Abstract:
An acetabular prosthesis including a ceramic shell having multiple layers and a polymer liner formed integrally with the shell. The acetabular prosthesis shell is formed of a ceramic wherein several layers with varying porosities and thicknesses are sintered together to form a unitary structure. A polymer inner liner forms a bearing within the shell to receive the ball of a femur or femur prosthesis. The polymer liner is formed in the ceramic shell and includes a plurality of portions which interdigitate into the pores of the inner porous layer of the ceramic shell.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to a method and apparatus for use in orthopedic surgery and more particular to a method and apparatus for providing a shell component incorporating a porous ingrowth material and liner for use during an orthopedic surgical procedure, particularly with respect to a prosthetic hip. 
     BACKGROUND OF THE INVENTION 
     A natural hip joint may undergo degenerative changes due to a variety of etiologies. When these degenerative changes become so far advanced and irreversible, it may ultimately become necessary to replace a natural hip joint with a prosthetic hip. To replace the hip, a prosthetic hip may be affixed to the femur in one of many ways generally known in the art. In addition to replacing the femoral portion of the hip joint, the acetabulum portion of the hip joint may also require replacement. If the acetabulum also needs to be replaced or repaired, all the remnants of the articular cartilage are generally removed from the acetabulum and an acetabular prosthesis which will accommodate the head or ball of the hip prosthesis is affixed to the acetabulum. It is generally known in the art to affix the acetabular prosthesis to the acetabulum by means of cement, screws, or other appropriate fixation mechanisms. 
     A typical acetabular prosthesis generally includes two (2) modular portions. In particular, the modular portions include an acetabular cup or shell and a synthetic liner or bearing wherein the liner is affixed within the acetabular cup through a locking mechanism. The modular acetabular prosthesis allows for numerous liners to be produced for a singular acetabular cup or vice versa. Other modular attachments may include additional fixation mechanisms to affix the acetabular cup to the patient. This enables a surgeon performing the procedure to determine which would fit best for the particular patient. Although a modular acetabular prosthesis performs its job adequately, it would be desirable to have an acetabular prosthesis with an acetabular cup and a liner which does not allow any movement relative to each other once implanted into the bone. 
     If the acetabular cup and the liner move, such as by micromotion, wear material may result. This wear material may migrate out of the acetabular cup or may enter into the articulating area of the hip joint prosthesis. Over time, these foreign materials may cause additional wearing beyond what is desirable in the acetabular prosthesis. Because of the wear material, it may also be desirable to have an acetabular prosthesis that does not include bores through the acetabular cup. The bores, which generally accept screws or other fixation devices, also allow foreign material or body fluids into the acetabular cup and wear material out of the acetabular cup, which may cause additional or accelerated degeneration of the acetabular prosthesis. 
     Therefore, it is desirable to provide a substantially non-modular acetabular prosthesis, such as an acetabular prosthesis which improves upon existing modular acetabular prostheses and does not suffer from the above mentioned disadvantages. 
     SUMMARY OF THE INVENTION 
     In accordance with the teachings of the present invention, a method and apparatus for providing a substantially non-modular acetabular cup which may include additional modular or non-modular flanges for use in orthopedic surgery is disclosed. The shell portion of the acetabular cup may be formed from a ceramic material that includes three integrally formed regions. In particular, a ceramic shell having an inner and outer porous region and therebetween a non-porous region. A liner may then be interdigitated into the inner porous region of the shell to be held firmly in place. The acetabular prosthesis is then placed into the bone of the ileum and the outer porous region allows bone to regrow to hold the acetabular prosthesis in place. When the shell is made of ceramic, the ceramic material is placed in a mold and then sintered to form the shell of the acetabular prosthesis. The bearing liner is formed by placing a polymer powder in the shell and through heat and pressure, is melted and formed into a solid bearing liner which interdigitates into the interior porous region of the shell of the acetabular prosthesis. 
     In a first embodiment, an acetabular prosthesis includes a rigid exterior shell that forms the acetabular cup and a bearing liner is formed and interdigitated on the inside, which include the regions that overlay the top edge of the cup. 
     In another embodiment, the bearing liner formed on the inside of the acetabular cup protrudes at an angle from the opening of the acetabular cup. In particular, the bearing liner would then allow the hip joint to have an angle which is different from the implantation angle of the acetabular cup. 
