Abstract:
A visual guidance system for use in a surgical procedure. The system includes an image processing unit programmed to receive a three-dimensional image of a surgical area, information on an instrument being used in the procedure, and a surgical plan including a planned trajectory that the instrument should follow. The image processing unit generates a derived representation of the surgical procedure depicting the movement of the instrument on the three-dimensional image. A graphical guidance indicator is disposed on the three-dimensional image with a center point of the graphical guidance indicator centered on the planned trajectory, the graphical guidance indicator being displayed orthogonal to the planned trajectory. The derived image and the graphical guidance indicator depict at least five degrees of freedom of motion of the instrument. The graphical guidance indicator is changed based on the movement of the instrument in accordance with the surgical plan.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to a surgical procedure and, more particularly, to a visual display system for providing a consolidated view in real-time of a surgical procedure with guidance to a surgical plan. 
       BACKGROUND 
       [0002]    There have been significant strides in surgical navigation systems in recent years. The current systems provide visualization of a surgical site on a computer monitor. Conventional systems operate by first creating a high-resolution three dimensional image of the anatomical region of interest. In dentistry, the preferred imaging modality is cone beam computed tomography, referred to herein as the “CT.” 
         [0003]    Using the image of the anatomical region, the surgeon creates a pre-operative plan in the area of operation identifying the desired location, depth and orientation of an implant on the CT image. 
         [0004]    During the operative procedure, a software system including tracking software, aligns or adjusts the position of the patient&#39;s jaw as observed in the operating suite with the pre-operative plan that is stored on the computer system. The system then detects the surgical tool (drill) and determines the location and orientation of the surgical tool with respect to the patient and the pre-operative plan. This information is then depicted on the screen as a representation. 
         [0005]    Using this information, in order to place an implant in a jaw, the surgeon will follow these steps: 
         [0006]    Find the position in the patient&#39;s jaw where the hole should be drilled into the bone. 
         [0007]    Place the drill tip in the proper position at the desired location, rotate the drill about the tip to the position the drill at the correct angular orientation. 
         [0008]    Drill into the tooth or bone, counteracting the drill&#39;s tendency to deviate from the desired trajectory as the bit twists into the bone. 
         [0009]    Continue along the planned trajectory until the drill tip is at the pre-planned depth within the bone. 
         [0010]    Failure to appropriately place the implant in the pre-planned location can result in a suboptimal aesthetic outcome or may injure the patient if the implant impinges on a nerve or erupts into surrounding anatomy if driven too deeply or in the wrong location. 
         [0011]    Because the surgeon is concerned with correct placement in three dimensions, most state of the art navigation systems simultaneously depict the instantaneous location of the surgical instrument with respect to the patient&#39;s anatomy from several different viewpoints. Referring to  FIG. 1 , a display from conventional navigation systems is shown. The display  10  includes a panoramic view  10 , typically of the entire jaw or at least the upper or lower jaw portion captured in an earlier scan. On the panoramic the current position of the drill  11  is represented as well as the depiction of the planned implant  12 . A live camera view  13  of the operating suite may be included. A computer generated comparative view  14  is displayed visually depicting the current displacement in buccal/lingual and mesial/distal directions of the drill  11  with respect to the planned implant  12 . A computer generated comparative view  15  is displayed visually depicting the current displacement of the angular orientation of the drill  11  with respect to the planned implant  12  along its long axis. 
         [0012]      FIG. 1  also includes a front view  16  and a side view  17  of the prior CT scans with the planned implant  12  and the current drill tip location  11  illustrated. These views also include a depiction of a nerve  18  in close proximity. The long axis  19  of the planned implant is illustrated depicting the positional misalignment in each plane. These views permit the surgeon to see a visual depiction of the current depth of the drill tip  11 . 
