Patent Publication Number: US-2022211441-A1

Title: Tracker For A Navigation System

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The subject application claims priority to and all the benefits of U.S. Provisional Patent App. No. 63/134,283, filed Jan. 6, 2021, the contents of which are hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     Surgical navigation systems assist users with locating objects in the surgical field. More particularly, optical navigation systems may employ light signals in order to track the position and orientation of the objects such as surgical instrumentation and patient anatomy. A localizer may cooperate with tracking elements on a tracker to determine the position and orientation of the objects. The tracking elements may be passive elements so as to reflect light, or active elements so as to emit the light itself. Other navigation systems may utilize electromagnetic or radiofrequency wavelengths. The tracker may be mounted to the surgical instrumentation with a suitable coupler, or to the patient anatomy with a suitable fastener. For example, a bone plate may be provided to secured to bony anatomy, after which the tracker is coupled to the bone plate with an arm or post. 
     The surgical field should be sterile, and surgical instrumentation that is reusable should be sterilized between surgical procedures. This is typically accomplished through autoclaving in which the instrumentation is subjected to pressurized steam for a duration. Owing to the materials forming the tracking elements, passive tracking elements are not well suited to endure repeated autoclaving without compromise of their light-reflecting qualities. For example, the passive tracking elements may include a reflective film that may deteriorate when repeatedly exposed to the pressurized steam. It is known to replace the tracking elements after each surgical procedure; however, doing so is cumbersome and associated with added expense. 
     Therefore, a need exists in the art for a tracker designed to overcome one or more of the aforementioned disadvantages. 
     SUMMARY 
     This Summary introduces a selection of concepts in a simplified form that are further described in the Detailed Description below. This Summary is not intended to limit the scope of the claimed subject matter and does not necessarily identify each and every key or essential feature of the claimed subject matter. 
     According to a first aspect, a tracker for a navigation system is provided. The tracker includes a tracker body that defines an opening extending through the tracker body, a lens and a tracking element positionable in the opening, and a plug to seal the opening at a first end, position the tracking element within the opening, and support the lens within the opening to seal the opening at a second end with the lens. 
     According to a second aspect, a tracker for a navigation system is provided. The tracker comprises: a tracker body defining a counterbore; a lens positionable within the counterbore; a plug comprising a plug body at least partially positionable within the counterbore; and a tracking element disposed between the lens and the plug body; wherein the plug is configured to be coupled with the tracker body to secure the lens and to define, with the lens, a sealed enclosure, wherein the tracking element is disposed within the sealed enclosure. 
     According to a third aspect, a tracker for a navigation system is provided. The tracker comprises: a tracker body having an upper surface and a lower surface opposite the upper surface, the tracker body defining an opening extending between the upper surface and the lower surface, and a lip surrounding the opening, and a complementary retention feature disposed between the upper surface and the lower surface; a lens configured to abut the lip; and a plug comprising a plug body, and a retention feature coupled to the plug body and configured to be removably coupled with the complementary retention feature of the tracker body, and wherein the plug is configured to support a tracking element, maintain abutment between the lens and the lip to provide a first seal for opening near the upper surface, and maintain abutment between the plug body and the tracker body to provide a second seal near the lower surface. 
     According to a fourth aspect, a tracker for a navigation system is provided. The tracker comprises: a tracker body defining an opening; a lens positionable to seal the opening; a tracking element; a sealing element configured to be compressed against the lens; and a plug comprising a plug body comprising an upper surface defining a groove and further defining a cavity separate from the groove, wherein the sealing element is configured to be disposed within the groove and the tracking element is configured to be disposed within the cavity. 
     According to a fifth aspect a navigation system includes the tracker according to any of the above aspects, and optionally, any of their corresponding implementations. The navigation system may include includes a camera include optical sensors configured to detect light reflected by the tracking element. 
