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© 1998 The Journal of Bone and Joint Surgery, Inc. Fixation of Acetabular Cups without Cement in Total Hip Arthroplasty. A Comparison of Three Different Implant Surfaces at a Minimum Duration of Follow-up of Five Years* , UPPER SADDLE RIVER, NEW JERSEY , INDIANAPOLIS, INDIANA , MOON TOWNSHIP, PENNSYLVANIAInvestigation performed at Indiana University School of Medicine, Indianapolis, and Clinical Measurement Corporation, Upper Saddle River
We evaluated 377 patients (428 hips) who had been managed, by a total of fourteen surgeons at twelve clinical sites in the United States and Europe, with a porous-coated press-fit acetabular cup, a hydroxyapatite-coated threaded screw-in cup, or one of two similar designs of hydroxyapatite-coated press-fit cups between April 1987 and November 1992. The same type of hydroxyapatite-coated femoral stem was inserted without cement in all patients. After a minimum duration of follow-up of five years (mean, 7.9 years; range, 5.3 to 9.1 years), one (1 per cent) of the 131 hydroxyapatite-coated threaded cups, two (2 per cent) of the 109 porous-coated press-fit cups, and twenty-one (11 per cent) of the 188 hydroxyapatite-coated press-fit cups had been revised because of aseptic loosening. A common radiographic sign of impending failure of the hydroxyapatite-coated press-fit cups was radiolucency at the interface between the implant and the subchondral bone beneath it. This radiolucency usually was seen initially more than two years after implantation. Radiographic evaluation of the 383 acetabular implants that were in situ at the time of the most recent follow-up showed that 123 (99 per cent) of the 124 hydroxyapatite-coated threaded cups, 101 (98 per cent) of the 103 porous-coated cups, and 139 (89 per cent) of the 156 hydroxyapatite-coated press-fit cups were stable with osseous ingrowth (as indicated by the absence of radiolucency at the interface and the absence of migration within the acetabulum). The probability of revision due to aseptic loosening was significantly greater for the hydroxyapatite-coated press-fit cups than it was for the hydroxyapatite-coated threaded cups or the porous-coated press-fit cups (p < 0.001 for both comparisons). Within the group of patients who had a hydroxyapatite-coated press-fit cup, the probability of revision due to aseptic loosening was significantly greater in association with a young age (p = 0.003), female gender (p = 0.02), the use of a femoral head with a diameter of thirty-two millimeters (p = 0.018), and the use of a thin polyethylene liner (p < 0.001). We found that the hydroxyapatite-coated threaded cups and the porous-coated press-fit cups continued to perform well more than five years after the operation. The hydroxyapatite-coated press-fit cups that were revised probably failed because the fixation interface beneath the cup could not sustain the tensile stresses that were imposed between the cup and the bone by the activity of the patient. Our data suggest that, in the specific biomechanical environment of the acetabulum, physical interlocking between the cup and the supporting bone beneath it may be a prerequisite for long-term stability.