     In yet another embodiment, the bearing liner extends a distance above the outer edge of the ceramic portion of the acetabular cup. In particular, the bearing liner creates a wall along the edge of the acetabular cup in a specific arcuate region. The wall creates an arcuate region in which the hip bone would not be able to rotate within the prosthesis. 
     In yet another embodiment, the non-porous middle region of the acetabular cup extends through and creates an upper collar above and around the outer and inner porous regions. Further formed in the collar are holes or indents to accept an impacter instrument. The impacter instrument is used to align the acetabular prosthesis during the surgical procedure and hold the instrument in place while the acetabular prosthesis is impacted into the bone portion. 
     In yet another embodiment, an upper collar of non-porous material extends down and over the outer porous region to provide fins which help align the acetabular prosthesis when it is being implanted into the bone. The fins allow the acetabular prosthesis to be placed properly in the acetabulum to receive the ball of the femur to form the hip joint. 
     In yet another embodiment, non-porous material extends through the bottom of the acetabular cup. That is, portions of the non-porous material extend through the outer porous region to form spikes near the bottom of the acetabular cup. These spikes formed by the non-porous material help secure the acetabular prosthesis in its final implanted position. In this way, a minimum amount of cement or other holding materials may be necessary to ensure that the acetabular prosthesis is affixed in the proper position. 
     Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiments of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein: 
     FIG. 1 is a top view of a non-modular acetabular cup according to a first embodiment of the present invention; 
     FIG. 2 is a cross-sectional view taken along line  2 — 2  of FIG. 1; 
     FIG. 3 is a plan view from the top of an acetabular cup including tool detents on the top thereof; 
     FIG. 3 a  is a cross-sectional view taken along lines  3 — 3  of FIG. 3; 
     FIG. 4 is a plan view from the bottom of the acetabular cup including portions to assist in molding; 
     FIG. 4 a  is a cross-sectional view taken along lines  4   a — 4   a  of FIG. 4; 
     FIG. 5 is a cross-sectional view of a second embodiment of the present invention; 
     FIG. 6 is a plan view from the top of a third embodiment of the present invention; 
     FIG. 6 a  is a cross-sectional view taken along line  6   a — 6   a  of FIG. 6; 
     FIG. 7 is a plan view from the bottom of a fourth embodiment of an acetabular cup according to the present invention; 
     FIG. 7 a  is a cross-sectional view taken long line  7   a — 7   a  of FIG. 7; 
     FIG. 8 is a plan view from the bottom of an acetabular cup according to a fourth embodiment of the present invention; 
     FIG. 8 a  is a cross-sectional view taken along line  8   a — 8   a  of FIG. 8; 
     FIG. 9 is a plan view of an acetabulum and reamer to prepare an acetabulum for an acetabular prosthesis according to the present invention; 
     FIG. 9 a  is a perspective view of a tool implanting the prosthesis according to the present invention; and 
     FIG. 10 is a perspective view of an implanted acetabular prosthesis according to the present invention with a hip prosthesis associated therewith. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The following description of the embodiment(s) are merely exemplary in nature and are in no way intended to limit the invention, its application, or uses. Furthermore, while the present invention is described in association with an acetabular cup for a hip joint, those skilled in the art will appreciate that the present invention may be incorporated into various orthopedic implants for a human body such as knee, shoulder, and other joints. Therefore, it is to be understood that the present illustrative embodiments are not meant to limit the present invention. 
     With reference to FIGS. 1 and 2, a first embodiment of the present invention is illustrated. An acetabular prosthesis is shown generally at  10 . The acetabular prosthesis  10  includes two main portions: an outer shell component  12  and an inner liner or bearing  14  affixed to the interior of the shell  12 . The liner  14  includes a hollow interior region  16  that forms the articulating surface which receives a natural or prosthetic femoral head. With particular reference to FIG. 2, the shell  12  generally includes three layers: an interior layer  18  that is porous and receives the liner  14  during formation, further described herein; an exterior porous layer  20  that forms the exterior of the shell  12  and engages the bone when implanted; and an intermediate non-porous layer  22  between the inner porous layer  18  and the outer porous layer  20 . It will be understood that the shell  12  may take any shape necessary for the particular orthopedic prosthesis, as mentioned above. The hemispherical or cup shape illustrated is exemplary only for the acetabular prosthesis  10  embodiment. The inner porous layer  18  forms the interior of the shell  12  and the outer porous layer  20  extends generally around the entire perimeter of the hemisphere and both terminate at a generally equal meridian of the shell  12 . However, the intermediate non-porous layer  22  extends and terminates at a different meridian above the meridian of the inner layer  18  and the outer layer  20 . In this way, the upper region of the shell  12  includes an upper rim or collar  24  which is a hard region formed of the non-porous material from the non-porous region  22 . This collar  24  helps in the implantation of the acetabular prosthesis  10 , as described herein. 