         [0013]    While conventional systems provide a combination of images that unambiguously show the orientation, depth and trajectory of a surgical instrument inside the patient with respect to a planned implant location, the multiple views that the surgeon must constantly comprehend can be exhausting to the surgeon. The surgeon must constantly shift focus between the different views, ensuring that every movement made to correct a deviation in one view does not adversely introduce error that can only be detected in a different view. These shifts in focus lead to user strain and can potentially lead to suboptimal outcomes. 
         [0014]    A need exists for an improved guidance system that depicts in real time the position and orientation of the surgical tool with respect to the patient&#39;s body and the planned implant trajectory. As a hole is drilled for the implant or as the implant is delivered into the bone, the system should clearly indicate if the surgeon is deviating from the planned trajectory in such a manner as to allow for straightforward correction. Additionally, upon the drill reaching the planned location within the body, the event should be readily apparent to the surgeon so further drilling can be stopped. The surgeon is trying to continuously maintain a proper position in the jaw with alignment to the planned trajectory all while drilling to the correct depth. It is therefore desirable that all the pertinent information is presented in a manner appropriate for simultaneous consumption by the surgeon. The current state of the art navigation systems fail to meet these basic objectives. 
       SUMMARY OF THE INVENTION 
       [0015]    The present invention is directed to a visual guidance system for use in a surgical procedure. The guidance system displays a derived image of the surgical procedure on a display. The guidance system includes a detection system or program for detecting and tracking a position and orientation of an instrument and tracking the position and orientation of a portion of a patient. The detection system registers the position and orientation of the portion of the patient to a prestored three-dimensional image of a portion of the patient. The detection system determines the location and orientation of an operating axis of the instrument relative to the prestored three-dimensional image. 
         [0016]    A display is provided for displaying the derived image of the surgical procedure. 
         [0017]    An image processing unit is included which is programmed to receive the prestored three-dimensional image, the instrument location and orientation, and a predetermined surgical plan including data representing a desired surgical result using the instrument for depiction on the prestored three-dimensional image and a planned trajectory to achieve the result. 
         [0018]    The image processing unit is programmed to generate a derived image of the surgical procedure including a three-dimensional visual representation of a portion of the instrument on the prestored three-dimensional image based on the location and orientation of the operating axis relative to a longitudinal axis of the planned trajectory. The derived image also includes a depiction of the planned trajectory on the prestored three-dimensional image. The derived image is formed orthogonal to the planned trajectory. 
         [0019]    The image processing unit is programmed to generate a graphical guidance indicator on the image display apparatus. The graphical guidance indicator is oriented so as to be depicted orthogonal to the planned trajectory. 
         [0020]    The image processing unit also sends the derived image to the display. The derived image and the graphical guidance indicator providing a visual depiction of at least five degrees of freedom of motion of the instrument relative to the surgical plan. 
         [0021]    The image processing unit changes the depiction of the graphical guidance indicator on the display based on the movement of the instrument in accordance with the surgical plan. 
         [0022]    In an embodiment, the surgical result is a drilled hole with a final depth. The surgical plan is a three-dimensional representation of the drilled hole. The system permits a user to select the three dimensional drilled hole shape. 
         [0023]    In one embodiment, the instrument is a drill with a drill bit, and the operating axis is a longitudinal axis of the drill bit. In this embodiment, the surgical procedure is an oral surgical procedure which involves drilling into bone of a tooth. 
         [0024]    The representation of the drill may include an annular cylindrical outer housing of a drill head and a concentric substantially cylindrical representation of a drill bit located within and spaced apart from the outer housing. 
         [0025]    In an embodiment, the graphical guidance indicator includes an aiming ring disposed around and centered on a longitudinal axis of the planned trajectory. The aiming ring provides a visual guide for a surgeon to use to locate the representation of the drill bit on the longitudinal axis of the planned trajectory. 
         [0026]    The graphical guidance indicator optionally includes an X mark centered on the longitudinal axis of the planned trajectory. The orientation of the X mark is associated with the orientation of the prestored three-dimensional image. 
