Patent Publication Number: US-6702822-B1

Title: Method and apparatus for fitting a prosthesis to a bone

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This is a divisional application of U.S. application Ser. No. 08/583,225 filed Jan. 4, 1996 now U.S. Pat. No. 6,152,963, the contents of which in its entirety is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to prostheses which are implanted in bone and, in particular, to an improved system for preparing a cavity for receiving the prosthesis and to improved prosthesis configurations for use with such cavities. 
     BACKGROUND OF THE INVENTION 
     A variety of prosthesis configurations have been proposed and used to implant prostheses in bone. See, for example, Noiles, U.S. Pat. No. 4,219,893 (see FIGS.  7 - 9 ); Meyer, U.S. Pat. No. 4,549,319; Noiles, U.S. Pat. No. 4,790,852; Penenberg et al., U.S. Pat. No. 4,808,185; Noiles, U.S. Pat. No. 4,846,839; Luman, U.S. Pat. No. 5,002,578; and the C-2 Conical Colla™ Hip System of the Kirschner Medical Corporation, Timonium, Md., 21093. 
     In overview, one or more cavities are prepared at the end of the bone for receiving the prosthesis. The prosthesis is then inserted into the prepared cavity and held in place by a mechanical fit or in some cases by bone cement. The initial fit between the prosthesis and the bone is critical to the long term success of the prosthesis especially with a mechanical fit. 
     Two fundamental criteria which a prosthesis and its cavity should meet are: (1) proper transfer of force from the prosthesis to the bone (see Meyer, U.S. Pat. No. 4,549,319); and (2) ready formation of the cavity so as to achieve an accurate fit with the prosthesis (see Noiles, U.S. Pat. No. 4,790,852). 
     With regard to the first criterium, force needs to be effectively transferred from the prosthesis to the cortical (hard/strong) bone and, in particular, to the cortical bone at the extreme end of the bone, e.g., the proximal end of the femur in the case of the femoral component of a hip joint, the distal end of the femur in the case of the femoral component of a knee joint, and the proximal end of the tibia in the case of the tibial component of a knee joint. 
     In the case of knee joints, instead of directly transferring force to the cortical bone, it is also common practice to have the prosthesis abut primarily cancellous bone and have the cancellous bone transmit force to the cortical bone. In such cases, the cancellous bone must have sufficient structural strength to sustain the loads imposed on it. 
     The application of forces of physiological magnitudes to bone fosters bone growth in the region where the forces are applied. The transfer of force to the cortical bone at the extreme end of the bone thus leads to bone growth in this critical region. If the end of the bone is not loaded, bone resorption can occur in this region. This leads to a diminished amount of bone which is undesirable in its own right and is particularly troublesome should the prosthesis fail and need to be replaced. 
     With regard to the second criterium, for repeatable success, the cavity for the prosthesis must be created in a precise and reproducible manner. The cavity preparation procedure preferably accommodates the anatomical variation between patients. Moreover, the surgical site does not favor complex procedures for preparing a cavity for implantation of a prosthesis in bone. 
     Prior techniques have achieved these two criteria to greater or lesser extents. Loading at the extreme end of the bone has not been a natural consequence of the prosthesis&#39; configuration in many cases. With regard to bone preparation, many prostheses require cavities whose configurations do not lend themselves to precise machining. The loading and bone preparation criteria have often led to compromises regarding other desirable criteria. For example, the ability to provide a one piece prosthesis which can be used in either the right or left hand bones of the patient has been difficult with prior prostheses. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing state of the art, it is an object of the invention to provide a prosthesis and cavity configuration which maximize the loading of the patient&#39;s hard bone at the extreme end of the bone in which the prosthesis is implanted. 
     It is an additional object of the invention to provide a cavity whose geometry can be readily machined in the patient&#39;s bone with a high degree of precision and which at the same time is a relatively close match to the shape of the patient&#39;s hard bone at the end of the bone, as for instance, a close match to the shape of the wall of the femur anterior to the calcar for a femoral hip prosthesis. 
     It is a further object of the invention to provide a one piece neutral (symmetric) prosthesis which can be used with both right and left bones. It is an additional object of the invention to achieve this goal with the removal of a minimum of hard bone. 
     It is a still further object of the invention to provide a prosthesis which can be implanted in either an anteverted, neutral, or retroverted orientation. In connection with this object, it is a further object to minimize the removal of bone for each of these orientations. 
     To achieve these and other objects, the invention provides a prosthesis for implantation in bone which has a bone engaging surface which comprises at least two cone-like bodies, the axes of which are non-collinear. In certain embodiments, the axes are parallel to one another, while in others the axes intersect. 
