Abstract
We observed early failure and radiographic signs of accelerated wear as early as one to three years after insertion, without cement, of a metal-backed acetabular component with a liner made of Hylamer (DePuy). This finding prompted us to review a larger cohort of patients in whom that liner had been used. Two hundred and thirty-three components with a Hylamer liner, evaluated at a minimum of two years, had a mean rate of wear of 0.27 millimeter per year compared with 0.12 millimeter per year for a contemporaneous group of fifty acetabular cups with a conventional ultra-high molecular weight polyethylene liner made by another manufacturer.At a mean of 3.2 years, we found a significant difference (p < 0.000000006) between the mean rate of wear (0.20 millimeter per year) when the Hylamer liner articulated with a DePuy modular cobalt-chromium femoral head and the mean rate (0.29 millimeter per year) when the liner articulated with an Osteonics modular cobalt-chromium femoral head. Radiographic evaluation revealed a significant correlation between the total linear wear and the prevalence of osteolytic lesions (r2 = 0.76, linear regression analysis). We found that wear of 1.5 millimeters or more could be detected by the unaided eye. Because of the positive correlation between osteolysis and wear of 1.5 millimeters or more, we defined a hip with a liner that had that amount of wear as a hip at risk.We concluded that the wear characteristics of a Hylamer liner in vivo are inferior to those of a conventional ultra-high molecular weight polyethylene liner. Also, the rate of wear of the liner is greater when the femoral head is from a manufacturer other than DePuy. A patient who has a total hip replacement that includes a Hylamer liner should be monitored frequently for signs of wear and osteolytic changes. Additional investigations, with longer durations of follow-up and larger populations, are needed to understand fully the importance of our findings.
Wear of the articulating surface has become one of the most important determinants of the longevity of a total hip replacement. The biological response to wear debris from conventional ultra-high molecular weight polyethylene has been identified as a major cause of osteolysis and its associated problems of loosening of the implant and loss of bone stock1,13. These problems have prompted a search for more durable forms of the polymer. Hylamer (DePuy Dupont Orthopaedics, Wilmington, Delaware) is a form of ultra-high molecular weight polyethylene with a higher crystallinity and thicker crystalline lamellae15. It is produced by a proprietary process that treats conventional Hoechst 415 GUR (Hoechst, Houston, Texas) ultra-high molecular weight polyethylene under high pressure and high temperature with controlled cooling15. By varying these treatment parameters, the manufacturer developed two forms of Hylamer for use in total joint arthroplasty: one (Hylamer) for acetabular liners and another, with a lower modulus of elasticity (Hylamer M), for the articulating surface of the tibial component in total knee replacement.
An initial in vitro study of the mechanical properties of Hylamer suggested that it might have better wear properties (a greater compressive yield strength) than conventional ultra-high molecular weight polyethylene15. A preliminary experiment with a hip-joint simulator, however, showed no difference in the wear behaviors of the two types of material17. To our knowledge, no other laboratory studies on the wear of Hylamer have been reported in a peer-reviewed journal.
We began to use metal-backed acetabular components with a Hylamer liner (Duraloc cup and Hylamer insert; DePuy, Warsaw, Indiana), without cement, in January 1991. During our routine clinical and radiographic follow-up at yearly intervals, we found radiographic signs of accelerated wear as early as one to three years postoperatively in some patients. This finding prompted us to review our experience with the Hylamer liner, and that review was the subject of a previous report4. In that study, two of us (M. J. C. and R. P.) and colleagues found that six of the 143 hips that had been followed for at least two years had been revised because of accelerated wear. The mean rate of wear of the revised Hylamer liners was 0.48 millimeter per year4. The long-term rate of linear wear of cemented acetabular components, as determined with use of radiographs, has ranged from 0.07 to 0.15 millimeter per year in series ranging from fifteen to 227 patients2,3,12,16,19,20,24, and the rate for acetabular components inserted without cement has ranged from 0.07 to 0.25 millimeter per year in series ranging from fifteen to 134 patients2,7,12,19. In our review4, there were no identifiable risk factors in the group of six patients who had revision because of wear of the liner: all of the patients were women, the mean weight was sixty-seven kilograms, the mean abduction angle of the acetabular component was 49 degrees, the minimum thickness of the Hylamer liner was eight millimeters, and all of the femoral heads were made of cobalt-chromium alloy and were twenty-eight millimeters in diameter. Because of the rate of failure of more than 4 per cent (six of 143) after a mean of 2.5 years of follow-up, we discontinued the use of acetabular cups with a Hylamer liner in December 1993; by that time, we had performed 391 primary hip replacements with components lined with this material.