         [0027]    The graphical guidance indicator may include a reticle centered on the longitudinal axis of the planned trajectory. The reticle provides a horizontal and vertical reference point relative to the image display. The reticle may include spaced apart tick marks representing distance relative to the prestored three-dimensional image. As such, the image processing unit may be programmed to adjust the spacing of the tick marks on the image display depending on a zoom level of the display of the prestored three-dimensional image. 
         [0028]    In one embodiment, the graphical guidance indicator includes a targeting ring disposed around and centered on the longitudinal axis of the planned trajectory. The targeting ring is preferably concentric with and located around the aiming ring. The targeting ring provides a visual guide for a surgeon to use to locate the representation of the drill head, so as to align the drill head with the longitudinal axis of the planned trajectory. 
         [0029]    It is contemplated that the image processing unit may receive data representing the location of a nerve in the prestored three-dimensional image. The image processing unit may depict a representation of the location of the nerve relative to the prestored three-dimensional image on the derived image. 
         [0030]    In an embodiment, the graphical guidance indicator includes a depth indicator adjacent to the targeting ring. The depth indicator is configured to be adjusted based on the movement of the drill bit along the planned trajectory. The depth indicator may be formed as a portion of a ring having a center coincident with the center of the targeting ring. The image processing unit changes the depth indicator by depicting a larger portion of the ring correlated to the depth of the drill bit in relation to a planned drilling depth. Alternately or in addition, the image processing unit can visually change the shading or coloring of the depth indicator on the image display based on the depth of the drill bit in relation to a planned drilling depth. 
         [0031]    In the event that the surgical plan involves different diameter drill bits, it is contemplated that the graphical guidance indicator can be visually changed to indicate when a different drill bit is required. For example, the aiming ring can be automatically resized to a diameter to accommodate the new drill diameter. 
         [0032]    It is also contemplated that the image processing unit can provide audible signals as the drill bit progresses along the planned trajectory toward the desired drilling depth. 
         [0033]    A method is also disclosed for providing visual guidance for a surgical instrument during a surgical procedure. 
         [0034]    The foregoing and other features of the invention and advantages of the present invention will become more apparent in light of the following detailed description of the preferred embodiments, as illustrated in the accompanying figures. As will be realized, the invention is capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and the description are to be regarded as illustrative in nature, and not as restrictive. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    For the purpose of illustrating the invention, the drawings show a form of the invention which is presently preferred. However, it should be understood that this invention is not limited to the precise arrangements and instrumentalities shown in the drawings. 
           [0036]      FIG. 1  illustrates a prior art computer display for monitoring and tracking a surgical procedure. 
           [0037]      FIG. 2  is a schematic representation of a system according to the present invention for displaying and tracking a surgical procedure. 
           [0038]      FIG. 3  is an illustration of a computer generated display for monitoring and tracking a surgical procedure according to the present invention. 
           [0039]      FIG. 4  is an enlarged illustration of a graphical interface used in the display of  FIG. 3 . 
           [0040]      FIG. 5  is an illustration of the graphical interface of  FIG. 4  with a portion of a representation of a surgical tool shown approaching a target site. 
           [0041]      FIG. 6  is another illustration of the graphical interface of  FIG. 4  with a portion of a surgical tool shown. 
           [0042]      FIG. 7  is an enlarged illustration of the graphical interface of  FIG. 6  with the portion of a surgical tool shown close to the target site. 
           [0043]      FIG. 8  illustrates the graphical interface of  FIG. 7  with the surgical tool aligned on a planned drilling site. 
           [0044]      FIG. 8A  illustrates the graphical interface of  FIG. 8  depicting a change in depth of a drill tip along a planned drill hole. 
           [0045]      FIG. 8B  illustrates the graphical interface of  FIG. 8  depicting a further change in depth of a drill tip along the planned drill hole. 
           [0046]      FIG. 8C  illustrates the graphical interface of  FIG. 8  depicting the drill tip reaching the desired drilling depth. 
           [0047]      FIG. 8D  illustrates the graphical interface of  FIG. 8  depicting a the drill tip exceeding the desired drilling depth. 
