well known methods in the art. In one embodiment, a pointer instrument P, such as disclosed in <CIT>, having its own tracker PT (see <FIG>), may be used to map the femur coordinate system FBONE and tibia coordinate system TBONE to the pre-operative images. These images are fixed in the femur coordinate system FBONE and tibia coordinate system TBONE.

During the initial phase of the procedure, the bone trackers <NUM>, <NUM> are firmly affixed to the bones of the patient. The pose (position and orientation) of coordinate systems FBONE and TBONE are mapped to coordinate systems BTRK1 and BTRK2, respectively. Given the fixed relationship between the bones and their bone trackers <NUM>, <NUM>, the pose of coordinate systems FBONE and TBONE remain fixed relative to coordinate systems BTRK1 and BTRK2, respectively, throughout the procedure. The pose-describing data are stored in memory integral with both manipulator controller <NUM> and navigation processor <NUM>.

The working end of the surgical instrument <NUM> (also referred to as energy applicator distal end) has its own coordinate system EAPP. The origin of the coordinate system EAPP may represent a centroid of a surgical cutting bur, for example. The pose of coordinate system EAPP is fixed to the pose of instrument tracker coordinate system TLTR before the procedure begins. Accordingly, the poses of these coordinate systems EAPP, TLTR relative to each other are determined. The pose-describing data are stored in memory integral with both manipulator controller <NUM> and navigation processor <NUM>.

Referring to <FIG>, a localization engine <NUM> is a software module that can be considered part of the navigation system <NUM>. Components of the localization engine <NUM> run on navigation processor <NUM>. In some versions, the localization engine <NUM> may run on the manipulator controller <NUM>.

Localization engine <NUM> receives as inputs the optically-based signals from the camera controller <NUM> and the non-optically based signals from the tracker controller <NUM>. Based on these signals, localization engine <NUM> determines the pose of the bone tracker coordinate systems BTRK1 and BTRK2 in the localizer coordinate system LCLZ. Based on the same signals received for the instrument tracker <NUM>, the localization engine <NUM> determines the pose of the instrument tracker coordinate system TLTR in the localizer coordinate system LCLZ.

The localization engine <NUM> forwards the signals representative of the poses of trackers <NUM>, <NUM>, <NUM> to a coordinate transformer <NUM>. Coordinate transformer <NUM> is a navigation system software module that runs on navigation processor <NUM>. Coordinate transformer <NUM> references the data that defines the relationship between the pre-operative images of the patient and the patient trackers <NUM>, <NUM>. Coordinate transformer <NUM> also stores the data indicating the pose of the working end of the surgical instrument <NUM> relative to the instrument tracker <NUM>.

During the procedure, the coordinate transformer <NUM> receives the data indicating the relative poses of the trackers <NUM>, <NUM>, <NUM> to the localizer <NUM>. Based on these data and the previously loaded data, the coordinate transformer <NUM> generates data indicating the relative position and orientation of both the coordinate system EAPP, and the bone coordinate systems, FBONE and TBONE to the localizer coordinate system LCLZ.

As a result, coordinate transformer <NUM> generates data indicating the position and orientation of the working end of the surgical instrument <NUM> relative to the tissue (e.g., bone) against which the instrument working end is applied. Image signals representative of these data are forwarded to displays <NUM>, <NUM> enabling the surgeon and staff to view this information. In certain embodiments, other signals representative of these data can be forwarded to the manipulator controller <NUM> to control the manipulator <NUM> and corresponding movement of the surgical instrument <NUM>.

Steps for determining the pose of each of the tracker coordinate systems BTRK1, BTRK2, TLTR in the localizer coordinate system LCLZ and systems and methods for determining the pose of the trackers <NUM>, <NUM>, <NUM> and the corresponding poses of the surgical instrument <NUM> with respect to the femur F and tibia T are described in greater detail in <CIT>, entitled "Navigation System Including Optical and Non-Optical Sensors".

In some embodiments, only one LED <NUM> can be read by the optical sensors <NUM> at a time. The camera controller <NUM>, through one or more infrared or RF transceivers (on camera unit <NUM> and trackers <NUM>, <NUM>, <NUM>), or through a wired connection, may control the firing of the LEDs <NUM>, as described in <CIT>. Alternatively, the trackers <NUM>, <NUM>, <NUM> may be activated locally (such as by a switch on trackers <NUM>, <NUM>, <NUM>) which then fires its LEDs <NUM> sequentially once activated, without instruction from the camera controller <NUM>.

One embodiment of trackers <NUM>, <NUM> is shown in <FIG>. Trackers <NUM>, <NUM> are configured to be attached to bone and fixed in position relative to the bone. As a result, movement of the bone results in corresponding and like movement of the trackers <NUM>, <NUM>. In some versions of the navigation system <NUM>, both trackers <NUM>, <NUM> comprise the same or substantially the same components. In other versions, the trackers <NUM>, <NUM> differ in one or more components. For simplicity, only tracker <NUM> will be described below, but it is understood that tracker <NUM> may be the same or substantially the same as tracker <NUM>.

Referring to <FIG>, tracker <NUM> includes a base for attaching to the patient's anatomy. The base can be attached directly to the anatomy being tracked or through other tissue. In the embodiment shown, the base is a bone plate <NUM> for attaching to the patient's bone - for instance the femur F. A plurality of fasteners secures the bone plate <NUM> in place. In one embodiment, the fasteners are bone screws <NUM>.

The bone plate <NUM> includes a plurality of protrusions for engaging the bone. In the embodiment shown, the protrusions are spikes <NUM>. Once the bone plate <NUM> is secured in place with one or more bone screws, the spikes <NUM> prevent rotation of the bone plate <NUM> relative to the bone.

An extension arm <NUM> is mounted to the bone plate <NUM>. The extension arm <NUM> has a base plate <NUM> that is secured to the bone plate <NUM> with one of the bone screws <NUM>. The extension arm <NUM> extends arcuately in a C-shape from the base plate <NUM> to a mounting end <NUM>.

A tracking head <NUM> is coupled to the mounting end <NUM> of the extension arm <NUM>. The tracking head <NUM> includes tracking elements. The tracking elements, in the embodiment shown and described, are the LEDs <NUM>, gyroscope sensor <NUM> (not shown), and accelerometer <NUM> (not shown). These tracking elements operate as previously described. In further embodiments, the tracking head <NUM> may include other types of tracking elements such as radio frequency receivers and/or transmitters, magnetic field sensors and/or generators, passive reflector balls, ultrasonic transmitters and/or receivers, or the like.

A connector assembly <NUM> couples the tracking head <NUM> to the extension arm <NUM>. The connector assembly <NUM> supports the tracking head <NUM> for movement in two degree of freedom. In the embodiment shown, the tracking head <NUM> is rotatably and tiltably mounted to the mounting end <NUM> of the support arm <NUM> via the connector assembly <NUM>.

Referring to <FIG>, the bone plate <NUM> is shown in greater detail. The bone plate <NUM> is generally triangular and concave between each spike <NUM> (see <FIG>). This concavity conforms to or otherwise accommodates the shape of bone or other tissue to which the bone plate <NUM> is to be secured.

The bone plate <NUM> has top and bottom surfaces <NUM>, <NUM> with three side surfaces <NUM>, <NUM>, <NUM>. The side surfaces <NUM>, <NUM>, <NUM> extend between the top and bottom surfaces <NUM>, <NUM>. The concavity of the bone plate <NUM> can be given by a radius of curvature R1 of the top surface <NUM> of from about <NUM> millimeters to about <NUM> millimeters and a radius of curvature R2 of the bottom surface <NUM> of from about <NUM> millimeters to about <NUM> millimeters (see <FIG>). In other embodiments, the radius of curvature R1 is from about <NUM> millimeters to about <NUM> millimeters and the radius of curvature R2 is from about <NUM> millimeters to about <NUM> millimeters.