The goals of acetabular reconstruction are the relief of pain, the restoration of joint function, the preservation of bone stock, and the maintenance of stability of the implant. In series comprising sixty-one to 330 total hip arthroplasties performed with cement, the rate of loosening of the acetabular component has ranged widely, from approximately 1 per cent to as high as 42 per cent8,9,13,14,33,35,37,39,41. The rate of loosening of femoral stems that are inserted with cement has been reported to decrease with time, whereas the converse appears to be true for acetabular components that are inserted with cement18,37,42. Clinical studies of acetabular components that were inserted without cement have yielded mixed results. Authors have previously reported that stable fixation of porous-coated metal-backed cups can be achieved by means of osseous ingrowth, provided that a stable press-fit of the implant is obtained intraoperatively1,20,23,27,33,35,36,46. Porous-coated acetabular shells are now used routinely for total hip replacement in the United States. Clinical studies comprising twenty to 160 patients have demonstrated rates of revision ranging from 0 to approximately 8 per cent after at least five years of follow-up19,28,30,42,45. However, a number of authors have reported failure of fixation due to osteolytic lesions in the bone stock supporting some of these porous-coated devices8,43,47. Threaded cups often have been associated with disappointing clinical results despite the apparent advantage that is provided by interlocking between the metal shell and the bone at the time of implantation. So-called first-generation threaded cups, which relied entirely on the maintenance of the interlocking between the bone and the threads that was achieved at the time of the operation, were associated with early failure of fixation2,3,9-11,21,34,39. Improved clinical results were reported in association with the use of so-called second-generation threaded implants, which had a porous coating for biological fixation as well as threads in the metal shell for mechanical fixation12,17,39. Pupparo and Engh, in a study of 144 patients, compared the clinical performance of threaded cups that had a porous coating with that of threaded cups of identical geometry that did not have such a coating39. Those authors found that the cups without the coating had a greater rate of failure in the first two years after the operation than did their porous-coated counterparts. They suggested that biological fixation as well as the intraoperative stability provided by the threads is necessary for the satisfactory clinical performance of threaded acetabular implants. We compared the clinical performance of three different types of implant surfaces used for acetabular components of similar geometry that were designed to be inserted without cement. All patients who were enrolled in the study had been followed for at least five years, and all had received the same type of femoral component. Thus, any measurable differences in the clinical or radiographic performance of the different acetabular components may be attributed to the different types of implant surfaces. The purpose of this study was to provide insight into the type of implant surface that may lead to long-term stable fixation of acetabular components inserted without cement during total hip arthroplasty.
Fourteen surgeons at twelve different clinical sites in the United States and Europe performed 464 total hip replacements in 409 patients between April 1987 and November 1992. By the time of the present study, 377 patients (428 hips) had been followed for at least five years. Of the thirty-two patients who were not included in the study, thirteen (fifteen hips) had been lost to follow-up, eleven (thirteen hips) had died before five years, and eight (eight hips) had had revision of the femoral stem before five years. The eight femoral revisions had been performed for reasons that were unrelated to failure of the acetabular component. Specifically, three femoral components had been revised because of pain in the thigh; two, because of infection; two, because of a postoperative fracture distal to the stem; and one, because of aseptic loosening. All patients received the same type of femoral component (Omnifit-HA; Osteonics, Allendale, New Jersey) and one of three main types of acetabular components (all made by Osteonics). The acetabular component for each patient was selected according to the preference of the operating surgeon. We divided the patients into groups depending on the type of acetabular component that was used. A porous-coated press-fit cup was used in 102 patients (109 hips); a hydroxyapatite-coated press-fit cup, in 168 patients (188 hips); and a hydroxyapatite-coated threaded cup, in 107 patients (131 hips). All acetabular components were made of commercially pure titanium and had a similar geometry, which was designed to load the periphery of the prepared acetabulum, and all had the same type of polyethylene liner and liner-locking mechanism. The surface of the porous-coated press-fit cups (Dual-Geometry Microstructured Cup) was designed to allow bone to grow into a double layer of titanium beads, which had a nominal diameter of 425 to 500 micrometers (Fig. 1, a). The conical peripheral geometry of these components was designed to preferentially load the periphery of the prepared acetabulum. Before the cup was inserted, the bone was prepared with a hemispherical reamer for the dome of the implant and then with a conical reamer for the periphery of the acetabulum.