           [0048]      FIG. 9  illustrates the different degrees of freedom provided with the graphical interface of  FIG. 8 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0049]    Referring to  FIGS. 2-10 , an embodiment of the present invention is depicted for providing a method and system for visually displaying an image guided surgical procedure. While the procedure depicted in the images is a dental procedure, it should be readily apparent that the system and method could be applied to any number of different surgical procedures where the insertion of a probe, needle, or treatment device to a specific location and depth inside the body, for example tumor biopsies and orthopedic device placement. 
         [0050]    One dental procedure that would benefit from precise guidance is the placement of dental implants within the jaw. The present system assists in (i) locating and depicting the correct location of the drill tip entry into the bone, (ii) depicting the direction and orientation of the drill tip, and (iii) providing information on the position of the drill tip within the anatomy even when not directly visible. If the drill tip&#39;s entry point aligns with the planned procedure and the drill is delivered into the bone in the proper trajectory, the third objective reduces to knowing the precise depth of the drill into the bone. 
         [0051]    To address the shortcomings of current surgical guidance systems, the inventors have developed a visual display and system that presents all the information required to align a surgical instrument in a precise location and orientation and then deliver it to a precise position within the patient according to a predetermined plan (e.g., following a preset trajectory.) 
         [0052]    The display  100  provides a continuously updated rendering of three-dimensional objects in the surgical area of interest with an overlay of a graphical guidance indicator  102  and a three-dimensional depiction of the surgical tool (e.g., drill)  104 . The vantage point of the three dimensional image scene  100  is chosen such that the virtual camera&#39;s image plane is perpendicular to the longitudinal axis of the preplanned hole that is to be drilled. As will become readily apparent, the present invention can work with any procedure where a planned trajectory or longitudinal axis is known. The planned trajectory is a surgical path that the instrument is to take to reach the planned end result (the final location). While in the illustrated embodiment the planned trajectory or surgical path is shown as a linear longitudinal axis, it should be readily apparent that, depending on the surgical procedure being undertaken, the trajectory could be a complex trajectory involving the use of multiple instruments, having different shapes, sizes and capabilities. The system is preferably designed such that the scene (displayed image)  100  may be panned, zoomed and rotated, but the orientation of the virtual camera (user&#39;s viewpoint) is maintained such that the image plane is perpendicular or orthogonal to the planned longitudinal axis (the planned trajectory). This permits straightforward drilling of the implant hole. If this perpendicular or orthogonal view were not used, a circle at the tip of the implant hole would appear elliptical, confounding efforts to align the drill tip along the implant hole&#39;s longitudinal axis. 
         [0053]    There are several objects and depictions in the guidance view scene  100 . A surgical plan is incorporated into the guidance indicators  102  and is depicted on the teeth/bones in the illustrated embodiment. To do this, the system  200  first requires that the surgeon create or input the surgical plan, including a planned implant location on the prior CT scan  202  of the surgical area. The surgical plan or implant location (which ego includes the location relative to the tooth/bone and the depth into the tooth/bone) is stored as a three dimensional representation  204 , preferably as a 3D polygonal mesh that is overlaid on the CT scan. The system  200  may include software drawing tools which permit the surgeon to draw a 3D representation of the planned hole to be drilled. Alternatively, the system may include stored 3D mesh representations of predetermined implant hole configurations. The surgeon can then select the desired 3D hole representation. It is also contemplated that the surgeon could select the desired implant and the system could automatically select one or more 3D mesh representations that would be needed to create a hole suitable for the implant selected. The mesh representation is preferably stored separately from the prior CT scan. The surgical plan includes a trajectory (for example, a longitudinal axis) of the planned implant hole. 
