Abstract:
An instrument for implanting patellar prostheses in various sizes includes a series of milling calipers of various sizes, a retaining instrument with a receptacle for one of the milling calipers matching the sizes of the patellar prostheses and milling bodies in various sizes. The retaining instrument is configured like forceps with two members, the front end of one of the members being provided with a retention plate for engagement with a front face of a patellar prosthesis, and the front end of the other one of the members being provided with a holder for a milling gauge.

Description:
REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is the national stage under 35 USC 371 of International Application No. PCT/EP2007/007492, filed Aug. 27, 2007, which claims Convention priority from European Patent Application No. 06021418.6, filed Oct. 12, 2006, the full disclosures of which are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The invention relates to a modular instrument set for implanting patella prostheses of various sizes, comprising a retaining instrument which is designed like forceps with two members, the front end of one member being provided with a retention plate for engagement with a front face of the patella, and the front end of the other member being provided with a guide for a milling gauge, and comprising several milling gauges which are available in various sizes matching the sizes of the patella prosthesis and are mounted exchangeably on the guide, and several milling bodies of various sizes, one each of which belongs to one of the milling gauges. 
       BACKGROUND OF THE INVENTION 
       [0003]    When a patient receives a knee prosthesis that ensures the articular function between femur and tibia, the implantation of a patella prosthesis is also generally indicated. Given the anatomy of the knee joint, the rear face of the patella bone cooperates with a front area of the femoral condyles during flexion of the knee. If, during implantation of a knee prosthesis, the natural condyles are replaced by condylar runners of the prosthesis, this causes contact between the natural patella and the condylar runners of the prosthesis. To avoid undesired contact between natural bone material and the artificial material of the prosthesis, generally of metal, an implant is provided on the rear face of the patella. This implant functions as an articulation surface and interacts with the corresponding mating surface on the condylar runners of the knee prosthesis. Because of this function, and because of the configuration of the natural patella, the patella prosthesis generally has a button-like shape. Its rear face acting as an articulation surface in most cases has a convex shape like a dome. Its front face for bearing on the natural patella bone is mostly flat, with optional anchoring pins. The implantation of the prosthesis involves bone material being removed by resection from the rear face of the natural patella bone, the thickness of the resected bone material corresponding approximately to the thickness of the patella prosthesis. This has the effect that functional joint anatomy can be preserved as far as possible. Resection of the patella bone is performed mainly by two techniques, namely sawing or milling. 
         [0004]    An instrument set for sawing is known from prior public use (Columbus Knee System from Aesculap Orthopedics). Here, the thickness of the patella bone is determined, and the patella bone is squeezed into retaining forceps. These engage laterally and on the front face of the patella bone. In doing so, there is a danger of possible damage to tissue structures, in particular ligaments, which are likewise connected laterally to the patella. The retaining forceps include a saw guide. In this way, the operating surgeon is able to resect bone material from the rear face of the patella using an oscillating saw. The thickness of the bone material that is to be resected can be determined by means of a height adjustment device arranged on the retaining instrument. Resection by means of an oscillating saw has the effect that chips created by the sawing may pass into the surrounding area and thereby deposit themselves on other tissue material. Complete removal is often not possible. Leaving the chips permanently in situ poses a risk of infections or other complications. Moreover, the slit-shaped guide for the oscillating saw offers only inadequate guiding in the area of the point of the saw blade where it removes material. If the operating surgeon does not hold the instrument exactly at right angles to the slit in the saw guide, the oscillating saw removes either too much or too little material from the patella. 
         [0005]    Instruments for milling the patella are also known, for example from U.S. Pat. No. 6,866,667. This includes a forceps-like instrument with a retention plate for supporting the front face of the patella on the front end of one member, and with a guide ring which is arranged on the front end of the other member and is designed to clamp the patella against the retention plate from the rear. The guide ring defines a zero position for a thickness measurement. The instrument further comprises an adjustable stop ring, which can be set to one of four different values depending on the size of the patella prosthesis intended for implantation. Depending on its setting, it permits a greater or lesser depth of insertion of a milling body through the guide ring, as a result of which bone material from the rear face of the patella is resected in accordance with the set thickness. It is the operating surgeon who sets the stop ring to the depth that is to be resected. This can lead to errors in setting. In particular, there is the danger of the stop ring being set to too great a depth, which depth belongs to a larger patella prosthesis than the one intended for implantation. Too much bone material would therefore be resected. 
