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The Journal of Bone and Joint Surgery 78:755-66 (1996)
© 1996 The Journal of Bone and Joint Surgery, Inc.

The Porous-Coated Anatomic Total Hip Prosthesis: Failure of the Metal-Backed Acetabular Component*

DONNA J. ASTION, M.D.{dagger}, NEW YORK, PAUL SALUAN, M.D.{ddagger}, BERNARD N. STULBERG, M.D.§, CLEVELAND, OHIO, CLARE M. RIMNAC, PHD¶ and STEPHEN LI, PHD¶, NEW YORK, N.Y.

Investigation performed at The Hospital for Special Surgery, New York City; University Hospitals of Cleveland, Case Western Reserve University, The Cleveland Clinic Foundation, and the Cleveland Center for Joint Reconstruction, Cleveland; and Northwestern University Medical School, Chicago


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 References
 
One hundred and ninety-nine total hip arthroplasties were performed, between 1983 and 1987, in 173 patients by three surgeons using the initial design of the porous-coated anatomic prosthesis. The acetabular component was a preassembled, metal-backed polyethylene device, with beads sintered to the metal backing to allow bone ingrowth and two pegs for initial fixation. Twenty-three acetabular components (12 per cent) failed because of either migration or severe osteolysis. The radiographic appearance of osteolysis was positively associated with the duration that the implant had been in situ (p < 0.001). The prevalence of osteolysis was also significantly greater in acetabular components with an outer diameter of fifty-five millimeters or less (a polyethylene thickness of 8.5 millimeters or less) (p = 0.03). Thirteen hips were revised at a mean of 69.5 months (range, thirty-three to ninety-one months) after the index operation. Examination of the retrieved acetabular components revealed extensive polyethylene damage on the articular and back surfaces of the liners. Cracks in the polyethylene rim of the liner and deformation of the anti-rotation notch in the polyethylene rim were common findings. The density of the polyethylene was greater than expected, and more particles than anticipated had not fused with the surrounding polyethylene. The results of this study suggest that factors related to both the design and the material contributed to the failure of these porous-coated anatomic acetabular components. CLINICAL RELEVANCE: Patients who have had a total hip arthroplasty with the initial design of the porous-coated anatomic acetabular component should be closely monitored clinically for the onset of pain and radiographically for the development of osteolytic lesions or migration of the component. These findings are associated with impending failure of the component and may warrant revision of this portion of the hip replacement.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 References
 
The use of metal-backed acetabular components for total hip arthroplasty was first recommended by Harris and Penenberg as a means of more uniformly distributing load to the cement mantle of the acetabulum15. It was hypothesized that this would increase the long-term fixation of acetabular components inserted with cement. The advent of fixation without cement through the use of porous-coated metallic interfaces extended this concept as a means of allowing acetabular fixation through bone ingrowth while avoiding direct apposition of ultra-high molecular weight polyethylene (hereafter referred to as polyethylene) to host bone. Direct apposition of bone to polyethylene, as occurs with the Morscher all-polyethylene design of the acetabular component (Isoelastic Acetabulum; Surgical Instruments, Bettlach, Switzerland), was unsuccessful because of severe abrasion of the polyethylene by bone and resultant osteolysis30,31.

Polyethylene debris from damage and wear of total joint components elicits deleterious biological reactions, including bone resorption38. Factors related to both the component and the patient are believed to influence polyethylene wear. The factors related to the component include the thickness of the polyethylene, the diameter of the femoral head, the material properties of the polyethylene, and the fit of the polyethylene liner into the metal backing4,23,26,29,41. The factors related to the patient include weight and activity level20,41.

The initial design of the porous-coated anatomic total hip prosthesis (Howmedica, Rutherford, New Jersey) was among the first to allow fixation without cement by means of bone ingrowth, and it was popular for primary total hip arthroplasty. The acetabular component was a preassembled, cobalt-chromium-alloy, metal-backed polyethylene device, with beads sintered to the metal backing to allow bone ingrowth and two pegs for initial fixation. The polyethylene liner was held in place by a central peg, which fit into a corresponding hole in the metal backing. A polyethylene rim extended past the rim of the metal backing. Rotation of the polyethylene liner in the metal backing was prevented by a tab on the metal rim, which corresponded to an anti-rotation notch on the polyethylene rim (Fig. 1). The porous-coated anatomic total hip replacement was first commercially available for implantation in 1983.