Two similar types of hydroxyapatite-coated press-fit components were used. The first type (Dual-Geometry HA Cup; ninety patients [ninety-six hips]) had the same conical peripheral shape as the porous-coated cups, whereas the second type (Dual-Radius HA Cup; seventy-eight patients [ninety-two hips]) had modification of the straight edge of the peripheral cone into a large peripheral radius to eliminate the need for conical reaming. Both types of implants were designed to peripherally load the acetabulum in a manner similar to that of the porous-coated cups. To enhance the peripheral interference fit that was achieved intraoperatively, the peripheral interface of the cup was machine-knurled to a depth of 200 micrometers in order to provide a roughened surface (Fig. 1, b) and the diameter across the opening of the cup was oversized by one millimeter compared with that of the matching reamer. It was expected that long-term stability of these cups would be achieved by means of the biological bond that has been reported to develop between the coating and the bone4,12,17,23,24,31. The initial stabilization of these implants and that of the porous-coated cups were enhanced, at the discretion of the surgeon, by additional underreaming of the acetabulum or with use of bone screws. The hydroxyapatite-coated threaded screw-in cups (Omnifit Threaded HA Cup) were ring-shaped and had a large central hole in the dome. The overall peripheral profile was the same as that of the other cups, but the periphery was threaded and the implant was screwed into the prepared acetabulum (Fig. 1, c). Immediate stability was achieved by means of interlocking between the bone and the threads. The hydroxyapatite coating was expected to enhance the interlocking by promoting attachment of the bone to the implant over time. All of the hydroxyapatite-coated cups in the present study had a fifty-micrometer-thick coating that consisted of more than 90 per cent hydroxyapatite. The coating had been plasma-sprayed7 onto the titanium substrate, which had been roughened previously by bead-blasting to a surface finish of three to four micrometers. Each patient was assessed preoperatively, in the early postoperative period (at five to seven weeks), at six months, at one year, and yearly thereafter. The clinical evaluation of each patient was performed by the operating surgeon with use of the Harris hip-scoring system26. Anteroposterior and lateral radiographs of the involved hip or hips were made at each visit. The anteroposterior radiographs were evaluated twice each year by three of us (W. N. C., J. A. D'A., and M. T. M.) who were blinded with regard to the corresponding clinical data. Three zones, based on those described by DeLee and Charnley16, were delineated so that apparent radiographic changes could be documented for each patient. The radiographs were evaluated for radiolucent lines, changes in apparent bone density, migration of the cup, and osteolysis. An osteolytic lesion was defined as any enclosed area of reduced bone density, independent of size. Any abnormal findings were confirmed, whenever possible, on the lateral radiographs. Any superior or medial migration of the acetabular component was measured on sequential radiographs. The stability of the acetabular component was determined with use of a modification of the criteria described by Engh et al.18. A component was designated as stable with osseous ingrowth if there were no radiolucent lines extending across 50 per cent or more of any zone and there was no measurable migration of the component, as stable with fibrous ingrowth if radiolucent lines were present in all three zones but the component had not migrated, and as unstable if there were radiolucent lines in all three zones and the cup had migrated three millimeters or more. Any migration of the femoral head into the polyethylene liner was evaluated with use of concentric circles to delineate the profiles of the head and the dome of the shell. Wear of the liner was indicated by eccentricity of the circles and was recorded simply as less than two millimeters or as two millimeters or more. The radiographs were digitized with use of a film scanner (VXR-8; Vidar Systems, Herndon, Virginia), and the digitized images were enhanced with use of commercially available software (Imagika; Clinical Measurement Corporation, Upper Saddle River, New Jersey). The enhancement process allowed selected regions of each image to be visually improved with use of edge detection and other algorithms. Visualization of the radiographic features in these selected regions also was improved by linear expansion of the gray scale of the digitized images. When applied retrospectively to a series of radiographs, the image-digitization and enhancement process allowed osseous features, such as reactive lines, to be seen more clearly. The computer program was used to confirm the visual radiographic findings. The performance of each type of implant surface was evaluated according to three indices. The first was the rate of aseptic failure, which was based on the number of components that had been revised because of aseptic loosening. The second was the rate of mechanical failure, which was based on the number of components that had been revised because of aseptic loosening and the number that were in situ but had been judged to be unstable according to radiographic criteria. The third was the combined rate of failure, which was based on the number of components that had failed mechanically and the number that had been revised because of polyethylene wear, osteolysis, or disabling pain. Implants that had been revised because of infection or dislocation were not included in the analysis of failure.