     The invention also provides surgical instruments for use in preparing the patient&#39;s bone to receive a prosthesis having the inventive configuration of its bone engaging surface. 
     As discussed in detail below, prostheses having the inventive configuration readily achieve the twin goals of high force transfer to the end of the bone and precise fit to a prepared cavity within the bone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a hip femoral prosthesis constructed in accordance with the invention. 
     FIG. 2 is a side view of the prosthesis of FIG.  1 . This view corresponds to an anterior view when the prosthesis is implanted in the left femur of a patient. 
     FIG. 3 is a medial view of the prosthesis of FIG.  1 . 
     FIG. 4 is a superior view of the prosthesis of FIG.  2 . 
     FIG. 5 is an inferior view of the prosthesis of FIG. 2, partially in section along lines  5 — 5  in FIG.  2 . This figure illustrates that the sum of the radii R 1  and R 2  of the bone-engaging surfaces of bodies  18  and  20  is greater than the distance D between axes  22  and  24  for at least one transverse cross-section through the prosthesis. 
     FIGS. 6 and 7 are superior views of the prosthesis of FIG. 1 implanted in a typical orientation in the left (FIG. 6) and the right (FIG. 7) femur of a patient. The views shown in these figures are referenced to the posterior aspect of the femoral condyles  10  of the knee. 
     FIGS. 8 and 9 are superior views of a patient&#39;s left femur which have been prepared to receive the prosthesis of FIG.  1 . FIG. 8 corresponds to the normal preparation of the bone so as to provide approximately 15° of anteversion of the neck of the femoral prosthesis relative to the femoral condyles  10 . FIG. 9 corresponds to a preparation of the bone which provides approximately 7° of retroversion of the neck of the femoral prosthesis relative to the femoral condyles  10 . 
     FIG. 10 shows an alternate construction of the prosthesis of the invention in which the axes of the two cone-like, bone-engaging bodies of the prosthesis intersect. 
     FIG. 11 shows an alternate construction of the prosthesis of the invention in which one of the cone-like, bone-engaging bodies of the prosthesis has a concave profile. 
     FIG. 12 shows a construction of the prosthesis of the invention suitable for use as the tibial component of a knee joint. 
     FIG. 13 shows an instrument for use in creating a cavity in a patient&#39;s bone for receiving the prosthesis of FIG.  1 . 
     FIGS. 14 and 15 are superior views of a patient&#39;s left femur. These figures compare the configurations of prepared cavities for receiving a prior art prosthesis (FIG. 14) and the prosthesis of FIG. 1 (FIG.  15 ). 
     FIGS. 16 and 17 are perspective views of the cavities of FIGS. 14 and 15, respectively. 
     FIG. 18 is a superior view of a patient&#39;s left femur prepared for orienting a prior art prosthesis so that it has a greater degree of anteversion than would be provided by the normal orientation of the prosthesis with respect to the anatomy of the patient&#39;s bone. 
     FIG. 19 is an anterior view of the femur of FIG. 18 along lines  19 — 19  in FIG.  18 . 
     FIG. 20 is a superior view of a patient&#39;s left femur prepared for orienting the prosthesis of FIG. 1 so that it has a greater degree of anteversion than would be provided by the normal orientation of the prosthesis with respect to the anatomy of the patient&#39;s bone. 
     FIG. 21 is an anterior view of the femur of FIG. 20 along the same direction as lines  19 — 19  in FIG.  18 . 
     The foregoing drawings, which are incorporated in and constitute part of the specification, illustrate the preferred embodiments of the invention, and together with the description, serve to explain the principles of the invention. It is to be understood, of course, that both the drawings and the description are explanatory only and are not restrictive of the invention. 
     The reference numbers used in the drawings correspond to the following: 
       10  femoral condyles of the knee 
       13  femoral hip prosthesis 
       14  neck 
       16  stem 
       18  first cone-shaped (cone-like) region of proximal bone-engaging surface 
       20  second cone-shaped (cone-like) region of proximal bone-engaging surface 
       22  axis of first cone  18   
       24  axis of second cone  20   
       26  line of intersection between cone  18  and cone  20   
       28  femoral bone 
       30  longitudinal axis of femoral bone 
       32  instrument for cutting cavity  200   
       34  body of instrument  32   
       36  bearing member of instrument  32   
       38  shaft of instrument  32   
       40  conical cutter of instrument  32   
       42  stop collar of instrument  32   
       44  calcar region of femur bone  28   
       46  line tangent to condyles  10   
       48  line through center of calcar region  44   
       50  line parallel to line  46   
       52  bone removal region of posterior wall of femur 
       54  bone removal region of anterior wall of femur 
       56  bone removal region of anterior wall of femur 
       58  concave profile of cone-like body 
       60  cone-like body of tibial prosthesis 
       62  cone-like body of tibial prosthesis 
       64  cone-like body of tibial prosthesis 
       180  conical cavity for cone  18   
       200  conical cavity for cone  20   
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Although the invention can be practiced with a variety of prostheses, a preferred application is to femoral hip prostheses. Accordingly, the initial description of the invention will be in terms of such a prosthesis, it being understood that this description is not intended to limit the scope of the invention. 
