Abstract:
A patellar resection instrument has an integral depth adjustment assembly that is easily, intuitively, and intra-operatively adjustable between a plurality of discrete positions. Each position corresponds to a particular, known depth of resection, and this depth may optionally be inscribed on the depth adjustment knob for easy visual selection and/or confirmation of the resection depth. The depth adjustment assembly provides positive tactile feedback as the stylus is moved between positions. The unique configuration of the depth adjustment assembly maintains the resection depth stylus in a chosen position throughout the patellar resection procedure, even if the procedure causes vibration or other forces to be transmitted to the patellar resection instrument.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to surgical instruments and, more particularly, to surgical instruments used in the resection of a patella during knee arthroplasty. 
     2. Description of the Related Art 
     Orthopedic prostheses are commonly utilized to repair or replace damaged bone and tissue in the human body. For example, a knee prosthesis may include a patellofemoral prosthesis designed to replace the natural patellofemoral groove, also called the femoral sulcus, formed in the distal portion of the femur. In total knee arthroplasty, for example, a femoral component designed to replace the entire distal portion of the natural femur includes an anterior flange which serves as a patellofemoral prosthesis. In some other surgical procedures, such as partial knee arthroplasty, a separate patellofemoral component can be implanted to replace the femoral sulcus. 
     A patellar prosthesis may also be used in knee arthroplasty procedures. The patella prosthesis replaces a portion of the natural patella, and is designed to articulate with the patellofemoral prosthesis. Alternatively, a surgeon may forego the use of a patellofemoral component and implant only the patellar prosthesis, which then articulates with the natural patellofemoral groove of the femur. In a typical implantation of a patellar prosthesis, the anterior portion of the natural patella is retained while the posterior, articulating portion of the patella is replaced with a prosthetic component. Replacing only the articulating surface of the patella preserves ligament connections between the natural patella and the surrounding anatomical structures. To make such a “partial” patellar replacement, the posterior portion of the patella is resected and the prosthetic patellar component is affixed to the resected surface. 
     To perform a resection of the posterior portion of the patella for receipt of a particular prosthesis, the depth of resection is controlled to remove a known amount of the natural bone stock of the anatomic patella. The chosen prosthesis replaces the removed bone stock to yield a desired prosthetic patellar articular surface. 
     SUMMARY 
     The present disclosure provides a patellar resection instrument with an integral depth adjustment assembly that is easily, intuitively, and intra-operatively adjustable between a plurality of discrete positions. Each position corresponds to a particular, known depth of resection, and this depth may optionally be inscribed on the depth adjustment knob for easy visual selection and/or confirmation of the resection depth. The depth adjustment assembly provides positive tactile feedback as the stylus is moved between positions. The unique configuration of the depth adjustment assembly maintains the resection depth stylus in a chosen position throughout the patellar resection procedure, even if the procedure causes vibration or other forces to be transmitted to the patellar resection instrument. 
     In one aspect thereof, the present invention provides a surgical instrument for use in resection of a patella, the instrument comprising: a cut guide defining a cut guide surface, the cut guide adapted to guide a cutting tool through at least a portion of the patella along the cut guide surface; and an integral depth adjustment assembly comprising: an adjustment body coupled to the cut guide; a stylus slidably coupled to the adjustment body, the stylus having a bone contacting surface movable between a plurality of discrete stylus positions, each position defining a predefined stylus distance between the cut guide surface and the bone contacting surface, whereby each stylus distance corresponds to a different resection depth of the patella when the patella is in contact with the stylus; and a biasing element biasing the stylus into one of the discrete positions when the biasing element is acted upon only by forces within the depth adjustment assembly. 
     In another aspect thereof, the present invention provides a surgical instrument for use in resection of a patella, the instrument comprising: an instrument body; means for fixing the patella to the instrument body; a cut guide having a cut guide surface; and resection depth selection means for selecting a predefined resection depth, the depth selection means integral with the instrument body, the depth selection means including a stylus having a bone contacting surface, the resection depth equal to a distance between the bone contacting surface and the cut guide surface. 