Clinical Results The mean Harris hip score26 for the entire series was 45 points (range, 7 to 81 points) preoperatively, 77 points (range, 38 to 100 points) at the early (five to seven-week) follow-up visit, 94 points (range, 52 to 100 points) at six months, 97 points (range, 53 to 100 points) at two years, and 93 points (range, 43 to 100 points) at five years or more. At the time of the latest follow-up visit, the patients who had a porous-coated press-fit cup had a mean Harris hip score of 92 points (range, 58 to 100 points), those who had a hydroxyapatite-coated press-fit cup had a mean score of 94 points (range, 43 to 100 points), and those who had a hydroxyapatite-coated threaded cup had a mean score of 98 points (range, 62 to 100 points). With the numbers available, the scores for the three groups of patients were not found to be significantly different. At the time of the most recent evaluation, 298 patients (79 per cent) were rated as having an excellent result; thirty-two (8 per cent), as having a good result; twenty-seven (7 per cent), as having a fair result; and twenty (5 per cent), as having a poor result26. Of the twenty patients who had a poor result, six were scheduled to have revision of the cup, four had severe osteolysis behind the cup, two had had recent revision of the cup, one had an unstable cup, and seven had limitation of function or had pain that was unrelated to the involved hip. After the exclusion of the twenty patients who had a poor result, 324 patients (86 per cent of the entire cohort) reported no pain in the hip and thirty-three (9 per cent of the entire cohort) reported pain that may have been related to the acetabular reconstruction. Of the thirty-three patients who had pain that may have been related to the procedure, eleven (3 per cent of the entire population) had pain in the buttock and twenty-two (6 per cent of the entire population) had pain in the groin. The pain in the buttock was classified as mild in eight patients, moderate in two, and marked in one; the most recent radiographs of the latter three patients showed excessive wear of the liner and osteolysis involving the bone supporting the acetabular implant. The pain in the groin was slight-to-mild in fourteen patients and moderate in eight. Of the eight patients who had moderate pain, four were scheduled to have revision of the cup, two had apparent osteolysis behind the cup, one had had revision of the cup, and one had pain of unknown etiology. (The pain in the latter patient subsequently resolved.) There was no apparent relationship between the presence of pain in either the buttock or the groin and the type of acetabular component that had been inserted.
Radiographic Results
An apparent increase in bone density was observed more frequently in zone I (proximal to the cup) than in zone II or III16. These regions of increased density were noted by the second postoperative year. By the fifth year, increased bone density in at least one zone was noted in association with ninety-one (88 per cent) of the 103 porous-coated press-fit cups, eighty-three (67 per cent) of the 124 hydroxyapatite-coated threaded cups, and 103 (66 per cent) of the 156 hydroxyapatite-coated press-fit cups. Increased bone density was noted significantly more often in association with the porous-coated press-fit cups than it was in association with either the hydroxyapatite-coated press-fit cups or the hydroxyapatite-coated threaded cups (p < 0.001 for both comparisons). The evaluation of radiolucent lines at the implant-bone interface revealed that 363 cups (95 per cent) could be classified as stable with osseous ingrowth; six (2 per cent), as stable with fibrous ingrowth; and fourteen (4 per cent), as unstable18. Of the 103 porous-coated components, 101 (98 per cent) were classified as stable with osseous ingrowth (Fig. 2); two (2 per cent), as stable with fibrous ingrowth; and none, as unstable. A review of earlier radiographs revealed that few changes occurred at the interface around the porous-coated cups after the second postoperative year. Of the 156 hydroxyapatite-coated press-fit implants, 139 (89 per cent) were classified as stable with osseous ingrowth (Fig. 3-A); four (3 per cent), as stable with fibrous ingrowth (Fig. 3-B); and thirteen (8 per cent), as unstable. All of the hydroxyapatite-coated press-fit components that were classified as unstable at the most recent evaluation had been classified as stable with osseous ingrowth at an earlier follow-up interval. Of the 124 hydroxyapatite-coated threaded implants, 123 (99 per cent) were classified as stable with osseous ingrowth (Fig. 4) and one (1 per cent) was classified as unstable. (The unstable cup had been inserted during a revision procedure, and it had not been fully seated in the bone at that time.) In general, the hydroxyapatite-coated threaded cups were surrounded by fewer radiolucent lines than were the other types of implants. Computerized magnification and enhancement of selected radiographs showed no signs of undiagnosed radiolucent lines adjacent to the threads; however, small radiolucent lines could have been hidden by the threads. Fewer osteolytic lesions were observed in association with the threaded cups than were observed in association with either the porous-coated cups or the two types of hydroxyapatite-coated press-fit cups (Table I).