Three spikes <NUM> are formed as integral extensions of surfaces <NUM>, <NUM>, <NUM>, <NUM>. Each spike <NUM> has a sharp tip <NUM> (see <FIG>) that is formed near the intersection of the bottom surface <NUM> and two adjacent side surfaces <NUM>, <NUM>, <NUM>. The bottom surface <NUM> extends arcuately along each spike <NUM> to the sharp tip <NUM> to form a gradual taper to the sharp tip <NUM>. The bottom surface <NUM> is generally concavely shaped between sharp tips <NUM>.

The sharp tips <NUM> are formed to cut through soft tissue, such as the periosteum, and pierce into bone when the bone plate <NUM> is secured to bone. When one or more of the sharp tips <NUM> pierce into bone, they, in conjunction with one or more of the bone screws <NUM>, prevent movement of the bone plate <NUM> relative to the bone.

The sharp tips <NUM>, when engaged in bone, also support the bone plate <NUM> to provide a space beneath the bone plate <NUM> and above the surface of the bone. In some cases, tissue such as muscle, ligaments, and the like may be present on top of the bone to which the bone plate <NUM> is to be secured. This tissue can be accommodated in this space without affecting the engagement of the sharp tips <NUM> in the bone.

Three openings <NUM> are defined through the bone plate <NUM> to receive the bone screws <NUM>. These three openings <NUM> have the cross-sectional configuration shown in <FIG>. One embodiment of this configuration is shown in <CIT>, in order to receive threaded heads <NUM> of the bone screws <NUM> shown in <FIG>.

Each of the openings <NUM> are defined about an axis A. Each opening <NUM> comprises a generally cylindrical throughbore <NUM> defined by inner surface <NUM>. The throughbore <NUM> is centered about the axis A.

An integral flange <NUM> is located in the throughbore <NUM> and directed radially inward toward axis A. The flange <NUM> is spaced from the top and bottom surfaces <NUM>, <NUM> of the bone plate <NUM>. This flange <NUM> is disposed annularly about axis A and generally perpendicular to axis A. The flange <NUM> tapers in cross-section from the inner surface <NUM> to an end surface <NUM>. The end surface <NUM> defines an opening (not numbered) that is cylindrical in shape. The taper of the flange <NUM> is symmetrically formed by upper and lower surfaces <NUM>, <NUM>. The upper surface <NUM> extends at an acute angle α from end surface <NUM> to inner surface <NUM>. The lower surface <NUM> extends at the same acute angle α from the end surface <NUM> to the inner surface <NUM>, but in the opposite direction.

Referring back to <FIG>, a recess <NUM> is defined in the top surface <NUM> of the bone plate <NUM>. The recess <NUM> is generally rectangular in shape for mating reception of the base plate <NUM> of the extension arm <NUM>. The base plate <NUM> is sized so that once located in the recess <NUM> the extension arm <NUM> is substantially prevented from rotation relative to the bone plate <NUM>.

A central opening <NUM> is located in the recess <NUM> and is defined through the bone plate <NUM>. The central opening <NUM> receives a bone screw <NUM> similar to the openings <NUM>, but has a different cross-section than openings <NUM>. The central opening <NUM> is a generally cylindrical throughbore of single diameter that is substantially perpendicular to the bone plate <NUM> at that location.

An axis C defines a center of the throughbore <NUM>, as shown in <FIG>. The openings <NUM> are spaced equidistantly from the axis C. Additionally, the openings <NUM> are spaced equally circumferentially about an imaginary circle defined through axes A of each of the openings <NUM> (see <FIG>).

Referring to <FIG>, a normal directional vector V to the bone plate <NUM> is defined downwardly along axis C toward the patient's anatomy when mounted. Inclined directional vectors I, arranged at acute angle β to the normal directional vector V, are defined downwardly along axes A. In one embodiment, these inclined directional vectors I are generally directed toward the normal directional vector V and cross the normal directional vector V at the same point INT along axis C. This orientation assists with preventing pull-out of the bone screws <NUM> from forces acting on the bone plate <NUM>.

The bone plate <NUM> may be formed of stainless steel, cobalt base alloys, bioceramics, titanium alloys, titanium, or other biocompatible materials.

Bone screws <NUM> are shown in <FIG>. In one embodiment, the bone screws are similar to those shown in <CIT>. Each of the bone screws <NUM> has self tapping threads <NUM> for engaging bone. The bone screws <NUM> also have the threaded heads <NUM> for engaging the flanges <NUM> of the openings <NUM>. The bone screws <NUM> may be of different sizes depending on the tissue to which they are being mounted. For instance, if the bone plate <NUM> is configured for mounting to a patella, smaller bone screws may be utilized. If the bone plate <NUM> is being mounted to the tibia, which is typically harder than other bones of the body, smaller bone screws may be used. For soft bone, longer bone screws that implant deeper into the bone may be desired.

<FIG> show the extension arm <NUM> in greater detail. The base plate <NUM> of the extension arm <NUM> defines a plate opening <NUM> that is the same in shape and size to the openings <NUM>. The plate opening <NUM> has the same features as openings <NUM> and will not be described further. Plate opening <NUM> receives a central fastener such as bone screw <NUM> in the same manner as openings <NUM>. In the embodiment shown, the base plate <NUM> is secured to the bone plate <NUM> by virtue of compression of the base plate <NUM> against the bone plate <NUM> when the bone screw <NUM> is fastened to bone through the plate opening <NUM> and central opening <NUM>.

An arcuate segment <NUM> extends from the base plate <NUM> to the mounting end <NUM>. A rib <NUM> is disposed partially on the base plate <NUM>, extends along the arcuate segment <NUM>, and ends at the mounting end <NUM>. The rib <NUM> provides additional rigidity to the extension arm <NUM> to prevent bending, buckling, twisting, or other deformation of the extension arm <NUM>.

A mounting surface <NUM> is located at the mounting end <NUM> of the extension arm <NUM>. The mounting surface <NUM> is configured to support the connector assembly <NUM> and tracking head <NUM>. The mounting surface <NUM> is generally planar. A threaded opening <NUM> is defined through the mounting end <NUM> for receiving a threaded adjustment fastener <NUM> (see <FIG>).

The extension arm <NUM> interconnects the bone plate <NUM> and the tracking head <NUM>. The extension arm <NUM> spaces the tracking elements (such as LEDs <NUM>) of the tracking head <NUM> from the bone plate <NUM>. The tracking elements are spaced in this manner to extend above the anatomy thereby improving line-of-sight potential between the tracking elements and the optical sensors <NUM> of camera unit <NUM>.

Referring to <FIG>, the extension arm <NUM> is generally C-shaped to define a tissue receiving area <NUM> between the tracker head <NUM> and the bone plate <NUM>. The tissue receiving area <NUM> is configured to receive soft tissue such as skin, fat, muscle, etc. above the bone plate <NUM> when the bone plate <NUM> is mounted to the bone.

The tissue receiving area <NUM> enables the user to retract soft tissue away from bone, mount the bone plate <NUM> directly to the bone, and then release the soft tissue back to a position above the bone plate <NUM>. Accordingly, the soft tissue is not required to be continually retracted during the entire surgical procedure. <FIG> shows layers of skin, fat, muscle, and fascia being located in the tissue receiving area <NUM> while the bone plate <NUM> is mounted to the femur F. In particular, the sharp tips <NUM> penetrate through the periosteum into the hard cortical bone of the femur F.

