Patent Description:
A tracking device may be used during a selected procedure to track a position of a selected object to which the tracking device is connected. The tracking device may be tracked with a selected tracking system, such as an optical tracking system that has a line of sight from one or more cameras to the selected tracking device. In positioning the tracking device on an item to be tracked, the tracking device may be used to evaluate the position of the item to which it is attached. Tracking systems are disclosed in <CIT>, <CIT>, <CIT> and <CIT>.

The invention provides a system as defined by claim <NUM>, that is able to fix a tracking device to a subject, such as a human subject. It is understood that the tracking device may be fixed to a nonhuman subject or nonliving subject for a selected procedure. The tracking device may be connected in or on a substantially small area at a selected location. In various embodiments, the tracking device may be fixed relative to a scalp or on a scalp of a subject near a location of a procedure, such as an otological, nasal, or similar procedure.

The tracking system may include a central fixation member or holding member and one or more auxiliary or radially positioned members. The radially positioned members may assist in stabilizing the tracking device relative to the central holding member. The assembly, once assembled and connected to the subject, may then be used to track the subject during a procedure.

With initial reference to <FIG>, in a procedure a navigation system <NUM> may be used by a user <NUM> to perform a selected procedure. The navigation system <NUM> may include various components that assist in navigating a procedure including a selected tracking system. The tracking system may include various components or portions such as various localizers. Various localizers may include an optical tracking system and/or an electromagnetic tracking system that includes a Tracking Coil Array (TCA) <NUM>. While an optical tracking system may be used with the TCA <NUM>, it is understood that only one tracking system, such as including the TCA <NUM>, may be used with the navigation system <NUM>.

The TCA <NUM> may include one or more conductive coils <NUM> positioned relative to a subject <NUM> on which a procedure is performed. In various embodiments, a procedure may be performed on or near a head <NUM> of the subject <NUM>. As discussed in further detail herein, a subject tracking device or assembly <NUM> may be connected to the subject <NUM>, such as to the head <NUM> of the subject <NUM>. The tracking assembly <NUM> may also be referred to as a Dynamic Reference Frame (DRF) or a patient tracker.

With continuing reference to <FIG>, the navigation system <NUM> may include various features or elements as discussed below. Generally, the navigation system <NUM> may be used to determine or track a position of an instrument <NUM> in a volume. The position may include both a three dimensional X,Y,Z location and orientation. Orientation may include one or more degrees of freedom, such as three degrees of freedom. Thus, a total of at least six degrees of freedom may be determined for the position of the instrument <NUM>.

Tracking the position of the instrument <NUM> may assist the user <NUM> in determining a position of the instrument <NUM>, even if the instrument <NUM> is not directly viewable by the user <NUM>. Various procedures may block the view of the user <NUM>, such as performing a repair or assembling an inanimate system, such as a robotic system, assembling portions of an airframe or an automobile, or the like. Various other procedures may include a surgical procedure, such as performing a spinal procedure, neurological procedure, positioning a deep brain simulation probe, or other surgical procedures on a living subject. In various embodiments, for example, the living subject may be a human subject <NUM> and the procedure may be performed on the human subject <NUM>. It is understood, however, that the instrument <NUM> may be tracked and/or navigated relative to any subject for any appropriate procedure. Tracking or navigating an instrument for a procedure, such as a surgical procedure, on a human or living subject is merely exemplary.

Nevertheless, in various embodiments, the surgical navigation system <NUM>, as discussed further herein, may incorporate various portions or systems, such as those disclosed in <CIT><CIT><CIT>and <CIT>; and <CIT>. Various components that may be used with or as a component of the surgical navigation system <NUM> may include an imaging system <NUM> that is operable to image the subject <NUM>, such as an O-arm® imaging system, magnetic resonance imaging (MRI) system, computed tomography system, etc. A subject support <NUM> may be used to support or hold the subject <NUM> during imaging and/or during a procedure. The same or different supports may be used for different portions of a procedure.

Image data may be acquired during a surgical procedure or acquired prior to a surgical procedure for displaying an image <NUM> on a display device <NUM>. The instrument <NUM> may be tracked in a trackable volume or a navigational volume that is produced by a transmitter antenna or transmitting coil array that is incorporated into the localizer <NUM>, as illustrated in <FIG>. The position of the instrument <NUM> may be tracked in the tracking volume relative to the subject <NUM> and then illustrated as an icon 32i with the display device <NUM>. In various embodiments, the icon 32i may be superimposed on the image <NUM> and/or adjacent to the image <NUM>. As discussed herein, the navigation system <NUM> may incorporate the display device <NUM> and operate to render the image <NUM> from selected image data, display the image <NUM>, determine the position of the instrument <NUM>, determine the position of the icon 32i, etc..