Rates of Failure
Eight of the 109 porous-coated press-fit cups were revised. One component was revised because of deep infection and therefore was not included in this analysis. Two components were revised because of aseptic loosening; thus, the rate of aseptic failure was 2 per cent. As no cup was found to be unstable at the time of the most recent radiographic evaluation, the rate of mechanical failure also was 2 per cent. Five components were revised because of clinical failure; specifically, four were revised because of excessive wear of the liner and one was revised because of pain. The combined rate of failure, therefore, was 6 per cent (seven components, including the two that had been revised because of aseptic loosening and the five that had failed clinically). Forty-one of the 188 hydroxyapatite-coated press-fit implants were revised or unstable. The rate of aseptic failure was 11 per cent (twenty-one implants), the rate of mechanical failure was 14 per cent (twenty-seven implants, including the twenty-one that had been revised because of aseptic loosening and an additional six that were classified as unstable at the most recent radiographic evaluation), and the combined rate of failure was 22 per cent (forty-one implants, including the twenty-seven that had failed mechanically, twelve that had been revised because of excessive wear of the liner, and two that had been revised because of pain). Comparison of the two types of hydroxyapatite-coated press-fit cups revealed that twelve Dual-Geometry HA components (13 per cent) had been revised because of aseptic loosening, one was mechanically loose, and eight had failed clinically (for a combined rate of failure of 22 per cent), whereas nine Dual-Radius HA components (10 per cent) had been revised because of aseptic loosening, five were mechanically loose, and six had failed clinically (for a combined rate of failure of 22 per cent). Five of the 131 hydroxyapatite-coated threaded implants were revised. One component was revised because of aseptic loosening, two were revised because of wear, and two were revised for reasons that were unrelated to the fixation (a deep infection in one hip and a dislocation in another). The latter two implants were not included in this analysis. Thus, the rate of aseptic failure was 1 per cent, the rate of mechanical failure was 2 per cent (two implants, including the one that had been revised because of aseptic loosening and one that had been unstable at the time of the most recent evaluation), and the combined rate of failure was 3 per cent (four implants, including the two that had failed mechanically and two others that had been revised because of wear). With the numbers available, no significant differences were found between the Dual-Geometry HA and Dual-Radius HA press-fit cups with regard to the rates of aseptic failure, mechanical failure, or clinical failure. Similarly, no significant differences were detected between the clinical performance of the hydroxyapatite-coated threaded cups and that of the porous-coated press-fit cups. However, compared with the overall group of hydroxyapatite-coated press-fit cups or with either the Dual-Geometry HA Cups or the Dual-Radius HA Cups separately, both the hydroxyapatite-coated threaded cups and the porous-coated press-fit cups had significantly lower rates of aseptic failure, mechanical failure, and clinical failure (p < 0.001 for all comparisons). No association was found between the presence or absence of supplemental fixation screws and any rate of failure for any group. Similarly, no association was found between migration of the femoral head into the polyethylene liner and the type of implant, the presence of osteolytic lesions, or the need for revision of the cup.