We continued our investigation and the follow-up of our patients in whom a Hylamer liner had been used in an effort to understand the factors that may have contributed to the early failure. Although all of the acetabular components had been manufactured by one company (DePuy), the femoral components had been manufactured by many companies. The purpose of the present study was to determine whether the early failure was caused solely by the suboptimum performance of the Hylamer material or if it was attributable in part to the liner articulating with the surface of a component manufactured by a company other than DePuy. We also sought to determine whether the radiographic finding of accelerated wear was a sufficient indication for revision of the prosthesis and, as a corollary, whether there was a correlation between the high rates of wear and radiographic evidence of osteolysis.
*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. No funds were received in support of this study.
†Department of Orthopedic Surgery, Brigham and Women's Hospital, 75 Francis Street, Boston, Massachusetts 02115.
‡Rockford Orthopedic Associates, 5668 East State Street, Suite 1500, Rockford, Illinois 61108.
Three hundred and ninety-one primary total hip arthroplasties in which a Hylamer acetabular liner was used as the bearing surface were performed at our institution between January 1991 and December 1993. All Hylamer liners were implanted in conjunction with a porous-coated titanium-alloy acetabular cup that was inserted without cement. Two hundred and thirty-three hips were evaluated at a minimum of two years postoperatively. Because most (184) of the 233 total hip arthroplasties included use of a twenty-eight-millimeter cobalt-chromium femoral head manufactured by one of two companies—DePuy or Osteonics (Allendale, New Jersey)—we focused on arthroplasties that included such a femoral head (Groups 1 and 2, Table I). We also studied the results of an additional seven arthroplasties that included a twenty-eight-millimeter DePuy alumina (ceramic) femoral head (Group 1a). In all hips, the modular femoral head and stem were made by the same manufacturer. There were a total of 191 hips in the three groups; the remaining forty-two of the 233 arthroplasties included a femoral head that was not twenty-eight millimeters or a femoral component that had been manufactured by a different company (Zimmer, Warsaw, Indiana; Howmedica, Rutherford, New Jersey; or Johnson and Johnson, New Brunswick, New Jersey).
We also compared the rate of wear of Hylamer with that of conventional ultra-high molecular weight polyethylene liners that had been used in total hip arthroplasties at our institution during a contemporaneous period. Between January 1990 and December 1992, 145 primary total hip arthroplasties were performed with insertion, without cement, of an Osteonics PSL acetabular component (porous-coated titanium alloy) lined with conventional ultra-high molecular weight polyethylene as well as insertion, with or without cement, of an Osteonics Omnifit femoral stem and a twenty-eight-millimeter-diameter modular cobalt-chromium femoral head. We reviewed retrospectively the results of the first fifty consecutive arthroplasties that had been followed clinically and radiographically for at least two years (Group 3, Table I).
All of the hip replacements in the present study were performed by five senior surgeons with use of a similar operative technique. Each total hip arthroplasty was performed with the patient in the lateral decubitus position and with use of a posterolateral approach without an osteotomy of the greater trochanter.
Demographic information regarding the patients was collected from our Total Joint Replacement Registry. Statistical comparisons among the three groups and among subgroups stratified according to the type of fixation of the femoral component (with or without cement) were made with analysis of variance and a two-tailed t test. Linear regression analysis was used to determine the extent to which the rate of wear correlated with the age and weight of the patient as well as the diameter and abduction angle of the acetabular cup. The number of revisions performed in each of the three groups also was documented.
Wear was measured independently by two of us (B. J. L. and M. J. C.) with use of the radiographic technique of Livermore et al. The amount of intrusion of the femoral head into the cup on the initial postoperative anteroposterior radiographs of the pelvis was compared with that on the radiographs that were made at the most recent follow-up examination. We found that the unaided eye could detect 1.5 millimeters of wear or more.