With reference to <FIG>, the localizer <NUM> may be an electro-magnetic (EM) localizer that is operable to generate electro-magnetic fields with a transmitting coil array (TCA) <NUM> which is incorporated into the localizer <NUM>. The TCA <NUM> may include one or more coil groupings or arrays. In various embodiments, more than one group is included and each of the groupings may include three coils, also referred to as trios or triplets. The coils may be powered to generate or form an electro-magnetic field by driving current through the coils of the coil groupings. As the current is driven through the coils, the electro-magnetic fields generated will extend away from the coils <NUM> and form a navigation domain or volume <NUM>, such as encompassing all or a portion of a head <NUM>, spinal vertebrae, or other appropriate portion. The coils <NUM> may be powered through a TCA controller and/or power supply <NUM>.

The navigation domain or volume <NUM> generally defines a navigation space or patient space. As is generally understood in the art, the instrument <NUM>, such as a drill, lead, etc., may be tracked in the navigation domain relative to a patient or subject with an instrument tracking device <NUM>. For example, the instrument <NUM> may be freely moveable, such as by the user <NUM>, relative to the DRF <NUM> that is fixed relative to the subject <NUM>. Both the tracking devices <NUM>, <NUM> may include tracking or sensing coils (e.g. conductive material formed or placed in a coil) that senses and are used to measure a magnetic field strength, etc. Due to the tracking device <NUM> connected or associated with the instrument <NUM>, relative to the DRF <NUM>, the navigation system <NUM> may be used to determine the position of the instrument <NUM> relative to the DRF <NUM>.

The navigation volume or patient space may be registered to an image space of the patient and the icon 32i representing the instrument <NUM> may be illustrated at a navigated (e.g. determined) and tracked position with the display device <NUM>, such as superimposed on the image <NUM>. Registration of the patient space to the image space and determining a position of a tracking device, such as with the tracking device <NUM>, relative to a DRF, such as the DRF <NUM> may be performed as generally known in the art, including as disclosed in <CIT><CIT><CIT>; and <CIT>; and <CIT>.

The navigation system <NUM> may further include a navigation processor system <NUM>. The navigation processor system <NUM> may include the display device <NUM>, the localizer <NUM>, the TCA controller <NUM>, and other portions and/or connections thereto. For example, a wire connection may be provided between the TCA controller <NUM> and a navigation processing unit <NUM>. Further, the navigation processor system <NUM> may have one or more user control inputs, such as a keyboard <NUM>, and/or have additional inputs such as from communication with one or more memory systems <NUM>, either integrated or via a communication system. The navigation processor system <NUM>, according to various embodiments, may include those disclosed in <CIT><CIT><CIT>and <CIT>; and <CIT>, and/or may also include the commercially available StealthStation® or Fusion™ surgical navigation systems sold by Medtronic Navigation, Inc. having a place of business in Louisville, CO.

Tracking information, including regarding the magnetic fields sensed with the tracking devices <NUM>, <NUM>, may be delivered via a communication system, such as the TCA controller <NUM>, which also may be a tracking device controller <NUM>, to the navigation processor system <NUM> including the navigation processor <NUM>. Thus, the tracked position of the instrument <NUM> may be illustrated as the icon 32i relative to the image <NUM>. Various other memory and processing systems may also be provided with and/or in communication with the processor system <NUM>, including the memory system <NUM> that is in communication with the navigation processor <NUM> and/or an imaging processing unit <NUM>. The image processing unit <NUM> may be incorporated into the imaging system <NUM>, such as the O-arm® imaging system, as discussed above. The imaging system <NUM> may, therefore, include various portions such as a source and a x-ray detector that are moveable within a gantry <NUM>. The imaging system <NUM> may also be tracked with a tracking device <NUM>. It is understood, however, that the imaging system <NUM> need not be present while tracking the tracking devices, including the instrument tracking device <NUM>. Also, the imaging system <NUM> may be any appropriate imaging system including a MRI, CT, etc. In various embodiments, the localizer may also include an optical camera system <NUM>. The optical camera system <NUM> may be used in conjunction with or alternatively to the localizer <NUM> for tracking the instrument <NUM>.