The bone plate <NUM> is firmly mounted in bone unicortically - meaning the bone screws <NUM> only penetrate the cortical bone layer once, from the outside.

The extension arm <NUM> may be formed of stainless steel, cobalt base alloys, bioceramics, titanium alloys, titanium, or other biocompatible materials.

Referring to <FIG>, the tracking head <NUM> includes the plurality of LEDs <NUM>, gyroscope sensor <NUM> (not shown), accelerometer <NUM> (not shown), and a transceiver (not shown) for receiving and transmitting signals to and from the camera unit <NUM> and/or navigation computer <NUM>. The tracking head <NUM> may also be connected to the navigation computer <NUM> via a wired connection as previously described.

The tracking head <NUM> includes a first hinge member <NUM> for mounting to the connector assembly <NUM>. The first hinge member <NUM> defines a non-threaded bore <NUM>.

The connector assembly <NUM> is shown in <FIG> and <FIG>. The connector assembly <NUM> includes a connector <NUM> for interconnecting the tracking head <NUM> and the extension arm <NUM>. The connector <NUM> includes a pair of second hinge members <NUM>. A space <NUM> is defined between the second hinge members <NUM> for receiving the first hinge member <NUM>. One of the second hinge members <NUM> has a non-threaded bore <NUM>, while the other has a threaded bore <NUM>. A threaded adjustment fastener <NUM> (see <FIG>) passes through the non-threaded bore <NUM> into the threaded bore <NUM>.

When tightening the adjustment fastener <NUM> the second hinge members <NUM> are drawn together to compress against the first hinge member <NUM>. This prevents movement of the first hinge member <NUM> relative to the second hinge members <NUM>. When the adjustment fastener <NUM> is loosened, the second hinge members <NUM> relax to a non-compressed position in which the first hinge member <NUM> is freely movable in the space <NUM>.

The tracking head <NUM> can be tilted relative to the bone plate <NUM> via the hinge created by the hinge members <NUM>, <NUM>. Tilting occurs in one degree of freedom about pivot axis P (see <FIG>). Pivot axis P is defined centrally through the bores <NUM>, <NUM>, <NUM>.

The connector <NUM> also has a rotational base <NUM>. The rotational base <NUM> is integral with the second hinge members <NUM>. The rotational base <NUM> has a flat bottom (not numbered) for mating with the mounting surface <NUM>.

The rotational base <NUM> defines an opening <NUM>. The opening <NUM> is shaped to receive a frusto-conical head (not separately numbered) of the adjustment fastener <NUM> (see <FIG>). The opening <NUM> also has a cylindrical bore <NUM>. The bore <NUM> is shaped so that a threaded shaft (not separately numbered) of the adjustment fastener <NUM> passes therethrough into the threaded opening <NUM> in the mounting end <NUM> of the extension arm <NUM>.

When tightening the adjustment fastener <NUM> the rotational base <NUM> is drawn against the mounting surface <NUM>. Friction between the bottom of the rotational base <NUM> and the mounting surface <NUM> prevents rotational movement of the connector <NUM> relative to the mounting surface <NUM>. When the adjustment fastener <NUM> is loosened, the connector <NUM> can be rotated freely relative to the mounting surface <NUM>. Thus, the tracking head <NUM> can be rotated relative to the bone plate <NUM>. Rotation occurs in one degree of freedom about rotational axis R (see <FIG>). Rotational axis R is defined centrally through bore <NUM> and threaded opening <NUM>.

Some of the tracking elements, such as the LEDs <NUM>, rely on line-of-sight with the optical sensors <NUM> to transmit tracking signals to the optical sensors <NUM>. As a result, these tracking elements are also referred to as line-of-sight tracking elements. These tracking elements must be within the field of view of the camera unit <NUM> and not be blocked from transmitting tracking signals to the camera unit <NUM>. When the signal path of one or more tracking elements is obstructed, an error message, in certain situations, may be generated.

The optical sensors <NUM> of the navigation system <NUM> are configured to receive signals from the LEDs <NUM>. The navigation system <NUM> controls activation of the LEDs <NUM>, as previously described, so that the navigation system <NUM> can anticipate when a signal should be received. When an anticipated signal from an LED <NUM> is not received one possibility is that the signal path is obstructed and the signal is blocked from being sent to the camera unit <NUM>. Another possibility is that the LED <NUM> is not functioning properly.

The navigation computer <NUM> determines that there is an error if any one of the optical sensors <NUM> fails to receive a signal from an LED <NUM>, even though other sensors <NUM> still receive the signal. In other embodiments, navigation computer <NUM> determines that there is an error if none of the optical sensors <NUM> receive the signal. In either case, when the navigation system <NUM> determines that there is an error based on the failure of one or more sensors <NUM> to receive signals from one or more LEDs <NUM>, an error signal is generated by the navigation computer <NUM>. An error message then appears on displays <NUM>, <NUM>. The navigation computer <NUM> also transmits an error signal to the tracker controller <NUM>.

An error indicator <NUM> is located on tracking head <NUM> in the embodiment shown in <FIG>. The tracker controller <NUM> activates the indicator <NUM> so that the user is aware of the error, e.g., that the signal from one or more of the LEDs <NUM> is blocked. Since the navigation computer <NUM> can determine which specific LED or LEDs <NUM> has failed to successfully transmit a signal to the optical sensor or sensors <NUM>, in some embodiments each LED <NUM> on the tracker <NUM> may have a separate indicator so that the user knows specifically which of the LEDs <NUM> are blocked.

The indicator <NUM> includes indicating light emitters such as indicating light emitting diodes (LEDs) <NUM>. The indicating LEDs <NUM> emit a first colored light when the tracker controller <NUM> receives the error signal from the navigation computer <NUM>, such as a red, yellow, or orange colored light. The indicating LEDs <NUM> emit a second colored light when no error signal is received or when an all clear signal is received from the navigation computer <NUM> after the error is cleared, such as a green or blue colored light. This indicates that the line-of-sight is not broken and is being maintained between at least a required number of the LEDs <NUM> and the optical sensor or sensors <NUM>. When an error is again detected the light changes from the second colored light to the first colored light. It should be appreciated that the indicating LEDs <NUM> may include separate indicating LEDs that are alternately activated based on the error status - one or more first colored indicating LEDs for error and one or more second colored indicating LEDs for no error. In other embodiments, the indicator <NUM> may include an LED or LCD display with error message and/or audible alerts when there is an error.

Tracking head <NUM> has a body <NUM> supporting the LEDs <NUM>. The body <NUM> defines openings (not numbered) covered by transparent windows <NUM>. The LEDs <NUM> are located inside the body <NUM> behind the windows <NUM> so that light from the LEDs <NUM> can be emitted through the windows <NUM> so that the light is visible to the user. The windows <NUM> may also include lenses (not shown) to provide desired lumination characteristics for the LEDs <NUM>.

In another embodiment shown in <FIG>, tracking head <NUM> supports first indicator LEDs <NUM> that emit orange colored light and second indicator LEDs <NUM> that emit green colored light. The tracking head <NUM> includes a top <NUM> supporting the LEDs <NUM>. Sapphire domes <NUM> cover the LEDs <NUM>. The tracking head <NUM> further includes a bottom <NUM>.