     According to a sixth aspect, a method of assembling a tracker for use with a navigation system is disclosed. A sealing element is disposed within a groove defined by a plug. A tracking element is disposed within a cavity defined by the plug. A masking spacer may be positioned within the cavity and to rest upon the tracking element. The lens may be positioned to rest upon the sealing element. The plug may be coupled to the tracker body. For example, the tracking element assembly, which includes the lens, the masking spacer, the tracking element, the sealing element, and the plug, may be rotated relative to the tracker body to engage complementary threads. In certain implementations, the tracker body defines an opening and a counterbore separated by a lip. The plug may be at least partially positioned within the counterbore. The sealing element may be compressed against the lens with abutment between the lens and the lip sealing the opening. 
     Any of the aspects may be combined in part, or in whole. Any of the aspects may be utilized, in part, or in whole, with any of the following implementations. 
     In certain implementations, the tracker, when assembled, is configured to prevent the tracking element from being exposed to fluid, for example, pressurized steam during autoclaving. The tracking element assembly includes a lens, the tracking element, and a plug configured to be coupled to the tracker body. The coupling may be a removable coupling, or a permanent joining. The tracking element may be a passive element or an active element. Preventing exposure of the tracking element to the fluid may eliminate the need to separately sterilize the tracking element, thereby extending the operational lifespan of the tracking element. In some implementations, the tracker is specifically a surgical navigation tracker to be attached to a surgical object, such as a patient, a tool, a robotic manipulator, a surgical table, an imaging device, a hand-held tool, a hand-held scanner or imager, or the like. 
     In certain implementations, the lens may be transparent, and tracking element may be disposed between the lens and the plug body so as to be visible through the lens. The tracker body may define an opening, and a counterbore in fluid communication with the opening. The plug body may be at least partially positionable within the counterbore. The plug, when coupled to the tracker body, secures the lens in place. The lens and the plug define a sealed enclosure within which the tracking element is disposed. 
     In certain implementations, an upper surface of the lens is transparent, and a lower surface of the lens includes a reflective coating to define the tracking element. In other words, a back of the lens may include the tracking element. The lens and/or the tracker body may otherwise include an anti-reflective coating. Alternatively, an upper surface of the plug may include a reflective coating to define the tracking element. The tracker body may include a lip separating the opening and the counterbore. The tracker body may include an upper surface including the lip and defining the opening, and an opposite lower surface defining the counterbore. The lip may include a chamfer extending inwardly away from an upper surface of the tracker body. The plug may support the lens in abutment with the lip to seal the counterbore. The counterbore may be circular, rectangular or any other constant or varying cross-sectional shape. 
     In certain implementations, the plug includes a shoulder extending radially outwardly from the plug body. The shoulder is positionable in sealing engagement with a widened portion of the counterbore. The plug body may further define a cavity, and the tracking element may be disposed within the cavity. A masking spacer may be disposed within the cavity and between the lens and the tracking element. The plug body may further define a groove. A sealing element may be disposed within the groove. The sealing element may include a thickness greater than a depth of the groove so as to be compressed against the lens. The cavity and the groove may be coaxially arranged. 
     In certain implementations, the tracker body includes a retention feature configured to be removably coupled with a complementary retention feature of the plug. The retention feature may be between the upper and lower surfaces of the tracker body, for example, be internal threads disposed within the counterbore. Engagement of the retention features may maintain abutment between the lens and the lip to seal the opening, and further maintain abutment between the plug body and the tracker body to provide another seal at the lower surface. The lens and the plug may be disposable within the counterbore from the lower surface. 
     In certain implementations, the tracker body further comprises a central portion, and an end portion wider than the central portion and including a flange. The flange may define the opening through which the tracking element is visible. There may be more than one flange, for example, three or more flanges. For example, the tracking body may include five flanges. The five flanges may be arranged between two end portions separated by the central portion in a generally pentagonal configuration. The tracker body may be plate-like in construction such that the tracking element assemblies are coplanar. Alternatively, the tracking element assemblies may be positioned and oriented in three-dimensions. The tracking element may be planar (e.g., discs), as shown, or hemispherical, spherical, or any other suitable geometry. The tracking element may be a passive element or an active element. 