     In yet another aspect thereof, the present invention provides a method of resecting a patella with a surgical instrument including a cut guide and an integral depth adjustment assembly, the depth adjustment assembly including: an adjustment body coupled to the surgical instrument; a stylus slidably coupled to the adjustment body, the stylus defining a plurality of discrete positions with respect to the adjustment body, each position corresponding to a resection depth of the patella; and a biasing element biasing the stylus into each of the discrete positions, the biasing element urging the stylus into one of the discrete positions when the biasing element is acted upon only by forces within the depth adjustment assembly, the method comprising: adjusting the depth adjustment assembly into one of the discrete positions to define a resection depth of the patella; contacting the patella with the stylus; while maintaining the contact between the patella and the stylus and after the step of adjusting the depth adjustment assembly, coupling the cut guide of the surgical instrument onto the patella; and resecting the patella using the cut guide. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above-mentioned and other features and advantages of this invention, and the manner of obtaining them, will become more apparent and the invention itself will be better understood by reference to the following descriptions of an embodiment of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a perspective view with a patellar resection instrument in accordance with the present disclosure, illustrating the instrument in use; 
         FIG. 2  is a top plan view of the patellar resection instrument shown in  FIG. 1 , illustrating open and closed configurations of the instrument; 
         FIG. 2A  is a partial, top plan view of the patellar resection instrument shown in  FIG. 2 , illustrating an engaged ratchet mechanism; 
         FIG. 2B  is a partial, top plan view of the patellar resection instrument shown in  FIG. 2 , illustrating a disengaged ratchet mechanism; 
         FIG. 3  is a side, elevation view of the patellar resection instrument shown in  FIG. 1 ; 
         FIG. 3A  is a partial side, elevation view of the patellar resection instrument shown in  FIG. 3 , illustrating the depth adjustment assembly in a lowered configuration; 
         FIG. 3B  is a partial side, elevation view of the patellar resection instrument shown in  FIG. 3 , illustrating the depth adjustment assembly in an intermediate configuration; 
         FIG. 3C  is a partial side, elevation view of the patellar resection instrument shown in  FIG. 3 , illustrating the depth adjustment assembly in a raised configuration; 
         FIG. 4  is a perspective, exploded view of the patellar resection instrument shown in  FIG. 1 ; 
         FIG. 5  is an exploded, perspective view of a depth adjustment assembly in accordance with the present disclosure; and 
         FIG. 5A  is a side, elevation, detail view of a scalloped region of the depth adjustment assembly shown in  FIG. 5 , in which the rounded profile of the scalloped region is shown as it would appear if unfolded into a planar configuration. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. The exemplification set out herein illustrates an exemplary embodiment of the invention, and such exemplification is not to be construed as limiting the scope of the invention in any matter. 
     DETAILED DESCRIPTION 
     Referring now to  FIG. 1 , patella resection instrument  10  includes a detent mechanism, such as depth adjustment assembly  12 , operable to move resection depth stylus  14  between a plurality of discrete positions relative to cut guide surfaces  16 . Each position corresponds to a known and enumerated resection depth D ( FIG. 3 ) that will result when instrument  10  is used to resect patella P. In the exemplary embodiment shown and described herein, resection depth D is the largest distance between the cut guide surfaces ( 16 ), formed in clamping portion  18  of instrument  10 , and bone contacting surface  60  of stylus  14  along a line substantially normal to surfaces  16 ,  60 . Although depth adjustment assembly  12  is described below as including scalloped region  70  ( FIGS. 3A-3C ) operable to adjust the height of stylus  14 , it is contemplated that other detent mechanisms may be used in accordance with the present disclosure, with a plurality of detents each defining a position of stylus  14 , and therefore defining resection depth D. 
     Prior to resection of the patella, depth adjustment assembly  12  is manipulated to set resection depth D to a particular desired value or distance. Stylus  14  is brought into contact with patella P, which is then clamped within clamping portion  18  while maintaining the patella/stylus contact. With patella P secured in clamping portion  18 , blade B ( FIG. 1 ) or other cutting tool is passed through cut slots  20 ,  20 ′ formed in clamp arms  26 ,  26 ′ of clamping portion  18  to resect patella P. After the resection is complete, the resected surface of the remaining portion of patella P is substantially coplanar with the lower cut guide surfaces  16 . For purposes of the following discussion, the “lower” cut guide surface  16  in each of clamp arms  26 ,  26 ′ is the cut guide surface furthest from bone contacting surface  60  of stylus  14 . Thus, the distance between bone contacting surface  60  of stylus  14  and lower cut guide surfaces  16  corresponds to resection depth D, i.e., the total depth of bone removed from patella P. 
