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The Effect of Total Hip Arthroplasty Cup Design on Polyethylene Wear Rate
The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 2005
The Effect of Total Hip Arthroplasty Cup Design on Polyethylene Wear Rate William G. Hamilton, MD, Robert H. Hopper Jr, PhD, Stuart D. Ginn, BS, Neil P. Hammell, BS, C. Anderson Engh Jr, MD, and Charles A. Engh, MD
Abstract: Using 743 total hip arthroplasties that included 6 hemispheric porouscoated cup designs, this study used a multiple linear regression to identify those factors that influenced polyethylene wear rates. Wear rates for each hip were based on serial head penetration measurement made with computer-assisted techniques. Implant factors associated with an increased wear rate included terminal sterilization with a non–cross-linking chemical surface treatment, a 4-mm lateralized liner, a cobalt-chrome femoral head, and a longer shelf life for liners c-irradiated in air. After accounting for these implant characteristics and patient factors, wear rates among the 6 cup designs were not significantly different ( P = .89). Although polyethylene wear is frequently characterized for specific implant designs, this study demonstrated that there are several common factors that influence polyethylene wear rates. Key words: primary porous-coated total hip arthroplasty, clinical outcome, polyethylene wear, effect of implant characteristics and cup design, effect of patient demographics. n 2005 Elsevier Inc. All rights reserved.
Although polyethylene wear after total hip arthroplasty (THA) is a multifactorial process influenced by implant and patient-related factors, implant design is commonly believed to play a fundamental role in the wear process. To assess the importance of cup design, this study sought to identify those factors that influenced polyethylene wear rates. In
the context of 6 different acetabular cup designs, our objective was to identify those generic and design-specific factors that were associated with polyethylene wear. Many studies have sought to identify the factors that have a significant influence on the wear process. Patient factors frequently noted in the literature include age at surgery, activity level, sex, weight, and preoperative diagnosis [1-4]. Among cementless acetabular components, implant factors that have been implicated in the wear process include cup design features related to shell holes, liner modularity, and locking mechanism [5]. In addition to the diameter and composition of the femoral head, polyethylene-related factors including liner material, fabrication method, thickness, terminal sterilization method, and oxidation associated with shelf aging after c-irradiation in air have also been associated with implant wear [1,2,4,6-18]. Surgical factors that have been implicated in the wear process include the surgeon and component placement [2,4,19]. Other factors including
From the Anderson Orthopaedic Research Institute, Alexandria, Virginia. Submitted January 15, 2005; accepted May 10, 2005. Benefits or funds were received in partial or total support of the research material described in this article. General research funding was provided by the Inova Health Services. Although no benefits or funds were received from any other commercial entity to support this research study, 3 of the authors serve as consultants for DePuy, a Johnson & Johnson company. Two of the authors receive royalties from DePuy and 1 owns Johnson & Johnson stock. Reprint requests: Robert H. Hopper, Jr, PhD, Anderson Orthopaedic Research Institute, Suite 200, 2501 Parkers Lane, P.O. Box 7088, Alexandria, VA 22307. n 2005 Elsevier Inc. All rights reserved. 0883-5403/05/1906-0004$30.00/0 doi:10.1016/j.arth.2005.05.007
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64 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005 accumulated debris, femoral head scratching, and in vivo oxidation have also been postulated as mechanisms that may accelerate wear over time [20-22]. Among the many clinical studies that have examined polyethylene wear, most investigators generally have reported factors found to be statistically significant instead of quantifying the magnitude of each factor’s influence on the wear rate in the context of other competing factors. Using several different cementless acetabular components, this study sought to quantify the relative importance of cup design in the context of patient factors and generic implant characteristics that could be quantified.
Materials and Methods For this institutional review board–approved retrospective study, we reviewed primary THAs derived from a database maintained at our institution. The study population included the 6 cup designs (Fig. 1) that were predominantly used for the primary THAs performed at our institution between 1985 and 1999. The modular Arthropor cup (Joint Medical Products, Stamford, Conn) featured a beaded, hemispheric titanium cup with holes for rim screws that locked the polyethylene liner and provided supplemental fixation for the cup. All Arthropor liners were sterilized using ethylene oxide, a non–cross-linking chemical surface treatment. The modular ACS Triloc+ cup
(DePuy, Warsaw, Ind) also featured a hemispheric beaded surface with rim screw holes. The polyethylene liner was sterilized with c-irradiation in air and had a nonuniformed thickness that was thinner in the region of the rim. The Harris-Galante cup (Zimmer, Warsaw, Ind) featured a hemispheric geometry with titanium mesh for bone ingrowth and cavitary holes for optional screw fixation with liners that were sterilized with c-irradiation in air. The Duraloc 1200 cup had 8 to 12 shell holes depending on the outer cup diameter, but we routinely implanted it without screws, so we transitioned to the Duraloc 100 which had a single central dome hole. The modular liners for the Duraloc 100 and 1200 cups were secured with a wire-locking ring that engaged a groove machined in the liner and shell. The liners used with the Duraloc cups included Enduron and highercrystallinity Hylamer (both by DePuy). We also used a 1-piece cup similar to the Duraloc with an Hylamer liner (28 mm inner diameter) that came preassembled in the shell. In the early 1990s, the Duraloc liners and all of the 1-piece cups were sterilized with c-irradiation in air at a standard dosage of 0.025 to 0.04 MGy. During the mid1990s, some liners were terminally sterilized at standard and low dosages in barrier packaging with an inert gas. Since the latter part of 1995, we have implanted Duraloc cups that were terminally sterilizing using gas plasma. In an effort to improve hip stability, we began to use 4-mm lateralized polyethylene liners with the
Fig. 1. Hemispheric porous-coated acetabular components used at our institution included the Arthropor (A), ACS Triloc+ (B), Harris-Galante (C), Duraloc 1200 (D), Duraloc 100 (E), and 1-Piece (F) cups.