     The entire shell  12  is formed into a single piece before the addition of the liner  14 . If the shell  12  is formed of ceramic, the following is an exemplary process to form the shell  12 . The ceramic which will become the non-porous region is first formed into a “green body” which is a compacted and molded ceramic powder. Once the “green body” is formed, the material which will become the porous layers is placed onto the inside and outside of the “green body.” The three layers are then placed into a form. Then all the layers are sintered in the form to produce the final ceramic shell  12 . The inner porous layer  18  and the outer porous layer  20  are formed during the sintering process. After sintering, however, each portion is integral with the others to form a firm ceramic shell  12 . Although the ceramic shell  12  includes three layers, that being an inner layer  18 , which is porous; an intermediate layer and an upper collar which is non-porous  22 ; and an outer layer  20  which is also porous, they are all integrally formed into one piece. It will also be understood that the shell  12  may be formed of other materials such as metal wherein the inner and outer surfaces of a metal shell may be made porous. Furthermore, other methods may be used to achieve similar ceramic formations not substantially different from the present invention. 
     Once the shell  12  is formed, then the liner  14  is integrated into the shell  12 . In particular, a portion of a polymer powder is placed within the shell  12 , which, after processing the powder, forms the liner  14 . Preferably, the polymer powder is Ultra High Molecular Weight Polyethylene (UHMWPE). After the UHMWPE powder is placed into the interior of the shell  12 , the entire system is placed into a molding apparatus. Briefly, the shell  12  is held in a lower plunger while an upper plunger presses down into the UHMWPE powder void  16  placed in the shell  12  and thereby forming the UHMWPE powder into the proper formation. Then the entire apparatus is heated to a suitable temperature to melt the UHMWPE powder. Pressure is also applied to the system to ensure that a proper formation of the liner  14  is created. As the UHMWPE powder melts, it flows into the pores created within the inner porous layer  18 . The flow of the UHMWPE into the pores interdigitates the UHMWPE such that when the polymer is cooled and solidified, the interdigitated portion will hold the liner  14  in place within the shell  12 . After a suitable time under the heat and pressure of the molding apparatus, the acetabular prosthesis  10  is removed and allowed to cool. The melted UHMWPE solidifies to form the liner  14  which includes liner collar regions  15  that extend above and on top of the non-porous collar  24 . The liner  14  is held within the shell  12  by the portions of the polymer that have interdigitated into the inner porous region  18 . 
     The acetabular prosthesis  10  may include an entirely non-modular formation. In particular, while the shell  12  includes several layers, the final product is completely integral and forms a seamless whole. Each of the layers are integral, making them unitary to substantially reduce or eliminate micromotion. Also the liner  14  interdigitates into the inner porous region  18 , thereby integrating the liner  14  into the shell  12 . The inner non-porous layer  22  also acts as a barrier to prevent the migration of the liner  14  through the shell  12 . The pores in the outer porous layer  20  provide places for bone ingrowth once the acetabular prosthesis  10  is surgically implanted into the patient. Furthermore, it is to be understood that each region of the acetabular prosthesis  10  may be varied in thickness depending upon the particular patient or application necessary. Additionally, the shell  12  may include modular attachments to assist in implantation. 
