Abstract:
An acetabular cup and method of securing the acetabular cup to an acetabulum so as to provide a bearing surface for a head portion of a femur is provided. The acetabular cup is shaped to provide a cementless, press-fit into a reamed acetabulum. The acetabular cup is formed of a body having a sidewall defining a radius from a center point of the annular rim to the sidewall wherein the radius increases in length from the apex to said annular rim. The method of securing the acetabular cup first includes reaming the acetabulum with a reamer having a head with a radius of curvature that is less than the radius from the center point to the annular rim of the acetabular cup.

Description:
[0001]    This application claims the benefit of co-pending U.S. patent application Ser. No. 09/678,032 filed Oct. 3, 2000 entitled Press Fit Acetabular Cup and Associated Method For Securing the Cup to an Acetabulum 
     
    
     
       TECHNICAL FIELD OF THE INVENTION  
         [0002]    The present invention relates generally to an acetabular cup and, more particularly, to a press-fit acetabular cup and associated method for securing the cup to an acetabulum.  
         BACKGROUND OF THE INVENTION  
         [0003]    During the lifetime of a patient, it may be necessary to perform a hip replacement procedure on the patient as a result of, for example, disease or trauma. The hip replacement procedure may involve a total hip replacement or a partial hip replacement. In a total hip replacement procedure, a femoral component having a head portion is utilized to replace the natural head portion of the thighbone or femur. The femoral component typically has an elongated intramedullary stem which is utilized to secure the femoral component to the patient&#39;s femur. In such a total hip replacement procedure, the natural bearing surface of the acetabulum is resurfaced or otherwise replaced with a cup-shaped acetabular component that provides a bearing surface for the head portion of the femoral component.  
           [0004]    Acetabular cups may be secured to the acetabulum in a number of different manners. For example, acetabular cups may be secured to the acetabulum by the use of bone cement. However, recent studies have speculated that it may be desirable to secure artificial components to natural bone structures without the use of bone cement. Hence, a number of press fit acetabular cups have been designed for securement to the acetabulum without the use of bone cement.  
           [0005]    In either case (i.e. cemented or cementless), the acetabulum is first reamed by the surgeon in order to create a cavity into which the acetabular cup is secured by the use of a surgical tool known as a reamer. It is often difficult for the surgeon to properly match the size of the reamer to the desired acetabular cup size.  
           [0006]    Although press fit acetabular cups have heretofore been referred to as being “generally hemispherical” in shape, such heretofore-designed cups, in reality, are sub-hemispherical in shape. In particular, as shown in the prior art drawing of FIG. 7, a heretofore designed acetabular cup  100  has an apex or dome  102  at a proximal end  104  thereof along with an annular rim  106  at a distal end  108  thereof. In between the dome  102  and the annular rim  106 , the prior art acetabular cup  100  has a sidewall  108  that has a convex proximal surface and a concave distal surface.  
           [0007]    However, as shown in FIG. 7, the configuration of the prior art acetabular cup  100  is sub-hemispherical. In particular, a “true” hemisphere  114  is shown in FIG. 7 as a phantom line overlay. As can be seen, a distal face  116  of the annular rim  106  does not, in fact, lie along the 180° surface (or loosely, the equator  118 ) of the hemisphere  114 , but rather is recessed away from the equator  18  by a relatively significant distance X. In fact, it is not uncommon for prior art cup designs to be recessed from the equator  118  of the cup by as much as 4-5 millimeters (i.e. X=4-5 mm).  
           [0008]    Such a configuration has a number of drawbacks associated therewith. For example, such a large recess distance X (i.e. 4-5 mm) renders it difficult for the surgeon to ream a properly sized cavity in the acetabulum. In particular, the cutting head of heretofore-designed reamers are typically configured as relatively true hemispheres. Hence, when a surgeon reams the patient&#39;s acetabulum, the surgeon has to “estimate” the approximate depth of the reamed recess. More specifically, if the surgeon reams all the way to the 180° surface or “equator” of the reamer, the annular rim  106  of the acetabular cup  100  will be recessed in the reamed cavity. Conversely, if the surgeon does not ream deeply enough (i.e. “under reams”), the acetabular cup  100  will not be fully seated in the reamed cavity of the acetabulum. In light of the fact that surgeons occasionally select a reamer that is slightly smaller in size than the acetabular cup to be implanted, under reaming may also disadvantageously lead to bone fracture of the acetabulum since excessive force is often utilized to insert the cup into the undersized (i.e. under reamed) cavity. Some of the early bone cemented cups did not suffer from this problem by being configured more closely as “true” hemispheres. However, as indicated above, such cups undesirably required the use of bone cement during implantation thereof.  