A light ring <NUM> is captured between the top <NUM> and bottom <NUM>. The indicator LEDs <NUM>, <NUM> are located inside the tracking head <NUM> within the light ring <NUM>, as schematically shown in <FIG>. The light ring <NUM> is preferably formed of white alumina material and has a rectangular ring shape. The light ring <NUM> illuminates either orange or green, depending on which of the indicator LEDs <NUM>, <NUM> are activated. The indicator LEDs <NUM>, <NUM> are in electronic communication with the tracker controller <NUM>, as shown in <FIG>. Weld rings <NUM> are located between the light ring <NUM> and the bottom <NUM> to facilitate assembly.

The light ring <NUM> is illuminated with the orange colored light from the first indicator LEDs <NUM> when the tracker controller <NUM> receives the error signal from the navigation computer <NUM>. The light ring <NUM> is illuminated with the green colored light from the second indicator LEDs <NUM> when no error signal is received or when an all clear signal is received from the navigation computer <NUM> after the error is cleared. When an error is again detected the light ring <NUM> changes from being illuminated green to being illuminated orange. The indicator LEDs <NUM>, <NUM> are alternately activated based on the error status - one or more first indicator LEDs <NUM> are activated for error conditions and one or more second indicator LEDs <NUM> are activated when no error conditions exist.

Alternating activation of the indicator LEDs <NUM>, <NUM> are carried out using the circuit shown in <FIG> shows an electrical schematic of the tracker controller <NUM> and the indicator LEDs <NUM>, <NUM>. The electrical schematic shows the electrical components that facilitate alternating activation/deactivation of the indicator LEDs <NUM>, <NUM>.

Each of the indicator LEDs <NUM>, <NUM> includes an anode <NUM> and a cathode <NUM>. In <FIG>, the anode <NUM> of each of the indicator LEDs <NUM>, <NUM> connects to a first voltage reference <NUM> and the cathode <NUM> of each of the indicator LEDs <NUM>, <NUM> connects to a second voltage reference <NUM>. In one embodiment, the first voltage reference <NUM> is +<NUM> VDC and the second voltage reference <NUM> is signal ground. The anode <NUM> of each of the indicator LEDs <NUM>, <NUM> may connect to the first voltage reference <NUM> separately, or in combination, as shown in <FIG>.

Switching elements M1, M2 respectfully connect between the cathodes <NUM> of the indicator LEDs <NUM>, <NUM> and the second voltage reference <NUM>. The switching elements M1, M2 control current flow through the indicator LEDs <NUM>, <NUM> thereby controlling operation of the indicator LEDs <NUM>, <NUM>. The switching elements M1, M2 may be further defined as transistors, and more specifically, N-channel MOSFETs. Each of the MOSFETs M1, M2 includes a gate, source, and drain. The gate of each MOSFET M1, M2 connects to the tracker controller <NUM>. The source of each MOSFET M1, M2 connects to the second reference voltage <NUM>, and more specifically, signal ground. The drain of each MOSFET M1, M2 connects to the cathode <NUM> of the respective indicator LED <NUM>, <NUM>. Resistors R1, R2 respectfully connect between the cathode <NUM> of each of the indicator LEDs <NUM>, <NUM> and the drain of each MOSFET M1, M2. The resistors R1, R2 limit current flow through the indicator LEDs <NUM>, <NUM> to suitable operating levels.

The tracker controller <NUM> controls activation/deactivation of the indicator LEDs <NUM>, <NUM>. The tracker controller <NUM> selectively controls the switching elements M1, M2 to allow or prevent current flow through the indicator LEDs <NUM>, <NUM>.

In one embodiment, the tracker controller <NUM> sends a first indicator control signal to the gate of the MOSFET M1. The tracker controller <NUM> may send the first indicator control signal in response to the tracker controller <NUM> receiving the error signal from the navigation computer <NUM>. The first indicator control signal causes the MOSFET M1 to form a closed circuit path between the source and the drain such that current freely flows through indicator LED <NUM> between the first and second voltage references <NUM>, <NUM> to illuminate indicator LED <NUM>.

In other embodiments, the tracker controller <NUM> sends a second indicator control signal to the gate of the MOSFET M2. The tracker controller <NUM> may send the second indicator control signal in response to the tracker controller <NUM> receiving an all clear signal or no error signal from the navigation computer <NUM>. In turn, the second indicator control signal causes the MOSFET M2 to form a closed circuit path between the source and the drain such that current freely flows through indicator LED <NUM> between the first and second voltage references <NUM>, <NUM> to illuminate indicator LED <NUM>.

The first and second indicator control signals may correspond to any suitable predetermined voltage or current for controlling MOSFETs M1, M2.

The tracker controller <NUM> may alternate activation/deactivation of the indicator LEDs <NUM>, <NUM>. In one embodiment, the tracker controller <NUM> sends the second indicator control signal to the gate of the MOSFET M2 to deactivate indicator LED <NUM>. According to one embodiment, the tracker controller <NUM> does so during activation of indicator LED <NUM>. To deactivate indicator LED <NUM>, the second indicator control signal causes the MOSFET M2 to form an open circuit between the source and the drain such that current is prevented from flowing through indicator LED <NUM> between the first and second voltage references <NUM>, <NUM>. Alternatively, the tracker controller <NUM> may send the first indicator control signal to the gate of the MOSFET M1 to deactivate indicator LED <NUM> during activation of LED indicator <NUM>.

In one embodiment, the indicator LEDs <NUM>, <NUM> may be selectively detachable from and attachable to the tracking head <NUM>. The tracking head <NUM> may include a printed circuit board (PCB) assembly (not shown) disposed therein with the indicator LEDs <NUM>, <NUM> being electrically connected to the PCB assembly. In <FIG>, the indicator LEDs <NUM>, <NUM> are coupled to a unit <NUM> that is detachable from and attachable to the PCB assembly. For simplicity, the unit <NUM> is represented in <FIG> by a dotted line surrounding indicator LEDs <NUM>, <NUM>. The unit <NUM> includes a first, second, and third terminal <NUM>, <NUM>, <NUM> for selectively connecting the indicator LEDs <NUM>, <NUM> to the PCB assembly. The first terminal <NUM> selectively connects the anode <NUM> of each of the indicator LEDs <NUM>, <NUM> to the first voltage reference <NUM>. The second and third terminals <NUM>, <NUM> selectively connect the cathode <NUM> of each of the indicator LEDs <NUM>, <NUM> to the MOSFETs M1, M2 for each respective indicator LED <NUM>, <NUM>.

Control of the LEDs <NUM> is carried out using the circuit shown in <FIG> shows an electrical schematic of the LEDs <NUM> connected to the tracker controller <NUM>. The electrical schematic shows the electrical components that facilitate control of the LEDs <NUM>.

In <FIG>, each of the LEDs <NUM> includes an anode <NUM> and a cathode <NUM>. The anode <NUM> of each of the LEDs <NUM> connects to a third voltage reference <NUM> while the cathode <NUM> of each of the LEDs <NUM> connects to a fourth voltage reference <NUM>. In one embodiment, the third voltage reference <NUM> is +<NUM> VDC and the fourth voltage reference <NUM> is signal ground. The anode <NUM> of each of the LEDs <NUM> may connect to the third voltage reference <NUM> separately, or in combination, as shown in <FIG>.