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 65
Duraloc cups in 1991. Although lipped liners were used in the early 1990s, neutral liners were used for most of the THAs during the mid-1990s. The use of lateralized liners progressively increased and by 1998, almost all cases were performed with 4-mm lateralized liners. The acetabulum was also reamed medially only enough to achieve a rim fit with the cup in approximately 458 of inclination. The combination of these techniques tended to lateralize the center of hip rotation. With the exception of the Hylamer liners used with the Duraloc and 1-Piece cups, the polyethylene material for all other cups was considered to be conventional. All modular cup designs in this study afforded the option to incorporate different femoral head sizes and liner geometries. Although the ACS Triloc+, Harris-Galante, and Duraloc 1200 cups featured shell holes for optional screw fixation, the acetabulum was routinely reamed to a diameter 1 mm less than the metal shell and the cup was press-fit without screws. Although most modular cup designs used a uniform metal shell thickness and accommodated larger outer cup diameters by increasing the polyethylene liner thickness, the Arthropor cup used a relatively thin polyethylene liner and increased the metal shell thickness for larger-diameter cups. The study population for this analysis included all THAs performed with Arthropor, ACS Triloc+, Harris-Galante, and Duraloc 100, 1200, or 1-Piece cups that had minimum 5-year x-ray follow-up, a postoperative x-ray, and at least 3 follow-up x-rays. ACS Triloc+ cups that sustained rim fractures were excluded. We also limited the study population to those THAs performed with modular femoral heads on an Anatomic Medullary Locking, (AML; DePuy), Prodigy (DePuy), or Solution (DePuy) stem. Hips with loose stems or cups were excluded. To exclude most of the head penetration associated with creep and bedding-in, the first follow-up x-ray included in our wear analysis was taken at least 0.75 years after surgery. A period of at least 4 years was required between the first and last follow-up x-rays. The study population was also restricted to those THAs with 28- or 32-mm femoral heads that had a conventional or Hylamer polyethylene liner. Total hip arthroplasties with liners that were terminally sterilized with c-irradiation in air were also excluded if the sterilization date was not known. Sterilization dates were determined by sending polyethylene catalog and lot numbers derived from implant stickers to the manufacturers. We identified 743 THAs (666 patients) that met our inclusion and exclusion criteria. During the
15-year period from 1985 through 1999 when these cases were done, 3814 primary THAs, including 568 Arthropor, 452 Triloc, 88 Harris-Galante, 231 Duraloc 1200, 1678 Duraloc 100, 77 1-Piece, and 720 other cups, were implanted at our institution. Tables 1 and 2 detail the implant and patient characteristics stratified by cup design. In most cases, the differences among implant and patient characteristics between the various cup designs were statistically significant. At our institution, anteroposterior pelvic x-rays are routinely taken with the beam centered on the pubic symphysis while the patient is supine with the legs internally rotated. Using the anteroposterior pelvic x-rays for each THA, 2-dimensional head penetration was determined for each follow-up x-ray relative to the immediate postoperative view with validated, computer-assisted techniques [23,24]. The same software was also used to determine the cup abduction and anteversion angle for each hip. A least-squares linear regression was used to determine the slope of the line that best fit the relationship between the magnitude of the wear vector and the time in situ [25]. The slope of the linear regression represented the wear rate of the polyethylene. Because at least 3 wear values were used to compute the wear rate for each THA, we also evaluated the standard error associated with the wear rate. To simultaneously assess the influence of implant, patient, and surgical factors on polyethylene wear rates, a multiple linear regression analysis was used with a stepwise variable entry and an inclusion P value of .05. Patient-related factors considered in our analysis included age at surgery, sex, weight, preoperative diagnosis, and whether a dislocation occurred. Generic implant factors included the presence of shell holes, liner geometry, femoral head composition and diameter, polyethylene liner material, sterilization technique, and shelf aging of liners sterilized with c-irradiation in air. All other implant features that were not captured by these generic factors were lumped into implant-specific variables that represented the cumulative effects of otherwise unquantified attributes unique to each cup design. These designspecific characteristics would include the resin used to manufacture the conventional polyethylene liners, proprietary polyethylene processing techniques, shell-liner conformity, liner locking mechanism, the finish of the shell’s interior surface, and other unique features. To acknowledge the possible influence of other variables, cup abduction and anteversion angles were incorporated as surgical factors and length of radiographic follow-up was
Overall Variable Sex Diagnosis
Head diameter (mm) Head material Poly material Liner geometry Terminal sterilization method
Dislocations
Categories Female Male Osteoarthritis Avascular necrosis Hip dysplasia Fracture/trauma Rheumatoid arthritis 28 32 CoCr Ceramic Conventional Enhanced (Hylamer) Neutral Lipped +4 mm c-Air Standard Dose c-Barrier Standard Dose c-Barrier low dose Gas plasma/ ethylene oxide None 1 or more
No. of THAs 387 356 591 58
Arthropor %
No. of THAs
52.1 47.9 79.5 7.8
93 87 136 21
38 28 28
5.1 3.8 3.8
403 340 519 224 576 167
ACS Triloc+
%
No. of THAs
51.7 48.3 75.6 11.7
51 43 76 2
6 6 11
3.3 3.3 6.1
54.2 45.8 69.9 30.1 77.5 22.5
41 139 164 16 180 0
314 326 103 444
42.3 43.9 13.9 59.8
50
Harris-Galante
%
No. of THAs
54.3 45.7 80.9 2.1
18 14 24 6
3 7 6
3.2 7.4 6.4
22.8 77.2 91.1 8.9 100.0 0.0
3 91 36 58 94 0
67 113 0 0
37.2 62.8 0.0 0.0
6.7
0
17
2.3
232 701 42
Duraloc 1200
%
No. of THAs
56.3 43.8 75.0 18.8
46 44 70 5
0 1 1
0.0 3.1 3.1
3.2 96.8 38.3 61.7 100.0 0.0
8 24 23 9 32 0
0 94 0 94
0.0 100.0 0.0 100.0
0.0
0
0
0.0
31.2
180
94.3 5.7
175 5
Duraloc 100
%
No. of THAs
51.1 48.9 77.8 5.6
168 149 264 21
7 3 5
7.8 3.3 5.6
25.0 75.0 71.9 28.1 100.0 0.0
28 62 56 34 63 27
15 17 0 32
46.9 53.1 0.0 100.0
0.0
0
0
0.0
100.0
0
97.2 2.8
89 5
1-Piece
%
No. of THAs
%
P
53.0 47.0 83.3 6.6
11 19 21 3
36.7 63.3 70.0 10.0
.64
18 9 5
5.7 2.8 1.6
4 2 0
13.3 6.7 0.0
31.1 68.9 62.2 37.8 70.0 30.0
293 24 226 91 207 110
92.4 7.6 71.3 28.7 65.3 34.7
30 0 14 16 0 30
100.0 0.0 46.7 53.3 0.0 100.0
25 64 1 90
27.8 71.1 1.1 100.0
177 38 102 198
55.8 12.0 32.2 62.5
30 0 0 30
100.0 0.0 0.0 100.0
0.0
0
0.0
50
15.8
0
0.0
0
0.0
0
0.0
17
5.4
0
0.0
0.0
0
0.0
0
0.0
52
16.4
0
0.0
94.7 5.3
30 2
93.8 6.3
85 5
94.4 5.6
294 23
92.7 7.3
28 2
93.3 6.7
.007
b.001 b.001 b.001
b.001
b.001
.49
66 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005
Table 1. Categorical Variables by Cup Design
Table 2. Continuous Variables by Cup Design Variable
Arthropor
743 2/85-5/99 58.9 F 11.6 (24-83) 42.8 F 7.4 (21-65)
180 2/85-1/91 57.2 F 12.7 (25-83) 44.9 F 6.2 (32-63)
18.6 F 7.6 ( 10 to 42) 0.61 F 0.82 (0-5.72) (444) 175 F 39 (80-365) 9.4 F 3.2 (5.0-18.3) 5.8 F 3.0 (3-22)
16.5 F 7.0 (1-39) None
0.12 F 0.11 ( 0.24 to 0.73) Standard error of 0.03 F 0.04 wear rate (mm/y) (0-0.52)
ACS Triloc+
Harris-Galante
Duraloc 1200
Duraloc 100
1-Piece
P
32 3/89-5/91 61.7 F 10.9 (34-82) 42.5 F 5.6 (32-53)
90 5/90-7/92 57.3 F 11.9 (24-80) 42.0 F 7.9 (24-61)
317 8/91-5/99 60.7 F 11.2 (25-83) 41.0 F 7.6 (21-65)
30 10/92-11/93 52.4 F 12.1 (25-70) 44.9 F 9.5 (26-60)
N/A N/A b.001 b.001
17.7 F 8.1 ( 10 to 35) 0.24 F 0.31 (0.02-1.53) (94) 172 F 39 (85-320) 175 F 40 (104-300) 12.9 F 3.1 (5.9-18.3) 10.6 F 2.67 (5.5-15.9) 8.1 F 4.0 (3-22) 6.0 F 2.8 (3-15)
18.2 F 6.7 (3-30) 0.42 F 0.43 (0-1.62) (32) 176 F 47 (96-260) 10.3 F 2.7 (5.7-14.1) 6.1 F 3.1 (3-15)
19.1 F 7.1 (5-36) 0.36 F 0.31 (0.01-1.53) (90) 176 F 40 (106-307) 9.1 F 2.1 (5.2-13.0) 5.4 F 2.1 (3-13)
19.9 F 7.8 (0-42) 1.00 F 1.06 (0.02-5.72) (198) 175 F 38 (80-365) 7.1 F 1.6 (5.0-12.4) 4.5 F 1.7 (3-14)
20.1 F 7.8 (6-36) 0.20 F 0.17 (0.02-0.76) (30) 175 F 28 (115-220) 9.0 F 1.2 (6.4-11.4) 5.2 F 1.6 (3-11)
b.001
0.18 F 0.11 ( 0.05 to 0.48) 0.03 F 0.03 (0-0.24)
0.08 F 0.07 ( 0.08 to 0.25) 0.03 F 0.02 (0-0.08)
0.08 F 0.08 ( 0.21 to 0.50) 0.02 F 0.06 ( 0-0.52)
0.12 F 0.12 ( 0.24 to 0.73) 0.04 F 0.04 (0-0.25)
0.11 F 0.05 (0.01-0.22) 0.02 F 0.01 (0-0.05)
94 8/87-5/90 58.8 F 9.4 (33-76) 44.8 F 6.1 (30-62)
0.08 F 0.07 ( 0.07 to 0.38) 0.03 F 0.02 (0-0.11)
Values are expressed as mean F SD (range). N/A indicates not applicable.