           [0009]    Another drawback associated with heretofore-designed press fit acetabular cups relates to the configuration of the outer shell. In particular, in an attempt to increase retaining forces, a number of acetabular cups have been designed with a flared rim (known as dual radius or “bubble” cups) or a frusto-conically shaped annular rim portion (known as dual-geometry cups). Although the configuration of such cups may generate relatively strong retention forces at the rim portion of the cup, surface contact and therefore retention forces are relatively small at the portions of the outer shell other than the rim portion, particularly in the dome area. Moreover, such reduced surface contact at the portions of the outer shell other than the rim portion reduces bone ingrowth in such portions.  
           [0010]    With the above-mentioned heretofore-designed press-fit acetabular cups, a two-part reaming process is typically necessary. The two-part reaming process involves reaming of the acetabulum using a reamer of a first size, then reaming the acetabulum using a reamer of a second size. The more reaming, the more likely that a problem will occur. For example, many conventional cementless acetabular cup systems use a cup that is two millimeters (2 mm) larger than the last reamer size used. Inserting this size cup into the undersized reamed acetabulum to accommodate this system is sometimes difficult, particularly with resistance in the dome area of the cup, which is also larger than the last reamer size used.  
           [0011]    What is needed therefore is an acetabular cup and associated method that overcomes one or more of the above-mentioned drawbacks.  
           [0012]    What is further needed is an acetabular cup and associated implant method that allows for the cup to be secured to the acetabulum without the use of bone cement.  
           [0013]    What is also needed is an acetabular cup and associated implant method that facilitates greater amounts of bone ingrowth relative to heretofore designed acetabular cups.  
           [0014]    What is still further needed is a cementless acetabular cup that utilizes a single reaming process for implant.  
         SUMMARY OF THE INVENTION  
         [0015]    The present invention is a press-fit acetabular cup and method of securing the acetabular cup to an acetabulum so as to provide a bearing surface for a head portion of a femur. The acetabular cup is shaped to provide a cementless, press-fit into a reamed acetabulum. The acetabular cup is formed of a body having a sidewall defining a radius from a center point of the annular rim to the sidewall wherein the radius increases in length from the apex to said annular rim. The method of securing the acetabular cup first includes reaming the acetabulum with a reamer having a head with a radius of curvature that is less than the radius from the center point to the annular rim of the acetabular cup.  
           [0016]    According to one embodiment of the present invention, there is provided an acetabular cup. The acetabular cup includes a body defining a dome having an apex and an annular rim. The dome is defined by an increasing radius sidewall that extends from the apex to the annular rim. The annular rim defines a plane having a center point. Wherein a radius from the center point to the annular rim has a first given length, 1 L , the center point to the apex defining a second given length, 2 L, and the second given length is less than the first given length.  
           [0017]    According to another embodiment of the present invention there is provided an acetabular cup. The acetabular cup includes a dome-shaped shell having an apex and an annular rim. The dome-shaped shell has a sidewall defining a radius from a center point of the annular rim to the sidewall, wherein the radius increases in length from the apex to the annular rim.  
           [0018]    According to yet another embodiment of the present invention, there is provided a method of securing an acetabular cup to an acetabulum. The method includes reaming a cavity of a first radius of curvature into an acetabulum with a reamer, the reamer including a reamer head having the first radius of curvature, and press fitting an acetabular cup into the reamed cavity, the acetabular cup having a body defining a dome having an apex and an annular rim, the dome defined by a gradually increasing radius sidewall that extends from the apex to the annular rim, the annular rim defining a plane having a center point, wherein a radius from the center point to the annular rim has a first given length, 1 L, the center point to the apex defining a second given length, 2 L, the second given length is less than the first given length, the first given length is greater than the first radius of curvature, and the second given length is approximately equal to the first radius of curvature.  