Switching elements M3, M4, M5, M6 respectively connect to the cathodes <NUM> of the LEDs <NUM>. The switching elements M3, M4, M5, M6 control current flow through the LEDs <NUM> thereby controlling operation of the LEDs <NUM>. The switching elements M3, M4, M5, M6 may be further defined as transistors, and more specifically, N-channel MOSFETs. Each of the MOSFETs M3, M4, M5, M6 includes a gate, source, and drain. The gate of each MOSFET M3, M4, M5, M6 connects to the tracker controller <NUM>. The source of each MOSFET M3, M4, M5, M6 ultimately connects to the fourth voltage reference <NUM>, and more specifically, signal ground. In <FIG>, the sources of the MOSFETs M3, M4, M5, M6 are connected to a shared line <NUM> in a parallel configuration. The drain of each MOSFET M3, M4, M5, M6 respectively connects to the cathode <NUM> of one of the LEDs <NUM>.

The tracker controller <NUM> controls activation/deactivation of the LEDs <NUM>. Mainly, the tracker controller <NUM> selectively controls the switching elements M3, M4, M5, M6 to allow or prevent current flow through the LEDs <NUM>.

In one embodiment, the tracker controller <NUM> receives an input signal indicating how the tracker controller <NUM> is to control any given LED <NUM> or combination of LEDs <NUM>. The tracker controller <NUM> may receive the input signal from the navigation computer <NUM>. In response, the tracker controller <NUM> sends an LED control signal to the gate of the MOSFET or MOSFETs M3, M4, M5, M6 connected to any given LED <NUM> or combination of LEDs <NUM>. In <FIG>, the tracker controller <NUM> sends four separate LED control signals and each LED control signal is sent to the gate of each MOSFET M3, M4, M5, M6. In one embodiment, the tracker controller <NUM> sequentially sends the control signals to the MOSFETs M3, M4, M5, M6 to sequentially control activation or deactivation of the LEDs <NUM>. The tracker controller <NUM> may send the LED control signals to the MOSFETs M3, M4, M5, M6 for controlling the LEDs <NUM> according various other configurations, sequences, cycles, or combinations.

If the input signal indicates that the tracker controller <NUM> is to activate any given LED <NUM>, the tracker controller <NUM> sends the LED control signal to the gate of the respective MOSFET M3, M4, M5, M6. Doing so causes the MOSFET M3, M4, M5, M6 to form a closed circuit path between the source and the drain of such that current freely flows through the respective LED <NUM> between the third and fourth voltage references <NUM>, <NUM> to illuminate the respective LED <NUM>.

If no input signal is received by the tracker controller <NUM> or when the input signal indicates that the tracker controller <NUM> is to deactivate any given LED <NUM>, the tracker controller <NUM> sends the LED control signal to the gate of the MOSFET M3, M4, M5, M6 to deactivate the given LED <NUM>. Mainly, the LED control signal causes the MOSFET M3, M4, M5, M6 to form an open circuit between the source and the drain of the MOSFET such that current is prevented from flowing between the third voltage reference <NUM> and signal ground <NUM> through the LED <NUM>.

The LEDs <NUM> may be detachable from and attachable to the tracking head <NUM>. In <FIG>, each LED <NUM> is coupled to a unit <NUM> that is detachable from and attachable to the PCB assembly. For simplicity, each unit <NUM> is represented in <FIG> by a dotted line surrounding each of the LEDs <NUM>. In one embodiment, each unit <NUM> includes a first and a second terminal <NUM>, <NUM> for selectively connecting each LED <NUM> to the PCB assembly. In <FIG>, the first terminal <NUM> selectively connects the anode <NUM> of each LED <NUM> to the third voltage reference <NUM>. The second terminal <NUM> selectively connects the cathode <NUM> of each LED <NUM> to drain of each respective MOSFET M3, M4, M5, M6.

In <FIG>, a voltage sensing circuit <NUM> is provided for measuring operating voltages of any given LED <NUM> or combination of LEDs <NUM>. The voltage sensing circuit includes a resistor R3 and a capacitor C1 that are arranged as a series RC circuit. The voltage sensing circuit <NUM> is generally connected between the LEDs <NUM> and the tracker controller <NUM>. Resistor R3 connects to the sources of the MOSFETs M3, M4, M5, M6 at the shared line <NUM> and capacitor C1 connects between resistor R3 and the fourth reference voltage <NUM>. In one embodiment, the voltage sensing circuit <NUM> measures operating voltage of the LEDs <NUM> between the third and fourth voltage reference <NUM>, <NUM>.

The voltage sensing circuit <NUM> sends to the tracker controller <NUM> a voltage sense signal representing a measured operating voltage of the LEDs <NUM>. In one embodiment, the voltage sensing circuit <NUM> protects the LEDs <NUM> from inappropriate voltage conditions. The tracker controller <NUM> may process the voltage sense signal and, in response, modify the LED control signal based on the value of the voltage sense signal. For instance, the tracker controller <NUM> may change the voltage of the LED control signal(s) if the tracker controller <NUM> determines that the voltage sense signal is above a predetermined threshold level. In another embodiment, the tracker controller <NUM> utilizes the voltage sensing circuit <NUM> for determining whether an LED <NUM> is malfunctioning. The tracker controller <NUM> can communicate the malfunction of the LED <NUM> to the navigation system <NUM> so that the navigation system <NUM> can anticipate such malfunction and respond accordingly. The voltage sensing circuit <NUM> may be implemented according to various other configurations and methods.

In <FIG>, a current sensing circuit <NUM> is provided for measuring operating currents of any given LED <NUM> or combination of LEDs <NUM>. The current sensing circuit <NUM> protects the LEDs <NUM> from inappropriate current conditions. The current sensing circuit <NUM> includes a resistor R4 and a capacitor C2 that are arranged as a series RC circuit. The current sensing circuit <NUM> is connected generally between the LEDs <NUM> and the tracker controller <NUM>. Resistor R4 connects to the sources of the MOSFETs M3, M4, M5, M6 at the shared line <NUM> and capacitor C2 connects between resistor R4 and signal ground <NUM>. In one embodiment, the current sensing circuit <NUM> measures total operating current of the LEDs passing between the third and fourth voltage reference <NUM>, <NUM>.

In one mode of operation, the current sensing circuit <NUM> provides to the tracker controller <NUM> a current sense signal. The current sense signal may be derived from the measured operating current of the LEDs <NUM>. The tracker controller <NUM> may process the current sense signal and determine whether the current sense signal conforms to a predetermined value. For instance, the tracker controller <NUM> may determine that the current sense signal is above a predetermined threshold voltage level.

A current limiting circuit <NUM> is further provided in <FIG> for limiting current provided to the LEDs <NUM>. In <FIG>, the current limiting circuit <NUM> includes an amplifier <NUM> for regulating the current through the LEDS <NUM>. The amplifier <NUM> includes a first and second input terminal <NUM>, <NUM> and an output terminal <NUM>. The current limiting circuit <NUM> includes a MOSFET M7 for controlling the current through the LEDs <NUM>. The MOSFET M7 includes a gate, a source, and a drain.

The first input terminal <NUM> of the amplifier <NUM> connects to the tracker controller <NUM>. The second input terminal <NUM> of the amplifier <NUM> connects to the gate and the source of the MOSFET M7. A capacitor C3 is included between the second input terminal <NUM> and the gate of the MOSFET M7. A resistor R5 is included between the second input terminal <NUM> and the source of the MOSFET M7. Resistor R6 connects between the source of the MOSFET M7 and the signal ground <NUM>. The output terminal <NUM> of the amplifier <NUM> connects to the gate of the MOSFET M7. Resistor R7 connects between the output <NUM> of the amplifier <NUM> and the gate of the MOSFET M7. The drain of the MOSFET M7 connects to the sources of the MOSFETs M3, M4, M5, M6 at the shared line <NUM>.