         [0054]    Also in this scene  100  is a computer generated representation of the drill  104  being used. The system  200  may include a database  206  of stored data representing multiple drills and drill bits. Based on the particular drill and drill bit being used (which can be inputted by the personnel at the time of the surgical procedure or could be automatically detected by the system, such as by optically detecting a code (e.g., bar or 2D graphical code) on the surgical tool or by detecting an RFID chip in the tool), the system  200  selects the appropriate graphical representation of the surgical drill and depicts it in the visual display  100 . In the illustrated embodiment, the drill may be depicted as a hand piece  106  with a cored-out head  108  and a drill bit  110  of a specific length and diameter. The drill bit&#39;s  110  representation reflects the physical characteristics of the drill bit that is currently attached to the drill. As mentioned, this data may be extracted from the database  206 . The data also includes an operating axis (which for a drill bit would be its longitudinal or drilling axis). During the course of the operation, the surgeon may use several drill bits to form the implant hole. By correctly depicting the drill bit length and diameter, the system permits accurate depiction of the drill depth into the jaw bone. By utilizing an image guidance or tracking system, such as the optical image guidance system developed by X-Nav Technologies, LLC and described in pending U.S. patent application Ser. Nos. 14/487,987, and 14/488,004, the disclosures of which are incorporated herein by reference in their entireties, the present invention can provide continuous real time data  208  representing the drill&#39;s position and orientation and the detected location of the surgical area, such as the location of the jaw bone. Those skilled in the art would readily appreciate that other guidance or tracking systems can be used, including electromagnetic tracking systems or mechanical arm encoding systems. The real time data  208  is used to accurately align the prior CT scan of the surgical area to the current surgical area. All the relevant data used by a computer program  210  that converts the data into representative images which are depicted the display  100 . While the preferred embodiment uses a tracker that determines a full 6 degrees-of-freedom rigid-body transform for the surgical tool, it is possible to use a 5 degrees-of-freedom tracker instead, providing only the tool&#39;s X, Y, Z location along with the tool&#39;s axis. 
         [0055]    Referring to  FIGS. 3 and 4 , the system  200  preferably controls features of the display  100  by tracking the movement of the drill  104  and the movement of the patient for adjusting the location of the planned implant hole. As the drill  104  is detected entering the general surgical area, the system  200  depicts the representation of the drill  104 , the trajectory and, optionally, the planned implant hole, on the display relative to the prior CT scan. The planned trajectory is surrounded by several visual cues or indicators that are configured to provide relevant information to the surgeon regarding the location, orientation and depth of the drill bit relative to the planned implant hole. As described below, these are all included in one combined active graphical guidance indicator  102 . 
         [0056]    As the surgical tool is brought toward the drilling location, the initial objective is to locate the drill tip  108  at the drilling location, which is depicted by an intermediate targeting ring  112  (which may be a certain color or shading, such as blue). The targeting ring  112  optionally surrounds cross-hairs  114 , generally forming an “X” or “+” in the illustrated embodiment (i.e., an X mark). The graphical guidance indicator  102  may also include a fixed reticle  116  with a center collocated on the planned trajectory or longitudinal axis of the planned implant hole. The reticle  116  provides a horizontal and vertical reference point with respect to the screen. Thus, it does not rotate as the image is rotated. Compare  FIGS. 5 and 6 . The X mark  114 , on the other hand, it tied to the scanned image and, while also centered on the planned trajectory or longitudinal axis of the planned implant hole, it will rotate relative to the reticle  116 . The ability to depict the rotation of the X mark  114  relative to the reticle  116  provides a visual sense of rotary motion. The reticle  116  may include tick marks  117  (shown in  FIG. 7 ) that are set to depict a prescribed distance, for example 1 mm. This gives the surgeon a sense of scale regardless of the zoom level of the system (i.e., the spacing of the tick marks  117  would change as the zoom level is changed. The level of accuracy required in certain procedures is a decision made by the surgeon and, therefore, providing this extra degree of visual information makes procedural decisions more accurate. The graphical guidance indicator  102  optionally includes numerical indicators  120  positioned around the targeting ring  112 . 