           [0019]    According to a further embodiment of the present invention, there is provided a method of securing an acetabular cup to an acetabulum. The method includes reaming a cavity of a first radius of curvature into an acetabulum with a reamer, the reamer including a reamer head having the first radius of curvature, and press fitting an acetabular cup into the reamed cavity, the acetabular cup having a dome-shaped shell having an apex and an annular rim, the dome-shaped shell having a sidewall defining a shell radius from a center point of the annular rim to the sidewall, wherein the shell radius gradually increases in length from the apex to the annular rim, and the shell radius at the annular rim is greater than the radius of curvature.  
           [0020]    The present cementless acetabular cup provides an enhanced peripheral press-fit with reduced loading in the dome area. This helps provide initial stability for bone ingrowth and long-term fixation. Further, a good peripheral fit may help protect against the migration of wear debris from a “pumping action” of the effective joint space.  
           [0021]    Additionally, reamer to cup dimensions of the present acetabular cup have a line-to-line fit in the dome area with a gradual increase in press-fit or size from the dome to the rim. This allows the present acetabular cup to achieve maximization of peripheral contact for stability while minimizing dome loading and helping reduce the risk of acetabular fracture.  
           [0022]    The present acetabular cup also accepts a polyethylene liner or insert such that the cup and liner achieve congruency therebetween without rim loading the liner. A positive locking mechanism and anti-rotation devices in the metal shell/body defining the acetabular cup secure the liner to the shell. Congruency and secure locking of the polyethylene liner work together to reduce micromotion at the shell/liner interface. Further, such high conformance between the shell and the liner results in efficient load transfer and reduce contact stresses. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    The various objects, features, and advantages of the present invention will become apparent and/or better understood by reference to the following descriptions of the embodiments of the present invention taken in conjunction with the accompanying drawing wherein:  
         [0024]    [0024]FIG. 1 is an exploded perspective view that shows an acetabular cup and associated bearing insert that incorporate the features of the present invention therein;  
         [0025]    [0025]FIG. 2 is an enlarged side elevational view of the acetabular cup of FIG. 1 with an imaginary true hemisphere superimposed thereon;  
         [0026]    [0026]FIG. 3 is a perspective view of a reamer that is utilized to ream the acetabulum of a patient prior to implantation of the acetabular cup of FIG. 1;  
         [0027]    [0027]FIG. 4 is a perspective view of the acetabulum subsequent to reaming with the reamer of FIG. 3;  
         [0028]    [0028]FIG. 5 is a view similar to FIG. 4, but showing the acetabular cup press fit into the cavity reamed into the acetabulum by the reamer;  
         [0029]    [0029]FIG. 6 is a diagrammatic view that shows an outline of the acetabular cup of FIG. 1 superimposed on an outline of the cavity reamed into the acetabulum by the reamer; and  
         [0030]    [0030]FIG. 7 is a side elevational view of a prior art acetabular cup. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    While the invention is susceptible to various modifications and alternative forms, a specific embodiment thereof has been shown by way of example in the drawings and will herein be described in detail. It should be understood, however, that there is no intent to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.  
         [0032]    Referring now to FIG. 1, there is shown a prosthetic hip assembly  10  for use in either a partial or total hip replacement procedure. The prosthetic hip assembly  10  includes an acetabular component or cup  12  and a bearing insert  14 . Collectively, the acetabular cup  12  and the bearing insert  14  provide an artificial bearing surface on which a natural or artificial head portion of a femur (not shown) may bear. In particular, as shall be discussed in greater detail, the acetabular cup  12  is implanted into a patient&#39;s acetabulum  16  (see FIGS. 4 and 5) such that the bearing insert  14  may then be positioned in an insert-receiving cavity  18  (see FIG. 1) defined in the acetabular cup  12 . The bearing insert  14  is preferably constructed from a polymeric material such as polyethylene or ultra-high molecular weight polypropylene (UHMWPE) thereby providing a desirable artificial surface on which the head portion of the femur may bear.  