     It is preferred that the inner porous region  18  has substantially all of its pores interdigitated by the liner  14 . Depending on the polymer or other material being used, the thickness of the inner porous layer  18  may be adapted to ensure that substantially all of the pores formed in the inner porous layer  18  are interdigitated. In particular, it is preferred that the thickness of the inner porous layer  18  be in the range of about 0.3 mm to 2 mm. It is also understood that the outer porous layer  20  may be varied in thickness depending upon the amount of bone ingrowth anticipated within the particular patient. Furthermore, the porosity of the inner porous layer  18  and the outer porous layer  20  may be adapted depending upon the type of polymer being used in the polymer liner  14  or the amount of bone ingrowth anticipated in the patient. Preferably, porosities for the inner porous layer  18  is between about 40 and about 70 percent, and preferably about 50 to about 60 percent, while the porosities of the outer layer  20  is in the range of about 40 to about 70 percent, and preferably about 60 to about 70 percent. The overall porosity percentage of the shell  12  is in a range between about 10 percent to 47 percent. Additionally, the outer porous region  20  may be impregnated or covered with a bio-active material, preferably bio-active glass, to encourage bone ingrowth. 
     With reference to FIGS. 3 and 3 a , the acetabular prosthesis  10  of the first embodiment may include detents or bores  26  in the collar  24 . The detents  26  help in the placement and positioning of an impacter or implantation tool  28 , illustrated more fully in FIG. 9 a . The impacter  28  includes a primary hemispherical head  30  which is complimentary to and inserted into the articulating region  16  of the acetabular prosthesis  10 . Fingers or legs  32  extend from the head  30  and engage the detents  26 . The legs  32  associate with the detents  26  to ensure that the impacter tool  28  is held steady and positioned properly for the implantation of the acetabular prosthesis  10 . Furthermore, the fingers  32  assist in the rotational positioning of the acetabular prosthesis  10  during implantation. When the shell  12  is formed of a ceramic, the collar  24  is able to receive the force of the legs  32  of the impacter tool  28  due to the fact that it is formed of the non-porous ceramic. Once the acetabular prosthesis  10  is properly positioned using the tool  28 , the acetabular prosthesis  10  is impacted into the bone of the patient undergoing the surgery, as is known in the art. 
     With reference to FIGS. 4 and 4 a , voids  34  may be formed in the outer porous region  20  of the cup  12  to assist in the molding and affixation of the liner  14 . That is, during the formation of the shell  12 , certain voids  34  may be left in the outer porous region  20  to assist in the mounting of the shell  12  during the formation and affixation of the liner  14 . In this way, the mounting portions or plungers of the apparatus to melt and form the liner  14  may engage the shell  12  without harming the outer porous region  20 . Therefore, the acetabular cup  12  will retain its integrity during the molding of the liner  14 . By creating the bores  34  in the outer porous region  20 , increased pressure may be placed upon the shell  12  during the molding of the liner  14 , since the plungers of the molding apparatus engage the non-porous layer  22 . This allows for an additional control of the molding process of the liner  14  and ensures a proper interdigitation of the liner  14  into the inner porous region  18 . The access to the non-porous region  22  are particularly helpful where the control of pressure or necessity of increased pressure exists. 
     With reference to FIG. 5, a second embodiment of the present invention is illustrated where portions similar to the first embodiment of FIG. 1 are given like numerals increased by 100. The acetabular prosthesis  110  includes a shell  112  having an inner porous layer  118 , an outer porous layer  120 , and a non-porous layer  122  there between. Additionally, a non-porous collar  124  extends above the inner porous layer  118  and the outer porous layer  120  to form an upper meridian of non-porous material. A liner  136  is affixed to the shell  112  as explained above. However, according to the second embodiment, the center line A of the liner  136  is not parallel to the center line B of the shell  112 . That being that the center line B of the hemisphere of the liner  136  is formed at an angle  a particular number of degrees from the center line A of the hemisphere of the shell  112 . The angle  will depend upon the particular circumstances of the patient, the surrounding ilium, and other factors for a proper fit of the femur head in the acetabular prosthesis  110 . This is used generally when the acetabular prosthesis  110  is inserted into a bone that is not in the proper alignment and cannot be repaired. Therefore, the liner  136  is tilted to properly accept and align the femoral prosthesis that is implanted into the hip joint. 
     The formation of the first embodiment of the acetabular prosthesis  10  as described above in relation to the first embodiment is the same in regard to the formation of the acetabular prosthesis  110  of the second embodiment. However, the molding of the liner  136  is modified only to ensure the proper placement of the liner  136  in its tilted orientation. It is also understood that the bores  26  and the voids  34  may be placed in the second embodiment to ensure proper placement and forming of the acetabular prosthesis  110 . In particular, the voids  34  in the outer porous region  120  are advantageous to ensure that enough pressure in the right direction is placed onto the polymer powder which will be formed into the liner  136  to ensure the proper tilt in forming the liner  136 . 