       SUMMARY OF THE INVENTION 
       [0006]    The object of the invention is to improve an instrument set of the type mentioned at the outset, so as to avoid the abovementioned disadvantages and in particular to counter the risk of setting too great a thickness relative to the size of the patella prosthesis. 
         [0007]    The solution according to the invention lies in an instrument set having the features of the invention as broadly described herein. Advantageous developments are the subject matter of the preferred embodiments. 
         [0008]    In a modular instrument set for implanting patella prostheses of various sizes, comprising a retaining instrument which is designed like forceps with two members, the front end of one member being provided with a retention plate for engagement with a front face of the patella prosthesis, and the front end of the other member being provided with a holder for a milling gauge, and comprising several milling gauges which are available in various sizes matching the sizes of the patella prostheses and are mounted exchangeably on the holder, and several milling bodies of various sizes, one each of which belongs to one of the milling gauges, provision is made, according to the invention, that size gauges with a circumferential marker edge are provided whose sizes are in each case adapted to one of the milling bodies and which are integrated into the milling gauges, and several thickness rings are provided which can each be placed on the milling body of matching size and whose thickness corresponds to that of the patella prosthesis associated with the milling body. 
         [0009]    Some of the terms used are explained below. 
         [0010]    An instrument designed like forceps is to be understood as an instrument whose two members are connected to each other via a joint arranged in a central area of the members. This will in most cases be a pivot joint, although it is also possible to provide a joint for parallel guiding. At their rear end, the members have a grip part which is designed to be held with force by an operating surgeon. 
         [0011]    Milling gauges are understood as elements which guide a milling body in at least one direction, preferably two directions, the milling bodies being movable in a third direction (mostly referred to as advance direction). 
         [0012]    Patella prostheses of various sizes is to be understood as meaning that the patella prostheses have different widths. Although the patella prostheses do not form part of the instrument set according to the invention, their width nevertheless defines the width of the associated milling body. It is generally the case that the number of milling bodies of different diameters provided in the instrument set will correspond to the number of patella prostheses of various sizes that are intended to be implanted by means of the instrument set. 
         [0013]    Thickness rings are understood as ring-shaped gauges that have a defined height. The height is the measure defined by the distance between the two end faces of the thickness ring. 
         [0014]    The invention is based on the concept of integrating a size gauge into the milling gauge and of using this combined milling and size gauge to allow the operating surgeon to visually determine whether the selected milling gauge suits the patella bone that is to be worked on. The combined milling and size gauge means that the determination of the correct prosthesis size and the guiding of the resection work are brought together within a single component. The operating surgeon can select the largest prosthesis size in question and fit the corresponding milling gauge from the instrument set onto the retaining instrument. By means of the size gauge integrated into the milling gauge, the operating surgeon can check, by comparing with the circumferential marker edge, whether the patella is sufficiently large for the selected size. If the circumferential marker edge is covered completely by the patella, then the patella can accept this size; if there is no coverage at certain points, this then means that the patella is too small for the selected size. In the latter case, the next smaller size must then be selected, and the procedure repeated using the next smaller milling gauge and the size gauge integrated therein. Finally, when the correct size is determined by means of the size gauge integrated in the milling gauges, the diameter of the milling body to be used is also established. Since there is a specific thickness ring belonging to each milling body, and since only this fits onto the respective milling body, the thickness of the bone material to be resected from the patella is also automatically established. 
         [0015]    The invention creates a seamless chain starting from the size gauge for selection of the correct prosthesis size and extending to the determination of the depth to which the milling body removes the bone material from the rear face of the patella. Therefore, with the modular instrument set according to the invention, neither measurement nor adjustment is necessary. By virtue of the invention, errors resulting from incorrect adjustment can no longer arise. Handling is thus made easier and also more reliable. 
         [0016]    In addition to the thickness ring, a displaceable stop ring is also expediently provided. It serves to form a depth stop directly on the milling body. In this way, a depth stop does not have to be provided on the retaining instrument. The arrangement of the stop ring directly on the milling body also has the advantage that imprecision in the positioning of the retaining instrument does not have any effect on the depth determination. To make adjustment of the stop ring easier, a clamping device is preferably provided on the stop ring. It permits simple positioning and securing of the stop ring in the desired position relative to the milling body. 