     As described below, an exemplary embodiment of patella resection instrument  10  includes springs  30 ,  64 ,  106 , which act upon various structures of patella resection instrument  10  to provide a biasing force. However, it is contemplated that any suitably biasing element may be used in lieu of any of springs  30 ,  64 ,  106 . Alternative biasing elements include elastomeric materials and magnetic elements, for example. 
     Referring to  FIGS. 1 and 4 , patella resection instrument  10  includes first and second instrument bodies  22 ,  22 ′ each having handles  24 ,  24 ′ and patellar clamp arms  26 ,  26 ′. Bodies  22 ,  22 ′ are joined by pivots  28 ,  28 ′ disposed between handles  24 ,  24 ′ and clamp arms  26 ,  26 ′, respectively. Torsion spring  30  spans pivots  28 ,  28 ′ and acts on both of instrument bodies  22 ,  22 ′ to bias handles  24 ,  24 ′ and clamp arms  26 ,  26 ′ away from one another and into an open configuration (solid lines of  FIG. 2 ). As discussed below, ratchet mechanism  32  ( FIGS. 2 and 4 ) operates to hold instrument  10  in a closed position (dashed lines of  FIG. 2 ) against the biasing force of torsion spring  30 . 
     Referring to  FIGS. 2 and 4 , one end of torsion spring  30  engages pivot  28 ′ of instrument body  22 ′ at aperture  31 ′ ( FIGS. 2 and 4 ), while the other end of torsion spring  30  engages pivot  28  of instrument body  22  at aperture  31  ( FIG. 4 ). Thus, torsion spring  30  is acted on, and may be compressed by handles  24 ,  24 ′ of instrument bodies  22 ,  22 ′ respectively. The travel of instrument bodies  22 ,  22 ′ toward the open position is limited by abutting contact between shoulders  29 A,  29 A′ formed near pivots  28 ,  28 ′ respectively, and by the corresponding opposite-side shoulders  29 B,  29 B′ ( FIG. 2 ). On the other hand, the travel of instrument bodies  22 ,  22 ′ toward the closed position is limited by abutting contact between shoulders  27 ,  27 ′ formed between clamp arms  26 ,  26 ′ and pivots  28 ,  28 ′ respectively. 
     Instrument bodies  22 ,  22 ′ are held together at pivots  28 ,  28 ′ by cooperation between threaded end  36  of adjustment body  46  (discussed below) of depth adjustment assembly  12  and bushing  38  threadably coupled thereto. On assembly, torsion spring  30  is placed between instrument bodies  22 ,  22 ′ at pivots  28 ,  28 ′ and threaded end  36  is passed through apertures  40 ,  40 ′ formed at pivots  28 ,  28 ′. Bushing  38  is then threadably coupled to threaded end  36  to pivotably couple bodies  22 ,  22 ′ to one another. In the exemplary embodiment illustrated in  FIG. 4 , a lower low-friction washer  42 , is placed between bushing  38  and instrument body  22 , and an upper low-friction washer  42 ′ is placed between depth adjustment assembly  12  and instrument body  22 ′ to facilitate smooth rotatable motion of instrument bodies  22 ,  22 ′ with respect to one another, and to facilitate smooth rotatable motion between depth adjustment assembly  12  and instrument body  22 ′. Low friction washers  42 ,  42 ′ may be coated with Teflon® material (Teflon® is a registered trademark of E. I. du Pont de Nemours and Company Corporation of Wilmington, Del.), for example. 
     Clamp arms  26 ,  26 ′ each include jaws  44  extending inwardly therefrom for firmly grasping patella P as described in detail below. Cut slots  20 ,  20 ′ are formed in clamp arms  26 ,  26 ′, respectively above jaws  44 , such that jaws  44  will remain engaged with patella P after the resection of same is complete. The lower surfaces of cut slots  20 ,  20 ′ are substantially coplanar when patella resection instrument  10  is assembled, thereby forming coplanar lower cut guide surfaces  16 . After the resection procedure is complete, lower cut guide surfaces  16  are also coplanar with the remainder of patella P. As a result of this coplanarity, lower cut guide surfaces  16  provide data for establishing the total resection depth D of patella P, as described in detail below. 