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  		Fig. 1 Views of the initial design of the porous-coated anatomic acetabular component (Howmedica, Rutherford, New Jersey). The polyethylene liner is held in place by a central peg of polyethylene (A), which fits into a corresponding hole in the metal backing (C). Rotation of the liner is prevented by a tab on the metal rim, which corresponds to an anti-rotation notch on the polyethylene rim (B).

 
We prospectively followed a series of patients who had received a porous-coated anatomic total hip prosthesis between November 1983 and December 1987. From this series, we studied a subset of patients who had had a primary total hip replacement with the initial design of the porous-coated anatomic total hip prosthesis. At the time of this study, there was a 12 per cent rate of failure (twenty-three of 199 hips) of the acetabular component. No other studies of which we are aware have documented such a high rate of failure for other designs of metal-backed acetabular components25,35-37.

This high rate of failure of the porous-coated anatomic acetabular component prompted us to perform the present study. Our experience with the porous-coated anatomic femoral component was not different from that reported previously by Kim and Kim18. The objective of the current study was to evaluate the clinical failures associated with the acetabular component and to discover the reasons for the failures through analysis of implants retrieved at the time of revision.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 References
 
We reviewed the experience of three surgeons at two tertiary-care centers where 338 primary porous-coated anatomic acetabular components of the initial design had been implanted in 300 patients between November 1983 and December 1987. Fourteen hips were excluded from the study because the acetabular component had been placed in conjunction with a structural bone graft, and thirty-one were excluded because a femoral component other than one of the porous-coated anatomic design had been used. Only hips in which the femoral component was clinically and radiographically stable were included. Two hundred and ninety-three hips (258 patients) met the criteria for inclusion. Ninety-three hips (eighty-four patients) were lost to follow-up, and one hip was in a patient who had died. Thus, 199 hips (68 per cent of those eligible) (173 patients) were available for evaluation. Patients who were operated on after 1985 were also part of an investigational device exemption.

There were eighty-seven women and eighty-six men; the mean age (and standard deviation) was 58 ± 14 years (range, nineteen to eighty-eight years). The mean duration of follow-up was 58 ± 18 months (range, nineteen to ninety-four months). There were 107 right hips and ninety-two left hips. The femoral component had been inserted without cement in 167 (84 per cent) of the 199 hips. The diagnosis at the time of the operation was degenerative osteoarthrosis in 127 hips (64 per cent), osteonecrosis in thirty-seven (19 per cent), and rheumatoid arthritis in nine (5 per cent); for the remaining twenty-six hips (13 per cent), the diagnoses included juvenile rheumatoid arthritis, mixed connective-tissue disorders, and post-traumatic osteoarthrosis.

Radiographic Analysis
Serial radiographs of all patients were evaluated with respect to the fixation and performance of the porous-coated anatomic acetabular component. The initial postoperative radiograph was compared with those made six months postoperatively and each year postoperatively. Fixation of the component was assessed by determining its migration over time. With use of the radiographs made immediately postoperatively, the performance of the component was assessed by determining the wear of the polyethylene liner, as indicated by the presence or progression of radiolucent lines about the component. The clinical symptoms associated with migration and wear of the component were also noted.

Migration of the acetabular component was determined radiographically with respect to changes in the horizontal angle, the vertical height, and the horizontal distance of the cup, as defined by Wixson et al.40. Vertical or horizontal migration of more than two millimeters or an increase in the horizontal angle of more than 2 degrees was considered an indicator of movement. Repeated measurements by the same one of us (P.S.) did not reliably identify differences of less than two millimeters or less than 2 degrees.

Radiographic evidence of polyethylene wear was assessed by noting the presence or progression of radiolucent lines about the acetabular component, in the six zones described by Davey and Harris and modified by Wixson et al.40. This method allows separate evaluation of the area of the pegs as well as the sections adjacent to the central hole in the metal backing. The time that the radiolucent lines appeared was recorded for each radiographic zone. Finally, as the degree of polyethylene wear may be related to the size of the femoral head and the corresponding outer diameter of the acetabular component, these two measurements were noted.