The radiographs were evaluated for the prevalence of osteolysis by one of us (R. P.), who was blinded to the rates of wear. Osteolysis was assessed on the anteroposterior radiographs of the pelvis and femur according to acetabular zones I, II, and III of DeLee and Charnley and according to femoral zones 1 through 7 of Gruen et al. We defined an osteolytic lesion as a discrete endosteal radiolucent area that was at least five millimeters long in any dimension. A correlation between the presence of osteolysis and the total linear wear was determined by linear regression analysis.
Stereomicroscopy was used in order to evaluate nine retrieved Hylamer liners for evidence of embedded particles as well as for wear of the articulating and non-articulating surfaces. Direct measurements of wear were performed on the polyethylene itself of four of the retrieved liners: the technique of Kabo et al. was used for three, and a coordinate-mapping machine was used for one. Eight retrieved prosthetic femoral heads were analyzed visually for the degree of scratching. Each femoral head was divided into three zones: the crown, a middle region, and the base. The amount of scratching in each zone was evaluated separately with use of a scale of 0 to 5 points: a score of 0 points indicated no scratching in the zone; 1 point, scratching in 1 to 20 per cent of the zone; 2 points, scratching in 21 to 40 per cent of the zone; 3 points, scratching in 41 to 60 per cent of the zone; 4 points, scratching in 61 to 80 per cent of the zone; and 5 points, scratching in 81 to 100 per cent of the zone. Samples of synovial tissue and of periprosthetic tissue obtained at the time of revision were examined with use of polarized light microscopy for the presence of birefringent particles consistent in appearance with polyethylene debris.
Radiographic Assessment of Wear
The mean rate of wear (and standard deviation) was 0.27 ± 0.19 millimeter per year for all 233 Hylamer liners that were followed for at least two years. Linear regression analysis revealed no correlation between the rate of wear and the duration of follow-up (between two and five years). Analysis of variance demonstrated that the combination of the head and liner had a significant effect (p < 0.000000006) on the rate of wear (Fig. 1). The mean rate of wear of the Hylamer liners that articulated with a DePuy cobalt-chromium head (Group 1) was 67 per cent greater (p < 0.015) than the mean rate for the conventional ultra-high molecular weight polyethylene liners that articulated with an Osteonics femoral head (Group 3) (0.20 compared with 0.12 millimeter per year). The mean rate of wear of the Hylamer liners that articulated with an Osteonics femoral head (Group 2) was almost two and one-half times greater (p < 0.000000001) than that of the conventional polyethylene liners that articulated with an Osteonics femoral head (Group 3) (0.29 compared with 0.12 millimeter per year); the rate of wear in Group 2 was also significantly different (p < 0.008) from that in Group 1. The rate of wear was the highest (0.33 millimeter per year) for the Hylamer liners that articulated with a DePuy alumina femoral head (Group 1a).
With the numbers available, no significant correlation could be detected between the rate of wear and the age or weight of the patient or the diameter or abduction angle of the acetabular cup—factors that can directly or indirectly affect wear. There was no significant difference among the groups with regard to the weight or gender of the patient or with regard to the diameter or abduction angle of the cup. The mean age did differ between Group 1 and Groups 2 (p < 0.001) and 3 (p < 0.02), but age did not correlate with the rate of wear (Table I). We also studied the method of fixation of the femoral component, as we thought that this might be a reflection of the level of activity (that is, cement probably was used in younger, more active patients). When each group was stratified according to the method of fixation, it was found that, on the average, the patients who had fixation of the stem without cement were younger (Table I). The difference between the rate of wear of the liners associated with stems fixed with cement and the rate for those associated with stems fixed without cement was found to be significant (p < 0.002) in Group 1 only (0.13 compared with 0.29 millimeter per year). In contrast, in Groups 2 and 3 the mean rate of wear was only slightly higher in association with stems fixed without cement (Table I); these differences were not found to be significant (p > 0.15 and 0.25, respectively).
Osteolysis
The percentage of hips with an osteolytic lesion in at least one femoral9 or acetabular6 zone increased as the total linear wear increased (r2 = 0.76, linear regression analysis) (Fig. 2-A). The mean number of zones with evidence of osteolytic lesions in each hip also increased with an increase in the total amount of wear (Fig. 2-B). Fifteen (65 per cent) of the twenty-three hips with a liner that had total linear wear of more than 1.5 millimeters had radiographic signs of osteolysis in at least one zone. We categorized a hip with a liner that had more than 1.5 millimeters of total wear as a hip at risk on the basis of the positive correlation between increased wear and osteolysis as well as the ability to determine that amount of wear with the unaided eye (Figs. 3-A, 3-B, 3-C and 3-D).