         [0057]    As discussed above, the scene is rendered in an orthographic projection, with the viewing plan being perpendicular to the planned trajectory or longitudinal axis of the planned implant hole. In such a rendering, the relative size of objects remains constant, regardless of their distance to the camera. This is in contrast to more conventional perspective projection, where closer objects appear larger. In the guidance view, the relative movements are quite small, so the depth cues that perspective projection provide minimal benefit and only serve to overwhelm the scene when it is close to the rendering camera (such as at the beginning of a surgical procedure where the drill could potentially fill the entire field of view. 
         [0058]    It is also contemplated that the system would permit the surgeon to adjust the view, such as by rotation, to allow the view to be similar to the way the surgeon is viewing the actual jaw. The X mark  114  depicts the precise location where the hole should be drilled. Thus, the X mark  114  is fixed with respect to the jaw/bone/tooth, even when the view is rotated (i.e., the X mark will rotate with the rotation of the image of the jaw, or the movement of the patient in the view). The present invention permits the X mark  114  to be oriented with 6 degrees of motion in order to accommodate implants that are not rotationally symmetric. The X mark  114  and surrounding targeting ring  112  provide visual cues regarding the planned implant hole location with respect to the surrounding jaw structure. When situated above the jaw, these indicators are rendered in a solid, opaque coloring. When parts of the X mark  114  or targeting ring  112  are located below the bone, the portion of those visual indicators is rendered as a semi-transparent or dashed element, thus providing the visual suggestion of depth when looking down. See, for example,  FIG. 6 . This would occur if the starting point for the planned implant hole is located below the surface in the CT scan, for example if a portion of the bone or a gum line must first be removed before commencing the planned hole drilling. 
         [0059]    As the surgical tool  104  gets closer to the surgical site (the planned implant hole)  102 , the system allows the view to be zoomed in. This can either be performed by the user selecting a zoom in/out feature, or could happen automatically, for example, based on the distance that the drill head  108  is from the surgical site  102 . As shown in the figures, the drill  104  and drill bit  110  are preferably drawn semi-transparent (translucent) so that the anatomy of the patient beneath the drill  104  and drill bit  110  can always be seen. It is contemplated that only the drill head  108  and drill bit  110  would be semi-transparent with the remainder of the surgical tool being opaque. Preferably, a small diameter or dot is depicted as tip  122  of the drill bit  110 . In one embodiment, the tip  122  has a diameter of about 0.35 mm. This assists the surgeon by providing a guide for the surgeon to place on the center of the X mark and the reticle  116  (i.e., on the planned trajectory), thereby aligning the drill tip  122  with the center of the planned implant hole. 
         [0060]    There are several visual cues or indicators that are part of the illustrated guidance indicator  102  and which facilitate the alignment of the drill bit  110  along the correct trajectory of the planned implant hole. The drill head  108  has a cylindrical outer diameter that is sized to fit within the inner diameter of the targeting ring  112 . This provides the surgeon with an initial goal for the correct placement of the drill head  108 . The guidance indicator  102  also preferably including an aiming ring  124  having a center coincident with the center of the reticle and the center of the targeting ring  112  such that the aiming ring  124  is concentric with the targeting ring  112 . The aiming ring  124  has an inner diameter that is preferably sized such that the outer diameter of the drill bit  110  fits within the inner diameter of the aiming ring  124  when they are properly aligned. As will become apparent, this permits alignment of the drill head  108  and the drill bit  110  with the planned trajectory and the planned implant hole being drilled. This provides a visual guide for a surgeon to use to locate the representation of the drill bit on the planned trajectory. In an alternate embodiment, the aiming ring  124  is depicted with a size and shape corresponding to the shape of the three-dimensional planned hole orthogonal to the longitudinal axis. Thus, the drill bit in this embodiment would be smaller than the depicted aiming ring  124 . 