b.001 .96 b.001 b.001 b.001 .02
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 67
No. of THAs Surgery dates Age at surgery (y) Abduction angle (8) Anteversion angle (8) c-Air shelf life (y) (no. of hips) Weight (lb) Follow-up (y) No. of follow-up x-rays Wear rate (mm/y)
Overall
68 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005
Fig. 2. Raw wear rate data demonstrate significant differences among the cup designs ( P b .001, analysis of variance) with the Arthropor liners (A) wearing at almost twice the rate of the ACS Triloc+ (B), Harris-Galante (C), Duraloc 1200 (D), Duraloc 100 (E), and 1-Piece cups (F). The error bars designate the 95% CIs associated with the mean wear rate for each cup design.
included to characterize potential changes in wear rates that occurred over time. Categorical variables were incorporated in the regression analysis using effects coding ( 1, 0, 1) [26]. Using this coding scheme, the regression equation allowed us to assess the difference between dichotomous categorical variables. When a category included only 2 groups, a single variable was created. When a category included 3 or more groups, an effectscoding variable was created to compare each group to all other THAs in that category. Interaction terms were created to accommodate the potential influence of shelf storage after c-irradiation in air. Separate terms were included for conventional and Hylamer liners. The magnitude of each factor’s effect was based on the best-fit regression model that accounted for the largest portion of the variance in the wear rate data. When an effect magnitude is reported, the 95% confidence interval (95% CI) is also noted. After identifying the factors that significantly influenced the wear rate, the wear data for each cup design were normalized to a 60-year-old, 170-lb man with a preoperative diagnosis of osteoarthritis and no history of dislocation who was implanted with a cobalt-chrome (CoCr) femoral head and a conventional c-sterilized neutral polyethylene liner that was not shelf-aged. A post hoc power analysis was also performed. Statistical analysis was done with SPSS (Statistical Package for the Social Sciences, Chicago, Ill).
Results Among the 32 factors incorporated in the multiple linear regression, 10 were found to be
statistically significant. Based on these 10 terms, the best-fit regression accounted for 29% (r = 0.54, r 2 = 0.29, P b .001) of the variance in the wear rate data. Five implant factors were found to have a statistically significant association with the polyethylene wear rate. The dominant factor was whether the polyethylene had been sterilized with c-irradiation. Those liners sterilized with c-irradiation wore 0.099 mm/y less ( P b .001; 95% CI, 0.082-0.115 mm/y) than non–cross-linked liners that were sterilized with gas-plasma or ethylene oxide. Although c-irradiation improved wear resistance, shelf aging of Hylamer liners sterilized in air tended to increase wear rates by 0.061 mm/y for each year of shelf storage ( P b .001; 95% CI, 0.040-0.082 mm/y2). The effect of shelf aging among conventional liners sterilized with c-irradiation in air increased the wear rate by only 0.014 mm/y for each year of shelf storage ( P = .02; 95% CI, 0.002-0.025 mm/y2). Using a 4-mm lateralized liner instead of a neutral or lipped component increased the wear rate by 0.041 mm/y ( P b .001; 95% CI, 0.021-0.061 mm/y). Using a ceramic femoral head instead of CoCr was associated with a 0.018 mm/y decrease in the wear rate ( P = .04; 95% CI, 0.001-0.034 mm/y). Cup design ( P z .21), manufacturer ( P N .78), femoral head diameter ( P = .21), the presence of one dome or multiple shell holes ( P = .68), and use of low or standard dosage among c-sterilized liners ( P = .47) did not have a significant influence on wear rate. Aside from the increased wear associated with the shelf-aged liners that were
Fig. 3. After controlling for terminal sterilization method, shelf aging of liners c-irradiated in air, femoral head material, liner lateralization, and patient demographics, the wear rates for the Arthropor (A), ACS Triloc+ (B), Harris-Galante (C), Duraloc 1200 (D), Duraloc 100 (E), and 1-Piece (F) cups were not significantly different ( P = .89, analysis of variance). The error bars designate the 95% CIs associated with the mean wear rate for each cup design.
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 69
c-irradiated in air, no other difference was found among Hylamer and conventional polyethylene liners ( P = .11) (Figs. 2 and 3). Five patient-related factors were found to have a statistically significant association with the polyethylene wear rate. A 1-year increase in age at the time of surgery was associated with a 0.003 mm/y decrease in wear rate ( P b .001; 95% CI, 0.0020.004 mm/y). This translated into a 0.03 mm/y decrease per decade increase in age. Each 10-lb increase in patient weight was also associated with a 0.003 mm/y decrease in the wear rate ( P = .002; 95% CI, 0.001-0.005 mm/y). Male sex was associated with a 0.025 mm/y increase in wear rate ( P = .002; 95% CI, 0.009-0.040 mm/y). One or more dislocations were associated with a 0.046 mm/ y increase in the wear rate ( P = .002; 95% CI, 0.016-0.076 mm/y). A preoperative diagnosis of osteoarthritis tended to increase the wear rate by 0.030 mm/y compared with other diagnoses ( P = .002; 95% CI, 0.011-0.048 mm/y). There was no significant difference in wear rates among patients with preoperative diagnoses of trauma ( P = .68), inflammatory arthritis ( P = .78), hip dysplasia ( P = .91), or osteonecrosis ( P = .99). Polyethylene wear rate was not significantly associated with cup abduction angle ( P = .75), anteversion ( P = .61), or length of radiographic follow-up ( P = .35). Based on the 743 THAs included in this analysis and the 10 factors that accounted for 29% of the variance in the wear rate data, this study had 90% power to detect an additional factor that accounted for at least 1% of the wear rate variance (an incremental r 2 of 0.01).