Revision of Total Hip Replacements with a Hylamer Liner
Since the initial report of the six hip arthroplasties that were revised early because of excessive wear4, an additional five replacements involving a Hylamer liner were revised because of wear. As with the initial six failed arthroplasties, there were no identifiable risk factors associated with the latter five (Table II). If we assume that no Hylamer liner failed in the patients who were followed for less than two years, the rate of revision because of wear was 3 per cent (eleven of 391), with a mean time to revision (and standard error of the mean) of 3.2 ± 0.21 years. A total of forty-two (18 per cent) of the 233 hips in the entire series had revision (eleven; 5 per cent) or were defined as being at risk (thirty-one; 13 per cent) (Table III).
Laboratory Analysis of Retrieved Liners
Stereomicroscopy of the Hylamer liners that were revised because of wear demonstrated an absence of particles embedded in the articulating surface. There were distinct worn and unworn regions on the articulating surface, with the worn regions appearing white and the unworn regions, yellowish. Some areas of the unworn region had machine marks. On the non-articulating surface, there was no evidence of motion, with machine marks still clearly visible, and there was deformation at the sites of screw-holes. This deformation was more pronounced in the area corresponding to the worn region on the articulating surface.
Both direct measurements (on the liner itself) and radiographic measurements of total linear wear were determined for three retrieved Hylamer liners. The direct measurements revealed 1.76, 1.71, and 1.27 millimeters of linear wear and the corresponding radiographic measurements, 1.88, 1.83, and 1.17 millimeters. The direct measurement of the fourth liner, for which radiographs were not available, demonstrated 1.63 millimeters of total linear wear. At the time of writing, direct measurements had not yet been made on the remaining seven liners in order to preserve the surface structures for future analysis. The high correlation between the direct and radiographic measurements of linear wear, although performed on only three retrieved liners, supported previous findings validating the radiographic technique for the measurement of wear8,20,24.
Visual analysis of eight retrieved femoral heads for the degree of scratching revealed no relationship between that variable and wear. One femoral head had a moderate amount of scratching (a score of 3 points), three had a trace amount of scratching (scores of 1 or 2 points), and four appeared to be in their original condition with no visible signs of wear.
Histological analysis of synovial and periprosthetic tissue revealed polyethylene particles, predominantly within macrophages and multinucleated foreign-body giant cells (Figs. 4-A and 4-B).
It would have been valuable to have been able to compare the rate of wear of the Hylamer acetabular liners with the rate of wear of conventional ultra-high molecular weight polyethylene liners in DePuy Duraloc acetabular components inserted without cement. However, this was not possible at our institution. Therefore, we chose the best control group available—conventional ultra-high molecular weight polyethylene liners in similar acetabular components (Osteonics PSL), inserted without cement, and articulating with an Osteonics Omnifit femoral head and stem (the same femoral component that was used with the Hylamer liners).
The mean rate of wear measured radiographically was 0.12 millimeter per year for the conventional ultra-high molecular weight polyethylene liners with a metal-backed acetabular component that had been inserted without cement (Group 3). That rate is consistent with the rate of wear of acetabular components inserted with cement2,3,12,16,19,20,24. In addition, the rate is at the low end of the range reported for the combination of a modular acetabular component (inserted without cement), a modular femoral stem (inserted with or without cement), and a cobalt-chromium head; these rates have been reported to be 0.072, 0.1497, 0.1719, 0.1512, and 0.2212 millimeter per year in series of fifteen to 134 patients. We also found a consistency between the radiographic and direct measurements of three liners. These facts support the validity of the high rates of wear that we found for Hylamer liners.