     In certain implementations, the lens may be transparent, translucent, or have any characteristic of light admittance. The tracking element, the lens and/or the plug may include a reflective coating. A discrete component constituting the tracking element may be eliminated with the coating being on the lens and/or the plug. The coating forming the tracking element is positioned between the upper surface of the lens and the plug body. In other words, the tracking element may be embedded on or between the lens and the plug. The tracking element is disposed within the sealed enclosure. In an alternative implementation, the lens may be opaque to light but transmissive to electromagnetic (EM) or radiofrequency (RF) wavelengths. The tracking element may be configured to reflect the EM or RF waves. For example, the tracking element may be an EM or RF disc, strip, or element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present disclosure will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
         FIG. 1  is a perspective view of one implementation of a surgical system. 
         FIG. 2  is a perspective view of a tracker, according to one implementation. 
         FIG. 3  is an exploded view of the tracker, according to one implementation. 
         FIG. 4  is a sectional view of the tracker of  FIG. 3  taken along section lines  4 - 4 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates an example of a surgical robotic system  10  for treating a patient. The robotic system  10  is shown in a surgical setting such as an operating room of a medical facility. In the embodiment shown, the robotic system  10  includes a manipulator  12  and a navigation system  20 . The navigation system  20  is arranged to track movement of various real objects in the operating room. Such real objects include, for example, a surgical tool  22 , a femur F of a patient, and a tibia T of the patient. The navigation system  20  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 tool  22  relative to virtual cutting boundaries (not shown) associated with the femur F and tibia T. 
     The navigation system  20  includes one or more computer cart assemblies  24  that houses one or more navigation controllers  26 . A navigation interface is in operative communication with the navigation controller  26 . The navigation interface includes one or more displays  28 ,  29  adjustably mounted to the computer cart assembly  24  or mounted to separate carts as shown. Input devices I such as a keyboard and mouse can be used to input information into the navigation controller  26  or otherwise select/control certain aspects of the navigation controller  26 . Other input devices I are contemplated including a touch screen, voice-activation, gesture sensors, and the like. 
     A surgical navigation localizer  34  communicates with the navigation controller  26 . In the embodiment shown, the localizer  34  is an optical localizer and includes a camera unit  36 . The camera unit  36  has a housing  38  comprising an outer casing that houses one or more sensors  40 . In some implementations at least two sensors  40  are employed, and preferably three or four. The sensors  40  may be separate charge-coupled devices (CCD). For example, three, one-dimensional CCDs may be employed. Alternatively, two-dimensional or three-dimensional sensors may be provided. The CCDs detect light signals, such as infrared (IR) signals. In alternative implementations, the localizer  34  may be an EM or RF localizer configured to emit EM and RF waves, respectively, and the camera unit  36  is configured to detect EM or RF signals. 
     The camera unit  36  may be mounted on an adjustable arm to position the sensors  40  with a field-of-view of the below discussed trackers that, ideally, is free from obstructions. The camera unit  36  includes a camera controller  42  in communication with the sensors  40  to receive signals from the sensors  40 . The camera controller  42  communicates with the navigation controller  26  through either a wired or wireless connection (not shown). Position and orientation signals and/or data are transmitted to the navigation controller  26  for purposes of tracking objects. The navigation controller  26  is loaded with software that converts the signals received from the camera unit  36  into data representative of the position and orientation of the objects being tracked. 
     The navigation system  20  is operable with a plurality of tracking devices  44 , also referred to herein as trackers. In the illustrated implementation, one of the trackers  44  is firmly affixed to the femur F of the patient, and another one of the trackers  44  is firmly affixed to the tibia T of the patient. Additionally, or alternatively, still another tracker  44  may be mounted to other tissue types or parts of the anatomy. The tracker  44  of the present disclosure to be described is particularly well suited for applications involving delicate anatomy in which the weight of the tracker  44  be minimized, such as surgery of the shoulder, the spine, or the like. An additional tracker  44  may be also be coupled to the manipulator  12 , as shown in  FIG. 1 . It is further contemplated that the tracker  44  may be coupled to any suitable surgical instrumentation, for example, a surgical power tool or a point probe for calibration or any non-powered tool. The tracker  44  may be utilized with the robotic system  10  or any other robotic surgical system, including hand-held robotic systems, table mounted robotic systems, etc. Additionally, or alternatively, the tracker  44  can be utilized by the navigation system without robotics to track other objects, such as imaging systems (e.g., CT, MRI, X-ray, etc.), soft tissue retractors, implants, the surgical table, cut guides, head-mounted devices, surgical drapes, mobile display devices (e.g., tablets), the practitioner or staff within the sterile field, and the like. 