         [0033]    As shown in FIG. 1, the bearing insert  14  has a number of keying tabs  20  defined therein. The keying tabs  20  are received into a number of corresponding keying slots  22  defined in the acetabular cup  12  to prevent rotation of the bearing insert  14  relative to the acetabular cup  12  when the bearing insert  14  is positioned in the insert-receiving cavity  18  of the cup  12 .  
         [0034]    As shown in FIGS. 1 and 2, the acetabular cup  12  includes a cup body or shell  24  that has a sidewall  26 . The body  24  may be made of any suitable material such as a titanium alloy. One such titanium alloy is Ti-6AI-4V. The sidewall  26  has a textured or porous outer surface. Such a textured or porous outer surface enhances bone ingrowth thereby facilitating long-term attachment of the acetabular cup  12  to the acetabulum  16 . Such a textured or porous outer surface may be a Porocoat® porous coating may be DePuy Orthopaedics of Warsaw, Ind.  
         [0035]    The sidewall  26  extends outwardly at a substantially constant radius R C  from an apex or dome  28  of the body  24  to an annular rim  30 . In particular, as shown in FIG. 2, an imaginary hemisphere  32  may be superimposed over the acetabular cup  12 . The imaginary hemisphere  32 , as with any true hemisphere, possesses an apex  34  and a great circle  36 . The great circle  36  is the circle that is defined by the intersection of the surface of a sphere by a plane that passes through the center of the sphere. In essence, a sphere that is bisected along its “equator” into two equal halves forms a great circle at the plane of bisection. Hence, the center point of the bisected sphere is the center point of the great circle of the hemisphere. Accordingly, every point along the surface of the imaginary hemisphere  32  (and hence every point on the outer surface of the sidewall  26 ) lies an equal distance (i.e. the radius R C ) from a center point  38  of the great circle  36  of the imaginary hemisphere  32 . Indeed, substantially every point on the sidewall  26  of the cup body  24  is positioned a distance that is equal to the radius R C  away from the center point  38  of the great circle  36 . It should be appreciated that the textured or porous outer surface of the sidewall  26  creates a somewhat irregular or “jagged” outer surface. Hence, as used herein, the term “sidewall”, when utilized in the context of “every point on the sidewall being positioned a distance equal to the radius (i.e. R C ) away from the center point of the great circle”, is intended to mean the average or mean height of the jagged outer surface of the sidewall thereby factoring out any slight fluctuations in the distance from the center point of the great circle caused by the textured or porous outer surface of the sidewall.  
         [0036]    The outer face of the annular rim  30  of the cup body  24  defines a segmental plane  40  (shown as a line in the side elevational view of FIG. 2) that intersects the imaginary hemisphere  32 . The segmental plane  40  is oriented substantially parallel to the great circle  36  and is spaced apart from the great circle  36  by a relatively small distance D. Hence, every point on the outer peripheral edge of the annular rim is spaced apart from the great circle  36  by the distance D. In one exemplary embodiment, distance D is between 0.5 and 2.0 millimeters. In a more specific embodiment, distance D is approximately 1 millimeter.  
         [0037]    As a result, the cup body  24  of the acetabular cup  12  is configured as a substantially true hemisphere. Indeed, with the exception of (1) the portion of the cup  12  near its apex  28  which is removed in order to facilitate a threaded aperture  42  which is utilized during implantation of the cup  12 , and (2) the portion of the sidewall  26  which would be present if the sidewall  26  was extended the distance D toward the great circle  32  of the imaginary hemisphere  32 , the cup body  24  is, in fact, configured as a true hemisphere. As shall be discussed below, such a configuration (i.e. that of a nearly true hemisphere) provides numerous advantages to the acetabular cup  12  relative to heretofore-designed cups.  