We followed the clinical course of 428 acetabular cups that had been implanted by fourteen surgeons in 377 patients; the minimum duration of follow-up for inclusion in the study was five years. All patients had received the same type of femoral stem and one of three main types of acetabular cups; the type of acetabular implant used in each patient depended on the preference of the operating surgeon. The patients were divided into three groups depending on whether they had received a porous-coated press-fit cup, a hydroxyapatite-coated threaded cup, or one of two similar types of hydroxyapatite-coated press-fit cups. The purpose of this study was to determine if significant differences in the clinical and radiographic results could be demonstrated among the three groups. The overall rate of revision because of aseptic loosening was 6 per cent (twenty-four implants); this rate is similar to those that have been reported in the literature6,8,9,13,14,27,30,33,35-40,46. Analyzed according to the type of cup, the rate of revision because of aseptic loosening was 11 per cent (twenty-one of 188) for the hydroxyapatite-coated press-fit cups, 2 per cent (two of 109) for the porous-coated press-fit cups, and 1 per cent (one of 131) for the hydroxyapatite-coated threaded cups. The rate of revision associated with the hydroxyapatite-coated press-fit cups was significantly higher than the rates associated with the porous-coated press-fit cups and the hydroxyapatite-coated threaded cups (p < 0.001 for both comparisons). The clinical and radiographic results associated with the hydroxyapatite-coated press-fit cups were generally unfavorable compared with the results associated with the hydroxyapatite-coated threaded cups and with those that we reported previously in association with hydroxyapatite-coated femoral stems10,15. A review of the manufacturer's records showed that all hydroxyapatite coatings on all implants met the manufacturer's standards for thickness (a nominal value of fifty micrometers), hydroxyapatite content (a minimum of 90 per cent), and overall quality and strength of attachment. Thus, the differences between the results associated with the hydroxyapatite-coated press-fit cups and those associated with the hydroxyapatite-coated threaded cups cannot be explained by differences in the quality of the hydroxyapatite coatings. All of the cups that were used in the present study were made of commercially pure titanium, had the same type of polyethylene liner and liner-locking mechanism, had the same internal geometry, and were designed to load the periphery of the reamed acetabulum. Thus, there were no differences in these design parameters that would account for the difference in performance between the hydroxyapatite-coated press-fit cups and the other two types of implants. The only remaining difference was the specific design of the bone-attachment interfaces of the different implants. Immediate fixation of the hydroxyapatite-coated threaded cups was achieved at the time of the operation by interlocking between the acetabular bone and the threads of the implant. Examination of the few clinically retrieved specimens and autopsy specimens that were available suggested that the hydroxyapatite-coated threaded cups tended to retain or perhaps to conduct bone into the threads over time4,5, in a fashion similar to the bone adaptation seen on examination of necropsy specimens from animals that had a hydroxyapatite-coated implant with surface indentations4,23,31. In the current study, intraoperative fixation of the porous-coated and hydroxyapatite-coated press-fit cups was provided by the hoop and compressive stresses in the bone around the acetabular opening, which was expanded during impaction of the implant. Long-term stability of the porous-coated press-fit cups was expected to be attained by the growth of bone into the coating, whereas that of the hydroxyapatite-coated press-fit cups was expected to be achieved by the osseointegrated bond that develops between the bone and the hydroxyapatite coating as demonstrated in in vivo studies4,12,17,23,24,31. The predominantly stable ingrown interfaces and the low rates of revision associated with the hydroxyapatite-coated threaded cups and the porous-coated press-fit cups suggest that the intended interlocking fixation of these implants in the acetabulum was achieved. The unstable interfaces and the higher rates of revision associated with the hydroxyapatite-coated press-fit cups suggest that the expected fixation of many of these implants was not achieved. It is possible that fixation between the bone and the coating of the hydroxyapatite-coated press-fit cups that were revised had never been achieved and the cups simply migrated. However, radiographs