Descriptive statistics were expressed as frequency counts and percentages for discrete clinical and radiographic data and as the mean, median, and standard deviation for continuous measures. Time-to-event analyses were performed with the Kaplan-Meier method17 to estimate the cumulative prevalence of osteolysis over the course of the follow-up. Ninety-five per cent confidence intervals were calculated at several time-points to express the precision of the estimates.

Retrieval Analysis
Of the 199 acetabular implants, thirteen had been revised (Table I) and ten had a revision pending because of failure. Eight of the revised components were available for analysis after retrieval. Of the five remaining components, one had been returned to the manufacturer, two were not located, and two were retrieved during a revision performed after the retrieval-analysis portion of the study had been completed. Of the eight components that were analyzed, three had been obtained from men and five, from women. The mean age of the thirteen patients in whom the acetabular component had been revised was 44 ± 15.2 years (range, twenty-four to sixty-seven years), and the mean weight was 74.2 ± 14.2 kilograms (range, 54.5 ± 100.0 kilograms). The mean duration that the implant had been in situ was 69.5 ± 20.5 months (range, thirty-three to ninety-one months). The acetabular component failed because of migration in five hips and because of osteolysis (believed to be due to wear of the polyethylene liner) in eight.


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  		TABLE I DATA ON THE THIRTEEN PATIENTS FROM WHOM THE ACETABULAR COMPONENT WAS RETRIEVED

 
The orientation of the polyethylene liner in the metal backing was known for all of the retrieved components since the liner fit into the metal backing in only one orientation. The orientation of the polyethylene liner in the patient was determined radiographically according to the position of the component.

Six polyethylene liners were examined visually and with a light microscope at a magnification of ten times. Both the articular surface and the surface adjacent to the metal backing were assessed. Gross examination included inspection of the rim for cracks and of the articular surface for evidence of machine marks. Machine marks indicated that the articular surface had not been heat-pressed (a process that results in a smooth, shiny surface finish of the polyethylene) after the polyethylene liner had been machined41. The anti-rotation notch in the polyethylene rim was inspected grossly for evidence of deformation. In addition, the metal backing and the polyethylene liner were reassembled to assess qualitatively the amount of motion between the liner and the metal backing when a rotary motion was applied manually.

The articular and back surfaces of the polyethylene liner were divided into quadrants (superior, inferior, anterior, and posterior), and each quadrant was rated with respect to damage (surface deformation, metal debris, pitting, scratching, burnishing, delamination, and abrasion) with a modification of the damage-scoring system described by Hood et al.16. The original damage-scoring system included an evaluation of polymethylmethacrylate debris but, since all of the retrieved components in the current study were matched with porous-coated anatomic femoral components that had been inserted without cement, metal debris was substituted for polymethylmethacrylate debris. A 0 to 3-point scale was used to describe each mode of damage. A score of 0 meant no damage; 1, damage of less than 10 per cent of the surface; 2, damage of 10 to 50 per cent; and 3, damage of more than 50 per cent. Within each quadrant, the scores for the modes of damage were added to determine the total score for that quadrant. Thus, the total damage score for each quadrant could range from 0 to 21 points and the total score for the surface could be a maximum of 84 points. With this additive scoring system, the damage scores were considered to be continuous and, therefore, parametric statistics were used. For comparative statistics, a p value of 0.01 was chosen for significance.

The extent of oxidative degradation of the polyethylene in seven of the eight components was determined by density-versus-depth measurements from a core taken through the area of the central peg. Sequential segments of polyethylene (approximately 200 micrometers thick) were sectioned from this core with a microtome, starting from the articular surface and extending to a depth of about three millimeters. The density of each section was measured in a density-gradient column of isopropanol and water1.