         [0038]    Referring now to FIG. 3, there is shown a cutting tool or reamer  50  associated with the prosthetic hip assembly  10 . The reamer  50  is utilized to ream or otherwise cut the acetabulum  16  in order to form a hemispherically shaped cavity  52  therein (see FIG. 4). The reamer  50  includes a cutting head  54  secured to a shaft  56 . The cutting head  54  includes a number of cutting projections  58  which are configured to engage and remove bone material from the patient&#39;s acetabulum  16 . The outer edges of the cutting projections  58  define the radius of the hemispherically shaped cutting head  54 . In one exemplary embodiment, the cutting projections  58  of the cutting head  54  define a true hemisphere. In particular, the general profile created by the cutting projections  58  (and hence the cavity created by the reamer  50 ) is that of a true hemisphere.  
         [0039]    Moreover, the radius of the cutting head  54  is preferably slightly smaller than the radius R C  of the acetabular cup  12 . In one exemplary embodiment of the present invention, the radius of the cutting head  54  is between one-half (0.5) and one and one-half (1.5) millimeters smaller than the radius R C  of the acetabular cup  12 . In a more specific exemplary embodiment of the present invention, the radius of the cutting head  54  is approximately one (1) millimeter smaller than the radius R C  of the acetabular cup  12 . For example, if the anatomy of a given patient requires the use of a fifty-six millimeter acetabular cup  12  (i.e. an acetabular cup having an outer diameter of 56 mm), the reamer  50  utilized to ream the patient&#39;s acetabulum  16  preferably has a cutting head diameter of fifty-four millimeters (i.e. the outer diameter of the cutting head is 54 mm). Such use of a smaller reamer  50  provides numerous advantages. For example, it has been found that such use of a slightly smaller reamer  50  creates a cavity  52  in the acetabulum  16  which provides preferable amounts of insertion resistance thereby firmly retaining the acetabular cup  12  upon press fit thereof into the cavity  52  without requiring insertion forces large enough to crack or otherwise break the acetabulum.  
         [0040]    Hence, as shown in FIG. 4, use of the reamer  50  to ream the acetabulum produces the cavity  52  having a radius R A . As described above, the radius of the cutting head  54  of the reamer  50  is preferably slightly smaller than the radius R C  of the acetabular cup  12 . Hence, the radius R A  reamed into the cavity  52  of the acetabulum  16  is likewise slightly smaller than the radius R C  of the acetabular cup  12 . Such a slight difference in radius size provides for enhanced implantation properties. In particular, as alluded to above, the acetabular cup  12  is configured to be press fit into the reamed cavity  52  of the acetabulum  16  without the use of bone cement. As such, the nearly true hemispheric shape of the acetabular cup  12  provides for constant contact with the reamed hemispherically shaped cavity  52  of the acetabulum  16  along the entire outer surface of the cup body  24 .  
         [0041]    Moreover, the presence of a slightly smaller radius R A  of the cavity  52  also causes a need for a slight increase in the insertion force (i.e. the press fit) as the cup  12  is implanted in the direction from the dome  28  to the annular rim  30 . This slightly increased resistance enhances the retention of the cup  12  when it is press fit into the acetabulum  16 . In addition, since the cavity  52  is reamed, for example, two (2) millimeters smaller in diameter relative to the acetabular cup  12  (i.e. R A  is 2 mm smaller than R C  ), an approximately one millimeter difference is created on each “side” of the annular rim  30 .  
         [0042]    Indeed, as shown in FIG. 6 according to one aspect thereof, a “graduated” or slightly increasing press fit is created as the acetabular cup  12  is implanted into the acetabulum  16 . Specifically, line-to-line contact exists between the cup  12  and the cavity  52  in the area near the dome  28  of the cup  12 . Hence, the press fit of the cup  12  into the cavity  52  gradually increases from approximately zero in the areas of such line-to-line contact (i.e. the dome  28 ) to a press fit that equals a distance P on each “side” of the cup  12  at the annular rim  30 . In an exemplary case, the distance P is one millimeter (1 mm) thereby creating an overall press fit of two millimeters (2 mm) at the annular rim  30  of the cup  12 . This increasing press fit provides for a reliable (i.e. stable) press fit of the acetabular cup  12  into the reamed cavity  52  thereby further enhancing the retention of the cup  12  in the reamed cavity  52 .  