In one mode of operation, the tracker controller <NUM> sends a current limiting signal to the current limiting circuit <NUM>. The tracker controller <NUM> may send the current limiting signal based on the value of the current sense signal provided by the current sensing circuit <NUM>. In <FIG>, the current limiting signal sent by the tracker controller <NUM> is received at the first input terminal <NUM> of the amplifier <NUM>. The amplifier <NUM> controls the MOSFET M7 based on the current limiting signal received from the tracker controller <NUM>. In one instance, the amplifier <NUM> deactivates each of the LEDs <NUM> in response to the current limiting signal. Specifically, the amplifier <NUM> delivers a signal from the output terminal <NUM> to the gate of MOSFET M7. Doing so causes MOSFET M7 to form an open circuit between the source and the drain. As such, current is prevented from flowing between the third and fourth voltage reference <NUM>, <NUM> thereby deactivating each of the LEDs <NUM>. In other embodiments, the amplifier limits current through the LEDs <NUM>, but does not entirely deactivate the LEDs <NUM>, in response to the current limiting signal.

The current sensing circuit <NUM> and the current limiting circuit <NUM> may be implemented according to various other configurations and methods.

In some embodiments, the tracker <NUM> may include four or more tracking LEDs <NUM> so that if the tracking signal from one of the LEDs <NUM> is obstructed, position and orientation data can still be obtained from the remaining LEDs <NUM>. In this instance, before any error signals are generated, the navigation computer <NUM> will first run through a complete tracking cycle. The complete tracking cycle includes sequentially activating all the LEDs <NUM> on the tracker <NUM> to determine if the optical sensors <NUM> receive tracking signals from at least three of the LEDs <NUM> in the tracking cycle. The error signal is then generated if an optical sensor <NUM> (or all optical sensors <NUM> in some embodiments) did not receive tracking signals from at least three LEDs <NUM> in the tracking cycle.

The navigation system <NUM> is configured to assist with positioning of the tracker <NUM> by the surgeon or other medical personnel. This assistance helps to place the tracking head <NUM> (or tracking heads <NUM>, <NUM>) in a desired orientation that provides line-of-sight between the LEDs <NUM> and the optical sensors <NUM> and helps to reduce line-of-sight errors that may otherwise be encountered during a surgical procedure.

Once the bone plate <NUM> is mounted to the bone, such as femur F, the tracking head <NUM> is movable relative to the bone plate <NUM> via the connector assembly <NUM>. In particular, the tracking head <NUM> is movable about pivot axis P and rotational axis R (see <FIG>). The connector assembly <NUM> allows movement of the tracking head <NUM> in these two degrees of freedom relative to the bone plate <NUM> to place the tracking head <NUM> in the desired orientation. This helps to provide the line-of-sight between the LEDs <NUM> and the optical sensors <NUM>.

Before navigation begins, the medical personnel are instructed to place the tracking head <NUM> in an initial orientation in which, visually, the tracking head <NUM> appears to be oriented so that the LEDs <NUM> will be within the line-of-sight of the optical sensors <NUM> and unobstructed throughout the surgical procedure. Once the user has placed the tracking head <NUM> in the initial orientation, the navigation computer <NUM> provides instructions to the medical personnel setting up the tracker <NUM> on how to further move the tracking head <NUM> to reach the desired orientation, if necessary.

The navigation computer <NUM> first determines the initial orientation of the tracking head <NUM> and whether the tracking head <NUM> is already in the desired orientation. If the tracking head <NUM> is not in the desired orientation, the navigation system <NUM>, through software instructions displayed on displays <NUM>, <NUM>, instructs the user to move the tracking head <NUM> relative to the bone plate <NUM> in one or more of the degrees of freedom to reach the desired orientation.

The tracker <NUM> includes an orientation sensor <NUM>, as shown in <FIG>. In one embodiment, the orientation sensor <NUM> is a three-axis gravity sensor <NUM> configured to measure gravity (acceleration) along three axes, the x-axis, y-axis, and z-axis.

The gravity sensor <NUM> is located inside the tracking head <NUM>. The gravity sensor <NUM> is operatively connected to the tracker controller <NUM>. The tracker controller <NUM> receives gravity measurements from the gravity sensor <NUM> with respect to the x-axis, y-axis, and z-axis. These signals are analyzed by the navigation system <NUM> to determine the current orientation of the tracking head <NUM> relative to gravity. It should be appreciated that the accelerometer <NUM>, in some embodiments, could be used as the gravity sensor <NUM>.

Referring to <FIG>, the x-axis and y-axis are oriented in a plane of the tracking head <NUM> that is parallel with a front face of the tracking head <NUM>. The z-axis is oriented perpendicular to the x-axis and y-axis. When the gravity measurement along the z-axis is zero, then gravity is not acting along the z-axis meaning that the x-y plane of the tracking head <NUM> is oriented vertically.

Referring to <FIG>, in one embodiment, the desired orientation includes having the tilt angle about pivot axis P and the rotational axis R set to angles resulting in the z-axis gravity measurement being zero.

Referring to <FIG>, the signals transmitted by the LEDs <NUM> are considered line-of-sight tracking signals that have a central signal axis <NUM> defining the center of a signal emitting region <NUM>. The desired orientation is further defined in one embodiment as the central signal axes <NUM> of the LEDs <NUM> being perpendicular to the direction of gravity, e.g., perpendicular to a vector <NUM> oriented in the direction of gravity.

In this embodiment, the navigation computer <NUM> is configured to instruct the user to adjust the tilt angle of the tracking head <NUM> until the z-axis gravity measurement is zero. This occurs when the x-y plane of the tracking head <NUM> is oriented vertically relative to the gravity vector <NUM>, as shown in <FIG>.

The instructions to the user to adjust the tilt angle are carried out by the navigation system <NUM> through displays <NUM>, <NUM>, in which the current orientation of the tracking head <NUM> is graphically represented. The current orientation is dynamically adjusted as the user changes the tilt angle. The desired orientation of the tracking head <NUM> is also shown graphically so that the user can visually determine how close the current orientation is to the desired orientation. When the current orientation is at the desired orientation, the displays <NUM>, <NUM> may flash green or have some other visual indicator that the tracking head <NUM> is in the desired orientation relative to gravity. It should be appreciated that the desired orientation may include predefined deviations from an ideal orientation in which the x-y plane is perfectly vertical relative to gravity, such as deviations of +/- ten percent, +/five percent, or +/- two percent.

In alternative embodiments, an LED on the tracking head <NUM> may indicate to the user when the current orientation is at the desired orientation by being activated to emit a green colored light. Alternatively, an audible indicator may be provided on the tracker <NUM> to indicate that the tracking head <NUM> is in the desired orientation.

Once the tilt angle is set so that the tracking head <NUM> is at the desired orientation relative to gravity (see, e.g., <FIG>), the adjustment fastener <NUM> of the connector assembly <NUM> is tightened so that the tracking head <NUM> is unable to tilt relative to the bone plate <NUM>. In some cases only tilt adjustment of the tracking head <NUM> may be necessary to place the tracking head <NUM> in the desired orientation.

In other cases, rotational adjustment about rotational axis R may be needed after the tilt adjustment to place the tracking head <NUM> in the desired orientation. In these cases, the desired orientation may include an additional rotational adjustment in which the gravity measurement along the z-axis moves from being approximately zero to being non-zero. Adjustment may also be iterative in which tilt adjustment is performed first to place the tracking head <NUM> vertically, then rotational adjustment is performed which causes the gravity measurement along the z-axis to be non-zero, and then further tilt adjustment is performed to place the z-axis back to approximately zero (i.e., to move the tracking head <NUM> back to vertical).