The mean rate of wear determined radiographically was 125 per cent greater for the entire group of 233 Hylamer liners (0.27 millimeter per year) than for the group of conventional polyethylene liners (0.12 millimeter per year); this difference was found to be significant (p < 0.000000009). An unexpected finding was that the mean rate of wear of the Hylamer liners articulating with a cobalt-chromium femoral head that was made by a manufacturer other than DePuy (0.29 millimeter per year) was 45 per cent higher than the mean rate for the Hylamer liners articulating with a DePuy cobalt-chromium femoral head (0.20 millimeter per year). It may be that the Hylamer liner is more sensitive to the variations, normally within acceptable limits, of the size22, surface finish, and composition (for example, the carbide content, as it can reflect itself in topographical features) of the femoral head. With use of analytical models to study the effect of a lack of congruency of the head and cup on stress in polyethylene1, previous investigators have shown that large increases in contact stress result from relatively small increases in radial clearance (the difference between the radius of the head and of the articulating surface of the cup). In a recent study in which adaptive finite element modeling was used, the initial rates of wear of conventional polyethylene were affected by the different tolerances of different sizes of the femoral head, although long-term volumetric wear was not affected significantly18. Finite element modeling also has shown the increase in stress in the liner expected as a result of the increased modulus of elasticity of Hylamer5,10. In one model, the increase in the contact stress in Hylamer (35 per cent greater than that in conventional ultra-high molecular weight polyethylene), due to its higher modulus of elasticity, was found to exceed the reported increase of 17 per cent in its yield strength10. The higher contact stress resulting from the higher modulus of Hylamer, coupled with an additional increase in stress resulting from the larger radial clearance associated with a femoral head from a manufacturer other than DePuy, may have led to increased wear in Group 2.
Within the entire group of total hip replacements involving only DePuy components, the mean rate of wear of the seven liners that articulated with an alumina femoral head was more than 65 per cent higher than the mean rate for the liners that articulated with a cobalt-chromium femoral head (0.33 compared with 0.20 millimeter per year). Additional investigation of a larger number of alumina femoral heads is needed to determine the importance of this finding.
It was not possible to determine the contribution of creep to the total linear intrusion of the femoral head into either type of liner (recorded as total linear wear). In previous investigations in which wear was measured radiographically, all of the reduction in the thickness of the polyethylene was attributed to wear12,20,21. Hylamer is less susceptible to creep than conventional ultra-high molecular weight polyethylene15; therefore, if creep did account for some of the linear intrusion of the head, our findings would be an underestimation of the difference between the rates of wear of Hylamer and conventional polyethylene liners. Furthermore, we showed that the rate of wear was independent of the duration of follow-up, which suggests that Hylamer does not have an initial so-called wear-in period with decreased wear afterward.
In an effort to determine whether the rates of wear were influenced by load or activity, patient-related factors such as diagnosis, gender, and weight were tested; with the numbers available, no significant effect was detected. We also could detect no significant effect of factors such as the thickness of the liner or the abduction angle of the cup. The mean age of the patients in Group 1 was significantly different from that in Group 3 (p < 0.02) and Group 2 (p < 0.001), but there was no significant difference (p > 0.08) between the mean ages in Groups 2 and 3. Moreover, linear regression analysis revealed no significant correlation between the rate of wear and age.
In only one group (Group 1) was the rate of wear of liners articulating with a femoral component that had been inserted without cement greater than the rate of wear of those articulating with a femoral component that had been inserted with cement.
With the numbers available for study, we could not detect a significant effect (p > 0.25) of the type of fixation of the femoral component (with or without cement) on the rate of wear in our control group (Group 3). This finding differs from that of a recent study12, in which the mean rate of wear of the acetabular cup, inserted without cement, was higher when the femoral stem had been inserted without cement (0.22 millimeter per year) than when the stem had been inserted with cement (0.15 millimeter per year). The authors of that study conjectured that cement ensheathing the femoral component "may help to absorb some of the stresses, and thus reduce the forces within the polyethylene." Moreover, they suggested that cement offers a better seal against titanium particles, which may act as third bodies, migrating from the stem into the joint space. There was no evidence of third-body wear in any of the liners that were revised in the present study, and none of the femoral heads demonstrated features that could be attributed to three-body wear. The femoral head with a moderate amount of scratching and one of the heads with a trace amount of scratching had been attached to a femoral stem that had been inserted with cement; the other two heads with a trace amount of scratching had been attached to a stem that had been inserted without cement. Another possible explanation for the effect of the type of fixation on the rate of wear in Group 1 is that stems are typically inserted without cement in younger, more active patients (Table I). However, with the numbers available, we found no significant correlation between the rate of wear and age in this subgroup. Moreover, in Group 2 the patients who had fixation of the femoral stem without cement did not have a significantly different rate of wear of the liner compared with the patients who had fixation of the stem with cement, even though the former patients were the youngest over-all and were 34 per cent younger than the patients who had fixation with cement (44.0 compared with 66.8 years). In Group 1, in which the type of fixation did have a significant effect on wear, the patients who had fixation of the stem without cement were only 28 per cent younger than the patients who had fixation without cement (47.7 compared with 65.5 years).