Information from all of the tracking devices may be communicated to the navigation processor <NUM> for determining a position of the tracked portions relative to each other and/or for localizing the instrument <NUM> relative to the image <NUM>. The imaging system <NUM> may be used to acquire image data to generate or produce the image <NUM> of the subject <NUM>. It is understood, however, that other appropriate imaging systems may also be used. The TCA controller <NUM> may be used to operate and power the TCA <NUM>, as discussed above.

With continuing reference to <FIG> and additional reference to <FIG>, the DRF <NUM> may be connected to the subject <NUM> to assist in performing a procedure or tracking a procedure relative to the subject <NUM>. In various embodiments, the DRF <NUM> may include various components that allow it to be connected relative to the head <NUM> of the subject <NUM> to assist in tracking the subject <NUM>, such as the head <NUM> thereof. The DRF <NUM>, for example, may include an anchor, such as a fixation screw or bolt <NUM> that is fixable to the head <NUM> of the subject <NUM>.

The anchor <NUM> may include a bone engagement portion or region <NUM> such as including an external thread <NUM>. The external thread <NUM> may be screwed into the skull of the subject <NUM> at the head <NUM>, or other appropriate location. The fixation screw <NUM> further includes a shaft <NUM> extending from the threaded or bone fixation portion <NUM> to a distal or tool engaging region <NUM>. The tool engaging region <NUM> may include a flange or protuberance, such as a conical region <NUM>. The conical region <NUM> may be similar to a counter-sink and assist in engaging the soft tissue of the subject <NUM>. The conical region <NUM> may act as a depth stop or limiter for engaging the anchor <NUM> into the subject <NUM> in addition to or alternatively to any depth limiter near the threads <NUM>.

Further the tool engaging region <NUM> may include a tool or driver connection or engagement <NUM>. In various embodiments, the driver engagement <NUM> may include a noncircular exterior or facet exterior, such as a hexagonal exterior. The tool engagement region <NUM> may allow for the anchor <NUM> to be engaged by a tool <NUM> to be driven into the head <NUM> of the subject <NUM>. It is understood, however, that the tool engagement region <NUM> may be formed in any appropriate configuration to be engaged by the selected tool <NUM>. For example, the tool engaging region <NUM> may include an internal portion to be engaged by the tool <NUM>.

The tool <NUM> may include a recess or depression <NUM> to engage or receive at least a portion of the screw <NUM>. The recess <NUM> allows the tool <NUM> to engage the tool engaging region <NUM>. The recess <NUM> may include an internal wall <NUM> that has a complementary shape or configuration to engage the tool engaging region <NUM> of the screw <NUM>. Accordingly, the tool <NUM> may allow the user <NUM> to drive the screw <NUM> into the head <NUM> of the subject <NUM>.

The screw <NUM> may further include a base or tracking device engagement region or portion <NUM>. The base engaging region <NUM> may include a selected configuration such as a protuberance portion <NUM>. The protuberance may be generally spherical or partially spherical in shape. The protuberance <NUM> may have a diameter or cross section distance <NUM> (e.g. diameter) greater than a cross sectional dimension <NUM> of an adjacent region or connection portion <NUM>. Thus, the protuberance <NUM> may extend out from the connection portion <NUM>. The shaft <NUM> may further have the diameter or cross-section <NUM> that is greater than the connection region dimension <NUM>. As discussed herein, therefore, the base or frame assembly <NUM> may engage the base engagement portion <NUM>, including the base interfering or engagement portion <NUM> to assist in holding the base or frame assembly <NUM> relative to the head <NUM>.

The DRF <NUM> includes the frame assembly <NUM> that includes a base or stabilization member or region <NUM>. The stabilization member <NUM> may be formed as one single member or as more than one member connected together. For example, the stabilization member <NUM> may be formed as two halves (e.g. two members molded separately) that are then fixed together with selected bolts or pins <NUM>. It is understood, however, that the stabilization member <NUM> need not be formed as two pieces or more than two pieces, but may be formed as a single piece.