Two randomly selected polyethylene liners were examined to evaluate the quality of the material in terms of the distribution and abundance of unconsolidated polyethylene particles. The liners were cut into quarters perpendicular to the articular surface. Sections, 100 to 200 micrometers thick, were made from each quarter with a Reichert-Jung microtome (Cambridge Instruments, Buffalo, New York). The number and size of the defects were determined with use of a color video camera (Sony CCD/RGB DXC-151, Montvale, New Jersey) attached to a light microscope with phase-fluorescence optics. Images were analyzed with use of Image-I software (Universal Imaging Systems, Westchester, Pennsylvania). Each section was visually divided into six zones, and each zone was analyzed individually at a magnification of thirty-five times. Each defect was manually identified and highlighted. The image-analysis software then determined the number of defects highlighted, the size and mean size of the defects, and the percentage of the total area of the scan covered by defects. For each zone, the number of defects and the mean percentage of the area covered by defects were determined.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
References
 
The patients who had a revision because of migration of the acetabular component had at least one distinctive symptom. When arising from a seated position, some patients initially had pain, whereas others needed to wait for a period of time until they felt that the hip was steady enough for them to begin walking. These patients also had slowly progressive pain on weight-bearing. The patients who had a revision because of osteolysis about the acetabular component had a sudden onset of clicking in the hip or pain on rising from a seated position, after a number of years of successful function.

Radiographic Results
Radiographically, a change in the horizontal angle of the cup was noted in thirty (15 per cent) of the 199 hips; in some hips, a vertical orientation was ultimately achieved (Figs. 2-A and 2-B). Of the thirteen components that had been revised, all five that had radiographic evidence of migration were found to be grossly loose intraoperatively, with no evidence of bone growth into the metal backing. The polyethylene liner and metal backing did not dissociate in any of these components. The eight components that did not have radiographic evidence of migration were not loose at the time of the revision.



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  		Figs. 2-A and 2-B: Radiographs of a fifty-six-year-old woman who had a total hip arthroplasty without cement on the left side for post-traumatic osteoarthrosis in January 1984; there were no complications. Fig. 2-A: Radiograph made three months postoperatively, showing an abduction angle of 57 degrees.

 


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  		Fig. 2-B Nine years after the index operation, the patient began to have pain in the hip that limited her activity. This radiograph, made in September 1994, shows that the abduction angle had increased to 105 degrees and there was an acetabular bone deficiency.

 
Osteolysis about the acetabular component often started as a radiolucent line in zone II that progressed to extensive osteolysis; this occurred in thirty-one (16 per cent) of the 199 hips (Fig. 3). Of the thirteen components that had been revised, eight had radiographic evidence of osteolysis behind the acetabular component before the onset of the symptoms. The position of the component was unchanged on the radiographs. At the time of the revision, wear of the polyethylene liner and rotational instability of the liner within the metal shell were observed. However, the metal-to-bone interface was believed to be mechanically stable.



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  		Fig. 3 Radiograph of a fifty-two-year-old woman, made approximately ninety-one months after a total hip replacement for acetabular dysplasia. There is a large osteolytic lesion (arrow) behind the acetabular component. She subsequently had a revision operation to replace this component.

 
Two hips had muscular weakness and pain that progressed from mild to severe over a short period of time. This was accompanied by a rapid deterioration of function, manifested by pain when starting to move, catching, and grating. Radiographically, eccentric wear of the cup was observed with osteolysis of the acetabulum. In one of these patients, a tumor-like lesion of the pelvis was identified by computerized tomographic scanning and arthrography. At the time of the revision, there was gross fracture of the polyethylene rim, dissociation of the polyethylene liner from the metal backing, and metallic debris from metal-on-metal wear. The tumor-like lesion was apparently due to osteolysis; histological analysis demonstrated that the lesion was filled with histiocytes, metallic debris, and polyethylene debris. In both patients, the implant consisted of a thirty-two-millimeter femoral head and an acetabular component with an outer diameter of forty-six millimeters.

Osteolysis was observed around twenty-five (22 per cent) of the 116 acetabular components with an outer diameter of fifty-five millimeters or less (a polyethylene thickness of 8.5 millimeters or less) and around two (6 per cent) of the thirty-one larger components that were used with a thirty-two-millimeter femoral head; the difference was significant (p = 0.03) (Fig. 4). This difference increased markedly between the sixth and seventh years after implantation (Fig. 5). The radiographic appearance of osteolysis was positively associated with the duration that the implant had been in situ (p < 0.001).