     The sensors  40  of the localizer  34  receive light signals from the tracker(s)  44 . In implementations in which the tracker  44  is a passive tracker, light emitted from the camera unit  36  is reflected by the tracker  44 . Additionally, or alternatively, the tracker  44  may be an active tracker in which, for example, light emitting diodes (LEDs) transmit light. The camera unit  36  receives optical signals from the tracker(s)  44  and outputs to the navigation controller  26  signals relating to the position of the tracking elements of the tracker(s)  44  relative to the localizer  34 . Based on the received optical signals, the navigation controller  26  generates data indicating the relative positions and orientations of the tracker(s)  44  relative to the localizer  34 . 
     Referring now to  FIG. 2 , the tracker  44  includes a tracker body  46  that may define the form factor or shape of the tracker  44 . The tracker body  46  may be of unitary of monolithic construction, as illustrated, or of subcomponents joined to one another. The tracker body  46  may be formed from any suitable material, such as steel or rigid plastic (e.g., thermoplastic polymer). In one example, the material is polyethylene terephthalate glycol. The material should be able to withstand pressures and temperatures associated with autoclaving. In an exemplary implementation, the tracker body  46  may be formed from a light plastic so as to minimize the overall weight of the tracker  44 , which, as mentioned, may facilitate its use with more delicate anatomy. More specifically, whereas the femur F, the tibia T, other long bones, the iliac crest, and the like may be sufficiently large to accommodate a larger fastener so as to support a heavier tracker, less robust anatomy such as the patella, vertebrae, bones of the foot, and the like may be better served with a smaller fastener and lighter tracker. The tracker  44  of the present disclosure advantageously realizes that benefit. 
     The tracker body  46  may comprise an anti-reflecting light material. In another example, the tracker body  46  may comprise features formed in or disposed the body  46  that are designed to reduce light reflection. Such features may include surface roughness (e.g., Rz in the range of 14-18), dimples or indentations, or an anti-reflective paint or material disposed or formed on/within the tacker body  46 . The tracker body  46  can be opaque, translucent or transparent to light. In implementations where the tracker body  46  is transparent, the tracker body  46  itself may form the lens with no discrete lens being further necessitated. 
     The tracker body  46  may define a coupler opening  47  configured to be removably coupled with an arm or post mounted to the object being tracked. An underside of the tracker body  46  may further define recesses configured to interface with the arm or post for confidently securing the tracker  44  to the object being tracked. 
     The form factor of the tracker body  46  may assume any suitable geometry so as to arrange a plurality of tracking element assemblies  50  in a desired spatial relationship.  FIG. 2  shows one example of the tracker body  46  including a central portion  52  that is elongate with a length greater than a width, a first end portion  54  having a width greater than the width of the central portion  52 , and a second end portion  56  also having a width greater than the width of the central portion  52 . The respective widths of the first end portion  54  and the second end portion  56  may be defined between flanges  58   a - 58   e  (collectively identified herein as  58 ) sized to accommodate the tracking element assemblies  50 . The illustrated implementation shows the first end portion  54  including two flanges  58   a ,  58   b , and the second end portion  56  including three flanges  58   c ,  58   d ,  58   e . The two flanges  58   a ,  58   b  are positioned opposite the central portion  52 , and the three flanges  58   c ,  58   d ,  58   e  are in a generally cruciform configuration. The resulting arrangement includes five of the flanges  58   a - 58   e  to which five of the tracking element assemblies  50  are coupled in a pentagonal configuration. Of course, fewer or more of the tracking element assemblies  50  are contemplated for which there could be fewer or more flanges  58 . For the central portion  52  is optional, wherein the first end portion  54  or the second end portion  56  is absent. The size of the flanges  58   a - 58   e  and their relative positioning relative to one another may be sufficient spaced apart for the sensors  40  to discretely detect the light signals from each of the tracking element assemblies  50 . It is further contemplated that the tracking element assemblies  50  may be coupled to the central portion  52  as well. 