Discussion Although the wear rates among the different cup designs were statistically different ( P = .001, Table 2), these differences could be accounted for based on generic implant factors and patient characteristics. The ACS Triloc+, Harris-Galante, and Duraloc 1200 cups wore at a mean rate of 0.08 mm/y (Table 2). These 3 groups had similar patient demographics and all liners were sterilized with c-irradiation in air. Although 30% of the Duraloc 1200 cups featured Hylamer liners that were sterilized with c-irradiation in air, the mean shelf age for these components was relatively short (0.29 years) and had a minor influence on the mean wear rate for this group, increasing it by 0.005 mm/y. A modestly higher mean wear rate of 0.11 mm/y among the 1-Piece cups can be
attributed to the younger mean patient age of 52.4 years and shelf aging of the Hylamer liners c-irradiated in air. Compared with other groups, the increased mean wear rate of 0.12 mm/y among the Duraloc 100 cups can be attributed to the use of Hylamer liners sterilized with c-irradiation in air with a mean shelf age of 0.79 years in 19% of cases (60 THAs), terminal sterilization with gas plasma among 16% of cases (52 THAs), and the use of 4 -mm lateralized liners among 32% of cases (102 THAs). The highest mean wear rate of 0.18 mm/y among the Arthropor cups can be explained almost entirely by the use of ethylene oxide, a non–cross-linking chemical surface treatment, for terminal sterilization. Although polyethylene wear is commonly characterized for specific implant designs, accounting for patient factors and generic implant characteristics revealed that the specific implant design did not play a prominent role in the wear process. Although our study population consisted of only 24% (743/3094) of the primary THAs performed at our institution with the 6 cup designs included in our analysis, this was primarily because of our inclusion criteria that required at least 3 follow-up x-rays to compute a wear rate for each hip. Although we did not analyze a consecutive series of patients, the integrity of our regression analysis technique only requires a distributed, unbiased sampling. Because the mean follow-up for this study was 9.4 years and implant wear is generally asymptomatic at intermediate follow-up, we do not believe that those patients returning for routine follow-up would have biased wear rates compared with other patients who returned less frequently. Although the multiple linear regression analysis allowed us to simultaneously assess the influence of many factors, several potential limitations should be considered. Because the study was not randomized, the factors potentially influencing wear were not equally represented among the cup designs comprising the study population. Although our regression analysis accounted for the potential influence of 32 factors, it did so only in the context of a linear relationship between these factors and the wear rate. It remains possible that some of the factors examined in this analysis are related to wear in a nonlinear fashion and that interactions between factors that were not incorporated in this analysis may also influence wear rates. However, in view of the small portion of the wear rate variance that each independent factor explains in our linear analysis, we consider it unlikely that introducing nonlinear terms or incorporating additional interaction effects by combining
70 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005 factors would account for a substantial portion of the variance in the wear rate data. Although a prospective study design would be preferable, we believe that a multiple linear regression analysis considering all eligible cases is superior to retrospectively matched groups that are more likely to mask persistent surgical biases without eliminating them. Reducing the study population to more homogeneous subgroups would also reduce the power of our analysis and limit our ability to compare different factors. Although this study demonstrated that the wear process is multifactorial and identified several factors that have relatively subtle influences on wear rates, the 10 factors identified as statistically significant accounted for only 29% of the variance in the wear data. As a consequence, we can identify those factors that are important to the wear of a population of THAs but cannot accurately predict the wear rate for an individual THA patient. The primary reason for the relatively low r 2 likely stems from the fact that implant usage was not precisely quantified among the factors we examined. Although several investigators have identified a significant correlation between age at surgery and activity level [27,28], a more precise measure of implant usage would likely improve the predictive value of our analysis. It could also simplify the relationship between patient factors and wear rate because the differences attributed to age, weight, sex, and diagnosis may all represent underlying differences in activity level. We expect that quantifying patient activity level and using longer follow-up data will enable us to refine future models and identify other factors that have a substantial influence on the wear rate. Unquantified factors potentially influencing wear, such as third-body debris, may account for some of the variance in the wear data. The uncertainty associated with the wear rate calculated for each hip also contributes to the variance that is unaccounted for by our model. The standard error, reported in Table 2, reflects the fact that serial head penetration values for an individual THA tend to deviate from a straight line. Although we used the computer-assisted techniques of Martell et al [23] and Devane et al [24] to measure radiographic head penetration, a prior study from our institution [29] found similar accuracy and reproducibility among these methods with good-quality radiographs. Deviations from a perfectly linear head penetration pattern could potentially stem from temporal changes in the head penetration rates, the presence of multiple wear vectors, or suboptimal x-ray quality. Failure to fully reduce the femoral
head when the supine x-ray was taken and accuracy limitations associated with using computerassisted techniques to measure femoral head penetration on clinical x-rays [24,30] could also introduce measurement errors. Despite these potential sources of nonlinearity, we found no relationship between length of radiographic follow-up and wear rate ( P = .35). Prior studies based on clinical data from our institution have also indicated that the long-term femoral head penetration rate tends to remain fairly constant [22,31]. We found that terminal sterilization with c-irradiation was associated with reduced wear rates for several different cup designs. This observation is consistent with clinical [7,9,32] and laboratory [13] findings that terminal sterilization with c-irradiation at nominal dosages of 0.025 to 0.04 MGy reduces polyethylene wear rates by approximately one half compared to non–cross-linking chemical surface treatments. In our study, non–cross-linked polyethylene sterilized by ethylene oxide or gas-plasma demonstrated a 0.099 mm/y increase in wear rate. Based on an average wear rate of slightly less than 0.10 mm/y for the c-irradiated liners included in this study, our findings confirm that terminal sterilization with c-irradiation reduced the wear rate of non–cross-linked polyethylene by about one half. Although 4-mm lateralized liners were associated with a wear rate that was increased by 0.041 mm/y, this increase may reflect the cumulative effects of cup and liner lateralization. Among the Duraloc cups that were implanted with lateralized liners, there was also a tendency to lateralize the cup to improve postoperative stability. We suspect that joint reactive force, determined by liner geometry and patient anatomy as well as the position of the cup and stem, may be the underlying factor. In future work, incorporating joint reactive force may help to better delineate the role of factors influencing wear in the context of the reconstructed joint geometry. Because c-irradiation dosage determines polyethylene cross-linking, we expected that liners sterilized at a low dosage would have higher wear rates than liners sterilized at a standard dosage. The absence of a relationship between sterilization dosage and wear rate may be explained, in part, by the small number of low-dosage liners included in this study, the relatively wide range of radiation exposures among the dosage groups, and the variability in actual radiation exposure for an individual liner during the terminal sterilization process. Cup modularity is commonly regarded as a potential source of increased wear. In this study,
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 71
however, we found no difference between the factory-preassembled 1-Piece cup and the other modular designs ( P = .86). This result suggests that differences in backside wear among the components we analyzed were probably not significant. Although we identified statistically different wear rates among several different cup designs, these differences were associated with patient demographics or generic implant characteristics including terminal sterilization technique, femoral head material, liner lateralization, and shelf aging. After accounting for differences in patient demographics and generic implant characteristics, we were unable to attribute improved wear performance to any particular design. Although it remains possible that some designs incorporated a combination of unique features, some of which may have decreased wear rates whereas others increased wear, the net effect of these unquantified factors was not associated with perceptible differences in wear performance. When considering new designs, surgeons should be skeptical of modifications or innovations that are purported to improve wear performance if the design features have never been shown to significantly influence polyethylene wear rates based on clinical data. We should also note that this study examined polyethylene wear, not osteolysis.
Acknowledgment The authors gratefully acknowledge the general research funding provided by Inova Health Care Services to support this study.
References 1. Devane PA, Horne JG. Assessment of polyethylene wear in total hip replacement. Clin Orthop 1999;369:59. 2. Devane PA, Horne JG, Martin K, et al. Threedimensional polyethylene wear of a press-fit titanium prosthesis. Factors influencing generation of polyethylene debris. J Arthroplasty 1997;12:256. 3. Joshi A, Ilchmann T, Markovic L. Socket wear in bilateral simultaneous total hip arthroplasty. J Arthroplasty 2001;16:117. 4. Schmalzried TP, Dorey FJ, McClung CD, et al. Factors contributing to the variability of short-term radiographic wear rates in total hip replacement. Orthop Trans 1998;22:81. 5. Eickmann TH, Green D, Donaldson T, et al. Backside wear in retrieved Hylamer liners. Presented at the 16th Annual Symposium of the International Society for Technology in Arthroplasty, San Francisco, CA, September 2003.
6. Collier JP, Sutula LC, Currier BH, et al. Overview of polyethylene as a bearing material: comparison of sterilization methods. Clin Orthop 1996;333:76. 7. Digas G, K7rrholm J, Nivbrant B, et al. Increased femoral head penetration using polyethylene sterilized with ethylene oxide. Presented at the European Federation of National Associations of Orthopaedics and Traumatology, Rhodes Congress, Greece; June 2001. 8. Donaldson T, Williams PA, Clarke IC, et al. Radiographic wear analysis of Hylamer cups. Differences with sterilization and storage methods. Presented at the 16th Annual Symposium of the International Society for Technology in Arthroplasty, San Francisco, CA, September 2003. 9. Hopper Jr RH, Young AM, Orishimo KF, et al. Effect of terminal sterilization with gas plasma or gamma radiation on wear of polyethylene liners. J Bone Joint Surg Am 2003;85:464. 10. Jin ZM, Dowson D, Fisher J. A parametric analysis of the contact stress in ultra-high molecular weight polyethylene acetabular cups. Med Eng Phys 1994;16:398. 11. Maxian TA, Brown TD, Pedersen DR, et al. The Frank Stinchfield Award. 3-Dimensional sliding/contact computational simulation of total hip wear. Clin Orthop 1996;333:41. 12. McKellop H, Lu B, Li S. Wear of acetabular cups of conventional and modified UHMW polyethylene compared on a hip joint simulator. Presented at the Ann Meet Orthop Res Soc; 1992. 13. McKellop H, Shen FW, Lu B, et al. Development of an extremely wear-resistant ultra high molecular weight polyethylene for total hip replacements. J Orthop Res 1999;17:157. 14. McKellop H, Shen FW, Lu B, et al. Effect of sterilization method and other modifications on the wear resistance of acetabular cups made of ultra-high molecular weight polyethylene. A hipsimulator study. J Bone Joint Surg Am 2000;82:1708. 15. Sychterz CJ, Engh Jr CA, Young AM, et al. Comparison of in vivo wear between polyethylene liners articulating with ceramic and cobalt-chrome femoral heads. J Bone Joint Surg Br 2000;82:948. 16. Sychterz CJ, Orishimo KF, Engh CA. Sterilization and polyethylene wear: clinical studies to support laboratory data. J Bone Joint Surg Am 2004;86:1017. 17. Vaughn BK, Dameron Jr TB, Bauer TW, et al. Early osteolysis following total hip arthroplasty with use of a Hylamer liner in combination with a modular ceramic femoral head. A case report. J Bone Joint Surg Am 1999;81:1446. 18. Yamauchi K, Hasegawa Y, Iwasada S, et al. Head penetration into Hylamer acetabular liner sterilized by gamma irradiation in air and in a nitrogen atmosphere. J Arthroplasty 2001;16:463. 19. Kligman M, Michael H, Roffman M. The effect of abduction differences between cup and contralateral acetabular angle on polyethylene component wear. Orthopedics 2002;25:65.