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  		Fig. 4 Graph showing the cumulative rate of osteolysis of the acetabular component over time as a function of the size of the cup when a thirty-two-millimeter femoral head was used (147 hips). The rate of osteolysis for cups with an outer diameter of fifty-five millimeters or less is significantly greater than that for the larger cups (p = 0.03).

 


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  		Fig. 5 Graph showing the cumulative rate of osteolysis (with 95 per cent confidence intervals) of the acetabular component for the entire series (199 hips). There is a significant increase in the rate of osteolysis after five years (p < 0.001).

 

Retrieval Analysis
Gross inspection of the metal backings revealed burnishing on the surface adjacent to the polyethylene liner. The retrieved polyethylene liners demonstrated a slightly eccentric appearance of the articular surface, with the major axis along the superior-inferior plane. The lip of the articular surface was grossly worn in six of the eight components, and in one the rim adjacent to the worn area was fractured and displaced (Fig. 6). Machine marks were seen on the articular and back surfaces of all eight polyethylene liners, indicating that these components had not undergone heat-pressing after machining. Defects in the polyethylene were visible as light patches on the surface of five components. These types of defects have been reported previously and have been identified as polyethylene particles that have not consolidated or fused adequately with the surrounding particles2,21.



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  		Fig. 6 Photograph of an acetabular liner, showing that the superior part of the rim was completely fractured off at the time of the revision operation.

 
Cracks were observed visually on the rim of seven of the eight components. These cracks were present around at least 50 per cent of the periphery of each cup. The concentration of the cracks varied along the circumference of the rim of each component; however, most cracks were located in the part of the rim adjacent to the weight-bearing areas corresponding to the area of greatest articular wear (as determined with the damage-scoring system16). Adjacent to the nonweight-bearing areas, the cracks were seen at the periphery and extended partially across the rim. Adjacent to the weight-bearing areas, the cracks were seen across the entire rim. In one component, the back surface of the polyethylene liner also had numerous cracks, which appeared to extend from the rim.

The walls of the anti-rotation notches in the polyethylene rim demonstrated deformation with multiple short cracks and a whiter color of the polyethylene (a consequence of the multiple small cracks in the polyethylene). When the polyethylene liner was reinserted into the metal backing, gross rotational motion of the liner in the backing was easily produced.

The scores for several of the modes of damage to the articular and back surfaces of the polyethylene liner were significantly different from zero (Table II). These included the scores for scratching, burnishing, and pitting on the articular surface (p < 0.01) and the score for burnishing on the back surface (p < 0.01). None of the total damage scores for any quadrant on either surface of the liner were significantly different from zero (Table III).


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  		TABLE II DAMAGE SCORES ACCORDING TO MODE16*

 

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  		TABLE III DAMAGE SCORES ACCORDING TO QUADRANT16*

 
The mean total damage score was 26.7 ± 3.8 points for the articular surface and 12.8 ± 3.2 points for the back surface (Table II). Although the sample size was small, certain significant correlations were found. The damage score for the articular surface was positively correlated with the damage score for the back surface (r2 = 87.5, p < 0.006). However, there was no significant difference between the total damage scores for the liners that failed because of migration of the acetabular component (24 ± 10.6 points) and those for the liners that failed because of wear (29 ± 14.6 points).

The density-versus-depth profiles of the articulating surface of seven components demonstrated no significant difference between the liners that failed clinically because of migration and those that failed because of osteolysis (Fig. 7). The density values ranged from 0.940 gram per cubic centimeter (a typical value for ultra-high molecular weight polyethylene) to as high as 0.967 gram per cubic centimeter (values normally associated with high-density polyethylene). The density values for all of these acetabular liners were greater than the maximum density recommended by the American Society for Testing and Materials for fabricated ultra-high molecular weight polyethylene2. In each case, the density was greater near the articulating surface than in the bulk of the liner.