     The tracking element assemblies  50  may be in a coplanar arrangement. The illustrated implementation shows the tracker body  46  being plate-like in construction with a relatively small thickness defined between an upper surface  60  opposite a lower surface  62 . The upper surface  60  may define openings  64  through which a respective one of the tracking element assemblies  50  is configured to be visible by the sensors  40 . With the tracking element assemblies  50  coupled to the tracking body  46  as illustrated in  FIG. 2 , the tracking element assemblies  50  are spaced apart from one another in two directions (x and y), but otherwise coplanar in the z-direction. In alternative implementations, the tracker body  46  may include geometric features in the z-direction, and the tracking element assemblies  50  may be suitably positioned and oriented in three dimensions relative to one another. The tracker  44  may include any number of tracking element assemblies  50 , and these assemblies need not necessarily be disposed in flanges  58  as specifically shown. One skilled in the art could construct the tracker body  46  to be any number of shapes wherein the tracking element assemblies  50  can remain readily visible. 
     With concurrent reference to  FIGS. 3 and 4 , the tracker  44  includes the tracking element assemblies  50  configured to be coupled to the tracker body  46 . One of the tracking element assemblies  50  is to be further discussed in detail with the disclosure applicable to any number of the tracking element assemblies  50  provided on the tracker  44 . The tracking element assembly  50  may include a lens  66 , a masking spacer  68 , a tracking element  70 , a sealing element  72 , and a plug  74 . When installed to the tracker body  46 , subcomponents of the tracking element assembly  50  define a sealed enclosure  76  for the tracking element  70 . The sealed enclosure  76  advantageously prevents the tracking element  70  from being exposed to gas and fluid, for example, pressurized steam associated with autoclaving. Therefore, the tracker  44  may be repeatedly autoclaved as a single unit without need to decouple the tracking element assembly  50 , and further without the need to replace the tracking element  70  after each use or few uses. Relative to known trackers, the tracker  44  of the present disclosure provides for a longer operational lifespan with less maintenance and less expense. Empirical data shows the tracker  44  may be confidently sterilized and reused for at least two-hundred duty cycles without degradation of the sealed enclosure  76  (and thus preservation of the wavelength-reflecting equalities of the tracking element  70 ). 
     As appreciated from the exploded aspects of  FIG. 3 , the tracking element assembly  50  includes a coaxial stacking of the subcomponents in a specific arrangement to achieve the sealed enclosure  76 . Further the specific arrangement of the subcomponents provides for simplified construction and intuitive coupling and decoupling of the tracking element assembly  50  and the tracker body  46 . More particularly, the plug  74  includes a retention feature  77  configured to be removably coupled with a complementary retention feature  78  of the tracker body  46 . For example, the retention feature  77  may be external threads on an outer diameter of the plug  74  and internal threads within a counterbore  80  of the tracker body  46 , or vice-versa. After the tracking element assembly  50  is assembled in a manner to be further described, the tracking element assembly  50  may be threadably secured to the tracker body  46 . The tracking element assembly  50  and the tracker body  46  cooperate to define the sealed enclosure  76 . Other suitable removably joining means are contemplated, for example, retaining rings, detents, defeatable interference fit, friction fit, elastic fit, and the like. It is further contemplated that the retention feature  78  need not be disposed within the counterbore  80  or otherwise internal to the tracker body  46 . Alternatively, the retention feature  78  may be external to the tracker body  46 . For example, a cap may be provided to secure the plug  74 , or the plug  74  may include a cap that is secured to the external retention feature. In another implementation, the opening  64  and the counterbore  80  need not be coaxial in which the tracker element assembly  50  is directed into the counterbore  80  from the lower surface  62  of the tracker body  46 . Rather, the tracker body  46  may define a side slot configured to slidably receive the tracker element assembly  50  whose dimensions may be modified to achieve the same. 