72 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005 20. Elfick APD, Smith SL, Unsworth A. Variation in the wear rate during the life of a total hip arthroplasty. J Arthroplasty 2000;15:901. 21. Fisher J, Firkins P, Reeves EA, et al. The influences of scratches to metallic counterfaces on the wear of ultra-high molecular weight polyethylene. Proc Inst Mech Eng [H] 1995;209:263. 22. Orishimo KF, Hopper Jr RH, Engh CA. Long-term in vivo wear performance of porous-coated acetabular components sterilized with gamma irradiation in air or ethylene oxide. J Arthroplasty 2003;18:546. 23. Martell JM, Berdia S. Determination of polyethylene wear in total hip replacements with the use of digital radiographs. J Bone Joint Surg Am 1997; 79:1635. 24. Devane PA, Bourne RB, Rorabeck CH, et al. Measurement of polyethylene wear in metal-backed acetabular cups. I. Three-dimensional technique. Clin Orthop 1995;319:303. 25. Sychterz CJ, Engh Jr CA, Yang AM, et al. Analysis of temporal wear patterns of porous-coated acetabular components: distinguishing between true wear and so-called bedding-in. J Bone Joint Surg Am 1999; 81:821.
26. Pedhazur EJ. Multiple regression in behavioral research: explanation and prediction. Philadelphia: Harcourt Brace; 1997. 27. McClung CD, Zahiri CA, Higa JK, et al. Relationship between body mass index and activity in hip or knee arthroplasty patients. J Orthop Res 2000;18:35. 28. Schmalzried TP, Szuszczewicz ES, Northfield MR, et al. Quantitative assessment of walking activity after total hip or knee replacement. J Bone Joint Surg Am 1998;80:54. 29. Sychterz CJ, Young AM, Engh CA. Effect of radiographic quality on computer-assisted head penetration measurements. Clin Orthop 2001;386:150. 30. Hui AJ, McCalden RW, Martell JM, et al. Validation of two and three-dimensional radiographic techniques for measuring polyethylene wear after total hip arthroplasty. J Bone Joint Surg Am 2003;85:505. 31. Hopper Jr RH, Young AM, Engh Jr CA, et al. Assessing the pattern of femoral head penetration after total hip arthroplasty. J Arthroplasty 2004; 19(Suppl 2):22. 32. Hopper Jr RH, Engh Jr CA, Fowlkes LB, et al. The pros and cons of polyethylene sterilization with gamma irradiation. Clin Orthop 2004;429:54.
The Effect of Total Hip Arthroplasty Cup Design on Polyethylene Wear Rate William G. Hamilton, MD, Robert H. Hopper Jr, PhD, Stuart D. Ginn, BS, Neil P. Hammell, BS, C. Anderson Engh Jr, MD, and Charles A. Engh, MD
Abstract: Using 743 total hip arthroplasties that included 6 hemispheric porouscoated cup designs, this study used a multiple linear regression to identify those factors that influenced polyethylene wear rates. Wear rates for each hip were based on serial head penetration measurement made with computer-assisted techniques. Implant factors associated with an increased wear rate included terminal sterilization with a non–cross-linking chemical surface treatment, a 4-mm lateralized liner, a cobalt-chrome femoral head, and a longer shelf life for liners c-irradiated in air. After accounting for these implant characteristics and patient factors, wear rates among the 6 cup designs were not significantly different ( P = .89). Although polyethylene wear is frequently characterized for specific implant designs, this study demonstrated that there are several common factors that influence polyethylene wear rates. Key words: primary porous-coated total hip arthroplasty, clinical outcome, polyethylene wear, effect of implant characteristics and cup design, effect of patient demographics. n 2005 Elsevier Inc. All rights reserved.
Although polyethylene wear after total hip arthroplasty (THA) is a multifactorial process influenced by implant and patient-related factors, implant design is commonly believed to play a fundamental role in the wear process. To assess the importance of cup design, this study sought to identify those factors that influenced polyethylene wear rates. In
the context of 6 different acetabular cup designs, our objective was to identify those generic and design-specific factors that were associated with polyethylene wear. Many studies have sought to identify the factors that have a significant influence on the wear process. Patient factors frequently noted in the literature include age at surgery, activity level, sex, weight, and preoperative diagnosis [1-4]. Among cementless acetabular components, implant factors that have been implicated in the wear process include cup design features related to shell holes, liner modularity, and locking mechanism [5]. In addition to the diameter and composition of the femoral head, polyethylene-related factors including liner material, fabrication method, thickness, terminal sterilization method, and oxidation associated with shelf aging after c-irradiation in air have also been associated with implant wear [1,2,4,6-18]. Surgical factors that have been implicated in the wear process include the surgeon and component placement [2,4,19]. Other factors including
From the Anderson Orthopaedic Research Institute, Alexandria, Virginia. Submitted January 15, 2005; accepted May 10, 2005. Benefits or funds were received in partial or total support of the research material described in this article. General research funding was provided by the Inova Health Services. Although no benefits or funds were received from any other commercial entity to support this research study, 3 of the authors serve as consultants for DePuy, a Johnson & Johnson company. Two of the authors receive royalties from DePuy and 1 owns Johnson & Johnson stock. Reprint requests: Robert H. Hopper, Jr, PhD, Anderson Orthopaedic Research Institute, Suite 200, 2501 Parkers Lane, P.O. Box 7088, Alexandria, VA 22307. n 2005 Elsevier Inc. All rights reserved. 0883-5403/05/1906-0004$30.00/0 doi:10.1016/j.arth.2005.05.007
63
64 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005 accumulated debris, femoral head scratching, and in vivo oxidation have also been postulated as mechanisms that may accelerate wear over time [20-22]. Among the many clinical studies that have examined polyethylene wear, most investigators generally have reported factors found to be statistically significant instead of quantifying the magnitude of each factor’s influence on the wear rate in the context of other competing factors. Using several different cementless acetabular components, this study sought to quantify the relative importance of cup design in the context of patient factors and generic implant characteristics that could be quantified.
Materials and Methods For this institutional review board–approved retrospective study, we reviewed primary THAs derived from a database maintained at our institution. The study population included the 6 cup designs (Fig. 1) that were predominantly used for the primary THAs performed at our institution between 1985 and 1999. The modular Arthropor cup (Joint Medical Products, Stamford, Conn) featured a beaded, hemispheric titanium cup with holes for rim screws that locked the polyethylene liner and provided supplemental fixation for the cup. All Arthropor liners were sterilized using ethylene oxide, a non–cross-linking chemical surface treatment. The modular ACS Triloc+ cup
(DePuy, Warsaw, Ind) also featured a hemispheric beaded surface with rim screw holes. The polyethylene liner was sterilized with c-irradiation in air and had a nonuniformed thickness that was thinner in the region of the rim. The Harris-Galante cup (Zimmer, Warsaw, Ind) featured a hemispheric geometry with titanium mesh for bone ingrowth and cavitary holes for optional screw fixation with liners that were sterilized with c-irradiation in air. The Duraloc 1200 cup had 8 to 12 shell holes depending on the outer cup diameter, but we routinely implanted it without screws, so we transitioned to the Duraloc 100 which had a single central dome hole. The modular liners for the Duraloc 100 and 1200 cups were secured with a wire-locking ring that engaged a groove machined in the liner and shell. The liners used with the Duraloc cups included Enduron and highercrystallinity Hylamer (both by DePuy). We also used a 1-piece cup similar to the Duraloc with an Hylamer liner (28 mm inner diameter) that came preassembled in the shell. In the early 1990s, the Duraloc liners and all of the 1-piece cups were sterilized with c-irradiation in air at a standard dosage of 0.025 to 0.04 MGy. During the mid1990s, some liners were terminally sterilized at standard and low dosages in barrier packaging with an inert gas. Since the latter part of 1995, we have implanted Duraloc cups that were terminally sterilizing using gas plasma. In an effort to improve hip stability, we began to use 4-mm lateralized polyethylene liners with the
Fig. 1. Hemispheric porous-coated acetabular components used at our institution included the Arthropor (A), ACS Triloc+ (B), Harris-Galante (C), Duraloc 1200 (D), Duraloc 100 (E), and 1-Piece (F) cups.