         [0017]    The stop ring is preferably designed congruent to the thickness ring. This ensures a large contact area between the end faces of stop ring and thickness ring that come to bear on each other. The congruent design also ensures easier handling. It is clear by intuition to the operating surgeon that the thickness ring has to be fitted onto the milling body (and of course it only fits onto one according to the invention) in such a way that it bears on the stop ring. This further counters the risk of incorrect handling. 
         [0018]    The retaining instrument designed like forceps expediently has a joint which is designed for linear guiding of the members. This is achieved by the retention plate on one member and the milling gauge on the other member moving linearly along an axis to each other, but otherwise not changing their orientation with respect to each other. This avoids the angle differences that would arise in the case of guiding along an arc of a circle in a simple pivot joint. This permits more precise guiding and, as a result, better protects the areas of the patella and adjoining ligaments that come into contact with the instrument according to the invention. 
         [0019]    Drill jigs of various sizes are expediently provided. They allow bores to be drilled into the milled rear face of the patella. These bores serve to receive anchoring pins of the patella prosthesis. In order to maintain a defined angle position, the drill jigs are expediently provided with an angle marking. It is particularly expedient if the latter is designed as a locking means, for example in the form of a pin which, in the correct position, engages in a matching recess on the milling gauge. In this way, the angle position is securely fixed. 
         [0020]    A pressing device is preferably provided that can be mounted on the holder. It is used for pressing the patella prosthesis onto the implantation site after the rear face of the patella has been resected and, if appropriate, after the bores have been drilled. The arrangement of the pressing device on the retaining instrument according to the invention has the advantage that the retaining instrument does not have to be removed when pressing the prosthesis in. This has advantages in cases where subsequent working proves necessary. 
         [0021]    A drill fixture is preferably provided which has a drill limited by a depth stop. This can be integrated into the milling gauge instead of the milling body. This may be expedient if, in the case of a very hard patella bone, the hard surface first has to be broken before the actual milling is carried out by means of the milling body. The depth stop on the drill ensures that the drill does not penetrate too deep and adversely affect the planicity of the surface intended to receive the patella prosthesis. It generally suffices if the depth stop is selected such that a sufficient distance remains in the case of the smallest size of patella prosthesis. If so desired, however, stops with different depths or displaceable stops can be provided in order to permit, for the larger patella sizes, a deeper breaking by means of the drill. 
         [0022]    According to a particularly expedient embodiment that is possibly deserving of independent protection, a saw jig module is provided which can be mounted on the holder of the retaining instrument. With the saw jig module, it is possible to resect the patella bone by sawing instead of by milling. This is advantageous especially if it transpires, during surgery, that resection with a milling device is inappropriate. It is then possible, without having to exchange the instrument set, to perform the resection by means of the saw jig module. For this purpose, guides are expediently provided on the size gauge module. They are of such a nature that the saw jig module can be safely mounted in a defined position on the size gauge module. The guide is preferably designed as a double guide, such that the saw jig module can be arranged on two sides of the size jig module. This allows the operating surgeon to choose the optimal position of the saw jig module according to the individual access conditions. 
         [0023]    The guide on the size gauge module is preferably designed such that the saw jig module is guided in a vertically adjustable manner on the size gauge module. In this way, the thickness of the material to be removed from the patella bone can be determined by a displacement of the saw jig module relative to the size gauge module. 