     FIGS. 1-5 show the structure of a femoral hip prosthesis  13  constructed in accordance with the invention. The prosthesis includes a neck  14  for receiving the ball (not shown) of the prosthesis and an elongated stem  16  which extends into the shaft of the patient&#39;s femur when the prosthesis is implanted. Neck  14 &#39;s orientation with respect to prosthesis  13  is preferably neutral with regard to anteversion/retroversion. That is, the prosthesis is preferably symmetric with regard to a longitudinal plane through the neck. As discussed below, this allows the prosthesis to be used with various anteversion/retroversion angles as well as with right and left femurs, thus reducing inventory requirements, i.e., there is less need to separately manufacture, ship, and store left, right, and special circumstance prostheses. 
     The bone-engaging surface of prosthesis  13  includes a first cone-shaped (cone-like) portion  18  and a second cone-shaped (cone-like) portion  20 . For ease of reference, these portions will be referred to herein as first cone  18  and second cone  20 . 
     As shown in FIG. 2, first cone  18  has an axis  22 , which corresponds in this case to the longitudinal axis of the prosthesis as defined by stem  16 , and second cone  20  has an axis  24  which is parallel to, but not collinear with, axis  22 . When this prosthesis is implanted, axis  22  is essentially aligned with longitudinal axis  30  of femoral bone  28  (see FIG.  17 ). 
     As shown in FIG. 3, cone  18  has an apical cone angle α and cone  20  has an apical cone angle β. The cone angles and spacings of axes  22  and  24  in FIGS. 1-5 are such that cones  18  and  20  intersect along line  26 . 
     A variety of cone angles and axis spacings can be used in the practice of the invention. In the case of a hip femoral prosthesis, cone angle α is preferably about 6°, cone angle β is preferably in the range between about 60° and about 120°, and the spacing between axes  22  and  24  is preferably chosen so that the apex of cone  20  lies in the vicinity of the surface of cone  18 . 
     As discussed fully below, one of the advantages of the invention is that it allows flexibility in the angular orientation of prosthesis  13  about the longitudinal axis of the patient&#39;s bone. Cone angle β is selected with this orientation feature in mind. 
     Specifically, larger cone angles produce a shallower cone  20  which allows more flexibility in angular orientation without sacrificing the integrity of the patient&#39;s bone. Smaller cone angles, on the other hand, provide more purchase into the end of the patient&#39;s bone, which may be required for some applications. Such smaller cone angles give less flexibility with regard to angular orientation. 
     The particular cone angles for any specific application can be determined by persons skilled in the art from the disclosure herein and the specific requirements of a particular application of the invention. In FIGS. 1-5, cone  20  is shown as having a representative cone angle of 90° which provides a substantial level of angular orientation flexibility in comparison to prior art prostheses (see discussion of FIGS. 14-21 below). 
     Implantation of prosthesis  13  in a patient&#39;s bone requires the preparation of two adjacent conical cavities  180  and  200  (see FIGS. 8 and 17) to receive cones  18  and  20 , respectively. Cavity  180  is aligned with the longitudinal axis  30  of femoral bone  28  and is prepared using a conventional conical reamer (see, for example, FIGS. 4-5 of U.S. Pat. No. 4,790,852). The longitudinal location of cavity  180  along axis  30 , is chosen with the ultimate location of prosthesis  13 , including cones  18  and  20 , along that axis in mind. Thus, the conical reamer used to prepare cavity  180  preferably includes means for indicating the depth of the reamer relative to the end of the patient&#39;s bone. 
     Although the foregoing discussion has been in terms of geometrical cones, it should be understood that cone-like bodies  18  and  20  are not limited to such shapes. Rather, each of these bodies needs to be generally cone-shaped and to have a form such that a cavity to receive the body can be generated by a cutting tool rotating about a fixed axis. 