During rotational adjustment, the LEDs <NUM> are being tracked by the optical sensors <NUM>. In particular, the navigation computer <NUM> is configured to determine, based on the signals received by the optical sensors <NUM>, which rotational orientation of the tracking head <NUM> provides the best line-of-sight to the LEDs <NUM>.

The desired rotational orientation can be determined by instructing the user to rotate the tracking head <NUM> through a maximum range of movement, e.g., <NUM> degrees, one or more times. While the tracking head <NUM> is rotated, the navigation computer <NUM> determines at which rotational positions (e.g., rotational angles) about axis R line of sight for each LED <NUM> is present and at which rotational positions there is no line of sight for each LED <NUM>. This may include rotating the tracking head <NUM> though its maximum range of movement at various positions of the knee joint, i.e., at the flexed and at the extended positions of the knee joint. This will determine a range of line-of-sight positions about axis R for each LED <NUM>. A best fit algorithm can then be used to determine the positions about axis R that best fits within the ranges of line-of-sight positions for all of the LEDs <NUM>.

The navigation computer <NUM> instructs the user, through the displays <NUM>, <NUM> to rotate the tracking head <NUM> until the current rotational position meets the desired rotational position determined by the navigation computer <NUM>. See, for instance, the desired rotational position shown in <FIG>, as compared to a current rotational position shown in <FIG>. Instruction to the user may be accomplished by showing a representation of the current rotational position and the desired rotational position on the displays <NUM>, <NUM> and dynamically changing the current rotational position as the user makes the adjustment toward the desired rotational position.

Like with adjusting the tilt angle, an LED on the tracking head <NUM> may indicate to the user when the rotational position is at the desired rotational position by being activated to emit a green colored light. Alternatively, an audible indicator may be provided on the tracker <NUM> to indicate that the current rotational position is at the desired rotational position. It should be appreciated that the desired rotational position may include predefined deviations from an ideal rotational position, such as deviations of +/- ten percent, +/- five percent, or +/- two percent.

When the desired rotational orientation is met, the adjustment fastener <NUM> of the connector assembly <NUM> is tightened so that the tracking head <NUM> is unable to rotate relative to the bone plate <NUM>. The tracking head <NUM> is now fixed from moving relative to the bone plate <NUM> and the bone.

In some embodiments, the LEDs <NUM> are being tracked by the optical sensors <NUM> during adjustment of both the tilt angle and rotational angle. In particular, the navigation computer <NUM> is configured to determine, based on the signals received by the optical sensors <NUM>, which tilt and rotational orientation of the tracking head <NUM> provides the best line-of-sight from the LEDs <NUM> to the optical sensors <NUM>.

In these embodiments, the desired orientation can be determined by instructing the user to tilt and rotate the tracking head <NUM> through their maximum ranges of movement, one or more times, either sequentially or alternately. While the tracking head <NUM> is tilted and rotated, the navigation computer <NUM> determines at which tilt and rotational positions (e.g., tilt and rotational angles) about axes P and R line of sight for each LED <NUM> is present and at which tilt and rotational positions there is no line of sight for each LED <NUM>. This will determine a range of line-of-sight positions about axes P and R for each LED <NUM>. A best fit algorithm can then be used to determine the positions about axes P and R that best fits within the ranges of line-of-sight positions for all of the LEDs <NUM>.

This process may be iterative and include several adjustments by the user about the axes P and R to find a suitable position for the tracking head <NUM>. In certain instances, the navigation computer <NUM> may be unable to identify an orientation in which line-of-sight is maintained for all of the LEDs <NUM> because of a poor initial orientation set by the user. In this case, the navigation computer <NUM> may first instruct the user to reorient the tracking head <NUM> so that the LEDs <NUM> visually appear to be facing the optical sensors <NUM> and then continue with measuring the orientation of the tracking head <NUM> through various movements to find the best fit that maintains the line-of-sight for all of the LEDs <NUM>.

In some cases, tilting adjustment may be processed first with the tracking head <NUM> being moved through its entire range of tilting motion, one or more times, and possibly at multiple knee positions including flexed and extended positions. The best fit tilt angle is then determined and the tilt angle is then fixed at the best fit tilt angle. The tracking head <NUM> can thereafter be moved through its entire range of rotational motion, one or more times, and possibly at multiple knee positions including flexed and extended positions. The best fit rotational angle is then determined and the rotational angle is then fixed at the best fit rotational angle.

In some embodiments, a third degree of freedom may be adjusted to a desired position, such as a height of the tracker <NUM>. In the embodiment shown only two degrees of freedom are adjusted due to the fact that the tracker <NUM> moves up and down as the surgeon flexes the knee joint. In this case, the LEDs <NUM> of the tracking head <NUM> may be raised or lowered relative to the optical sensors <NUM> without breaking the line-of-sight between the LEDs <NUM> and sensors <NUM>.

A screw driver <NUM> is shown in <FIG>. The screw driver <NUM> is used to place the bone screws <NUM>.

Referring to <FIG>, the screw driver <NUM> includes an integrated impactor <NUM> that punches each bone screw <NUM> into the bone prior to screwing. When the screw driver <NUM> is pressed against the bone screw <NUM>, the impactor <NUM> stores energy in a rear spring <NUM>. The energy is eventually released as a force that drives the bone screw <NUM> into the bone. The impactor <NUM> starts in a rest state in which no energy is stored, is operated to gradually increase the stored energy, and then is actuated to release the energy.

The screw driver <NUM> includes a body. The body comprises a nose tube <NUM>, middle tube <NUM>, and rear cap <NUM>. The nose tube <NUM>, middle tube <NUM>, and rear cap <NUM> are separate parts that are releasably connected together for purposes of assembling internal components. It should be appreciated that in other embodiments, the nose tube <NUM>, middle tube <NUM>, and rear cap <NUM> may be permanently fixed together.

The nose tube <NUM> has a generally conical distal end <NUM>. The nose tube <NUM> extends from its distal end <NUM> to an externally threaded proximal end <NUM>. The nose tube <NUM> is hollow. The nose tube <NUM> defines a proximal bore <NUM> and distal bore <NUM>. The proximal bore <NUM> is larger in cross-sectional area than the distal bore <NUM>. The proximal bore <NUM> is circular in cross-section and the distal bore <NUM> is hexagonal in cross-section.

The middle tube <NUM> is generally cylindrical. The middle tube <NUM> has an internally threaded distal end <NUM> and an externally threaded proximal end <NUM>. The internally threaded distal end <NUM> threads onto the externally threaded proximal end <NUM> of the nose tube <NUM>.

The rear cap <NUM> has a top <NUM>. The rear cap <NUM> extends distally from the top <NUM> to an internally threaded section <NUM>. The internally threaded section <NUM> threads onto the externally threaded proximal end <NUM> of the middle tube <NUM>.

The impactor <NUM> includes a hammer <NUM> disposed in the middle tube <NUM>. The hammer <NUM> is generally cylindrical in shape. The rear spring <NUM> is disposed between the rear cap <NUM> and the hammer <NUM>. The rear spring <NUM> biases the hammer <NUM> distally. Compression of the rear spring <NUM> can be adjusted by loosening or tightening the rear cap <NUM> to decrease or increase the stored energy of the hammer <NUM>.