         [0024]    In order to arrange the saw jig module in the correct position on the size gauge module, a depth-measuring device is expediently provided. It is designed to probe the thickness of the natural patella bone and, on the basis of this, to determine the height of the saw jig module such that, during sawing, bone material is removed to a thickness that corresponds to the thickness of the patella prosthesis to be implanted. For this purpose, the depth-measuring device has a probe which protrudes down into the opening formed by the size gauge. This probe is expediently designed as a probe star, with a plurality of probe bodies arranged in a radiating pattern. The probe bodies differ in terms of their dimensions, with each probe body being assigned to one size of the patella prostheses. The extent of the probe bodies in the radial direction corresponds to the thickness of the respective patella prosthesis. By rotating the star such that the probe body for the size intended for implantation points in the direction of the patella retention plate, the depth-measuring device is correctly adjusted automatically, without any further manual maneuvers. The probe device then simply has to be moved together with the saw jig module along the guide, until the probe device bears on the rear face of the patella bone. In this way, the position of the saw jig module is determined, and thus the plane for the bone resection by means of a saw. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The invention is explained below with reference to the attached drawing, which depicts an advantageous illustrative embodiment of the invention and in which: 
           [0026]      FIG. 1  shows a view of an illustrative embodiment of the instrument according to the invention; 
           [0027]      FIG. 2  shows a detail of the instrument from  FIG. 1 ; 
           [0028]      FIG. 3  shows size gauges of the instrument set to which the instrument according to  FIG. 1  belongs; 
           [0029]      FIG. 4  shows milling bodies of the instrument set in two different sizes; 
           [0030]      FIG. 5  shows thickness rings of different sizes and a milling body; 
           [0031]      FIG. 6  shows drill jigs of the instrument set according to the invention; 
           [0032]      FIG. 7  shows a patella prosthesis for implantation using the instrument set; 
           [0033]      FIG. 8  shows the determination of a correct prosthesis size using the instrument set according to the invention; 
           [0034]      FIG. 9  shows the insertion of a milling body for determination of a starting position; 
           [0035]      FIG. 10  shows the patella being milled; 
           [0036]      FIG. 11  shows a tip of the patella being broken; 
           [0037]      FIG. 12  shows the patella prosthesis being pressed in; 
           [0038]      FIG. 13  shows a perspective view of a saw jig for the instrument set according to the invention; and 
           [0039]      FIG. 14  shows the instrument set with the saw jig mounted on the patella. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0040]    An instrument set according to the invention comprises a retaining instrument  1 , a plurality of milling gauges  3 , each of them with an integrated size gauge  2 , a corresponding number of milling bodies  4  adapted in size to the milling and size gauges  2 ,  3 , thickness rings  5  assigned in each case to one of the milling bodies  4 , a corresponding number of drill jigs  6  and pressing-in devices  7 , a drill  8 , and a saw jig module  9 . 
         [0041]    Patella prostheses are provided in different sizes for implantation. One example is shown in  FIG. 7 . The patella prosthesis  99  comprises a support body with a convex front face and several pegs on the rear face. 
         [0042]    The retaining instrument  1  is designed like forceps, with two members  11 ,  12  arranged movably about a pivot joint  10 . Both members are designed as grips in their rear section, and at the rear end there is a locking device  14  for fixing the retaining instrument  1  in a clamped position. The locking device  14  comprises a rod  140  which is arranged pivotably on the member  11  and which has saw teeth  141  on its front face. This engages in a corresponding edge (not shown) of a recess at the rear tip of the member  12 . A V-shaped leaf spring  144  is provided for applying a force that drives the members  11 ,  12  apart. 
         [0043]    The front end of the members  11 ,  12  is designed as a linear guide  15 . It comprises a guide sleeve  152  in the front end of the member  12 , in which a branch  161  of an L-shaped retention bracket  16  is guided longitudinally displaceably in a transverse axis of the instrument  1 . Transverse axis is understood here as an axis that lies perpendicular to the longitudinal axis  100  of the retaining instrument  1 , which extends from front to rear through the pivot joint  10 . The other branch  162  of the retention bracket  16  extends forward parallel to the longitudinal axis  100  and has a retention plate  17  at its front end. This has an annular edge  170 , with a plurality of spikes  171  distributed uniformly thereon and pointing toward the main axis  100  of the instrument. The retention plate  17  is designed to grip and fix the rear aspect of the patellar tendon when the retaining instrument  1  is fitted in place. The end of the branch  161  remote from the retention plate  17  is connected to the front end of the member  11  via a pivot bearing  165 . The pivot bearing  165 , in its simplest form, can be designed as a curved slit  116  at the front end of the member  11 , the shaft being guided on a screw  166 . The curvature and orientation of the slit  116  are chosen such that, in addition to the pivoting movement, length compensation is also achieved. A multi-purpose holder  18  is formed at the front end of the member  12  along with the guide sleeve  152 . It serves to ensure that the size and milling gauges  2 ,  3 , the pressing module  7  and the saw jig module  8  are mounted exactly in position on the instrument  1 . The multi-purpose holder  18  has a stepped configuration and comprises an upper section directed toward the member  11 , the material thickness decreasing in the direction of the center axis  100 . This section is designated as thinner section  182 , while a section adjoining this in the direction of the member  12  is designated as thicker section  183 . 