The stabilization portion or member <NUM> may include a various or selected number of legs or arms, such as a first arm <NUM>, a second arm <NUM>, and a third arm <NUM>. Each of the three arms <NUM>-<NUM> may be positioned at a distance spaced radially from a central portion or hub region <NUM>. In various embodiments, the central portion <NUM> may surround an axis <NUM>. The axis <NUM> may be a central axis of the central portion <NUM> and may be aligned and/or extend from a central axis of the anchor <NUM>. For example, each of the legs <NUM>, <NUM>, and <NUM> may be positioned at about <NUM> degrees about or around the central axis <NUM>. It is understood, however, that more than three legs or arms may be provided or that the stabilization portion <NUM> is formed as a single continuous edge or region that may engage the head <NUM>. Further, each of the legs <NUM>-<NUM> may extend or have a portion that extends generally parallel with the axis <NUM>. In various embodiments, the legs <NUM>-<NUM> may include a first portion that is connected to the center <NUM> and extends perpendicular to the axis <NUM> and a second portion that extends from the first portion generally parallel to the axis <NUM>.

In various embodiments, as illustrated in <FIG>, each of the legs <NUM> - <NUM> includes a scalp engagement projection or member including a first projection <NUM>, a second projection <NUM>, and a third projection <NUM>. Each of the three projections <NUM> - <NUM> may be formed or provided as individual members. Each projection member <NUM>-<NUM> may include portions such as a distal tip which may include a sharp or penetrating region 174a-178a (e.g. about <NUM> to about <NUM> in length) and an arm engaging region, such as a externally threaded portion 174b-178b. Accordingly, it is understood that the projections <NUM> - <NUM> may be provided as separate members, such as those including externally threaded portions that are engaged into the respective arms <NUM> - <NUM>. It is understood, however, that the projections <NUM> - <NUM> may otherwise be formed such as being molded into the arms <NUM> - <NUM>, press fit into the arms <NUM> - <NUM>, or other appropriate fixation. Further the projections <NUM> - <NUM> may be formed integrally and as one member with each of the respective arms <NUM> - <NUM>. Further, it is understood, that the stabilization portion <NUM> may be formed as a continuous member (e.g. a cylinder) rather than including separate and spaced apart arms <NUM>-<NUM>.

In various embodiments, the stabilization portion <NUM> may be formed of a selected polymer, such as a substantially ridged polymer. The projections <NUM>- <NUM> may be formed of a selected hard material, such as a metal (e.g. stainless steel, titanium, etc.) that are fitted into the respective arms <NUM> - <NUM>. It is further understood, however, that the stabilization portion <NUM> may be formed as a unitary or single member, such as formed of a selected ridged material (e.g. a metal) where all of the portions are milled from the single piece block. For example, a single piece of stainless steel may be milled to include the stabilization portion <NUM> including each of the arms <NUM> - <NUM> and projection portions <NUM> - <NUM> having the features as discussed above.

The DRF <NUM> further includes a central passage through the center <NUM> to allow a tracking portion or member, such as an elongated member, <NUM> to pass through the passage. The tracking portion <NUM> may include or be an elongated member that extends into a portion of the stabilization base <NUM> to be fixed therein and/or connected thereto. In addition, or alternatively thereto, the tracking portion <NUM> may extend to an anchor or screw engaging portion or region <NUM>.

The screw engaging region <NUM> includes a side opening or bore <NUM> that is complementary to a shape to the base engaging portion <NUM> of the anchor <NUM>. The anchor engaging portion <NUM> may further include a taper or engagement portion <NUM> that has a diameter smaller than the diameter <NUM> of the anchor portion <NUM> such as complementary to the diameter or cross dimension <NUM> of the tapered or connection portion <NUM>. Further, a bore or passage may be formed through an end of the anchor engaging region <NUM>. Therefore, the anchor engaging region <NUM> may be moved over and/or around the ball <NUM> to engage or hold the ball <NUM> passed through the opening <NUM> in the anchor engaging portion <NUM>. The tapered portion <NUM> may then engage a surface of the ball <NUM> by passing through or over the tapered region <NUM> of the anchor <NUM>. An installation of the device <NUM> as discussed further herein, the brief discussion here illustrates an engagement of the stabilization base <NUM> to the anchor <NUM>.