     With reference to FIGS. 6 and 6 a , a third embodiment of the present invention is described where portions similar to the first embodiment have similar numerals increased by 200. The acetabular prosthesis  210  includes a ceramic cup  212  which has an inner porous layer  218 , an outer porous layer  220 , and an intermediate non-porous layer  222  in between. Additionally, a non-porous collar  224  creates an upper meridian on the top of the shell  212 . Formed within the shell  212  and interdigitated into the inner porous region  218  is a liner  238 . The liner  238  defines an inner articulating region  216  which receives the ball portion or femoral head of a hip joint. According to the third embodiment, one arc portion of the liner  238  forms a wall  240 . The wall  240  ensures that the femoral head is not able to dislocate from the inner articulating region  216  of the bearing liner  238  during impingement of the femoral stem with the acetabular prosthesis  210 . When the femur is allowed to move to extreme ranges of motion, the likelihood of dislocating the femoral head from the liner  238  increases. Therefore, the wall  240  increases the range needed to dislocate the femoral head thereby decreasing the likelihood of dislocations. The surgeon may place wall  240  at any position during implantation that the surgeon believes such an extreme range may occur. 
     With references to FIGS. 7 and 7 a , a fourth embodiment of the present invention is illustrated where elements similar to the first embodiment have like numerals increased by 300. An acetabular prosthesis  310  includes a shell  312  which has an interior porous layer  318 , an exterior porous layer  320 , and a non-porous layer  322  therebetween. Additionally, a non-porous collar  324 , integral with the inner non-porous region  322 , forms an upper meridian of the shell  312 . Formed within the hemispherical void of the shell  312  and interdigitated into the inner porous region  318  is a liner  314 . Formed within the liner  314  is an articulating region  316  to receive the ball portion of the hip joint. According to the fourth embodiment, portions of the non-porous layer  322  form spikes  342  that extend down and through the outer porous layer  320 . The spikes  342  are formed in the non-porous layer  322  during the initial formation process of the shell  312 . In this way the spikes  342  are integral with the shell  312  thus forming a unitary cup  312 . When the shell  312  is formed of ceramic during formation of the “green body,” the spikes  342  are molded into the compacted ceramic powder. 
     The non-porous spikes  342  help assist in the implantation of the acetabular prosthesis  310  into the patient. It is to be understood that the acetabular prosthesis  310  may include detents similar to that described above ( 26  in FIG.  3 ). In this way when the acetabular prosthesis  310  is being implanted into the patient, the spikes  342  are driven into the bone of the acetabulum to help secure the acetabular prosthesis  310  in position. Additionally, the acetabular prosthesis  310  may include voids  34  to assist in formation of the shell  312  and spikes  342 . The spikes  342  ensure that the implanted acetabular prosthesis  310  is not allowed any movement post operatively. Additionally, the spikes  342  assist in the fixation of the acetabular prosthesis  310  to the bone of the ileum. 
     With reference to FIGS. 8 and 8 a , a fifth embodiment of the present invention is illustrated, where portions similar to those described according to the first embodiment have like numerals increased by 400. An acetabular prosthesis  410  includes a ceramic cup  412  which has an inner porous layer  418  and an outer porous layer  420  with a non-porous layer  422  therebetween. Formed within and interdigitated into the inner porous layer  418  is a liner  414  which defines the articulation region  416  which receives the ball portion of the hip joint. Additionally, a collar  424  of non-porous material forms the upper meridian of the shell  412 . 
     At the upper meridian of the shell  412  are fins or protrusions  444  of non-porous material. In particular, the fins  444  are extensions of the collar  424  of the shell  412 . Again, when the shell  412  is formed of ceramic, the fins  444  are formed during the sintering process of the non-porous ceramic powder. The fins  444  help position the acetabular prosthesis  410  in the acetabulum of the patient during the operative procedure. This helps ensure that proper alignment of the acetabular prosthesis  410  occurs during implantation. Additionally, the fins  444  assist the acetabular prosthesis  410  in maintaining its correct orientation within the patient after implantation and further prevents rotation of the acetabular prosthesis  410 . 