         [0061]    Referring to  FIG. 7 , the surgeon moves the drill head  108  toward the targeting ring  112 . As shown, the shaded portion of the drill head  108  depicts the sidewall  108   S  of the drill head. Similarly, the shaded portion of the drill bit  110  depicts the sidewall  110   S  of the drill bit. Since the view in  FIG. 7  is set to be an orthogonal view looking directly down on the drill head  108 , the sidewalls  108   S    110   S  of the drill head and drill bit should not be visible in that view if the drill head  108  is properly oriented with the drill bit&#39;s longitudinal axis lying along the axis of the hole being drilled (i.e., the planned trajectory). Thus, the view in  FIG. 7  quickly informs the surgeon that the drill head  108  and drill bit need to be rotated. Referring now to  FIG. 8 , the view illustrates the drill head  108  rotated so that the sidewalls  108   S ,  110   S  are no longer visible. The drill head  108  is also properly positioned inside the targeting ring  112 . The bit  110  is also located within the aiming ring  124  thus providing a clear visualization to the surgeon that the drill bit  110  is properly aligned with the planned trajectory for the planned implant hole. 
         [0062]    Once appropriately aligned, the surgeon can begin drilling into the jaw. The desired depth into the bone is determined during preoperative planning and is part of the planned implant hole data  204  provided to the surgical guidance program  210 . The present invention preferably provides a visual cue as part of the guidance indicator  102  that informs the surgeon on the depth. Referring to  FIG. 8 , a depth indicator  126  is preferably depicted as a ring around the outside of and concentric with the targeting ring.  FIG. 8  illustrates the depth indicator  126  prior to drilling into the bone. The depth indicator  126  is preferably unfilled or lightly shaded so as to also be translucent. Once drilling begins and the tip of the drill bit begins to enter the bone, the program  210  begins to fill the depth indicator  126 , preferably with an initial color or shading  128 , such as yellow, in real time as the depth increases into the bone. See,  FIGS. 8A and 8B . This provides visual guidance to the surgeon as to how far into the bone the drill has progressed. The numeric depth markers  120  positioned circumferentially about the depth indicator  126 , show the depth into the bone that the drill has progressed. The system can be programmed such that the scale numbering and/or the units used can be changed during the planning stage. For example, in the illustrated embodiment, each numerical depth marker  120  indicates 3 mm in depth into the bone up to 12 mm. For a planned hole that is not as deep, each numeric depth marker  120  may only represent 1 mm in drilled depth. The planned desired depth is preferably shown on the depth indicator (in the illustrated embodiment, it is 12 mm). To anticipate over-drilling, the depth indicator  126  may also include additional numeric depth markers that are depicted in a different color and that may indicate the depth past the planned depth, e.g., −2 mm. The numeric depth markers  120  can either be regularly spaced or may correlate to specific meaningful measurements. For example, an implant hole is typically not drilled using a single size drill bit  110 . Instead, successively larger bits are used. The marks on the depth indicator can therefore represent the depth each drill bit  110  should be inserted. The sequence of drill bits  110  and associated depths can be preoperatively set by the surgeon at the time of planning or the sequence may be determined from a database that correlates a selected implant model with the surgical tool sequence. 
         [0063]      FIG. 8B  depicts how a change in the drill bit diameter by a surgeon can affect the guidance indicator. In this embodiment, a smaller drill bit  110  was attached to the drill  104 . As such, the aiming ring  124  is preferably automatically updated to a smaller diameter that correlates to the diameter of the smaller drill bit  110 . The depth indicator  126  is shown with additional shading or coloring  128  to depict the current depth. As the drilling progresses, the coloring of the depth indicator  126  preferably changes to highlight to the surgeon where the depth is compared to the planned drilling depth. For example, the color may progress in the order: yellow (keep advancing) (slanted hatching in  FIG. 8B ), green (the drill is at the pre-planned depth) (dotted shading in  FIG. 8C ), and red (the drilling has gone too deep) (heavy shading in  FIG. 8D ). Of course other coloring or shading may be used. Audible indictors may also be provided, such as an alarm when the planned depth is exceeded. It is also contemplated that an audible signal or verbal signal can be provided at predetermined depths of the drill tip into the bone. 