         [0044]    In order to protect against accidental loosening or displacement, a locking screw  181  is provided on the respective insert or module. 
         [0045]    The size and milling gauges  2 ,  3  are provided in different sizes, of which  FIG. 3  shows two from a set of, for example, four in total. They are composed principally of an annular sleeve  20  and of a retainer clip  30  that extends radially outward from the latter. The retainer clip  30  has two retaining branches  31 ,  32  that border the U-shaped space  33  for receiving the multi-purpose holder  18  with an exact fit. Moreover, arranged at the outer ends of the branches  31 ,  32 , there are mutually facing projections  34 ,  35  that enclose the multi-purpose holder  18  in the assembled state. A seat for the locking screw  181  is arranged on at least one of the branches. The width between the projections  34 ,  35  is dimensioned such that it is sufficient to permit passage of the thinner section of the multi-purpose holder  18 , but prevents passage of the thicker section. On its periphery, the annular sleeve  20  has two pairs of guide grooves  21  which are arranged at an angle to each other and extend in the axial direction along the full height of the annular sleeve  20 . A clamping screw  23  is arranged in a thread between the two guide grooves  21  of each pair. This device is used for securing additional modules, as will be explained in more detail below. On the top surface of a transition area between the annular sleeve  20  and the retaining branch  30  there is a distinctive size marking  24  (for example the numbers  1 ,  2 ,  3  or  4 ) which clearly identifies the respective dimension of the size and milling gauge  2 ,  3 . A particularly important role is played by the circumferential inner lower edge  22  of the annular sleeve  20 . It is designed as a marker edge. This means that its contour is designed to exactly match the outer contour of the patella prosthesis  99 . This means that the marker edge  22  exactly delimits the area on which the patella prosthesis  99  is to be implanted. For each size of the patella prostheses  99 , there is a size and milling gauge  2 ,  3  whose marker edge  22  exactly follows the outer contour of the respective patella prosthesis  99 . This ensures a clear association between the respective size and milling gauge  2 ,  3  and a size of the patella prostheses  99 . This allows the operating surgeon to make a simple and reliable choice of the correct size of patella prosthesis  99  and, at the same time, allows him to safely establish the elements of the instrument set according to the invention that are to be used to implant these sizes. Preferably starting with the largest size, the operating surgeon holds the size gauges  2  in succession over the patella bone on which the patella prosthesis  99  is to be implanted. It is then simply necessary to check whether or not the marker edge  22  is completely covered by the patella bone. If this is not the case, the patella bone is too small for the size being checked, and the next smaller size is then to be chosen, with this procedure being repeated until finally the size gauge  2  is found in which the marker edge  22  is covered completely by the patella bone. In this way, the correct size of the patella prosthesis is found. This size gauge  2  can now be secured on the retaining instrument  1  by means of the multi-purpose holder  18 . Since, according to the invention, the size gauge  2  is at the same time also the milling gauge  3 , to which only one milling body  4  precisely fits, this therefore ensures at the same time that the correct milling gauge is used and thus also the correct milling body  4 . This eliminates errors that are caused by accidentally using too large a milling body  4  and, consequently, by removing the wrong amount of bone material irretrievably. 
         [0046]    When the size and milling gauge  2 ,  3  of the correctly defined dimension is fixed on the retaining instrument  1 , the rear face of the patella bone can be prepared for implantation of the patella prosthesis  99  of the selected size. In cases where there are hard and very osseous rear surfaces of the patella bone, provision can optionally be made to use the drill  8  to break the tip on the rear face of the patella and thus make access easier for the milling bodies  4 . At its front section, the drill  8  has a helical area  80  which is designed in the normal manner for a drill. This area is adjoined to the rear by a collar  81 . The face  82  of the latter directed toward the helical section  80  functions as an abutment surface against a top face of an abutment plate  86 . The abutment plate  86  is designed as a circular disk and lies like a cover on the annular sleeve  20  of the selected size and milling gauge  2 ,  3 . It has a central opening which is dimensioned for passage of the helical section  80  of the drill but is smaller than the diameter of the collar  81 . In this way, during drilling, the drill  8  can engage in the annular sleeve  20  only to a depth at which the collar  81  bears with its face  82  on the plate  86 . The height of the annular sleeve  20  is chosen as a function of the dimension of the respective size and milling gauge  2 ,  3  such that the drill  8 , on reaching the abutment, only removes the upper tip area of the patella bone. This ensures that the drill  8  does not remove bone material beyond the actual implantation depth. 