     Turning now to  FIG. 5 , depth adjustment assembly  12  includes resection depth stylus  14 , adjustment body  46 , and adjustment knob  48 . Stylus  14  includes pivot body  50 , pin  52 , knob mounting shaft  54 , cap mounting shaft  56 , and stylus extension  58 . Stylus extension  58  includes bone contacting surface  60  along a lower end thereof. Pin  52  is received within longitudinal aperture  61  formed in lower shaft  62  of adjustment body  46 , with spring  64  disposed between shoulder  63  atop shaft  62  and lower shoulder  51  of pivot body  50 . Spring  64  biases stylus  14  upwardly away from adjustment body  46  to facilitate discrete adjustment of assembly  12 , as discussed in detail below. Pivot body  50  and spring  64  are received within head portion  66  of adjustment body  46 . In order to accommodate stylus extension  58 , head portion  66  includes slot  68 . Head portion  66  of adjustment body  46  further includes scalloped region  70 , which forms the basis for the discrete adjustment of stylus  14  to select resection depth D, as described in detail below. 
     With pivot body  50  received in head portion  66  of adjustment body  46 , knob mounting shaft  54  is positioned within aperture  57  formed through knob  48 . Similarly, cap mounting shaft  56  may be received within aperture  81  formed through knob cap  80  once knob cap  80  is positioned within counterbore  82  formed in knob  48  (discussed further below). The close fit between shafts  54 ,  56  and apertures  57 ,  81 , respectively, centers knob  48  upon pivot body  50  of stylus  14 . Thus, knob  48  remains axially aligned on stylus  48  as knob  48  is rotated to adjust resection depth D. As discussed below, knob  48  and stylus  14  move upwardly and downwardly together as resection depth D is adjusted, such that shafts  54 ,  56  remain in contact with apertures  57 ,  81  respectively through the range of adjustment for depth adjustment assembly  12 . 
     Knob  48  includes skirt  72  having aperture  74  sized to receive pin  76 . When knob  48  is mounted to knob mounting shaft  54  of stylus  14 , skirt  72  partially covers head portion  66  of adjustment body  46 , thereby allowing pin  76  to engage scalloped region  70 . On assembly, knob  48  is held down against the biasing force of spring  64  to align aperture  74  in skirt  72  with scalloped region  70 . Pin  76  is then pushed through aperture  74  and into scalloped region  70  (see, for example,  FIG. 3B ). Pin  76  can be interference-fit through aperture  74  or welded in place, for example. When knob  48  is subsequently released, spring  64  urges pin  76  into the nearest adjacent trough  71  ( FIG. 5A ) formed in scallops  78 . More specifically, pin  76  is acted upon by spring  64  (via pivot body  50 ), which pushes upwardly on knob  48 . This spring-biased interaction between knob  48  and scalloped region  70  of adjustment body  46  forms the basis for discrete adjustment of patella resection depth D, and also provides tactile feedback indicative of such adjustment (as described below). 
     Referring still to  FIG. 5 , knob cap  80  is received within counterbore  82  formed in top surface  84  of knob  48 . On assembly, knob cap  80  is fixed to cap mounting shaft  56  of stylus  14  (such as by welding), and rotatably received within counterbore  82  of knob  48 , so that knob  48  is rotatable with respect to stylus  14  and adjustment body  46 , but is only axially moveable to the extent permitted by cooperation of pin  76  with scalloped region  70  (as described below). More particularly, upper shoulder  53  receives a corresponding surface within knob  48  (not shown), which are biased into abutting engagement by spring  64  when knob  48  is assembled to stylus  14  as discussed below. 