     In an alternative implementation, the tracking element assembly  50  may be fixedly secured to the tracker body  46 . In other words, the tracking element assembly  50  is assembled, and the plug  74  and the tracker body  46  are permanently joined to one another through spin welding or another suitable joining process. With the sealed enclosure  76  preserving or extending the wavelength-reflecting qualities of the tracking element  70 , the longer operational lifespan may justify making the tracker  44  discardable after a durable number of duty and sterilization cycles. In other words, once the sealed interfaces begin to become compromised, the entire tracker  44 —including the tracker body  46  and the tracking element assembly  50 —may simply be discarded. 
       FIG. 4  illustrates the upper surface  60  of the tracker body  46  defining the opening  64 , and the lower surface  62  of the tracker body  46  defining the counterbore  80  in fluid communication with the opening  64 . The opening  64  may have a diameter less than a diameter of the counterbore  80 . The opening  64  and the counterbore  80  may be separated by a lip  82  of the tracker body  46 . The lip  82  may include the upper surface  60  such that with the lens  66  secured against the lip  82 , the lens  66  is near the opening  64 . In such an arrangement, light reflected by the tracking element  70  positioned just below the lens  66  is detectable by the sensor  40  at a wider field of view (angle relative to the upper surface  60 ). To that end, the lip  82  may include a chamfer  84  to further maximize the field of view of the tracker  44  as represented by the bolded arrow in  FIG. 4 . 
     The coaxial stacking of the subcomponents of the tracking element assembly  50  will now be described with continued reference to  FIGS. 3 and 4 . The plug  74  may be disc-like in construction and define a groove  86  within an upper surface  88 . The sealing element  72 , for example, an O-ring, is sized to be seated within the groove  86 . More particularly, the sealing element  72  may be slightly thicker than a depth of the groove  86  such that, when assembled, the sealing element  72  is compressed between the lens  66  and the plug  74 . The compression prevents ingress of fluid between the lens  66  and the plug  74 , thereby facilitating the sealed enclosure  76 . The sealing element  72  and the upper surface  88  of the plug  74  may define a platform upon which the lens  66  rests during assembly of the tracking element assembly  50 . 
     The plug  74  may further define a cavity  90  located radially inwardly from the groove  86 . In other words, an annular barrier  92  may separate the groove  86  and the cavity  90  that are coaxial with one another. A depth of the cavity  90  is sized to accommodate certain subcomponents of the tracking element assembly  50 , namely the masking spacer  68  and the tracking element  70 , and a base surface  94  of the plug  74  is configured to support subcomponents of the aforementioned subcomponents. The tracking element  70  rests upon the base surface  94 , and the masking spacer  68  rests upon the tracking element  70 . The masking spacer  68 , for example, a ring washer, is sized to a gap between the tracking element  70  and the lens  66  so as to prevent movement of the tracking element  70  within the sealed enclosure  76 . In implementations in which the tracker  44  is an optical tracker, the masking spacer  68  defines an aperture  96  and is configured to create an accurate area on the reflective surface of the tracking element  70 . In other words, the masking spacer  68  covers a portion of the reflective surface of the tracking element  70 , and the remaining portion of the reflective surface that is visible through the aperture  96  is known with precision. Further, the masking spacer is generally thin in construction so as to prevent shadowing the reflective surface at angles off normal. Further, outer diameters of each of the masking spacer  68  and the tracking element  70  may be sized to slightly less than an inner diameter of the cavity  90 . As best shown in  FIG. 4 , the sealed enclosure  76  may be bounded from below by the base surface  94 , from above by a lower side of the lens  66 , and from the sides by the annular barrier  92 . The plug  74  is at least partially disposed in the counterbore  80 , and the tracking element  70  is disposed in the sealed enclosure  76 . It is understood that the tracking element  70 , whether as a discrete component or as a coating on the lower surface of the lens  66 , may be positioned at any location in the component stack-up between the upper surface of the lens  66  and the plug  74 . 