Preliminary testing of the wear of Hylamer, with a reciprocating pin-on-flat technique5 and cyclic loading in a hip-simulator17, yielded conflicting results: the rate of wear of Hylamer was greater than that of conventional ultra-high molecular weight polyethylene in one study5 and no difference was found in the other17. The fact that the study with the hip-simulator did not demonstrate a greater rate of wear of Hylamer indicates that the laboratory test does not adequately model certain in vivo conditions or that a biological environment may have a more adverse effect on the wear characteristics of Hylamer than on those of conventional ultra-high molecular weight polyethylene. Additional analysis of the in vivo behavior of Hylamer is necessary in order to examine this issue better.
In the present study, there was a correlation between the total linear wear determined by a two-dimensional method and the presence of osteolytic lesions. Previous investigators found a correlation between the amount of wear determined with a three-dimensional technique and osteolysis but could not find one between wear determined with a two-dimensional method and osteolysis7. We noted that the prevalence of osteolytic lesions increased with an increase in the total linear wear. Additionally, we found that the number of zones with osteolytic lesions in each hip increased as the total linear wear increased. Because of the increased rates of wear of Hylamer liners, such liners may be associated with a higher risk of the development of osteolytic lesions, which makes revision operations more difficult. This increased risk may be due to the large numbers of Hylamer particles of a phagocytosable size (less than ten micrometers) that are released by the wear process. It is not yet known if the size and shape of Hylamer particles differ substantially from those of particles released by wear of conventional polyethylene. Preliminary histological examination of periprosthetic tissue obtained during revision arthroplasty revealed that the characteristics of osteolytic lesions induced by Hylamer were similar to those of lesions induced by particles of conventional polyethylene: macrophages were abundant, and multinucleated foreign-body giant cells also were present. However, although there were similarities between the cellular composition of the lesions induced by Hylamer and conventional polyethylene, as well as between the particles of the two types of material, there may be important differences that could be reflected by differences between the osteolytic responses to Hylamer and conventional polyethylene. There is work in progress to address this issue.
We categorized hips with a liner that had more than 1.5 millimeters of total linear wear as being at risk because of the positive correlation that we found between such wear and osteolysis and because of the ability to observe this level of wear with the unaided eye. We recommend that, during the follow-up of patients who have a Hylamer liner, a template should be used to track the migration of the femoral head into the liner and to aid in the identification of an accelerated rate of wear before the wear can be detected by the unaided eye. As part of our follow-up routine, we used Müller templates23. Our current indications for revision are accelerated wear associated with osteolysis or pain (presumably due to synovitis), or both. When a patient has no evidence of osteolysis or pain but has at least 1.5 millimeters of wear of the liner, we recommend that the patient be informed and radiographs be made at six-month intervals.
On the basis of the findings of the present study and the rates of wear reported in the literature, we concluded that the performance of Hylamer acetabular liners after primary total hip arthroplasty is inferior to that of conventional ultra-high molecular weight polyethylene liners. The rate of wear of Hylamer is greater than that of conventional polyethylene, and the increased wear is exacerbated by the use of a femoral head from a different manufacturer. We documented a correlation between the total linear wear and osteolysis. If a Hylamer liner wears at an accelerated rate, a critical level of wear may be reached earlier, thereby necessitating premature revision to prevent the bone loss caused by osteolytic lesions. Because our study was limited to patients at one institution, we urge that more investigations of the clinical performance of Hylamer be performed in larger populations and with longer periods of follow-up. Until the findings of such studies have been reported, we urge close follow-up of patients who have a Hylamer liner, particularly those who have more than 1.5 millimeters of wear. Such patients should be followed at six-month intervals for clinical or radiographic evidence of synovitis (pain) or osteolysis.
NOTE: The authors thank F. C. Ewald, M.D.; D. T. Reilly, M.D.; C. B. Sledge, M.D.; and W. H. Thomas, M.D., for allowing them to include their patients in the present study and for their valuable discussions. They also thank the Brigham Orthopedic Foundation for its support.
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