         [0047]    The four differently dimensioned milling bodies  4  in the depicted illustrative embodiment each have an upper section  40  and a lower section  41 . The upper section  40  is designed like a sleeve and has, on its inside  4 , radially inwardly directed projections  48  (see  FIG. 9 ) as coupling elements for a drive tool  48  (see  FIG. 10 ). The milling body  4  is driven via these coupling elements. The upper section  40  has the same width in all the milling bodies  4 . The lower section  41  has a cylinder shape with milling teeth  42  on its bottom end face. The diameter of the lower section  41  differs depending on the dimension of the respective milling body  4 . It corresponds in each case to the diameter of the different sizes of patella prosthesis  99 . Apart from a slight play that is provided to avoid jamming, this diameter corresponds to the internal diameter of the annular sleeve  20  of the respectively associated size and milling gauge  2 ,  3 . A stop ring  43  is pushed onto the lower section  41 . It is guided longitudinally displaceably, with the aid of a clamping screw, in an axial groove  45  on the jacket of the lower area  41 . The stop ring can be pushed to any desired position along the groove  45  and can be fixed there by means of the clamping screw  44 . The internal diameter of the stop ring  43  is chosen such that it sits practically free of play on the lower section  41 ; its external diameter is chosen so as to coincide with that of the annular sleeve  20  of the associated size and milling gauge  2 ,  3 . For milling, the milling head  4  is inserted into the previously selected size and milling gauge  2 ,  3  until its end face with the milling teeth  42  bears on the tip of the rear face of the patella bone. From this position, the depth of bone removal is to be determined that is equal to the thickness of the support element of the patella prosthesis  99 . The milling body  4  is intended to remove material to a thickness corresponding to the thickness of the support body of the patella prosthesis  99 . 
         [0048]    A respective thickness ring  5  is provided for each size of patella prosthesis  99 . Its height corresponds in each case to the thickness of the support body of the patella prosthesis  99 , for which reason the height is also designated as “thickness”. The internal diameter of the inner rings  5  is in each case chosen so as to coincide with the external diameter of the lower area  41  of the respective milling body  4  belonging to the respective size of patella prosthesis  99 . This ensures a clear and unmistakable allocation between thickness rings  5  and milling bodies  4  with respect to the respective size of patella prosthesis  99 . Provision is made that the thickness ring  5  assigned to the selected milling body  4  is pushed from underneath onto the lower section  41  and is brought into contact with the stop ring  43 . The selected milling body  4 , with the thickness ring  5  pushed on, is then inserted into the allocated size and milling gauge  2 ,  3  and pushed forward until the end face of the lower section  41  with the milling teeth  42  bears on the tip of the rear face of the patella bone. The stop ring  43  is then moved down toward the annular sleeve  20  until the thickness ring  5  abuts against the top surface of the annular ring  20 . In this position, the clamping screw  44  of the stop ring  43  is tightened. The depth of the milling to be carried out with the milling body  4  is thus determined from the zero point defined by the height of the tip of the patella bone and from the thickness, as represented by the thickness ring  5 , of the patella prosthesis that is to be implanted. The stop ring  43  thus functions now as a depth stop. The milling body  4  is detached from the milling and size gauge in order to remove the thickness ring. It is then fitted back in place and can be connected to the drive tool. The milling can then be carried out until the milling body  4  has been advanced to the extent that the stop ring  43  bears on the top surface of the annular sleeve  20  (see  FIG. 9 ). The maximum milling depth corresponding to the thickness of the selected size of patella prosthesis  99  is reached in this way. Errors of the kind that could arise by accidentally milling too deeply are reliably ruled out in this way. 