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  		Fig. 7 Graph showing the density of the polyethylene as a function of depth from the articular surface for seven retrieved acetabular liners. The profile of each liner is depicted as a separate line with a distinct symbol.

 
Light microscopy of two liners revealed that the polyethylene was inhomogeneous, as evidenced by alternating light and dark bands extending from the back surface to the articular surface as well as by multiple unconsolidated polyethylene particles throughout the thickness of the liner (Fig. 8). The dark bands were found to be concentrated areas of unconsolidated polyethylene particles, and they followed the contours of the components at a depth of approximately one millimeter. In regions outside of the dark bands, unconsolidated particles accounted for 1.2 ± 0.84 and 2.3 ± 0.78 per cent of the total area for the two cups. In the dark-banded zones, however, unconsolidated particles accounted for 3.6 ± 0.60 per cent to 15 ± 1.61 per cent of the total cross-sectional area. The mean diameter for the unconsolidated particles was 40 ± 9.5 micrometers (range, eighteen to 110 micrometers).



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  		Fig. 8 Light photomicrographs of sections from two acetabular liners. The small circles within the material are unconsolidated particles of polyethylene. The dark bands near the outer and inner surfaces of the components are regions of concentrated unconsolidated particles (x 71).

 

Discussion
The primary purpose of this study was to evaluate a large series of patients who had received a porous-coated anatomic acetabular implant and to attempt to understand the reasons for the high rate of failure of this device. Another objective was to alert physicians to the high rate of failure and to the clinical and radiographic presentations of such failure.

The extensive and progressive osteolysis seen about the acetabular components was most likely the result of the biological reaction to particulate debris generated from polyethylene wear. Analysis of the retrieved liners demonstrated wear of both the back surface (adjacent to the metal backing) and the articular surface. The deformation seen at the anti-rotation notch in the polyethylene rim, and the motion observed between the liner and the metal backing when the two were reassembled, suggested that movement between the liner and the metal backing occurred in situ. Thus, this anti-rotation mechanism, which was intended to prevent rotation of the polyethylene liner in the metal backing, appeared to be inadequate. The presence of fine metallic debris on the back surface of five of the six polyethylene liners demonstrated that fine metallic debris was also generated by this relative motion. Thus, compared with polyethylene acetabular components inserted with cement, metal-backed acetabular components provide an additional surface for the generation of polyethylene debris. Furthermore, the presence of holes in the metal backing may allow direct access of the debris to the bone. Osteolysis about the component can then occur by a mechanism similar to that postulated by Schmalzried et al. for acetabular components inserted with cement38. In the current study, the debris from this additional surface may have contributed to the osteolysis.

The finding that osteolysis was significantly more prevalent around acetabular components with a polyethylene liner that was 8.5 millimeters thick or less (an outer diameter of the acetabular component of fifty-five millimeters or less) (p = 0.03) and the finding that the radiographic appearance of osteolysis was positively associated with the duration of the implant in situ (p < 0.001) suggest that the most severe problem with respect to the porous-coated anatomic acetabular component is wear of the polyethylene liner. Thus, it can be expected that the prevalence of osteolysis about porous-coated anatomic components will continue to increase with time. We saw the prevalence rise rapidly in our patients after the implant had been in place for seventy-two months. By eighty-four months, more than 50 per cent of our patients had evidence of osteolysis at the acetabular interface (Fig. 5).