     It is to be understood that each of the embodiments may include portions of the other embodiments as described above. That is each embodiment is not exclusive to itself. For example, the acetabular prosthesis  10  according to the first embodiment may also include the fins  444  as described in relation to the fourth embodiment to assist in the alignment of the acetabular prosthesis  10 . Additionally, each of the embodiments may include the voids  34 , as described in conjunction with the first embodiment, to help in the formation of the liner. 
     The method for implanting the acetabular prosthesis  10  will now be described with reference to FIGS. 9 and 9 a . It will be understood that the method for implanting the other preferred embodiments of the acetabular prosthesis disclosed herein will also follow a similar procedure. It is also understood that the embodiments described above may be varied only slightly to create other orthopedic prosthesis such as knee, shoulder, wrist, hand, neck, or other joints, particularly any articulating joints. These variations, however, do not remove them from the breadth of the present invention. 
     It will be understood that while the acetabular prosthesis  10  disclosed herein are discussed in engaging the acetabulum or any region of the acetabulum, these components may engage just the acetabulum or any region of the acetabulum in the surrounding pelvis such as the ilium, pubis, and ischium or engaging the other bone anatomy of the patient. Once an x-ray has been taken of the hip or hip prosthesis that is to be replaced, a suitably sized acetabular prosthesis  10  is selected. Once the proper acetabular prosthesis  10  is chosen, a suitably sized hip prosthesis (shown in FIG. 10) to fit within the intramaduliary canal of a host femur is also chosen. The hip prosthesis may include many different types of hip prosthesis generally known in the art and available to physicians. Once all the proper prosthesis have been chosen, a surgical incision is made and the hip joint is dislocated to expose the acetabulum  430 . The head of the femur may also be resected if a hip prosthesis is to be implanted into the femur. 
     Once the acetabulum  430  has been exposed, it may be necessary to remove degenerated bone, cartilage, or, if performing a revisionary prosthesis, the cement of the previous acetabular cup may need to be reamed out. In this case, a reamer  432  driven by a motor  434  is used to remove the degenerated bone cartilage or other material from the acetabulum  430 . Furthermore, the reamer  432  insures that the acetabulum  430 , which is to receive that acetabular prosthesis  10 , is the proper form and shape to receive the acetabular prosthesis  10 . 
     Once the acetabulum  430  has been prepared by the reamer  432 , the acetabular prosthesis  10  is implanted. The acetabular prosthesis  10  may include bores  26  to position the tool and provide additional support for impacting the acetabular prosthesis  10  into the acetabulum  430 . Additionally the impacter  28  may include one or more guide legs  436  which engage the pelvis or other landmarks of the patient to insure proper alignment of the impacter  28  and thereby of the acetabular prosthesis  10 . Once the proper alignment is determined and checked using the guide legs  436 , the impacter tool  28  is driven down into the acetabular prosthesis  10  thereby setting the acetabular prosthesis  10  into the acetabulum  430 . Once the acetabular prosthesis  10  is impacted into the acetabulum  430 , it is held in place by friction or other cementing materials which were placed in the acetabulum  430  after being prepared by the reamer  432 . 
     With reference to FIG. 10, a fully implanted acetabular prosthesis  10  is shown implanted into the acetabulum  430 . Additionally, a femur  438  is shown to include a hip prosthesis  440  which when returned to the acetabulum  430  includes a ball joint  442  which rides within the liner  14  of the acetabular prosthesis  10 . It is understood that the head  442  of the hip prosthesis  440  rides within the liner  14  of the acetabular prosthesis  10 . 
     The non-porous region of the present invention provides an impenetrable barrier to foreign materials. In particular, after implantation of the acetabular prosthesis according to the present invention, foreign materials would not be able to flow into the acetabular prosthesis. Additionally, since the inner liner is interdigitated into the shell, no internal motion may occur of the liner. Therefore, the liner does not wear as quickly as a liner which is not held as firmly in place. In particular, the liner is affixed to the inner porous region of the shell through hundreds of digits which have interdigitated into the pores. Since this is the case, the liner is held in place at nearly every point along the interface with the inner porous region. Additionally, since the liner is not allowed to move, there is also less of a chance that foreign material would make its way into the acetabular prosthesis post operatively. Therefore, wear is reduced and integrity of the prosthesis is heightened. It is to be understood, however, that any porous substance may achieve these results. As an example, a shell formed of titanium may have pores formed into it which would allow interdigitation of the liner. 
     The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.