         [0043]    Particularly, and aside from the near true hemispherical aspect of the acetabular cup  12 , the geometry of the acetabular cup  12  may be described in an alternative manner. Due to the distance P, the sidewall  26  that extends from the dome  28  to the annular rim  30  also increases in radius from the dome  28  to the annular rim  30 . More particularly, the sidewall  26  gradually increases in radius, as measured from a center point  38  (see FIG. 2), from the dome  28  to the annular rim  30 . The radius increases such that a radius from the center point  38  to the annular rim  30  is greater than the radius from the center point  38  to the apex of the dome  28 . Preferably, the radius from the center point  38  to the annular rim  30  is between 0.5 mm to 1.5 mm greater than the radius from the center point  38  to the apex of the dome  28 . More preferably, the radius from the center point  38  to the annular rim  30  is approximately 1.0 mm greater than the radius from the center point  38  to the apex of the dome  28 .  
         [0044]    Moreover, as described above, since both the reamed cavity  52  and the acetabular cup  12  are preferably configured as nearly true hemispheres having similar sizes, the outer surface of the sidewall  26  of the cup  12  contacts the reamed hemispherically shaped cavity  52  of the acetabulum  16  along substantially all of the surface of the cavity  52 . Hence, the outer porous surface of the sidewall  26  of the acetabular cup  12  “scratches” or otherwise slightly abrades substantially all of the wall surface of the cavity  52  as the cup  12  is press fit into the cavity  52 . This slight abrading facilitates bone ingrowth into the porous outer surface of the acetabular cup  12 .  
       Operation of the Present Invention  
       [0045]    In operation, the prosthetic hip assembly  10  of the present invention is utilized in the performance of either a total or partial hip replacement procedure in order to provide an artificial bearing surface for either a natural or artificial head portion of the femur without the use of bone cement. As shown in FIG. 4, the reamer  50  is first utilized to ream or otherwise cut the acetabulum  16  in order to form the hemispherically shaped cavity  52  therein. In particular, the surgeon rotates the reamer  50  such that the cutting projections  58  of the cutting head  54  engage and remove bone material from the patient&#39;s acetabulum  16 . As described above, the radius of the cutting head  54  is preferably slightly smaller than the radius R C  of the acetabular cup  12 . For example, the reamer  50  utilized by the surgeon generally has a cutting radius that is approximately one (1) millimeter smaller than the radius R C  of the acetabular cup  12  that is to be implanted. For example, if the anatomy of the patient requires the use of a fifty-six millimeter acetabular cup  12  (i.e. an acetabular cup having an outer diameter of 56 mm), the reamer  50  utilized to ream the patient&#39;s acetabulum  16  preferably has a cutting head diameter of fifty-four millimeters (i.e. the outer diameter of the cutting head is 54 mm).  
         [0046]    Hence, as shown in FIG. 4, once the surgeon has utilized the reamer  50  to ream the acetabulum  16 , the hemispherically shaped cavity  52  (having a radius R A ) is formed. Thereafter, the surgeon implants the acetabular cup  12  into the reamed acetabulum  16 . In particular, the acetabular cup  12  is press fit into the reamed cavity  52  of the acetabulum  16  by the surgeon without the use of bone cement. During such press fitting, the true hemispherical shape of the acetabular cup  12  provides for constant contact with the reamed hemispherically shaped cavity  52  of the acetabulum  16  along the entire outer surface of the cup body  24 . Moreover, the presence of a slightly smaller radius R A  of the cavity  52  also causes a need for a slight increase in the insertion force as the cup  12  is implanted in the direction from the dome  28  to the annular rim  30 . This increases the retention of the cup  12  when it is press fit into the acetabulum  16 .  
         [0047]    Moreover, since the cavity  52  is reamed, for example, two (2) millimeters smaller in diameter relative to the acetabular cup  12  (i.e. R A  is 2 mm smaller than R C ), an approximately one millimeter difference is created on each “side” of the annular rim  30 . This slight difference provides for a reliable (i.e. stable) press fit of the acetabular cup  12  into the reamed cavity  52 .  