     The cone-like shape is important because it allows the prosthesis to reach out toward the hard bone in the region of the end of the patient&#39;s bone. That is, it gives the prosthesis a longitudinal cross-section at the end of the bone which is similar to the longitudinal profile of the hard bone at that end. A spherical shape of the type used in U.S. Pat. No. 4,808,185, does not have this property. 
     The ability to be received in a cavity formed by a cutting tool rotating about a fixed axis is important because it means an excellent fit can be achieved between the prosthesis and the cavity under the real world conditions which exist in the operating room. 
     FIG. 11 illustrates a body  20  having such a cone-like shape. Body  20  of this figure has a concave longitudinal profile  58 , which can even more closely correspond to the inside surface of the hard bone in some cases than a true geometrical cone. This would not be true for a convex longitudinal profile. Accordingly, the prostheses of the invention have cone-like shaped bodies or regions whose longitudinal profiles are either straight or concave. 
     For the more general case of cone-like bodies, as opposed to bodies which are true cones, the relative shapes of the bodies can be describe in terms of their overall longitudinal profiles, rather than their cone angles. In general terms, cone-like body  20 &#39;s surface area and diameter decreases faster than those of cone-like body  18  in moving away from the end of the bone in which they are implanted. 
     In most cases, the transverse cross-section of the prosthesis and the cavity in the region of the end of the bone will include two intersecting circular parts with displaced centers. An alternate transverse cross-section comprises a circle and an ellipse. This cross-section arises when axes  22  and  24  of cone-like bodies  18  and  20  intersect, as opposed to being parallel, as shown in FIG.  10 . These cross-sections can be characterized as having a wasp-waisted configuration, a nipped in the waist configuration, or a configuration which includes a cusp. The cusp can be rounded out if desired. 
     Cavity  200  is preferably prepared using instrument  32  shown in FIG.  13 . The instrument has a body  34  whose outer surface includes a conically shaped portion which seats in conical cavity  180 . 
     Body  34  carries bearing member  36 . Shaft  38 , which carries cutter  40  at its distal end, is rotatable and slidable within bearing member  36 . Shaft  38  is rotated and advanced into the patient&#39;s bone by conventional means, such as, a T-handle (not shown). Stop collar  42  is mounted on shaft  38  and defines the end point of the advance of cutter  40  so that the spatial relationship of cavities  180  and  200  matches that of cones  18  and  20 . 
     The instrument of FIG. 13 can be used to prepare a cavity in the patient&#39;s bone for various orientations of the neck  14  of the prosthesis relative to the remaining calcar region  44  of the patient&#39;s femur. FIGS. 8-9,  15 ,  17 , and  20 - 21  illustrate some of the possibilities. 
     In each of these figures, the remaining calcar region  44  is shown having a degree of anteversion of about 7° relative to line  46  which is tangent to condyles  10 . That is, line  48  which passes through the center of calcar region  44  and intersects longitudinal axis  30  is rotated 7° counterclockwise relative to line  50  which is parallel to line  46  and also intersects longitudinal axis  30 . (This geometric construction is for purposes of illustration only since, as is well known in the art, there is a considerable variation in version angles and anatomy in human hips.) For ease of reference, a prosthesis whose neck  14  is aligned with line  48  will be referred to as having an anteversion of 7°. 
     The average anteversion of the natural femoral head of the femur is greater than 7° because the natural neck turns in a forward direction as it rises from the calcar region. For many patients, the anteversion of the natural femoral head is in the range of about 12° to about 15°. Accordingly, in practice, it is generally desired to orient the neck  14  of prosthesis  13  at some greater amount of anteversion than that of the remaining calcar region  44 , e.g., between about 12° and about 15° counterclockwise from line  50  for a left femur. 
     This generally preferred orientation of the neck  14  of prosthesis  13  is illustrated in FIGS. 6-8. Specifically, FIG. 6 shows implantation of prosthesis  13  in the patient&#39;s left femur at 15° anteversion and FIG.  7  shows implantation in the right femur, again at 15° anteversion. FIG. 8 is the bone preparation for the implantation of FIG.  6 . The bone preparation for the implantation of FIG. 7 is the mirror image of that of FIG.  8 . 
     A cavity for use in providing a relatively extreme orientation of neck  14  of prosthesis  13  is shown in FIG.  9 . In this case, the neck of the prosthesis when implanted is retroverted by 7° with respect to line  50 . Although such an orientation is generally unlikely, it may be needed for some patients. It should be noted that some removal of the posterior wall of the femur is likely to occur during preparation of the bone for this orientation of the prosthesis (see region  52  in FIG.  9 ). However, due to the shallowness of cone-like cavity  200 , the remaining bone still provides a strong structural support for the prosthesis. 
     A cavity for use in providing another relatively extreme orientation of neck  14  of prosthesis  13  is shown in FIG.  20 . In this case, the neck of the prosthesis when implanted is anteverted by more than 15° with respect to line  50 . Again, some removal of the wall of the femur is likely to occur during preparation of the bone for this orientation of the prosthesis, specifically, removal of a small portion of the anterior wall is likely to occur (see region  54  in FIGS.  20  and  21 ). Again, however, the remaining bone still provides a strong structural support for the prosthesis because the flare of the bone in region  54  is in the same direction as the flare of the cone  20 . This is particularly so because the force from the prosthesis to the bone in the calcar region is directed posteriorly where the external wall of the femur is still intact. It should be noted that the orientation of FIG. 20 will be more common than the orientation of FIG.  9 . 
     Significantly, a single neutral prosthesis can be used for all of the orientations shown in FIGS. 6-9 and  20 , as well as for a variety of orientations within and beyond those illustrated. As discussed above, prosthesis  13  is preferably symmetric with respect to neck  14 , i.e., the prosthesis has neutral version. Through the use of a cone  20  which is relatively shallow, such a neutral version prosthesis can be used for both the right and left femurs as illustrated in FIGS. 6 and 7, and for the relatively extreme orientations of the prosthesis as illustrated by FIGS. 9 and 20. Specifically, the shallow cone  20  allows for angular variation about axis  30  of the placement of the prosthesis in the calcar region of the bone without compromising the bone&#39;s structural strength or the fixation of the prosthesis. 
     This “shallowness” aspect of the invention is illustrated in FIGS. 14-17 which show a prepared femur for receiving the prosthesis of the invention (FIGS. 15 and 17) and a prepared femur for receiving a prior art prosthesis (FIGS.  14  and  16 ). As illustrated in these figures, the bone-engaging surface of the prosthesis of the invention in the calcar region tends to be as much “on the bone” as “in the bone” because of the shallowness of cone  20 . This geometry further encourages the favorable loading of the bone at the end of the bone. 
     For the prior art prosthesis, on the other hand, the bone-engaging surface of the prosthesis in the calcar region is clearly “in the bone”. As a result, rotation of this part of the prior art prosthesis to provide additional anteversion causes the removal of an unacceptable amount of the anterior wall of the femur as shown in FIG. 19 (see  56  in FIG.  19 ). 
     Put another way, if the geometry of the prior art prosthesis were to be used in a one piece neutral prosthesis and that prosthesis were to be oriented in a more anteverted orientation than that of the calcar region, more critically placed bone would have to be removed than for the comparable prosthesis constructed in accordance with the geometry of the present invention. 
     The ability to use a single neutral prosthesis for a variety of orientations is a significant advantage of the invention because it reduces the costs of manufacture, shipping, and storage relative to the use of right-handed and left-handed prostheses. Moreover, the prosthesis of the invention provides greater latitude of orientation compared to single orientation left and right hand prostheses. 
     Further, the invention provides this multiple orientation advantage in a single piece prosthesis, as opposed to a modular prosthesis (see U.S. Pat. No. 4,790,852). It should be noted that the two cone geometry of the invention can be used with modular prostheses, if desired. 
     Although it is not preferred, the present invention can be used in connection with left and right handed prostheses with anteverted necks if desired. 
     FIG. 12 shows application of the invention to a tibial knee prosthesis. In this case, the prosthesis includes three cone-like bodies  60 ,  62 , and  64 . As shown in this figure, bodies  62  and  64  have the same shape. Different shapes can be used for these bodies if desired. 
     In addition to hip joints and tibial components of knee joints, the invention can also be used for various other joints, such as, the humeral component of a shoulder joint prosthesis, the femoral component of a knee prosthesis, and the like. 
     The prosthesis can be constructed of various biocompatible materials suitable for implantation now known or subsequently developed. For example, it can be made of a cobalt-chromium-molybdenum alloy (see ASTM-F75 and ASTM-F799) or a titanium alloy such as Ti-6Al-4V (ASTM-F136). The cone-like, bone-engaging bodies of the prosthesis can include surface texturing, such as the steps shown in the figures. These surfaces can also be porous coated, plasma sprayed, chemically modified, or the like to enhance fixation. Similarly, the surfaces can be coated with bone growth stimulating materials such as hydroxylapatite. 
     Although preferred and other embodiments of the invention have been described herein, additional embodiments may be perceived by those skilled in the art without departing from the scope of the invention as defined by the following claims.