     In an exemplary embodiment, top surface  84  of knob  48  includes resection depth markings  88  indicative of resection depth D ( FIG. 3 ). Markings  88  are spaced on knob  48  to correspond to the rotational orientation of knob associated with each of troughs  71 , such that each discrete position of stylus  14  is represented by an individual mark. Markings are indexed to a reference mark, which may be formed on stylus extension  58  of stylus  14 , or knob cap  80 , for example. After assembly of instrument  10 , any discrepancy between the distance measured between lower cut guide surfaces  16  and bone contacting surface  60  of stylus  14  and the intended resection depth D (indicated by the numerical values of markings  88 ) can be corrected by machining away material from bone contacting surface  60  until the distance exactly matches the numerical values of resection depth markings  88 . When the exact match is found, this distance is equal to resection depth D. In an exemplary embodiment, extra material is provided on stylus extension  58  at bone contacting surface  60  to ensure sufficient material is present for removal by the above-described calibration process. 
     As best seen in  FIGS. 2 ,  2 A,  2 B and  4 , patella resection instrument  10  further includes ratchet mechanism  32  for holding clamping portion  18  in a closed configuration (shown in dashed lines in  FIG. 2 ). Referring to  FIG. 4 , ratchet mechanism  32  includes arcuate rack  90  having toothed concave surface  92  formed thereon. Arcuate rack  90  is fixed to handle  24 ′ at one end, such as by pins  94  passing through apertures  96  formed through handle  24 ′ and arcuate rack  90 , with the other end of rack  90  free. As patella resection instrument  10  is actuated between opened and closed positions ( FIG. 2 ), the free end of arcuate rack  90  extends into slot  98  formed in handle  24 . Release lever  100  is pivotably connected to handle  24  at ratchet pivot  101  ( FIG. 2 ), and includes release lever slot  102  ( FIG. 4 ) through which the free end of arcuate rack  90  passes on its way into slot  98 . Referring to  FIGS. 2A and 2B , ratchet pivot  101  is formed by the abutting contact of pivot pin  101 A with release lever pivot end  101 B, with such contact maintained by the biasing force of spring  106 . Release lever  100  is captured to instrument bodies  22 ,  22 ′ by arcuate rack  90 , which is surrounded on all sides by release lever slot  102 . In alternative embodiments, release lever  100  may be hingedly connected to one of instrument bodies  22 ,  22 ′ or otherwise connected in any suitable manner. 
     As best seen in  FIGS. 2A and 2B , release lever slot  102  includes pawl  104 , which is located and shaped to selectively engage toothed concave surface  92  as arcuate rack  90  passes through release lever slot  102 . More particularly, pawl  104  is releasably engaged with toothed surface  92  when lever  100  is spaced from handle  24  ( FIG. 2A ), and becomes disengaged when lever  100  is pulled toward handle  24  ( FIG. 2B ). To maintain ratchet mechanism  32  in a “normally engaged” configuration, spring  106  biases release lever  100  away from handle  24 . The teeth formed on toothed concave surface  92  cooperate with pawl  104  to allow surface  92  to pass through aperture  102  in lever  100  as instrument  10  is closed, even if lever  100  is under the biasing force of spring  106 . However, the teeth on surface  92  cooperate with pawl  104  to prevent any opening movement of rack  90  back through aperture  102  when pawl  104  is engaged with surface  92 , as shown in  FIG. 2A . For example, the teeth of toothed surface  92  may present an angled surface to pawl  104  which can be “climbed” to allow instrument  10  to be freely manipulated from an open to a closed configuration. On the other hand, the teeth of toothed surface  92  may present a vertical or reverse-angled surface which locks pawl  104  against surface  92 , thereby only allowing instrument  10  to be reconfigured from a closed to an open position if release lever  100  is actuated (against the biasing force of spring  106 ) to disengage pawl  102  from toothed concave surface  92  as shown in  FIG. 2B . 
     Although ratcheting mechanism  32  is described as the mechanism for locking instrument  10  in a closed configuration, it is contemplated that other locking devices may be used in accordance with the present disclosure. Such locking mechanisms might include other ratchet mechanism configurations, cam/follower systems, and physical stops such as cotter pins and fasteners. 
     In use, patella resection instrument  10  can be set to a particular resection depth D for removal of a known quantity of patella P. Resection depth D is first set to a desired level by manipulation of depth adjustment assembly  12 , and instrument  10  is subsequently affixed to patella P. Advantageously, the novel design of depth adjustment assembly  12  reliably maintains resection depth stylus  14  in the originally set position throughout the resection of patella P, even where vibrations, shocks or other forces are transmitted to instrument  10  during the resection process. 