     The plug  74  may further include a plug body  98 , and a shoulder  100  extending radially outwardly from the plug body  98 . With continued reference to  FIG. 4 , the plug body  98  includes the retention feature  77  at a first outer diameter, and the shoulder  100  may be at a second outer diameter greater than the first outer diameter. The counterbore  80  may complementarily include a widened portion  102  having an inner diameter greater than the inner diameter of the retention features  78  of the tracker body  46 . The shoulder  100  is configured to be positioned in sealing engagement with the widened portion  102  of the counterbore  80  when the plug  74  is coupled to the tracker body  46 . The sealing engagement between the shoulder  100  and the tracker body  46  is configured to accurately locate the plug  74  within the tracker body  46 . The sealing engagement may also further prevent ingress of fluid within the subcomponents of the tracking element assembly  50 , thereby further facilitating the sealed enclosure  76 . 
     As mentioned, the tracking element assembly  50  and its subcomponents facilitate intuitive assembly of the tracker  44 . A method of assembling the tracker  44  may include disposing the sealing element  72  within the groove  86  of the plug  74 . The tracking element  70  may be disposed within the cavity  90  of the plug  74 , and the masking spacer  68  may be disposed within the cavity  90  to rest on the tracking element  70 . The steps of disposing the tracking element  70  and the masking spacer  68  may be performed before or after the step of disposing the sealing element  72  within the groove  86 . The lens  66  is disposed on the platform defined by the sealing element  72  and the upper surface  88  of the plug  74 . The tracking element assembly  50  may then be supported with one hand of the user, and the other hand of the user may support the tracker body  46  in the orientations generally shown in  FIG. 3 . The tracking element assembly  50  is moved towards the lower surface  62  of the tracker body  46  to be at least partially positioned within the counterbore  80 . The complementary retention features  77 ,  78  engage one another, and the plug  74  is rotated to further advance the tracking element assembly  50  into the counterbore  80 . The tracking element assembly  50  may be considered fully installed once the lens  66  is in abutment with the lip  82 , and further in sealing engagement with the same owing to the resilient forces of the sealing element  72  under compression. The tracking element assembly  50  may further be considered fully installed once the shoulder  100  is in sealing engagement with the widened portion of the counterbore  80 . 
     The tracker  44  may be coupled to the arm or post that itself was previously secured to the patient with known techniques. The surgical procedure may follow. The sealed enclosure  76  maintains the sterility of the tracking element  70  (and the masking spacer  68  and other internal features of the subcomponents). Following the conclusion of the surgical procedure, the tracker  44  may be decoupled from the arm or post. Without requiring further manipulation of the tracker  44 , the tracker  44  may be autoclaved to sterilize all exposed subcomponents and their geometries. The sealed enclosure  76  prevents ingress of the pressurized steam, thereby preserving the light-reflecting properties of the tracking element  70 . This method may be repeated as many times as viable. Only once the sealed interfaces begin to become compromised (or after a predetermined number of uses) may it be appropriate to replace the tracking element  70  and/or the sealing element  72 , and doing so is intuitive and follows the aforementioned method in reverse. In the alternative implementation previously introduced in which the tracking element assembly  50  and the tracker body  46  are permanently joined, the tracker  44  may be discarded once the sealed interfaces begin to become compromised or after the predetermined number of uses. 
     The foregoing description is not intended to be exhaustive or limit the invention to any particular form. The terminology which has been used is intended to be in the nature of words of description rather than of limitation. Many modifications and variations are possible in light of the above teachings and the invention may be practiced otherwise than as specifically described. In one implementation, the lens  66  may include features configured to be tracked by the sensors  60  such that the tracker  44  may not include a discrete tracking element  70 . For example, the lens  66  may include an upper surface configured to be positioned against the lip  82 , and a lower surface opposite the upper surface. The upper surface may be transparent, and a reflective coating may be disposed on the lower surface. The reflective coating is in the sealed enclosure. In other words, the upper surface and a thickness of the lens  66  prevents the fluid associated with autoclaving from coming into contact with the reflective coating. Alternatively, the lens  66  may be formed a layer of reflective material in another suitable construction.