In addition to the resulting thinner polyethylene, the large (thirty-two-millimeter) diameter of the femoral head of many of the porous-coated anatomic total hip prostheses may have accelerated the generation of polyethylene debris. In a large series of total hip arthroplasties in which femoral heads of three different diameters were compared, there was a higher rate of revision of the acetabular component (2.7 per cent; thirteen of 487 hips) when a femoral component with a thirty-two-millimeter head had been used than when a femoral component with either a twenty-two or a twenty-eight-millimeter head had been used (approximately 1 per cent [forty-four of 4576 hips] and approximately 0.5 per cent [two of 520 hips], respectively)29. In a study by Livermore et al., radiographic analysis demonstrated greater volumetric wear of the acetabular cup in association with thirty-two-millimeter femoral heads than in association with heads that had a smaller diameter23. This finding is consistent with the larger surface area from which debris can be generated, as compared with that for acetabular components with an inner diameter of twenty-eight or twenty-two millimeters. It is also consistent with the increased range of motion that occurs with the thirty-two-millimeter femoral head as well as the increased sliding distance thought to occur with the thirty-two-millimeter components28. (The sliding distance is the sum of the distances of relative travel between the contact areas of the femoral ball and the polyethylene liner. For a given femoral head, the distances traversed by individual points vary with location. For instance, a point on the equator of the ball travels farther than a point away from the equator in a gait cycle. It can be estimated that, for femoral heads of different diameters, the ratio of corresponding sliding distances is the ratio of the diameters of the heads. Thus, for the same range of motion, the sliding distance for a thirty-two-millimeter head is 1.14 times greater than that for a twenty-eight-millimeter head [thirty-two divided by twenty-eight] and 1.45 times greater than that for a twenty-two-millimeter head [thirty-two divided by twenty-two]).

In the porous-coated anatomic components in the current study, the combination of the thinner polyethylene liner and a thirty-two-millimeter femoral head exacerbated the problem of wear. With a femoral head diameter of thirty-two millimeters, the maximum contact stress in the polyethylene of the acetabular component increases as the thickness of the component decreases3, similar to what has been reported in association with a femoral head diameter of twenty-eight millimeters5. Other stresses that are thought to influence polyethylene wear, such as maximum shear stress and maximum principal stress, generally also increase as the thickness of the polyethylene component decreases. It is therefore not surprising that the increased stress in the thinner polyethylene components articulating with the thirty-two-millimeter femoral head in our study exacerbated the already detrimental factor of increased sliding distance and increased abrasive wear.

Our analysis of the retrieved porous-coated anatomic components demonstrated several important findings. Visual inspection of the surfaces of the polyethylene liner revealed machine marks on the portion of the surface without wear. As stated earlier, the presence of machine marks means that heat-pressing was not used after manufacturing. Therefore, in contrast to the experience with porous-coated anatomic tibial knee components, in which heat-pressing of the polyethylene articular surface contributed to its delamination41, heat-pressing of the polyethylene was not a contributing factor in the failure of these porous-coated anatomic acetabular components.

To our knowledge, the cracks in the rim that were observed on the retrieved porous-coated anatomic acetabular liners are not a common finding. Finite element analysis of the acetabular component suggested that there are large compressive stresses between the rim of the polyethylene and that of the metal backing when the inner surface of the polyethylene liner was loaded to simulate the joint forces at the hip during the single-limb stance phase of gait19. This finding is supported by the greater number of cracks in the rim of the polyethylene liner adjacent to weight-bearing regions. If the compressive stresses between the rim of the polyethylene and that of the metal were sufficiently large to cause plastic deformation, a residual tensile stress state could occur and result in fatigue cracks32.

The increased density seen near the weight-bearing surfaces of the analyzed components was presumably secondary to oxidative degradation35, as these components had not been heat-pressed. Eyerer and Ke13 as well as Bostrom et al.7 showed that, with time, polyethylene components that have been sterilized with gamma radiation undergo an aging process that increases density near the surface. In the study by Bostrom et al., the densities measured within each component were greater than the maximum density of 0.944 gram per cubic centimeter for fabricated polyethylene that was recommended by the American Society for Testing and Materials2 (Fig. 7). Density values of greater than 0.940 gram per cubic centimeter for ultra-high molecular weight polyethylene that has not been specifically processed to provide higher crystallinity (such as that manufactured by Hylamer, DePuy-Dupont Orthopaedics, Warsaw, Indiana) indicate oxidation of the polyethylene22. The increase in the density of the polyethylene from oxidative degradation is worrisome, as there is an associated increase in the elastic modulus19, leading to a more brittle and presumably a less abrasion-resistant material14.