     Prior to attachment of clamping portion  18  to patella P, resection depth D ( FIG. 3 ) is set by manipulation of depth adjustment assembly  12 . More particularly, resection depth D is set by rotating knob  48  to engage knob pin  76  within a selected one of troughs  71  among scallops  78 . For example, referring to  FIG. 3C , stylus  14  may be placed in a raised configuration to define resection depth D R , such that a relatively large amount of patella P would be resected. In this raised configuration, pin  76  engages the uppermost trough  71  in scalloped region  70 . As shown in  FIGS. 3A-3C , scalloped region  70  includes a plurality of troughs  71 , with each successive trough at a different position relative to other adjacent troughs  71 . In the illustrated embodiment, troughs  71  successively index upwardly from a lowest position ( FIG. 3A ) to a highest position ( FIG. 3C ), with each intermediate position slightly higher than the previous one. In an exemplary embodiment illustrated in  FIG. 5A , each position indexes downwardly by a trough-to-trough distance  75  of approximately 0.50 mm (0.0197 in). Scalloped region  70  includes seven troughs  71 , yielding a net adjustability  75 ′ ( FIG. 5A ) of resection depth stylus  14  of about 3.0 mm (0.1181 in). However, it is contemplated that any level of total adjustment with any number of intermediate indexed positions may be chosen as required or desired for a particular application. 
     Referring to  FIG. 3B , depth adjustment assembly  12  is shown in an intermediate configuration yielding resection depth D I  between bone contacting surface  60  of stylus  14  and lower cut guide surfaces  16  of clamping portion  18 . To adjust depth adjustment assembly  12  from the raised configuration shown in  FIG. 3C  to the intermediate configuration shown in  3 B, knob  48  is pressed downwardly against the biasing force of spring  64  to dislodge knob pin  76  from the uppermost trough  71  ( FIG. 3C ). Knob  48  is then rotated, with pin  76  riding smoothly along ramped surface  73 ′ of scalloped region  70  until markings  88  indicate a desired resection depth, such as intermediate resection depth D I . Adjustment knob  48  is then released. In the absence of any user manipulation or other force external to depth adjustment assembly  12 , spring  64  biases pin  76  into the nearest trough  71  of scallops  78 . Alternatively, knob  48  can simply be rotated, causing pin  76  to follow the surface of scallops  78  to “click” into position at each of troughs  71  once the external force, i.e., the rotation of knob  48 , is removed. Put another way, the absence of external manipulation forces on depth adjustment assembly  12  (i.e., via knob  48 ) renders all forces acting on spring  64  internal to depth adjustment assembly  12  (i.e., the counterbalancing forces from shoulders  51 ,  63  of pivot body  50  and lower shaft  62  respectively). When only these internal forces act on spring  64 , spring  64  will bias pin  76  into the nearest adjacent trough  71 , thereby placing stylus  14  in one of the discrete adjustment positions defined by scalloped region  70 . 
     As a result of the relatively lower position of trough  71  in this intermediate position, resection depth stylus  14  has moved downwardly from the position shown in  FIG. 3C  and pivot body  50  has pushed further into head portion  66  of adjustment body  46  against the biasing force of spring  64 . More particularly, the abutting engagement between an inner surface of knob  48  (not shown) and upper shoulder  53  of pivot body  50  causes any downward movement of knob  48  to be transferred to stylus  14 . As the position of knob  48  is lowered or raised by engaging pin  76  with a lower or higher trough  71 , the spring-biased abutting engagement between knob  48  and pivot body  50  ensures that stylus  14  will also lower or raise correspondingly. Alternatively, ramped surface  73  formed on knob  48  ( FIGS. 3B and 3C ) may contact stylus extension  58  directly to urge stylus  14  downwardly as pin  76  of knob  48  is moved into engagement with troughs  71  having different heights. In this case, a space is formed between upper shoulder  53  of pivot body  50  and the adjacent surface within knob  48 . 