         [0064]    As shown in  FIG. 9 , the present invention provides six degrees of positional/directional freedom depicted in a single view. The X/Y coordinates show the position in the plane in which the image is formed. The orthogonal Z direction is depicted by the depth indicator. The orientation/angulation of the drill  104  is given by the alignment of the hollowed-out drill head  108  within the view. The pitch and yaw visible in this view depict the correct angulation of the drill with respect to the planned drill path. The roll of the drill around the drill axis gives the user an orientation of the drill with respect to the body itself so that the drill head  108  is not floating in space. This helps to quickly convey to the surgeon how to correct the orientation/location of the drill. As should be apparent from the above discussion, the present invention is tracking the x, y, z pitch and roll of the tool. However, in the context of the display, the image depicted is the x, y, pitch and roll and yaw of the surgical tool in the view. The depth of the tool is the depicted by the depth indicator (either visually or audibly). 
         [0065]    In one embodiment, the system  200  permits the surrounding operating area (e.g., the bone) to be modified to make it more transparent and depict obstacles within the scene, such as nerves. The distance to the nerves along the implant trajectory can be added on the wrap-around depth indicator by the software in the based on a known distance, or could be added based on the current projected trajectory of the drill bit  110  and a known or detected location of the nerve. This can all be done in an automated fashion. Any conventional method for detecting nerves in CT scans or in a patient can be used. 
         [0066]    The system  200  may also be configured to change the colors or shading of various cues of the guidance indicator  102 , e.g., the reticle or the inner aiming ring, when the physician is too far from the correct planned drilling angle. 
         [0067]    It should be apparent that the wrap-around depth indicator  126  is only one embodiment for visually depicting the depth of the drill bit. For example, the view could be modified such that there is a visual indicator that resizes to show depth. For instance, the aiming ring  124  could start large and shrink to the diameter of the drill bit at the correct depth. Alternately, the targeting ring can be adjusted to get smaller as the depth advances until the targeting ring overlies the aiming ring, thus indicating that the desired depth has been achieved. 
         [0068]    Any of the visual components that are displayed could be rendered with different lighting, shading, color, and fill effects within the scope of the present invention. 
         [0069]    Some of the unique features of the invention are a third-person oriented, realistic view of the surgical area, including a graphical representation of the drill and the jaw in the display rendered as how they generally appear provide the user with a sense that the display is a real-life depiction, which makes the user feel more comfortable that what they are seeing is happening live. Also, providing these objects in the specific orientation of looking down on the implant hole&#39;s longitudinal axis is unique. The system provides a view outside of the surgical area at the point where the tip of the drill bit is located. The system also preferably depicts the drill, drill bit, jaw, nerves and other components to scale, updated in real time and rendered in the an orthographic projection. 
         [0070]    By including the depth indicator  126  as part of the guidance indicator  102  allows the surgeon to maintain attention on the target (the planned implant hole) without the need to shift focus to determine the drill&#39;s current depth. 
         [0071]    The system or systems described herein may be implemented on any form of computer or computers. The system of the present invention may include a software program stored on a computer and/or storage device (e.g., mediums), and/or may be executed through a network. The method may be implemented through program code or program modules stored on a storage medium. 
         [0072]    For the purposes of promoting an understanding of the principles of the invention, reference has been made to the preferred embodiments illustrated in the drawings, and specific language has been used to describe these embodiments. However, no limitation of the scope of the invention is intended by this specific language, and the invention should be construed to encompass all embodiments that would normally occur to one of ordinary skill in the art. 
         [0073]    The particular implementations shown and described herein are illustrative examples of the invention and are not intended to otherwise limit the scope of the invention in any way. For the sake of brevity, conventional electronics, control systems, software development and other functional aspects of the systems (and components of the individual operating components of the systems) may not be described in detail. 
         [0074]    Finally, the use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. Numerous modifications and adaptations will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention.