Patent Publication Number: US-2007106392-A1

Title: Acetabular cup locking mechanism

Description:
BACKGROUND OF THE INVENTION  
      The present invention relates to acetabular cup replacement assemblies, and more particularly to an acetabular cup replacement assembly which includes a single shell that allows for the interchangeability between metallic/ceramic and polyethylene inserts.  
      Total hip replacement surgery typically entails the removal and replacement of the femoral head portion of the femur, as well as the resurfacing and replacement of the acetabular cup. In both cases, prosthetic implants are utilized to replace the removed bone portions. Although these types of surgeries have become rather common place, surgeons are often faced with decisions during surgery relating to the particular prosthetic implants utilized, in addition to their respective orientation and positioning. For example, acetabular cup replacements may require a surgeon to first implant a shell portion and thereafter select different cup inserts and position them with respect to the shell portion. Several attempts at providing beneficial acetabular cup assemblies have been developed heretofore. Two examples of such acetabular cup assembly designs are disclosed in U.S. Pat. Nos. 6,475,243 and 6,610,097, the disclosures of which are hereby incorporated by reference herein.  
      Acetabular cup assemblies do indeed provide benefits to surgeons, but they are not without their own drawbacks. In certain designs, surgeons utilizing and implanting such shell and insert combinations often have a difficult time ensuring that the two components are properly aligned. Frequently, the relatively small working space which is available to a surgeon during a hip replacement prevents the complete view of the replacement components. The previously designed acetabular cup replacements simply have not offered a solution to this problem. In addition, in certain other designs, the mating of the shell and insert portions of acetabular cup replacements is such that movements of the two components, although best described as micro-movements, can develop over time. This may be caused by differences in the tolerances of the two components, and is clearly not desired for long term implantation into a patient.  
      Therefore, there exists a need for an acetabular cup replacement assembly which allows for easy assembly and alignment in situ, and provides a substantially non-moveable coupling between a shell portion and insert.  
     SUMMARY OF THE INVENTION  
      A first aspect of the present invention is a prosthetic cup assembly. A first embodiment of this assembly may include a metallic shell having an outer shell surface and an inner shell surface, the outer shell surface being adapted for insertion into the acetabulum of a patient and the inner shell surface including a plurality of scallops and a female taper. The assembly may also include a metallic insert having an outer insert surface, the outer insert surface being adapted for insertion into the inner shell surface of the shell, the outer insert surface including a plurality of lobes adapted for engagement with the scallops and a male taper adapted for engagement with the female taper. Preferably, the dimensional relationship between the insert and the shell is such that the insert may be inserted into the shell from an axially aligned position or from an angled position, and insertion of the outer insert surface within the inner shell surface allows fixable attachment of the insert to the shell.  
      In accordance with this first aspect of the present invention, the shell may be constructed of many different materials including cobalt chrome alloys, stainless steel and titanium. In one preferred embodiment, the shell may include twelve scallops and the insert may include twelve lobes. The shell may further include a circumferential groove, twelve diametrical recesses and at least one barb. The insert may further include twelve rim portions and a centralizing chamfer. Still further, the outer shell surface may include bone growth inducing surfaces. The insert may be inserted into the shell in situ.  
      A second aspect of the present invention is another prosthetic cup assembly. In accordance with one embodiment of this second aspect, the assembly may include a metallic shell having an outer shell surface and an inner shell surface, the outer shell surface being adapted for insertion into the acetabulum of a patient and the inner shell surface including a plurality of scallops, a circumferential groove and at least one barb. The assembly may further include a polymeric insert having an outer insert surface, the outer insert surface being adapted for insertion into the inner shell surface of the shell, the outer insert surface including a plurality of lobes, a circumferential bead and a plurality of stepped sections. Preferably, the plurality of lobes are adapted for engagement with the scallops, the circumferential bead is adapted for engagement with the circumferential groove, and the at least one barb are adapted for engagement with the plurality of stepped sections upon insertion of the outer insert surface within the inner shell surface, thereby fixable attaching the insert to the shell.  
      Yet another aspect of the present invention is a kit for use in hip replacement surgery. The kit may include at least one shell, the shell having an outer shell surface and an inner shell surface, the outer shell surface being adapted for insertion into the acetabulum of a patient and the inner shell surface including a plurality of scallops, a female taper, a circumferential groove and at least one barb. The kit may further include at least one metallic insert having an outer metallic insert surface being adapted for insertion into the inner shell surface of the shell, the outer metallic insert surface including a plurality of lobes and a male taper, where the plurality of lobes are adapted for engagement with the scallops and the male taper is adapted for engagement with the female taper upon insertion of the outer insert surface within the inner shell surface, thereby fixably attaching the insert to the shell. The kit may also include at least one polymeric insert having an outer insert surface being adapted for insertion into the inner shell surface of the shell, the outer insert surface including a plurality of lobes, a circumferential bead and a plurality of stepped sections, where the plurality of lobes are adapted for engagement with the scallops, the circumferential bead is adapted for engagement with the circumferential groove, and the at least one barb are adapted for engagement with the plurality of stepped sections upon insertion of the outer insert surface within the inner shell surface, thereby fixably attaching the insert to the shell. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      A more complete appreciation of the subject matter of the present invention and the various advantages thereof can be realized by reference to the following detailed description in which reference is made to the accompanying drawings in which:  
       FIG. 1  is a perspective view of an acetabular cup replacement assembly in accordance with the present invention.  
       FIG. 2  is a top perspective view of a shell portion of the acetabular cup replacement assembly of  FIG. 1 .  
       FIG. 3  is a side perspective view of a metallic/ceramic insert of the acetabular cup replacement assembly of  FIG. 1 .  
       FIG. 4  is a side perspective view of a polymeric insert of the acetabular cup replacement assembly of  FIG. 1 .  
       FIG. 5  is a side perspective view of the metallic/ceramic insert of  FIG. 3  in one angular orientation with respect to the shell portion of  FIG. 2 .  
       FIG. 6  is a side cross sectional view of the metallic/ceramic insert of  FIG. 3  in another angular orientation with respect to the shell portion of  FIG. 2 .  
       FIGS. 7   a - 7   d  are side cross sectional views of the metallic/ceramic insert of  FIG. 3  being inserted into the shell portion of  FIG. 2  from yet another angular orientation.  
       FIG. 8  is an enlarged view of the shell portion of  FIG. 2 , showing a barb located in the interior of the shell portion.  
       FIG. 9  is an enlarged view of the polymeric insert of  FIG. 4 , showing a step located on the exterior surface of the insert.  
       FIG. 10  is a cross sectional view of the polymeric insert of  FIG. 4  partially located within the shell portion of  FIG. 2 , just prior to engagement.  
       FIGS. 11   a - 11   d  are enlarged cross sectional views of the mating portions of the shell portion of  FIG. 2  and the polymeric insert of  FIG. 4 , in sequential mating positions.  
       FIG. 12  is a cross sectional view of the shell portion of  FIG. 2  with the polymeric insert of  FIG. 4  attached thereto. 
    
    
     DETAILED DESCRIPTION  
      Referring to the drawings, wherein like reference numerals refer to like elements, there is shown in  FIG. 1 , an acetabular cup replacement assembly designated generally by reference numeral  10 . As shown in that figure, assembly  10  includes a shell portion  12  and a metallic/ceramic insert  14  or a polymeric insert  16 , which may be used alternatively in shell  12 . In other words, assembly  10  allows a surgeon to utilize a common shell portion  12  with either the two inserts  14  and  16 , thereby providing a convenient assembly for implantation into a patient. Of course, a surgeon may only implant either metallic/ceramic insert  14  or polymeric insert  16  within shell portion  12  at a given time. It is noted that metal/ceramic insert  14  may be a metal shell itself having a ceramic bearing fixedly connected thereto, such as by press fitting. Alternatively, metallic/ceramic insert  14  may be constructed solely of metal or ceramic. Determination of whether to utilize metallic/ceramic insert  14  or polymeric insert  16  may depend on several factors relating to the patient&#39;s age, activity level and/or anatomy, and is best left up to the discretion of the surgeon performing the surgery. Often times, this determination is made pre-operation, but may also be made during the procedure. In either case, the aforementioned inserts are preferably adapted to receive and cooperate with the ball of a femoral head replacement. As such, inserts and femoral head replacements may be offered in combinations.  
      Shell portion  12  is depicted from its open end in  FIG. 2 . In certain embodiments, shell portion  12  may be constructed of various metals, including biocompatible metals such as cobalt chrome alloy, stainless steel and titanium. In the embodiment shown in the figures, shell portion  12  includes an outer surface  18  and an inner surface  20 . Outer surface  18  is preferably sized and configured to be implanted within a previously prepared acetabulum, and may include bone growth inducing surfaces. Such preparation of the acetabulum is well known to those of ordinary skill in the art. On the contrary, inner surface  20  is preferably sized and configured to receive the aforementioned inserts  14  and  16 , and includes several elements for mating/locking with the inserts. More particularly, inner or interior surface  20  of the preferred shell  12  includes twelve recesses, dimples or scallops  22   a - l , a female taper  24 , a circumferential groove  26 , barbs  28   a - d  (shown in more detail in  FIG. 8 ), and diametrical recesses  30   a - l . Scallops  22   a - l  may be rounded recesses which are adapted to cooperate with similar rounded projections or lobes. Female taper  24  is essentially a sloped side wall as is well known in the art, such as a “Morse” taper. Groove  26  may extend around an inner circumference of shell  12  and is preferably sized and configured to receive a corresponding projection, such as an element of polymeric insert  16 . Four barbs  28   a - d  may be relatively sharp projections designed to cut into certain material, such as polymeric material. Finally, diametrical recesses  30   a - l  are recessed areas located between scallops  22   a - l,  which create a stepped configuration of the walls between the scallops. Each of the above elements will be discussed more fully below with regard to their cooperation with the aforementioned inserts  14  and  16 .  
      As is best shown in  FIG. 3 , metallic/ceramic insert  14  includes an outer portion  32  and an inner portion  33 . In one preferred embodiment, like that shown in the figures and mentioned above, outer portion  32  is constructed of a biocompatible material, such as stainless steel or titanium. Contrarily, inner portion  33  may be constructed of a ceramic material or the like. Alternatively, providing an entirely metal or ceramic insert  14  is contemplated. As is briefly mentioned above, inner portion  33  may be sized and configured to mate with the ball portion of a femoral head replacement component. Thus, the surface of inner portion  34  should be such that proper motion can be restored and/or retained in the hip joint. Like inner surface  20  of shell  12 , outer portion  32  may include several elements for mating/locking with the shell. Specifically, the outer surface of outer portion  32  may have twelve projections or lobes  34   a - l  for cooperation with the above discussed scallops  22   a - l , rim portions  36   a - l  for use in extraction of insert  14  from shell  12  (e.g. —by providing a gap which allows for an instrument to pry insert  14  from shell  12 ), a male taper  38  for cooperation with the above mentioned female taper  24  of shell  12 , and a centralizing chamfer  40  to aid in the alignment of insert  14  with respect to shell  12 . Once again, each of the elements of insert  14  will be discussed more fully below in the discussion relating to the mating of shell  12  and insert  14 .  
       FIG. 4  depicts the remaining component of assembly  10 , polymeric insert  16 . Like metallic/ceramic insert  14 , insert  16  includes an inner surface  42  for cooperation with the head of a corresponding femoral replacement component. Once again, inner surface  42  should be sized and configured so as to restore/allow proper motion in the hip joint to be replaced. However, unlike metallic/ceramic insert  14 , insert  16  is a single pieced element, with an outer surface  44  having several elements necessary for mating and locking with shell  12 . Specifically, outer surface  44  may include twelve projections or lobes  46   a - l , twelve stepped sections  48   a - l  (shown in more detail in  FIG. 9 ) located between lobes  46   a - l , a circumferential bead  50  extending around a portion of the outer surface, a centralizing chamfer  52  to aid in the alignment of insert  16  with respect to shell  12 , and a circumferential rim portion  54  for further locking of insert  16  into shell  12 . As with the above discussed components of assembly  10 , the elements of polymeric insert  16  will be better understood when taken in conjunction with the below description of the cooperation between shell  12  and insert  16 .  
       FIGS. 5-7  depict three separate methods of operatively connecting shell  12  and metallic/ceramic insert  14 . Preferably, this connection is such that the two components are substantially locked together, so as to prevent most, if not all, movement between the two. As was alluded to above, the basic locking feature between shell  12  and insert  14  may be a standard taper lock between female taper  24  of shell  12  and male taper  38  of insert  14 . This type of locking between two substantially circular or conical elements is well known in the engineering arts, and has been utilized in previously acetabular cup assemblies. However, it is a potential problem in previous incarnations of acetabular cup assemblies that the shell and insert become canted or misaligned during insertion of the insert into the shell. Although basic engineering principles dictate that taper locks should preferably include a length of engagement which is equal to twice the taper diameter, in order to ensure proper alignment, such is not possible in the relatively short maximum section of taper that can exist in an acetabular cup design. Thus, the above noted canting problem may occur, thereby leaving an insert locked into an angled position with respect to the shell. In fact, one way to ensure that the inserts and shells of previous acetabular cup replacements become properly engaged with one another is to align the respective axis of the two components and thereafter insert the insert purely by movement in the direction of the aligned axis. However, this is often not possible within the relatively small constraints of a hip replacement surgery, especially one conducted in accordance with minimally invasive surgery (MIS) techniques. Such a small operating space also often prevents a surgeon from visually inspecting the connection between the shell and insert. Acetabular cup assembly  10  of the present invention is designed so as to prevent the above problems associated with canting or misalignment of shell  12  and insert  14 . More particularly, the cooperation between scallops  22   a - l  and lobes  34   a - l  of insert  14  ensures proper alignment of the two components, which ultimately results in proper locking therebetween.  
      As mentioned above, shell  12  and insert  14  can be attached together in three different fashions. The first of these attachment methods is illustrated in  FIG. 5 , where insert  14  is essentially aligned axially with shell  12  along an axis L. This method is like the aforementioned prior art methods of connecting inserts with implanted shells, and as discussed above, may not be possible in relatively small operating spaces. Nevertheless, it is indeed one method of attaching insert  14  to shell  12 . As shown in  FIG. 5 , subsequent to being axially aligned with shell  12 , insert  14  is placed into the shell in the direction of arrow A. In this position, lobes  34   a - l  either align with scallops  22   a - l , or the lobes rest on the top surface  56  (shown in  FIGS. 2 and 5 ) of shell  12 . In the latter case, a rotation of insert  14  is required in order to have lobes  34   a - l  and scallops  22   a - l  align. Given the fact that in the preferred embodiment twelve lobes and scallops exist on the respective insert  14  and shell  12 , the maximum amount of rotation required to align the lobes and scallops will be approximately 15 degrees. Once lobes  34   a - l  and scallops  22   a - l  are properly aligned, insert  14  may be locked into shell  12  with a couple of swift blows using an impaction tool (not shown). This operation essentially forces male taper  38  into female taper  24 , and keeps shell  12  and insert  14  from becoming dislodged from one another absent certain specific forces being applied thereto. Centralizing chamfer  40 , although primary shaped to soften the edges of insert  14 , may also aid in keeping insert  14  properly aligned with shell  12  during this insertion procedure. In the fully assembled state, the chamfer is designed to provide clearance with the bottom of shell  12 , and may further limit canting of the insert with respect to the shell.  
      The second method of attaching shell  12  and insert  14 , where the two parts are initially misaligned, is depicted in  FIG. 6 . In this method, insert  14  is first arranged so that some of lobes  34   a - l  initially rest on top of top surface  56  of shell  12 . A distance d between the start of female taper  24  and top surface  56  is preferably such that a trailing edge  58  of male taper  38  of insert  14  always clears the remainder of the geometry of shell  12 . This allows for insert  14  to naturally establish coaxial alignment with shell  12 , upon further insertion of trailing edge  58  into shell  12 . In other words, distance d allows for portions of insert  14  to be placed atop shell  12  at any angle and the remainder of the insert to be swung into the shell. This may allow a surgeon to connect shell  12  and insert  14  together without having to align them first axially. Once trailing edge  58  is completely inserted into shell  12 , lobes  34   a - l  rest on the top surface  56 , as discussed above in relation to the first method of inserting insert  14  into shell  12 . Similar to the above first method, this position requires a maximum 15 degree rotation of insert  14  with respect to shell  12 , in order to have lobes  34   a - l  align with scallops  22   a - l . Thereafter, an impaction tool or the like may be utilized to force male taper  38  into female taper  24 , as is also discussed above. Once again, centralizing chamfer  40  may aid in keeping insert  14  properly aligned with shell  12 .  
      The third method of attaching a misaligned shell  12  and insert  14  is depicted in  FIGS. 7   a - 7   d . The major difference between this third method and that of the second method is that some of lobes  34   a - l  are initially aligned and inserted into scallops  22   a - l . In this position, insert  14  and shell  12  are initially in contact at points  60 ,  62  and  64 , and there preferably exists a clearance between a leading edge  66  of male taper  38  and an adjacent portion of female taper  24  (the clearance being depicted by the circled section C). This clearance is formed because of the ratio between a height X of each lobe  34   a - l  and a diameter Y, both of insert  14 , and a diameter Z of female taper  24  of shell  12 . Each of these dimensions is best depicted in  FIG. 7   a . In addition, it is noted that each scallop  22   a - l  may have a depth X′, which is preferably slightly larger than height X of each lobe  34   a - l . This may also play into the formation of clearance C. It is noted that other factors, such as tolerances taper angles, fillet sizes, etc., may also affect the relationship between insert  14  and shell  12  and the formation of clearance C. Nonetheless, in one preferred embodiment, the relationship between X, Y and Z is such that the following equation applies: 
 
 R* ( Y−Z )/2= X  
 
 where R is between 1.4 and 1.6 
 
      However, the above equation is merely one such equation for providing the proper relationship between shell  12  and insert  14 . For example, another equation for providing the proper relationship between X, Y and Z could read as follows: 
 
1.33 X= ( Y−Z ) 
 
 (i.e. —where R is 1.5) 
 
      Given the above discussion relating to the relationship between shell  12  and insert  14 , it is to be understood that the two components can be assembled together from the position shown in  FIG. 7   a . Upon an application of a force F, insert  14  preferably moves in a cam action in the directions D 1 , D 2  and D 3 . These directions are all depicted in  FIG. 7   a , and the movement of insert  14  with respect to shell  12  is depicted in  FIGS. 7   b - 7   d . Thus, upon the application of force F, insert  14  naturally establishes coaxial alignment with shell  12  (shown in  FIGS. 7   b - 7   c ), whereupon insert  14  may be seated within shell  12  (shown in  FIG. 7   d ) through the utilization of the above mentioned impaction tool and method of using same. The aforementioned second and third methods of attaching shell  12  and insert  14  are particularly useful for use in relatively small operating spaces, such as those typically present in MIS procedures. In addition, the inclusion a scallops  22   a - l  and lobes  34   a - l  allows a surgeon to be sure that insert  14  has been properly seated within shell  12 , without visually confirming same. There simply is no possibility of insert  14  becoming implanted in a canted fashion within shell  12 , because of these aligning aids.  
      With regard to the attachment of shell  12  and polymeric insert  16 , it is first noted that assembly  10  of the present invention is designed so as to allow insert  16  to be inserted into shell  12  in any of the same fashions described above in connection with insert  14 . Thus, insert  16  may be initially positioned in vertical alignment with shells  12 , at an angle such that some of lobes  46   a - l  of insert  16  rest on top surface  56  of shell  12 , or at angle such that some of lobes  46   a - l  are aligned and inserted into scallops  22   a - l . Once again, this provides a surgeon with the opportunity to work in relatively small operation areas, such as those present during MIS procedures. Although the initial insertion of polymeric insert  16  into shell  12  may be done in a similar fashion to that of metallic/ceramic insert  14 , the ultimate seating of insert  16  is quite different. However, like that of insert  14 , the connection between shell  12  and insert  16  is preferably such that rotational and/or vertical or push out movements, as well as micro-movements are prevented between the two components. Therefore, both inserts  14  and  16  are preferably capable of being inserted either axially or at an angle between their axis, and are prevented from substantially all movement with respect to shell  12 .  
      To achieve this cooperation and attachment between shell  12  and insert  16 , assembly  10  of the present invention utilizes a series of sequentially stepped locking features. More particularly, connection between shell  12  and insert  16  is such that the above mentioned elements of each component are designed so as to achieve proper constraints while continuing to be easy to assembly.  FIG. 10  depicts shell  12  with insert  16  in an aligned position during the assembly process. This position is similar to the naturally established coaxial alignment discussed above in relation to the third method of attaching insert  14  to shell  12 . It is noted that centralizing chamfer  52  may aid in this alignment, as described above. However, it is noted that the insert may at least initially be aligned in different fashions, including axial alignment or an angled nature. From the position depicted in  FIG. 10 , the next step is to apply a force from an impaction tool (not shown) in order to put insert  16  in its final locked position within shell  12 . During this application of force, the locking mechanisms between the two components go through several sequential steps.  FIGS. 11   a - d  depict these steps during the attachment procedure.  
      First, upon application of a force by the aforementioned impaction tool, insert  16  moves in a direction depicted by arrow B in  FIG. 10  and lobes  46   a - l  of insert  16  engage scallops  22   a - l  of shell  12  to establish alignment and ensure concentricity between the two components. It is noted that a rotation, similar to that discussed above in the discussion on insert  14 , may be required to initially properly align insert  16  and shell  12 . Once again, the maximum amount of rotation should be no more than approximately 15 degrees. Upon further application of a force, as is best shown in  FIG. 11   b , insert  16  continues to move in the direction of arrow B and circumferential bead  50  begins to hit female taper  24  of shell  12 , thereby beginning to compress the bead. At the same time, and continuing upon further application of a force from the impaction tool, circumferential rim  54  of insert  16  begins to engage recess  30   a  so as to compress at least a portion of the insert to thereby form an interference fit between the two components. This is also seen in  FIG. 11   b , and continues in  FIGS. 11   c  and  11   d.    
      It is noted that application of a force from an impaction tool or the like may require the surgeon to operate the tool in intervals. In other words, the tool may act like a standard hammer, where successive engagements with insert  16  may further push the insert into shell  12 . Beginning in  FIG. 11   c , the aforementioned four barbs  28   a - d  (only barb  28   a  is depicted in  FIGS. 11   a - d ) start to cut into the wider section of stepped sections  48   a - l  aligned therewith (only section  48   a  is depicted in  FIGS. 11   a - d ). This engagement creates yet another interference fit which aids in the prevention of micro-movements between insert  16  and shell  12 . It is noted that relationship between the barbs and the stepped sections is preferably such that the barbs only engage the wider section of the stepped section. This should also result in the barbs not engaging any portion of metal insert  14 . Finally, as shown in  FIG. 11   d , once insert  16  is fully inserted into shell  12 , circumferential bead  50  slips into groove  26  in a well known manner, and the previously compressed bead  50  expands to establish a primary locking device. This locking device prevents vertical movement of insert  16  with respect to shell  12 . In the position shown in  FIG. 11   d , and more fully depicted in  FIG. 12 , insert  16  is fully seated within shell  12 . Essentially, the spherical diameters of the two components substantially match so as to allow this seating.  
      It is noted that the embodiment depicted in the figured and discussed in this detailed description is merely one such embodiment in accordance with the present invention. As such, variations of any of the components of assembly  10  may fall within the scope of the present invention. For example, although specifically sized and shaped components and elements are shown and described in the present application, such components and elements may vary in size and shape. In one instance, it is contemplated to provide differently shaped lobes and corresponding scallops. Specifically, rather than the rounded lobes and scallops shown in the drawings, it is noted that rectangular lobes and scallops can be utilized in accordance with shell  12  and either insert  14  or  16  of the present invention. In addition, although specific materials for constructing assembly  10  are discussed herein, many different types of materials can be utilized. For example, insert  16  is described herein as being constructed of a polymeric material, such as Ultra High Molecular Weight Polyethylene. However, given the operation of insert  16  and its cooperation with shell  12 , it is contemplated to provide any relatively flexible material to construct the insert.  
      Although not specifically set forth herein, it is to be understood that assembly  10  of the present invention may be utilized during any type of hip surgery known in the art. This includes total hip surgery methods performed prior to the time of the present application, as well as those performed in the future. Typically, these types of surgeries entail accessing the hip joint of a patient, resecting the end of the femur and replacing such with a femoral replacement prosthesis, and resurfacing the acetabulum. Assembly  10  of the present invention is preferably utilized in conjunction with this resurfaced acetabulum. Initially, shell  12  is implanted in the resurfaced acetabulum, and either insert  14  or  16  is thereafter inserted within and attached to shell  12  in accordance with the above descriptions. In this regard, it is noted that kits may be provided for allowing a surgeon to choose differently sized shells  12  and corresponding inserts  14  or  16 . The required size of shell  12  may differ depending upon the anatomy of the patient and/or the amount of damage to the acetabulum, as well as many other factors.  
      Finally, as the joints of the hip and shoulder are substantially similar, it is contemplated to provide a replacement assembly in accordance with the present invention for use in a shoulder replacement surgery. Obviously, such an assembly would have to be sized and configured to fit within the particular shoulder anatomy of a patient.  
      Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the the principles and applications of the present invention. It is therefore to be understood that numerous modifications may be made to the illustrative embodiments and that other arrangements may be devised without departing from the spirit and scope of the present invention as defined by the appended claims.