In our experience, the inhomogeneity seen in the polyethylene of the porous-coated anatomic acetabular components, as manifested by numerous randomly distributed and concentrated bands of unconsolidated particles, is not unique, but the levels of unconsolidated particles in these polyethylene liners were higher than those previously noted6,21,27,39. (Examinations with use of this method have revealed polyethylene liners of other designs of components in which no unconsolidated particles were detectable.) The quantitative contribution of these defects is not clear at this time, although the high concentration of defects in the banded zones could be expected to be detrimental to the mechanical properties of the polyethylene in these regions. This detrimental effect was suggested in a study of retrieved Charnley acetabular cups in which there was a 100 per cent rate of correspondence between the presence of banded defects and fracture of the polyethylene component9. Thus, material factors, such as oxidative degradation and unconsolidated particles, probably contributed to the early failures of the components in our study.

In our study, 12 per cent (twenty-three) of the 199 acetabular components failed. Other authors have reported failure of the initial design of the porous-coated anatomic acetabular component. Brien et al.8 and Ries et al.34 described four and three patients, respectively, in whom the polyethylene liner of the one-piece porous-coated anatomic acetabular component dissociated from the metal backing. Ries et al. also noted that the central peg and the polyethylene rim fractured. Cooper et al. reported on four patients who had received a porous-coated anatomic total hip prosthesis of the initial design and who had radiographic evidence of large osteolytic areas in the acetabulum and the proximal part of the femur11. The rates of wear of the components in that study were two to four times higher than those described by Livermore et al.23 in association with thirty-two-millimeter femoral heads. All of the patients of Cooper et al. had received a thirty-two-millimeter femoral head and an acetabular component with an outer diameter of fifty-five millimeters or less (a polyethylene thickness of 8.5 millimeters or less).

McCoy et al., using radiographic measurements and calculations, found measurable polyethylene wear (a mean rate of 1.85 millimeters) after twenty-seven (84 per cent) of thirty-two Charnley total hip replacements24. The mean age of their patients was sixty years, and the duration of follow-up was more than ten years. Survival analysis revealed a 96 per cent rate of survival of the acetabular component at fifteen years24. In other reports on total hip arthroplasty with use of cement in younger patients (mean age, 30.5 to 53.8 years), the rates of revision of the acetabular component were 2 per cent (one of forty-five hips)10, 6 per cent (six of 100 hips)12, and 3 per cent (three of 103 hips)33. The performance of the porous-coated anatomic acetabular component (a 12 per cent rate of failure in our series) falls short of the experience with acetabular components inserted with cement in older24 and comparable younger patient populations10,12,33.

The results of the current study suggest that patients who have had a total hip arthroplasty with the initial design of the porous-coated anatomic acetabular component should be closely monitored clinically for the onset of pain and radiographically for the development of osteolytic lesions or migration of the component. These clinical and radiographic findings may be associated with failure of the acetabular component and may indicate the need for revision. Our data also suggest that the performance of all designs of metal-backed acetabular components should be followed over time, as several features related to the design and the material of the initial design are generic. These features include thin polyethylene in conjunction with a thirty-two-millimeter femoral head, a load-bearing polyethylene rim, an insufficient anti-rotation device, polyethylene with multiple fusion defects, and an additional interface between the metal backing and the polyethylene liner at which additional polyethylene debris can be generated.

NOTE: The authors gratefully acknowledge the assistance of Donald Bartel, Ph.D.; Kirk Easley, M.S.; Mark Froimson, M.D.; Robert Klein, B.S.; S. David Stulberg, M.D.; and Richard L. Wixson, M.D.


   Footnotes
 
*No benefits in any form have been received or will be received from a commercial party related directly or indirectly to the subject of this article. Funds were received in total or partial support of the research or clinical study presented in this article. The funding sources were National Institutes of Health Grants AR 38905, AR 40191, and AR 01876 and the Clark Foundation.

{dagger}The Hospital for Joint Diseases, 301 East 17th Street, New York, N.Y. 10003.

{ddagger}Department of Orthopaedic Surgery, The Cleveland Clinic Foundation, 9500 Euclid Avenue, Cleveland, Ohio 44195.

§Cleveland Center for Joint Reconstruction, St. Vincent Charity Hospital, 2322 East 22nd Street, Cleveland, Ohio 44115.

¶Department of Biomechanics, The Hospital for Special Surgery, 535 East 71st Street, New York, N.Y. 10021.


   References
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 References
 

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