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 65
Duraloc cups in 1991. Although lipped liners were used in the early 1990s, neutral liners were used for most of the THAs during the mid-1990s. The use of lateralized liners progressively increased and by 1998, almost all cases were performed with 4-mm lateralized liners. The acetabulum was also reamed medially only enough to achieve a rim fit with the cup in approximately 458 of inclination. The combination of these techniques tended to lateralize the center of hip rotation. With the exception of the Hylamer liners used with the Duraloc and 1-Piece cups, the polyethylene material for all other cups was considered to be conventional. All modular cup designs in this study afforded the option to incorporate different femoral head sizes and liner geometries. Although the ACS Triloc+, Harris-Galante, and Duraloc 1200 cups featured shell holes for optional screw fixation, the acetabulum was routinely reamed to a diameter 1 mm less than the metal shell and the cup was press-fit without screws. Although most modular cup designs used a uniform metal shell thickness and accommodated larger outer cup diameters by increasing the polyethylene liner thickness, the Arthropor cup used a relatively thin polyethylene liner and increased the metal shell thickness for larger-diameter cups. The study population for this analysis included all THAs performed with Arthropor, ACS Triloc+, Harris-Galante, and Duraloc 100, 1200, or 1-Piece cups that had minimum 5-year x-ray follow-up, a postoperative x-ray, and at least 3 follow-up x-rays. ACS Triloc+ cups that sustained rim fractures were excluded. We also limited the study population to those THAs performed with modular femoral heads on an Anatomic Medullary Locking, (AML; DePuy), Prodigy (DePuy), or Solution (DePuy) stem. Hips with loose stems or cups were excluded. To exclude most of the head penetration associated with creep and bedding-in, the first follow-up x-ray included in our wear analysis was taken at least 0.75 years after surgery. A period of at least 4 years was required between the first and last follow-up x-rays. The study population was also restricted to those THAs with 28- or 32-mm femoral heads that had a conventional or Hylamer polyethylene liner. Total hip arthroplasties with liners that were terminally sterilized with c-irradiation in air were also excluded if the sterilization date was not known. Sterilization dates were determined by sending polyethylene catalog and lot numbers derived from implant stickers to the manufacturers. We identified 743 THAs (666 patients) that met our inclusion and exclusion criteria. During the
15-year period from 1985 through 1999 when these cases were done, 3814 primary THAs, including 568 Arthropor, 452 Triloc, 88 Harris-Galante, 231 Duraloc 1200, 1678 Duraloc 100, 77 1-Piece, and 720 other cups, were implanted at our institution. Tables 1 and 2 detail the implant and patient characteristics stratified by cup design. In most cases, the differences among implant and patient characteristics between the various cup designs were statistically significant. At our institution, anteroposterior pelvic x-rays are routinely taken with the beam centered on the pubic symphysis while the patient is supine with the legs internally rotated. Using the anteroposterior pelvic x-rays for each THA, 2-dimensional head penetration was determined for each follow-up x-ray relative to the immediate postoperative view with validated, computer-assisted techniques [23,24]. The same software was also used to determine the cup abduction and anteversion angle for each hip. A least-squares linear regression was used to determine the slope of the line that best fit the relationship between the magnitude of the wear vector and the time in situ [25]. The slope of the linear regression represented the wear rate of the polyethylene. Because at least 3 wear values were used to compute the wear rate for each THA, we also evaluated the standard error associated with the wear rate. To simultaneously assess the influence of implant, patient, and surgical factors on polyethylene wear rates, a multiple linear regression analysis was used with a stepwise variable entry and an inclusion P value of .05. Patient-related factors considered in our analysis included age at surgery, sex, weight, preoperative diagnosis, and whether a dislocation occurred. Generic implant factors included the presence of shell holes, liner geometry, femoral head composition and diameter, polyethylene liner material, sterilization technique, and shelf aging of liners sterilized with c-irradiation in air. All other implant features that were not captured by these generic factors were lumped into implant-specific variables that represented the cumulative effects of otherwise unquantified attributes unique to each cup design. These designspecific characteristics would include the resin used to manufacture the conventional polyethylene liners, proprietary polyethylene processing techniques, shell-liner conformity, liner locking mechanism, the finish of the shell’s interior surface, and other unique features. To acknowledge the possible influence of other variables, cup abduction and anteversion angles were incorporated as surgical factors and length of radiographic follow-up was
Overall Variable Sex Diagnosis
Head diameter (mm) Head material Poly material Liner geometry Terminal sterilization method
Dislocations
Categories Female Male Osteoarthritis Avascular necrosis Hip dysplasia Fracture/trauma Rheumatoid arthritis 28 32 CoCr Ceramic Conventional Enhanced (Hylamer) Neutral Lipped +4 mm c-Air Standard Dose c-Barrier Standard Dose c-Barrier low dose Gas plasma/ ethylene oxide None 1 or more
No. of THAs 387 356 591 58
Arthropor %
No. of THAs
52.1 47.9 79.5 7.8
93 87 136 21
38 28 28
5.1 3.8 3.8
403 340 519 224 576 167
ACS Triloc+
%
No. of THAs
51.7 48.3 75.6 11.7
51 43 76 2
6 6 11
3.3 3.3 6.1
54.2 45.8 69.9 30.1 77.5 22.5
41 139 164 16 180 0
314 326 103 444
42.3 43.9 13.9 59.8
50
Harris-Galante
%
No. of THAs
54.3 45.7 80.9 2.1
18 14 24 6
3 7 6
3.2 7.4 6.4
22.8 77.2 91.1 8.9 100.0 0.0
3 91 36 58 94 0
67 113 0 0
37.2 62.8 0.0 0.0
6.7
0
17
2.3
232 701 42
Duraloc 1200
%
No. of THAs
56.3 43.8 75.0 18.8
46 44 70 5
0 1 1
0.0 3.1 3.1
3.2 96.8 38.3 61.7 100.0 0.0
8 24 23 9 32 0
0 94 0 94
0.0 100.0 0.0 100.0
0.0
0
0
0.0
31.2
180
94.3 5.7
175 5
Duraloc 100
%
No. of THAs
51.1 48.9 77.8 5.6
168 149 264 21
7 3 5
7.8 3.3 5.6
25.0 75.0 71.9 28.1 100.0 0.0
28 62 56 34 63 27
15 17 0 32
46.9 53.1 0.0 100.0
0.0
0
0
0.0
100.0
0
97.2 2.8
89 5
1-Piece
%
No. of THAs
%
P