showed a stable interface with osseous ingrowth for the first two to three years postoperatively, before a progressive radiolucent line became evident in the subchondral bone beneath the cup and the fixation began to fail in this region. Finite element analyses of the interface stresses around acetabular implants that have been inserted without cement may help to explain why these implants failed. Numerical models have predicted that, when a smooth metal-backed cup is bonded to bone in the acetabulum, very high local tensile stresses occur at the fixation interface beneath the cup as physiological loads are applied to the construct32,40. It has been suggested that such tensile stresses could lead to failure of fixation beneath the cup, that the continued application of physiological loads will cause motion and distraction between the component and the osseous structures beneath it, and that progressive loosening at the interface and failure of fixation may occur40. The progressive radiolucent lines seen in zone III adjacent to many of the hydroxyapatite-coated press-fit cups that went on to have a revision may be the radiographic manifestation of this predicted mode of failure. Conversely, the lack of radiolucent lines in zone III adjacent to the hydroxyapatite-coated threaded cups may mean that mechanical interlocking between the threads and the bone prevents such failure. The importance of non-progressive radiolucent lines in subchondral osseous structures, seen in association with approximately 16 per cent of the porous-coated press-fit cups, remains unknown. Long-term follow-up will be necessary to determine if these radiolucent lines are an early sign of failure at the interface or whether they simply represent stable ingrowth of fibrous tissue in this highly stressed region. The rate of osteolysis that was associated with the hydroxyapatite-coated threaded cups, which had a large hole in the dome, was significantly lower than those associated with the porous-coated press-fit cups and the hydroxyapatite-coated press-fit cups, which did not have such a hole (p < 0.001 for both comparisons). Thus, our data are not consistent with suggestions that a hole or holes in an acetabular shell may allow polyethylene debris to penetrate the fixation interface and cause osteolytic changes in the supporting bone43,44,47. In the current study, the highest prevalence of osteolytic lesions (12 per cent) was noted in the region above the dome of the hydroxyapatite-coated press-fit cups. These implants also were associated with the highest prevalence of peripheral radiolucent lines. The lowest prevalence of osteolytic lesions (0 per cent) was observed at the periphery of the threaded cups. These implants were associated with the lowest prevalence of radiolucent lines. Thus, our data support the suggestion that pathways that allow wear particles to migrate from the periphery to the dome are a factor in the development of osteolysis around acetabular components25,29. Many authors have reported a relationship between failure of the acetabular component and use of a femoral head with a diameter of thirty-two millimeters9,22,35,41,45,47. In the current study, we observed a significant relationship between the use of a thirty-two-millimeter femoral head and the eventual failure of a hydroxyapatite-coated press-fit cup (p = 0.018). Other variables that were related to failure of these cups were the age and gender of the patient and the thickness of the polyethylene liner. Specifically, compared with the entire cohort, the patients who had revision of a hydroxyapatite-coated press-fit cup were younger (mean age, forty-four years compared with fifty years; p = 0.003) and were more likely to have received a thin polyethylene liner (mean thickness, 8.6 millimeters compared with 10.5 millimeters; p < 0.001). Within the group of patients who had a hydroxyapatite-coated press-fit cup, female patients were more likely to have a revision than male patients were (58 per cent of the patients who had a revision of a hydroxyapatite-coated press-fit cup were female compared with only 36 per cent of the entire group of patients who originally had been managed with such a cup; p = 0.02). With the numbers available, we could detect no significant relationships between the revision of a hydroxyapatite-coated press-fit cup and the height or weight of the patient, the diagnosis, or the use of bone screws. The low rates of failure that were observed in association with the hydroxyapatite-coated threaded cups and the porous-coated press-fit cups precluded valid statistical analysis of the relationship between such variables and the failure of these implants. We found the porous-coated press-fit cups and the hydroxyapatite-coated