In various embodiments, the anchor <NUM> may otherwise engage the tracking portion <NUM>. For example, the anchor <NUM> may include a thread (e.g. an external thread) and the tracking portion <NUM> includes an internal thread. The anchor <NUM> and the tracking portion <NUM> may, therefore, be threadably engaged together. Further, the anchor <NUM> and the tracking portion <NUM> may be formed as one piece or member. For example, the tracking portion <NUM> may include an end that is formed as the bone engaging portion <NUM>. In various embodiment, the tracking portion <NUM> may taper or have a dimension at a selected end similar or identical to those of the anchor discussed above. Thus, the tracking portion <NUM> may not need the anchor <NUM> as a separate member and the tracking portion <NUM> may be the only portion to have the bone engaging portion <NUM> to engage the subject <NUM>.

The DRF <NUM> further includes a rotatable knob or nut <NUM> that includes an internal thread <NUM> that engages an external thread <NUM> on a shaft <NUM> of the tracking portion <NUM>. The knob <NUM> may include an abutment surface <NUM> that is configured to engage at the center <NUM> of the stabilization base <NUM>. By rotating the knob <NUM> in a selected direction, such as a clockwise direction, the knob <NUM> may move along the shaft <NUM> generally in the direction of arrow <NUM> along the axis <NUM>. The surface <NUM> of the knob <NUM> will abut or engage the center <NUM> of the stabilization portion <NUM> causing the stabilization portion <NUM> to move along the shaft <NUM>. As discussed further herein, if the anchor engagement portion <NUM> is engaging the anchor <NUM> movement of the knob <NUM> will cause the stabilization base <NUM> to move along the axis <NUM> in the direction of arrow <NUM> toward a surface or scalp of the head <NUM>.

With continuing reference to <FIG> and additional reference to <FIG> the tracking portion <NUM> includes a distal end <NUM>. The distal end <NUM> may include various features or elements, such as a tracking device <NUM>. The tracking device <NUM> may be fit within the distal end <NUM>. In various embodiments, the tracking device <NUM> may be placed in a portion, such as a distal hollowed or empty end, of the shaft <NUM>. The tracking device <NUM> may include one or more coils that are configured to sense a field generated in the navigation volume <NUM>, as illustrated in <FIG>. The field may be sensed by the tracking device <NUM> as discussed above. Accordingly, a position of the tracking device <NUM> may be determined relative to other tracking device, such as the tracking device <NUM> on the instrument relative to the instrument <NUM>.

The tracking device <NUM> may be formed with the distal portion <NUM>, such as molding the tracking portion <NUM> as a member and molding therein the tracking device <NUM>. Alternatively, the tracking portion <NUM> may be formed as separate pieces and the tracking device <NUM> may be positioned within the distal region <NUM>. Further, the tracking device <NUM> may communicate within the navigation system <NUM> wirelessly, such as wirelessly transmitting a signal, or transmitting a signal over a wire, such as a wire communication <NUM>. It is understood, however, that the wire communication <NUM> may represent both the wired or wireless communication. Nevertheless, the distal portion <NUM> may include a bore or passage <NUM> to allow passage of the wire <NUM>.

The distal portion <NUM> may further include a distal or terminal projection or point <NUM>. In various embodiments, the distal portion <NUM> may include a first point 234a and a second point or projection 234b. The projections <NUM> may be used to cut a selected material, such as a surgical drape <NUM>. Further, the distal portion <NUM> may include a groove or catch portion <NUM> that may assist in holding or capturing a portion of the drape <NUM> (illustrated in <FIG>) in the groove <NUM>. It is understood that the capture region may include the groove <NUM> and/or include a protuberance or ledge to assist in capturing the drape <NUM>.

Accordingly, as discussed further herein, a surgical drape may be moved over or past, such as by being pressed over, the projections <NUM> and slid down an exterior wall <NUM> (which may be tapered and conical) of the distal portion <NUM> to engage the groove <NUM> or other appropriate drape capture portion. The external wall <NUM> may be substantially smooth and selectively tapered, at an angle <NUM> relative to the central axis <NUM>. In various embodiments, the angle <NUM> may be about <NUM> degrees to about <NUM> degrees, including about <NUM> degrees to about <NUM> degrees, further including about <NUM> degrees to about <NUM> degrees. The distal portion <NUM> may, therefore, generally form or define a truncated cone. Also, the distal portion <NUM> may be formed as a single portion or member with the shaft <NUM> or formed separately and connected to the shaft <NUM>. The distal portion <NUM> may form a stop or limiter to movement of the knob <NUM>.