53.0 47.0 83.3 6.6
11 19 21 3
36.7 63.3 70.0 10.0
.64
18 9 5
5.7 2.8 1.6
4 2 0
13.3 6.7 0.0
31.1 68.9 62.2 37.8 70.0 30.0
293 24 226 91 207 110
92.4 7.6 71.3 28.7 65.3 34.7
30 0 14 16 0 30
100.0 0.0 46.7 53.3 0.0 100.0
25 64 1 90
27.8 71.1 1.1 100.0
177 38 102 198
55.8 12.0 32.2 62.5
30 0 0 30
100.0 0.0 0.0 100.0
0.0
0
0.0
50
15.8
0
0.0
0
0.0
0
0.0
17
5.4
0
0.0
0.0
0
0.0
0
0.0
52
16.4
0
0.0
94.7 5.3
30 2
93.8 6.3
85 5
94.4 5.6
294 23
92.7 7.3
28 2
93.3 6.7
.007
b.001 b.001 b.001
b.001
b.001
.49
66 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005
Table 1. Categorical Variables by Cup Design
Table 2. Continuous Variables by Cup Design Variable
Arthropor
743 2/85-5/99 58.9 F 11.6 (24-83) 42.8 F 7.4 (21-65)
180 2/85-1/91 57.2 F 12.7 (25-83) 44.9 F 6.2 (32-63)
18.6 F 7.6 ( 10 to 42) 0.61 F 0.82 (0-5.72) (444) 175 F 39 (80-365) 9.4 F 3.2 (5.0-18.3) 5.8 F 3.0 (3-22)
16.5 F 7.0 (1-39) None
0.12 F 0.11 ( 0.24 to 0.73) Standard error of 0.03 F 0.04 wear rate (mm/y) (0-0.52)
ACS Triloc+
Harris-Galante
Duraloc 1200
Duraloc 100
1-Piece
P
32 3/89-5/91 61.7 F 10.9 (34-82) 42.5 F 5.6 (32-53)
90 5/90-7/92 57.3 F 11.9 (24-80) 42.0 F 7.9 (24-61)
317 8/91-5/99 60.7 F 11.2 (25-83) 41.0 F 7.6 (21-65)
30 10/92-11/93 52.4 F 12.1 (25-70) 44.9 F 9.5 (26-60)
N/A N/A b.001 b.001
17.7 F 8.1 ( 10 to 35) 0.24 F 0.31 (0.02-1.53) (94) 172 F 39 (85-320) 175 F 40 (104-300) 12.9 F 3.1 (5.9-18.3) 10.6 F 2.67 (5.5-15.9) 8.1 F 4.0 (3-22) 6.0 F 2.8 (3-15)
18.2 F 6.7 (3-30) 0.42 F 0.43 (0-1.62) (32) 176 F 47 (96-260) 10.3 F 2.7 (5.7-14.1) 6.1 F 3.1 (3-15)
19.1 F 7.1 (5-36) 0.36 F 0.31 (0.01-1.53) (90) 176 F 40 (106-307) 9.1 F 2.1 (5.2-13.0) 5.4 F 2.1 (3-13)
19.9 F 7.8 (0-42) 1.00 F 1.06 (0.02-5.72) (198) 175 F 38 (80-365) 7.1 F 1.6 (5.0-12.4) 4.5 F 1.7 (3-14)
20.1 F 7.8 (6-36) 0.20 F 0.17 (0.02-0.76) (30) 175 F 28 (115-220) 9.0 F 1.2 (6.4-11.4) 5.2 F 1.6 (3-11)
b.001
0.18 F 0.11 ( 0.05 to 0.48) 0.03 F 0.03 (0-0.24)
0.08 F 0.07 ( 0.08 to 0.25) 0.03 F 0.02 (0-0.08)
0.08 F 0.08 ( 0.21 to 0.50) 0.02 F 0.06 ( 0-0.52)
0.12 F 0.12 ( 0.24 to 0.73) 0.04 F 0.04 (0-0.25)
0.11 F 0.05 (0.01-0.22) 0.02 F 0.01 (0-0.05)
94 8/87-5/90 58.8 F 9.4 (33-76) 44.8 F 6.1 (30-62)
0.08 F 0.07 ( 0.07 to 0.38) 0.03 F 0.02 (0-0.11)
Values are expressed as mean F SD (range). N/A indicates not applicable.
b.001 .96 b.001 b.001 b.001 .02
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 67
No. of THAs Surgery dates Age at surgery (y) Abduction angle (8) Anteversion angle (8) c-Air shelf life (y) (no. of hips) Weight (lb) Follow-up (y) No. of follow-up x-rays Wear rate (mm/y)
Overall
68 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005
Fig. 2. Raw wear rate data demonstrate significant differences among the cup designs ( P b .001, analysis of variance) with the Arthropor liners (A) wearing at almost twice the rate of the ACS Triloc+ (B), Harris-Galante (C), Duraloc 1200 (D), Duraloc 100 (E), and 1-Piece cups (F). The error bars designate the 95% CIs associated with the mean wear rate for each cup design.
included to characterize potential changes in wear rates that occurred over time. Categorical variables were incorporated in the regression analysis using effects coding ( 1, 0, 1) [26]. Using this coding scheme, the regression equation allowed us to assess the difference between dichotomous categorical variables. When a category included only 2 groups, a single variable was created. When a category included 3 or more groups, an effectscoding variable was created to compare each group to all other THAs in that category. Interaction terms were created to accommodate the potential influence of shelf storage after c-irradiation in air. Separate terms were included for conventional and Hylamer liners. The magnitude of each factor’s effect was based on the best-fit regression model that accounted for the largest portion of the variance in the wear rate data. When an effect magnitude is reported, the 95% confidence interval (95% CI) is also noted. After identifying the factors that significantly influenced the wear rate, the wear data for each cup design were normalized to a 60-year-old, 170-lb man with a preoperative diagnosis of osteoarthritis and no history of dislocation who was implanted with a cobalt-chrome (CoCr) femoral head and a conventional c-sterilized neutral polyethylene liner that was not shelf-aged. A post hoc power analysis was also performed. Statistical analysis was done with SPSS (Statistical Package for the Social Sciences, Chicago, Ill).
Results Among the 32 factors incorporated in the multiple linear regression, 10 were found to be
statistically significant. Based on these 10 terms, the best-fit regression accounted for 29% (r = 0.54, r 2 = 0.29, P b .001) of the variance in the wear rate data. Five implant factors were found to have a statistically significant association with the polyethylene wear rate. The dominant factor was whether the polyethylene had been sterilized with c-irradiation. Those liners sterilized with c-irradiation wore 0.099 mm/y less ( P b .001; 95% CI, 0.082-0.115 mm/y) than non–cross-linked liners that were sterilized with gas-plasma or ethylene oxide. Although c-irradiation improved wear resistance, shelf aging of Hylamer liners sterilized in air tended to increase wear rates by 0.061 mm/y for each year of shelf storage ( P b .001; 95% CI, 0.040-0.082 mm/y2). The effect of shelf aging among conventional liners sterilized with c-irradiation in air increased the wear rate by only 0.014 mm/y for each year of shelf storage ( P = .02; 95% CI, 0.002-0.025 mm/y2). Using a 4-mm lateralized liner instead of a neutral or lipped component increased the wear rate by 0.041 mm/y ( P b .001; 95% CI, 0.021-0.061 mm/y). Using a ceramic femoral head instead of CoCr was associated with a 0.018 mm/y decrease in the wear rate ( P = .04; 95% CI, 0.001-0.034 mm/y). Cup design ( P z .21), manufacturer ( P N .78), femoral head diameter ( P = .21), the presence of one dome or multiple shell holes ( P = .68), and use of low or standard dosage among c-sterilized liners ( P = .47) did not have a significant influence on wear rate. Aside from the increased wear associated with the shelf-aged liners that were
Fig. 3. After controlling for terminal sterilization method, shelf aging of liners c-irradiated in air, femoral head material, liner lateralization, and patient demographics, the wear rates for the Arthropor (A), ACS Triloc+ (B), Harris-Galante (C), Duraloc 1200 (D), Duraloc 100 (E), and 1-Piece (F) cups were not significantly different ( P = .89, analysis of variance). The error bars designate the 95% CIs associated with the mean wear rate for each cup design.