threaded cups to be substantially more successful than the hydroxyapatite-coated press-fit cups in terms of the rate of revision and the presence of a stable interface with osseous ingrowth. The hydroxyapatite-coated threaded implants, which were screwed into the acetabulum at the time of the operation, had immediate mechanical interlocking with osseous structures and were the most successful type of implant that we investigated. At a minimum of five years after the operation, 123 (99 per cent) of 124 threaded implants continued to exhibit radiographic signs of a stable interface with osseous ingrowth and only one such implant had been revised because of aseptic loosening. The porous-coated press-fit cups also performed well; at a minimum of five years postoperatively, 101 (98 per cent) of 103 implants had radiographic evidence of a stable interface with osseous ingrowth and only two such implants had been revised because of aseptic loosening. However, the 16 per cent prevalence of radiolucent lines in zone III suggests that the interfaces around these implants may lack durability over time. The relatively smooth hydroxyapatite-coated press-fit cups, which relied on osseointegration between the coating and the bone for stability, had apparently stable interfaces at two to three years, but, by five years, twenty-one of these implants had been revised because of aseptic loosening. Design features that were common to all three types of implants, including the type of material (titanium), the type of liner, and the type of locking mechanism, were ruled out as factors that may have contributed to failure. The hydroxyapatite coating per se also was ruled out as a causative factor because the manufacturer's records demonstrated consistency of the coatings among all of the implants and because successful results were achieved with use of the threaded cups and the femoral stems. Indeed, comparison of the results achieved with the different types of hydroxyapatite-coated implants suggests that a hydroxyapatite coating is most effective when primary stability in bone is achieved by the mechanical design of the implant itself. Our radiographic findings, in conjunction with published predictions of the state of stress at bonded acetabular implant interfaces32,40, strongly suggest that the failure of fixation of the smooth hydroxyapatite press-fit cups was due to the magnitude of the tensile stresses that were applied by activity of the patient to an interface that was unable to tolerate such stresses over time. We observed a significant relationship between the use of a thirty-two-millimeter femoral head and the need to revise a hydroxyapatite-coated press-fit cup because of aseptic loosening (p = 0.018). Other variables that were found to be related to revision of these cups were a young age, female gender, and the use of a thin polyethylene liner. There was no relationship in any group between migration of the femoral head and the type of cup. This finding suggests that none of the coatings that were studied can be implicated specifically with regard to polyethylene wear. In conclusion, we found that the short-term (two to three-year) results after acetabular reconstruction may be an unreliable indicator of long-term success. The findings of the present study suggest that the attainment and maintenance of interlocking between the implant and the bone may be necessary for the stable fixation of an acetabular component that is inserted without cement. Such interlocking may be especially important between the implant and the subchondral bone beneath it because distraction between the shell and the bone is most likely to occur at this location.
NOTE: The authors thank the following investigators who contributed to this study: Benjamin Bierbaum, M.D., Boston, Massachusetts; Michael Christie, M.D., Nashville, Tennessee; Omar Crothers, M.D., Portland, Maine; Joseph Dimon, III, M.D., Atlanta, Georgia; Vincent Eilers, M.D., St. Paul, Minnesota; William Jaffe, M.D., New York, N.Y.; Randall Lewis, M.D., Washington, D.C.; David Mattingly, M.D., Boston, Massachusetts; William Stillwell, M.D., Smithtown, New York; Anthony Unger, M.D., Washington, D.C.; and Richard Zimmerman, M.D., Portland, Oregon. They also thank Anne Serekian, M.S., Mahwah, New Jersey, for help in the preparation of this manuscript.
*One or more of the authors has received or will receive benefits for personal or professional use 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 source was the Osteonics Corporation, Allendale, New Jersey.
#Osteonics Corporation, 59 Route 17, Allendale, New Jersey 07401. **University Hospital Maastricht, Peterdebyelan 25, NL 6202 AZ Maastricht, The Netherlands.
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