     Referring to  FIG. 3A , depth adjustment assembly  12  may be similarly reconfigured into a fully lowered configuration to define resection depth D L  between bone contacting surface  60  of stylus  14  and lower cut guide surfaces  16  of clamping portion  18 . In the lowered configuration of  FIG. 3A , pin  76  is received by the lowermost trough  71  of scallops  78 , such that knob  48  urges stylus  14  into its lowest position via contact with upper shoulder  53  of pivot body  50 . Pivot body  50  is also more fully received within head portion  66  of adjustment body  46 , such that spring  64  is in a fully or nearly fully compressed state. Depth adjustment assembly  12  may, in the exemplary embodiment shown, be reconfigured into any of the seven discrete positions defined by troughs  71  in a similar manner. 
     Once the surgeon has selected a desired depth of resection D for patella P, clamping portion  18  of instrument  10  may be affixed to patella P to prepare for the resection procedure. Patella P is first exposed using conventional retraction procedures and/or other surgical methods. Patella P is typically everted, i.e., rotated away from femur F and tibia T to expose the articular surface to be resected, as shown in  FIG. 1 . With clamping portion  18  in an open configuration (solid lines of  FIG. 2 ), bone contacting surface  60  of resection depth stylus  14  is placed onto the exposed articular surface of patella P. 
     Patella resection instrument  10  is then aligned so that lower cut guide surfaces  16  are oriented with respect to patella P, as desired by the surgeon. While maintaining contact between bone contacting surface  60  and patella P, handles  24 ,  24 ′ are squeezed together to bring clamping portion  18  into a closed configuration (dashed lines of  FIG. 2 ). In order to maintain contact with the desired “high point” of patella P during this clamping procedure, depth adjustment assembly  12  may pivot with respect to pivots  28 ,  28 ′ of instrument bodies  22 ,  22 ′ respectively. Such pivoting is made possible because lower shaft  62  of adjustment body  46  is not rotatably fixed within apertures  40 ,  40 ′ ( FIG. 4 ). Additionally, as noted above, low-friction washers  42 ,  42 ′ facilitate smooth rotation of depth adjustment assembly  12  with respect to bodies  22 ,  22 ′. 
     Upon contact with patella P, jaws  44  rigidly fix clamping portion  18  (and therefore, instrument  10 ) to patella P. As noted above, ratchet mechanism  32  allows handles  24 ,  24 ′ to be freely squeezed together into the closed configuration against the biasing force of torsion spring  30 , but prevents clamping portion  18  from reverting back to the open configuration under such biasing force. Thus, clamping portion  18  is maintained in the fixedly coupled state with patella P until release lever  100  is actuated at the end of the resection procedure. 
     With clamping portion  18  now firmly engaging jaws  44  about the periphery of patella P, and resection depth stylus  14  in contact with patella P and set to resection depth D, blade B (or any suitable cutting instrument) is passed into one of cut slots  20 ,  20 ′, with one of lower cut guide surfaces  16  used to maintain blade B at the appropriate resection height. Once blade B has passed through the other of cut slots  20 ,  20 ′ and is in contact with both lower cut guide surfaces  16  at both of clamp arms  26 ,  26 ′, blade B may be moved throughout cut slots  20 ,  20 ′ to complete the resection of patella P and fully dislodge the removed portion of patella P from the newly resected surface on the remainder of patella P. 
     Because resection depth D is defined by lower cut guide surfaces  16  of cut slots  20 ,  20 ′, and the removed portion of patella P is the “upper” portion thereof in the context of instrument  10 , lower cut guide surfaces  16  are coplanar with the resected surface of the remainder of patella P after the resection operation is complete. Thus, the thickness of blade B does not affect resection depth D, which is the distance between bone contacting surface  60  of stylus  14  and lower cut guide surfaces  16 , and is equal to the total amount of bone removed from patella P. Put another way, resection depth D may be said to be the total distance from lower cut guide surfaces  16  to bone contacting surface  60 , when measured along a line normal to the planar lower cut guide surfaces  16 . 
     With the resection of patella P now complete, release lever  100  is actuated against the biasing force of spring  106  to release pawl  104  from rack  92 . When so released, the biasing force of torsion spring  30  is allowed to move clamp arms  26 ,  26 ′ away from one another, reconfiguring clamping portion  18  to an open configuration. The newly resected surface of patella P may then be used to implant a prosthetic patellar component in accordance with conventional methods. 