A receiving hole <NUM> is formed in the hammer <NUM>. The receiving hole <NUM> is disposed about a central axis of the hammer <NUM>. The receiving hole <NUM> is cylindrically shaped. A cross bore <NUM> is formed in a direction perpendicular to the receiving hole <NUM> (see <FIG>).

The impactor <NUM> includes a trigger <NUM> located in the cross bore <NUM>. The trigger <NUM> is semi-cylindrical in shape. The trigger <NUM> has a flat bottom <NUM> (see <FIG>). A throughbore <NUM> is defined in the trigger <NUM>. When the impactor <NUM> is in the rest state, a leaf spring <NUM> biases the trigger <NUM> to a position in which the throughbore <NUM> is offset to the receiving hole <NUM>, i.e., they are misaligned.

A driving rod <NUM> ultimately receives the energy stored in the impactor <NUM> to drive in the bone screws <NUM>. The driving rod <NUM> has a cylindrical shaft <NUM> with a proximal end <NUM>. The proximal end <NUM> is shaped for mating reception within the receiving hole <NUM> of the hammer <NUM>.

A boss <NUM> is located on the proximal end <NUM> of the driving rod <NUM> to form a shoulder <NUM> (see <FIG>). In the rest state of the impactor <NUM>, the shoulder <NUM> engages the flat bottom <NUM> of the trigger <NUM>. In this state, the boss <NUM> protrudes into the throughbore <NUM> defined in the trigger <NUM>, but the shoulder <NUM> prevents the cylindrical shaft <NUM> from entering the throughbore <NUM>.

The driving rod <NUM> has a hexagonal shaft <NUM> with a distal end <NUM>. The hexagonal shaft <NUM> mates with the hexagonal-shaped distal bore <NUM> of the nose tube <NUM> so that rotation of the body by the user also rotates the hexagonal shaft <NUM>. The distal end <NUM> has features adapted to engage heads of the bone screws <NUM> to rotate and drive the bone screws <NUM> into the bone when the body of the screw driver <NUM> is rotated.

A collar <NUM> is fixed to the hexagonal shaft <NUM>. The collar <NUM> prevents the driving rod <NUM> from falling out of the nose tube <NUM>. The collar <NUM> is cylindrical in shape to mate with the proximal bore <NUM> of the nose tube <NUM>.

A spring cap <NUM> is centrally disposed inside the middle tube <NUM> between the nose tube <NUM> and the hammer <NUM>. The spring cap <NUM> is press fit inside the middle tube <NUM>.

A rod spring <NUM> is disposed about the cylindrical shaft <NUM> of the driving rod <NUM>. The rod spring <NUM> acts between the spring cap <NUM> and the collar <NUM> of the driving rod <NUM> to return the screw driver <NUM> to the rest state after the hammer <NUM> is actuated. The spring cap <NUM> defines a throughbore (not numbered) for receiving the cylindrical shaft <NUM> of the driving rod <NUM> and centering the cylindrical shaft <NUM> inside the middle tube <NUM>.

The screw driver <NUM> is pressed against the bone screw <NUM> by gripping the rear cap <NUM> and/or middle tube <NUM> and urging them distally. The bone screw <NUM> is thus pressed against the bone. At the same time, the driving rod <NUM> travels proximally in the middle tube <NUM>. The shoulder <NUM> pushes the trigger <NUM> proximally while the leaf spring <NUM> keeps the trigger <NUM> throughbore <NUM> misaligned with the receiving hole <NUM> of the hammer <NUM>.

The middle tube <NUM> includes an inclined inner surface <NUM>. The inclined inner surface <NUM> engages the trigger <NUM> when the trigger <NUM> reaches the inclined inner surface <NUM>. When this occurs, the inclined inner surface <NUM> acts like a cam to push the trigger <NUM> in a manner that centers the throughbore <NUM> of the trigger <NUM> and places the throughbore <NUM> into alignment with the receiving hole <NUM> of the hammer <NUM>. Likewise, the proximal end <NUM> of the driving rod <NUM> is now aligned to fit within the throughbore <NUM>, which allows the trigger <NUM> to slide down the driving rod <NUM> thereby releasing the hammer <NUM>. The hammer <NUM> then moves forward, propelled by the rear spring <NUM>.

Because the receiving hole <NUM> in the hammer <NUM> has a predefined depth, the boss <NUM> on the proximal end <NUM> of the driving rod <NUM> eventually bottoms out in the receiving hole <NUM> and the force of the hammer <NUM> is transmitted into the driving rod <NUM> and the bone screw <NUM> punching the bone screw <NUM> into the bone.

In one embodiment, when each of the trackers <NUM>, <NUM>, <NUM> are being actively tracked, the firing of the LEDs occurs such that one LED <NUM> from tracker <NUM> is fired, then one LED <NUM> from tracker <NUM>, then one LED <NUM> from tracker <NUM>, then a second LED <NUM> from tracker <NUM>, then a second LED <NUM> from tracker <NUM>, and so on until all LEDs <NUM> have been fired and then the sequence repeats. This order of firing may occur through instruction signals sent from the transceivers (not shown) on the camera unit <NUM> to transceivers (not shown) on the trackers <NUM>, <NUM>, <NUM> or through wired connections from the navigation computer <NUM> to the tracker controller <NUM> on each of the trackers <NUM>, <NUM>, <NUM>.

The navigation system <NUM> can be used in a closed loop manner to control surgical procedures carried out by surgical cutting instruments. Both the instrument <NUM> and the anatomy being cut are outfitted with trackers <NUM> such that the navigation system <NUM> can track the position and orientation of the instrument <NUM> and the anatomy being cut, such as bone.

In one embodiment, the navigation system <NUM> is part of a robotic surgical system for treating tissue. In some versions, the robotic surgical system is a robotic surgical cutting system for cutting away material from a patient's anatomy, such as bone or soft tissue. The cutting system could be used to prepare bone for surgical implants such as hip and knee implants, including unicompartmental, bicompartmental, or total knee implants. Some of these types of implants are shown in <CIT>, entitled, "Prosthetic Implant and Method of Implantation".

In one embodiment, the navigation system <NUM> communicates with a robotic control system (which can include the manipulator controller <NUM>). The navigation system <NUM> communicates position and/or orientation data to the robotic control system. The position and/or orientation data is indicative of a position and/or orientation of instrument <NUM> relative to the anatomy. This communication provides closed loop control to control cutting of the anatomy such that the cutting occurs within a predefined boundary.

Claim 1:
A navigation system (<NUM>) for tracking an object, the navigation system (<NUM>) comprising:
a tracking device (<NUM>, <NUM>, <NUM>) being attachable to the object and including a tracking head (<NUM>, <NUM>, <NUM>) having a tracking element (<NUM>, <NUM>, <NUM>) configured to produce a tracking signal, and wherein the tracking head (<NUM>, <NUM>, <NUM>) supports an error indicator (<NUM>) including at least one light emitter (<NUM>, <NUM>);
a sensor (<NUM>) configured to receive the tracking signal from the tracking element (<NUM>, <NUM>, <NUM>) to determine a position of the object; and
a computing system (<NUM>) configured to:
- determine whether the sensor (<NUM>) received the tracking signal from the tracking element (<NUM>, <NUM>, <NUM>) and
- generate an error signal when the sensor (<NUM>) does not receive the tracking signal, and
said computing system being characterized in that it is further configured to control the error indicator (<NUM>) such that:
the at least one light emitter (<NUM>, <NUM>) is activated to emit a colored light in response to absence of the error signal; and
the at least one light emitter (<NUM>, <NUM>) is deactivated in response to generation of the error signal.