         [0048]    In addition, during press fitting of the acetabular cup  12  into the reamed cavity  52 , the outer porous surface of the sidewall  26  of the acetabular cup  12  “scratches” or otherwise slightly abrades substantially the entire wall surface of the cavity  52 . As described above, this slight abrading facilitates bone ingrowth into the porous outer surface of the acetabular cup  12 . Moreover, the similar configuration and size of the acetabular cup  12  and reamed cavity  52  allows the cup  12  to be fully seated into a position in which the annular rim  30  is substantially flush mounted with the distal surface of the acetabulum  16  (see FIG. 5) without requiring significant amounts of “estimating” by the surgeon during reaming of the bone.  
         [0049]    Once the acetabular cup has been press fit into the cavity  52  defined in the acetabulum  16 , the bearing insert  14  is installed. In particular, the bearing insert  14  may then be positioned in the insert-receiving cavity  18  (see FIG. 1) defined in the acetabular cup  12 . As described above, the keying tabs  20  of the bearing insert  14  are received into the corresponding keying slots  22  defined in the acetabular cup  12  to prevent rotation of the bearing insert  14  relative to the acetabular cup  12 . Once installed in such a manner, the bearing insert  14  provides a desirable artificial surface on which the artificial or natural head portion of the femur may bear.  
         [0050]    Hence, as described herein, the prosthetic hip assembly  10  of the present invention provides numerous advantages over heretofore-designed assemblies. For example, the prosthetic hip assembly  10  of the present invention may be utilized to secure the acetabular cup to the acetabulum without the use of bone cement. Moreover, use of a constant radius, nearly true hemispherically-shaped acetabular cup provides for enhanced performance characteristics such as resistance to loosening and instability since the configuration of the cup distributes loads more evenly across the entire outer surface of the cup relative to heretofore designed dual-geometry or bubble cups. In addition, the use of a constant radius cup prevents the development of gaps near the flared outer rim surfaces of heretofore-designed dual-geometry or bubble cups. It is known that the presence of such gaps not only prevents bone ingrowth, but also undesirably facilitates the formation of lysis in the bone positioned near the rim of the cup.  
         [0051]    In addition, since both the reamer  50  and the acetabular cup  12  are configured as nearly true hemispheres having similar sizes, the surgeon is not required to estimate the approximate depth as to when the acetabulum has been reamed deeply enough. This is a significant improvement over the use of heretofore-designed sub-hemispherical cups in which the surgeon must do such estimating.  
         [0052]    Moreover, since both the reamer (and hence the reamed cavity  52 ) and the acetabular cup  12  are configured as nearly true hemispheres having similar sizes, the outer surface of the sidewall  26  of the cup  12  contacts the reamed hemispherically-shaped cavity  52  of the acetabulum  16  along substantially all of the surface of the cavity  52 . As described above, this feature causes the outer porous surface of the sidewall  26  of the acetabular cup  12  to “scratch” or otherwise slightly abrade substantially all of the wall surface of the cavity  52  as the cup  12  is press fit into the cavity  52 . Such slight abrading advantageously facilitates bone ingrowth into the porous outer surface of the acetabular cup  12 .  
         [0053]    Further, the similar configuration and size of the acetabular cup  12  and the reamed cavity  52  also allows the cup  12  to be fully seated into a position in which the annular rim  30  is substantially flush with the distal surface of the acetabulum  16  (see FIG. 5) without requiring significant amounts of “estimating” by the surgeon during reaming of the bone.  
         [0054]    Moreover, use of a reamer that is slightly smaller than the acetabular cup provides preferable amounts of resistance thereby firmly retaining the acetabular cup upon press fit thereof into the cavity without requiring insertion forces large enough to crack or otherwise break the acetabulum.  
         [0055]    While the invention has been illustrated and described in detail in the drawings and foregoing description, such an illustration and description is to be considered as exemplary and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes and modifications that come within the spirit of the invention are desired to be protected.  
         [0056]    There is a plurality of advantages of the present invention arising from the various features of the prosthetic hip assembly and associated method described herein. It will be noted that alternative embodiments of the prosthetic hip assembly and associated method of the present invention may not include all of the features described yet still benefit from at least some of the advantages of such features. Those of ordinary skill in the art may readily devise their own implementations of a prosthetic hip assembly and associated method that incorporate one or more of the features of the present invention and fall within the spirit and scope of the present invention as defined by the appended claims.