         [0049]    The milling body  4  can now be removed, and bores for securing purposes can be provided by means of the drill jigs  6 . The drill jigs  6  are of a mushroom-like shape, with a lower section  61  whose diameter is adapted to the internal diameter of the annular sleeve  20  of the respective size and milling gauge  2 ,  3 . An upper section  60  projects like a collar and has a diameter corresponding to the external diameter of the annular sleeve  20 . Three through-bores  62 , offset by 120°, are arranged on the top surface of the drill jig  6  and extend through the upper section  60  under the lower section  61 . Depending on the dimension of the drill jig, a countersink  66  can be provided surrounding the respective bore  62 . It is designed to function as a depth stop for a drill engaged through the bores  62 . This ensures that the drill can reach a predefinable depth relative to the underside of the upper section  60  functioning as reference surface. Drilling the securing holes for the pegs of the patella prosthesis  99  too deep can be easily and reliably avoided in this way. Arranged on the reference surface there is a pin  64 , which engages in a corresponding recess on the size gauge/milling gauge  2 ,  3  and functions as an angle marking. 
         [0050]    With the removal of the drill jig  6 , the patella bone is ready for implantation of the patella prosthesis  99  of the selected size. The thickness of the removed bone material corresponds exactly to the thickness of the support element of the patella prosthesis  99 . The depth of the holes corresponds to the length of the securing pegs  96  of the patella prosthesis  99 . In contrast to conventional instrument sets, there is no danger of the milling or drilling being too deep. By virtue of the instrument set according to the invention, it is possible instead to ensure that the thickness of the removed bone material corresponds exactly to the size of the patella prosthesis  99 , such that a more reliable and anatomically correct positioning of the patella prosthesis  99  is achieved. 
         [0051]      FIGS. 13 and 14  show the saw jig module  9 , which is provided for carrying out the alternative resection method of sawing instead of milling. The saw jig module  9  has an L-shaped main body with a base branch  91  and a guide branch  92 . On its side facing away from the guide branch  92 , the base branch  91  has guide ledges  93  which are designed for form-fit engagement in the guide grooves  21  of the combined milling/size gauge  2 ,  3 . They permit a height-adjustable arrangement of the saw jig module  9  on the milling/size gauge  2 ,  3 . A slit  95  is arranged extending parallel between the guide ledges  93 . The clamping screw  23  can be engaged through this slit  95  in order, when tightened, to lock the saw jig module  9  in its position. The guide branch  92  has a guide slit  94  which defines a cutting plane for a saw blade  97 . The cutting plane is oriented here such that its normal vector extends in the direction of the guide ledges  93 , with the result that, when the saw jig module is assembled, the cutting plane lies parallel to the upper edge of the retention plate  170 . 
         [0052]    Arranged on the base branch  91 , on the end directed away from the guide branch  92 , there is a probe star  96 . It has four probe bodies  98  which are offset by 90° and which point in the radial direction. They each have a different length, with one of the probe bodies  98  being assigned in each case to a size of the prosthesis  99 . The different lengths reproduce the different thicknesses of the prostheses  99  in the different sizes. The probe star  96  is arranged to be rotatable, such that one of the probe bodies  98  can be brought in each case into a position pointing in the direction of the guide slit  94 . This probe body is the active probe body. Its identification corresponds to that of the used size/milling gauge module  2 ,  3 . 
         [0053]    During the operation, the surgeon simply has to move the saw jig module  9  down along its guide formed by the guide ledges  93  running in the guide grooves  21 , until the probe body  98  belonging to the size/milling gauge  2 ,  3  to be used bears with its outer surface on the top of the rear face of the patella bone that is to be resected. The correct position of the saw jig module  9  is found in this way. The clamping screw  23  can be fixed. The guide slit  94  for the saw blade  97  is then located at the correct height, namely at a distance, corresponding to the thickness of the patella prosthesis  99 , below the highest point of the patella bone. The resection by sawing can now be carried out in a simple way by inserting the saw blade  97  into the guide slit  94  and performing the sawing action. The instrument with the saw jig module  9  according to the invention does not have to be withdrawn, and instead the sawing can be performed directly in one procedure. 
         [0054]    The guide grooves  21  are arranged on the size/milling gauge  2 ,  3  in such a way that the saw jig module  9  protrudes rearward on both sides at a 45° angle with respect to the longitudinal axis  100  of the instrument. This arrangement represents the best possible compromise between accessibility and favorable guiding of the saw blade. Therefore, with the saw jig module  9  according to the invention, the sawing can be carried out easily and accurately, and the possibilities of error are reduced to a minimum.