     Alternatively, instrument  10  may remain engaged with patella P to facilitate holding patella P during implantation of the prosthetic patellar component. To allow unfettered exposure of the resected surface, stylus  14  may be swiveled to either side by pivoting depth adjustment assembly  12  with respect to pivots  28 ,  28 ′ of bodies  22 ,  22 ′ respectively in the same manner as the pivoting of stylus  14  during the clamping of clamping portion  18  upon patella P (described above). 
     Advantageously, depth adjustment assembly  12  of instrument  10  defines a plurality of discrete depth adjustment values corresponding to known resection depths, such that resection depth D can be set to a particular value by simply manipulating knob  48  until markings  88  on top surface  84  thereof indicate that resection depth D has been set. This method of setting resection depth D is efficient and intuitive, such that a surgeon may quickly and efficiently adjust resection depth D intra-operatively. Moreover, depth adjustment assembly  12  is integral to instrument  10 , in that assembly  12  remains rotatably attached to instrument bodies  22 ,  22 ′ throughout the use of instrument  10 . Depth adjustment assembly  12  is also integral to instrument  10 , in that no parts of depth adjustment assembly  12  are separable from the remainder of instrument  10  (i.e., instrument bodies  22 ,  22 ′) during use. Depth adjustment assembly  12  being integral to instrument  10  provides ease of use, storage and transport, prevents assembly  12  from coming loose from instrument  10  during the surgical procedure, and ensures that depth adjustment assembly  12  is always together with the rest of instrument  10 . 
     Also advantageously, depth adjustment assembly  12  of instrument  10  may provide tactile and audible feedback to indicate movement between discrete depth adjustment values, in contrast to a “continuously adjustable” (i.e., threadably adjusted) system lacking predefined distances defining discrete patella resection depths. If a surgeon wishes to adjust a continuously adjustable system to a particular resection depth, the resection depth adjuster is carefully and precisely rotated to the position corresponding to the desired depth of resection. Thus, changing the resection depth a very small amount involves a very small movement of the depth adjuster in a continuously adjustable system. On the other hand, depth adjustment assembly  12  allows for the desired resection depth D to be predefined (i.e., by the locations of troughs  71  formed in adjustment body  46 , as discussed above). Rather than carefully and precisely manipulating assembly  12  with small movements of knob  48 , the surgeon can simply adjust knob  48  into one of the discrete, predefined resection depth positions with a quick, relatively large movement. The precision of the final resection depth D comes from the automatic movement adjustment assembly  12  into the desired position, rather than from manual fine-tuning. 
     More particularly, knob pin  76  ( FIG. 5 ) may ride over scallops  78  of scalloped region  70  as knob  48  is rotated, such that pin  76  “snaps” into respective troughs  71  of scallops  78 . With each such “snap,” a surgeon using instrument  10  can both feel and hear knob  48  engaging a new discrete, predefined resection depth position under the force provided by spring  64 . Thus, depth adjustment assembly  12  eliminates ambiguity as to whether stylus  14  is in a particular discrete position, because spring  64  will bias pin  76  fully into one of recesses  71  in scalloped region  70 . Because there are a finite number of recesses  71 , such as seven recesses in the exemplary embodiment disclosed herein, simply releasing knob  48  ensures that resection depth stylus is in one of the discrete predefined positions. 
     A further advantage of instrument  10  is that vibrations and other forces transmitted throughout the components of instrument  10  during a resection procedure will not cause depth adjustment assembly  12  to loosen, nor will resection depth stylus  14  change position absent manipulation of knob  48 . Spring  64  biases pin  76  firmly into a respective one of troughs  71 , and troughs  71  are sufficiently deep to ensure that vibrations and other forces transmitted to depth adjustment assembly  12  during normal usage of instrument  10  will result in pin  76  becoming dislodged from the chosen trough  71  corresponding to a desired resection depth D. Moreover, a surgeon can view resection depth markings  88  at any time during use of instrument  10  to visually verify that stylus  14  remains in the chosen position. 
     While this invention has been described as having an exemplary design, the present disclosure can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the scope of the appended claims.