Further the distal portion <NUM> may include a recess or depression <NUM> having a wall <NUM>. The depression <NUM> may be a registration depression or divot. In use, as discussed above, the DRF <NUM> may be registered to the patient by tracking an instrument relative to the divot <NUM>. Further other instruments may be calibrated or registered relative to the DRF <NUM> by positioning a selected portion of the instrument, such as distal tip of the instrument <NUM>, in the divot <NUM>. The depression <NUM> may allow instruments to be registered or calibrated relative to the DRF <NUM> during a selected procedure.

With continuing reference to <FIG> and additional reference to <FIG>, a process for connecting the DRF <NUM> to the subject <NUM> is described and illustrated. As discussed above, the tool <NUM> may be used to engage the anchor <NUM> as illustrated in <FIG>. The anchor <NUM> may be positioned into a portion of the skull having a selected bone thickness. In various embodiments, an appropriate or a selected bone thickness may be about <NUM> millimeters (mm) to about <NUM>, or greater; further including about <NUM> to about <NUM>, and further including about <NUM>.

To gain access to the skull bone, an incision <NUM> may be made through a skin layer of the subject <NUM> in the head at a selected appropriate location. The incision <NUM> may have a dimension or length of less than about <NUM>, including less than about <NUM>, including less than about <NUM>, and further including about <NUM>. The incision may also be referred to as a stab wound or puncture. Further, the incision <NUM>, in various embodiments may not be required for selected purposes. The anchor <NUM>, or at least the bone engaging portion <NUM>, may be passed through the incision <NUM> into the bone below the tissue. The tissue, such as the skin of the subject <NUM>, need not be retracted as the bone engagement portion <NUM> is tapered and the thread <NUM> may engage the bone below the tissue. A cross section dimension, such as a diameter of the anchor <NUM> may include the cross sectional dimension <NUM> that is less than the incision length. For example, the cross sectional dimension <NUM> of the anchor <NUM>, such as the shaft <NUM>, may be less than about <NUM>, including less than about <NUM>, and may be about <NUM> to about <NUM> in diameter. Accordingly, the anchor <NUM> may be passed through a selectively sized incision, such as one that is about <NUM> in length or less, such as less than <NUM> in length. In various procedures, a <NUM> incision may not require any suturing after a procedure is completed to ensure appropriate or selected healing of the subject <NUM>.

With reference to <FIG>, the anchor <NUM> may extend a selected distance above the surrounding tissue such that the base engaging portion <NUM> is about <NUM> centimeters (cm) to about <NUM>, including a distance <NUM>, above the surface of the surrounding tissue on the head <NUM> of the subject <NUM>. In this way the anchor <NUM> is fixed to the head <NUM> of the subject <NUM> and is prepared to receive the stabilization base <NUM> and the associated tracking portion <NUM>.

With continuing reference to <FIG> and additional reference to <FIG>, the engagement region <NUM> may be extended a maximum distance relative to the central portion <NUM> of the base <NUM>. In various embodiments, the knob <NUM> may be bottomed out on a bottom surface or projection <NUM> (illustrated in <FIG>) of the distal portion <NUM>. Accordingly, the anchor engaging region <NUM> is spaced a furthest distance from the center <NUM> and the distal tips 174a - 178a are spaced a maximum distance from the head <NUM>.

As illustrated in <FIG>, the base engaging portion <NUM> may be passed through the opening <NUM> into the engaging region <NUM>. After engaging or capturing the base engaging portion <NUM> in the anchoring engaging portion <NUM>, the knob <NUM> may be rotated to engage and move the stabilization base <NUM> toward the head <NUM> generally in the direction of the arrow <NUM>. After a selected amount of rotation of the knob <NUM>, the stabilization base <NUM> is moved toward and is configured and does engage the head <NUM> in a selected manner as illustrated in <FIG>.

The base <NUM> may engage the head <NUM> to a selected degree. For example, the projections <NUM> - <NUM> may be pushed into a scalp of the subject <NUM> a selected amount. In various embodiments, the distal points 174a - 178a may pierce the external skin of the subject, but not push through the bone surface of the head <NUM>. It is understood, however, that the projections <NUM> - <NUM> may extend through the tissue of the subject and engage the bone of the head <NUM>. However the projections <NUM> - <NUM> generally have a diameter, even a maximum diameter, less than that of the anchor <NUM> (i.e. diameter <NUM>). Accordingly, even if the projections <NUM> - <NUM> pierce the skin of the subject <NUM>, sutures or additional closing techniques may not be required following a procedure.

Once the stabilization base <NUM> is engaging the head <NUM>, the DRF <NUM> is stabilized relative to the head <NUM>. The stabilization base <NUM>, therefore, is generally three-dimensionally locked or stabilized relative to the head <NUM>. Further, the combination of the stabilization base engaging the head <NUM> due to movement along the portion <NUM> that engages the spherical engaging portion <NUM> may allow stabilization of the base <NUM> even if the base is not aligned and/or moving directly along an axis 100a of the anchor <NUM>. Thus, the base have engage and be stabilized relative to the head <NUM> when it is at an angle 160a relative to the axis 100a of the anchor <NUM>. It is understood, however, that the engaging region <NUM> of the anchor <NUM> and the engaging region <NUM> may be keyed such that a single or one orientation is achieved between the two during stabilization of the base <NUM>.

Regardless, the engagement of the base <NUM> allows the tracking device <NUM> in the tracking portion <NUM> to be used to track a position of the head <NUM>. The tracking device <NUM> may be registered to image data, such as the image <NUM> displayed on the display device <NUM> (see <FIG>). Tracking the instrument <NUM> relative to the DRF <NUM> may then be possible once both are tracked with the tracking system and the procedure may be navigated with the navigation system, as discussed above.

Further, after the base <NUM> is engaged to the head <NUM> to stabilize the DRF <NUM>, the surgical drape <NUM> may be easily and/or efficiently placed over a portion of the DRF <NUM>. For example, prior to registration the drape <NUM> may be pushed over the projections <NUM>, as illustrated in <FIG>. The drape <NUM> may be pushed over the projections <NUM> and a small cut may be formed in the drape <NUM>. The drape <NUM> may generally be formed of an elastic or slightly elastic material which may be passed over the exterior conical surface <NUM> of the distal end <NUM> and rebound or be biased to engage the groove <NUM>. Accordingly, the drape <NUM> may be held relative to the distal end <NUM> over the subject <NUM>. Further piercing the drape <NUM> may allow for access to the divot <NUM> without interference of any drape or other sterile coverings such as for registration of the subject <NUM> and/or calibration of the instrument <NUM>.

As discussed above, the DRF <NUM> will be formed of selected materials. In various embodiments the tracking portion <NUM>, the knob <NUM>, and the stabilization base <NUM> may be formed of substantially single use materials, such as entirely or substantially molded or machined polymer materials. Accordingly, the DRF <NUM> may be sterilized prior to the procedure and therefore need not to be covered with the drape <NUM> to ensure sterility of a remote incision or surgical area. Further, the DRF <NUM> may be sterilized for a single use and be disposed after the selected procedure.

The DRF <NUM>, as disclosed herein, may be positioned on the subject <NUM> to perform a procedure, such as the one that is navigated with the navigation system <NUM>, by the user <NUM>. Moreover the DRF <NUM> may be stabilized and fixed relative to the subject <NUM> with substantially a single incision or only a single incision, such as the single incision <NUM>. The single incision <NUM> may be sized such that additional or post procedure closures (e.g. sutures) are not needed for complete healing of the subject <NUM>. The stabilization base <NUM> may be engaged to the head <NUM> without substantial or any trauma to the subject <NUM>. Accordingly the DRF <NUM> may be used on the subject <NUM> during a procedure with minimal or no trauma to the subject <NUM> to attach the DRF <NUM>.

Claim 1:
A system for fixing a tracking device (<NUM>) to a subject (<NUM>), comprising:
an anchor (<NUM>) extending along a first axis between a first end and a second end, wherein the first end includes a base engaging portion (<NUM>) and the second end includes a subject engaging portion (<NUM>) that is configured to be passed through an incision in a subject to engage a bone;
an elongated member (<NUM>) having an anchor engaging portion (<NUM>) to engage the anchor;
a stabilization base (<NUM>) having a passage, wherein the elongated member is operable to pass through the passage such that the stabilization base is operable to move toward the anchor along the elongated member; and
a knob (<NUM>) to abut the stabilization base and move the stabilization base toward the subject along the elongated member;
wherein the stabilization base includes a plurality of legs (<NUM>, <NUM>, <NUM>), each leg including a projection (<NUM>, <NUM>, <NUM>) configured to engage the subject to three-dimensionally lock the elongated member relative to the subject; and
characterised in that the base engaging portion includes a spherical portion (<NUM>) operable to engage the complementarily shaped anchor engagement portion of the elongated member.