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 69
c-irradiated in air, no other difference was found among Hylamer and conventional polyethylene liners ( P = .11) (Figs. 2 and 3). Five patient-related factors were found to have a statistically significant association with the polyethylene wear rate. A 1-year increase in age at the time of surgery was associated with a 0.003 mm/y decrease in wear rate ( P b .001; 95% CI, 0.0020.004 mm/y). This translated into a 0.03 mm/y decrease per decade increase in age. Each 10-lb increase in patient weight was also associated with a 0.003 mm/y decrease in the wear rate ( P = .002; 95% CI, 0.001-0.005 mm/y). Male sex was associated with a 0.025 mm/y increase in wear rate ( P = .002; 95% CI, 0.009-0.040 mm/y). One or more dislocations were associated with a 0.046 mm/ y increase in the wear rate ( P = .002; 95% CI, 0.016-0.076 mm/y). A preoperative diagnosis of osteoarthritis tended to increase the wear rate by 0.030 mm/y compared with other diagnoses ( P = .002; 95% CI, 0.011-0.048 mm/y). There was no significant difference in wear rates among patients with preoperative diagnoses of trauma ( P = .68), inflammatory arthritis ( P = .78), hip dysplasia ( P = .91), or osteonecrosis ( P = .99). Polyethylene wear rate was not significantly associated with cup abduction angle ( P = .75), anteversion ( P = .61), or length of radiographic follow-up ( P = .35). Based on the 743 THAs included in this analysis and the 10 factors that accounted for 29% of the variance in the wear rate data, this study had 90% power to detect an additional factor that accounted for at least 1% of the wear rate variance (an incremental r 2 of 0.01).
Discussion Although the wear rates among the different cup designs were statistically different ( P = .001, Table 2), these differences could be accounted for based on generic implant factors and patient characteristics. The ACS Triloc+, Harris-Galante, and Duraloc 1200 cups wore at a mean rate of 0.08 mm/y (Table 2). These 3 groups had similar patient demographics and all liners were sterilized with c-irradiation in air. Although 30% of the Duraloc 1200 cups featured Hylamer liners that were sterilized with c-irradiation in air, the mean shelf age for these components was relatively short (0.29 years) and had a minor influence on the mean wear rate for this group, increasing it by 0.005 mm/y. A modestly higher mean wear rate of 0.11 mm/y among the 1-Piece cups can be
attributed to the younger mean patient age of 52.4 years and shelf aging of the Hylamer liners c-irradiated in air. Compared with other groups, the increased mean wear rate of 0.12 mm/y among the Duraloc 100 cups can be attributed to the use of Hylamer liners sterilized with c-irradiation in air with a mean shelf age of 0.79 years in 19% of cases (60 THAs), terminal sterilization with gas plasma among 16% of cases (52 THAs), and the use of 4 -mm lateralized liners among 32% of cases (102 THAs). The highest mean wear rate of 0.18 mm/y among the Arthropor cups can be explained almost entirely by the use of ethylene oxide, a non–cross-linking chemical surface treatment, for terminal sterilization. Although polyethylene wear is commonly characterized for specific implant designs, accounting for patient factors and generic implant characteristics revealed that the specific implant design did not play a prominent role in the wear process. Although our study population consisted of only 24% (743/3094) of the primary THAs performed at our institution with the 6 cup designs included in our analysis, this was primarily because of our inclusion criteria that required at least 3 follow-up x-rays to compute a wear rate for each hip. Although we did not analyze a consecutive series of patients, the integrity of our regression analysis technique only requires a distributed, unbiased sampling. Because the mean follow-up for this study was 9.4 years and implant wear is generally asymptomatic at intermediate follow-up, we do not believe that those patients returning for routine follow-up would have biased wear rates compared with other patients who returned less frequently. Although the multiple linear regression analysis allowed us to simultaneously assess the influence of many factors, several potential limitations should be considered. Because the study was not randomized, the factors potentially influencing wear were not equally represented among the cup designs comprising the study population. Although our regression analysis accounted for the potential influence of 32 factors, it did so only in the context of a linear relationship between these factors and the wear rate. It remains possible that some of the factors examined in this analysis are related to wear in a nonlinear fashion and that interactions between factors that were not incorporated in this analysis may also influence wear rates. However, in view of the small portion of the wear rate variance that each independent factor explains in our linear analysis, we consider it unlikely that introducing nonlinear terms or incorporating additional interaction effects by combining
70 The Journal of Arthroplasty Vol. 20 No. 7 Suppl. 3 October 2005 factors would account for a substantial portion of the variance in the wear rate data. Although a prospective study design would be preferable, we believe that a multiple linear regression analysis considering all eligible cases is superior to retrospectively matched groups that are more likely to mask persistent surgical biases without eliminating them. Reducing the study population to more homogeneous subgroups would also reduce the power of our analysis and limit our ability to compare different factors. Although this study demonstrated that the wear process is multifactorial and identified several factors that have relatively subtle influences on wear rates, the 10 factors identified as statistically significant accounted for only 29% of the variance in the wear data. As a consequence, we can identify those factors that are important to the wear of a population of THAs but cannot accurately predict the wear rate for an individual THA patient. The primary reason for the relatively low r 2 likely stems from the fact that implant usage was not precisely quantified among the factors we examined. Although several investigators have identified a significant correlation between age at surgery and activity level [27,28], a more precise measure of implant usage would likely improve the predictive value of our analysis. It could also simplify the relationship between patient factors and wear rate because the differences attributed to age, weight, sex, and diagnosis may all represent underlying differences in activity level. We expect that quantifying patient activity level and using longer follow-up data will enable us to refine future models and identify other factors that have a substantial influence on the wear rate. Unquantified factors potentially influencing wear, such as third-body debris, may account for some of the variance in the wear data. The uncertainty associated with the wear rate calculated for each hip also contributes to the variance that is unaccounted for by our model. The standard error, reported in Table 2, reflects the fact that serial head penetration values for an individual THA tend to deviate from a straight line. Although we used the computer-assisted techniques of Martell et al [23] and Devane et al [24] to measure radiographic head penetration, a prior study from our institution [29] found similar accuracy and reproducibility among these methods with good-quality radiographs. Deviations from a perfectly linear head penetration pattern could potentially stem from temporal changes in the head penetration rates, the presence of multiple wear vectors, or suboptimal x-ray quality. Failure to fully reduce the femoral
head when the supine x-ray was taken and accuracy limitations associated with using computerassisted techniques to measure femoral head penetration on clinical x-rays [24,30] could also introduce measurement errors. Despite these potential sources of nonlinearity, we found no relationship between length of radiographic follow-up and wear rate ( P = .35). Prior studies based on clinical data from our institution have also indicated that the long-term femoral head penetration rate tends to remain fairly constant [22,31]. We found that terminal sterilization with c-irradiation was associated with reduced wear rates for several different cup designs. This observation is consistent with clinical [7,9,32] and laboratory [13] findings that terminal sterilization with c-irradiation at nominal dosages of 0.025 to 0.04 MGy reduces polyethylene wear rates by approximately one half compared to non–cross-linking chemical surface treatments. In our study, non–cross-linked polyethylene sterilized by ethylene oxide or gas-plasma demonstrated a 0.099 mm/y increase in wear rate. Based on an average wear rate of slightly less than 0.10 mm/y for the c-irradiated liners included in this study, our findings confirm that terminal sterilization with c-irradiation reduced the wear rate of non–cross-linked polyethylene by about one half. Although 4-mm lateralized liners were associated with a wear rate that was increased by 0.041 mm/y, this increase may reflect the cumulative effects of cup and liner lateralization. Among the Duraloc cups that were implanted with lateralized liners, there was also a tendency to lateralize the cup to improve postoperative stability. We suspect that joint reactive force, determined by liner geometry and patient anatomy as well as the position of the cup and stem, may be the underlying factor. In future work, incorporating joint reactive force may help to better delineate the role of factors influencing wear in the context of the reconstructed joint geometry. Because c-irradiation dosage determines polyethylene cross-linking, we expected that liners sterilized at a low dosage would have higher wear rates than liners sterilized at a standard dosage. The absence of a relationship between sterilization dosage and wear rate may be explained, in part, by the small number of low-dosage liners included in this study, the relatively wide range of radiation exposures among the dosage groups, and the variability in actual radiation exposure for an individual liner during the terminal sterilization process. Cup modularity is commonly regarded as a potential source of increased wear. In this study,
Effect of Cup Design on Polyethylene Wear ! Hamilton et al 71
however, we found no difference between the factory-preassembled 1-Piece cup and the other modular designs ( P = .86). This result suggests that differences in backside wear among the components we analyzed were probably not significant. Although we identified statistically different wear rates among several different cup designs, these differences were associated with patient demographics or generic implant characteristics including terminal sterilization technique, femoral head material, liner lateralization, and shelf aging. After accounting for differences in patient demographics and generic implant characteristics, we were unable to attribute improved wear performance to any particular design. Although it remains possible that some designs incorporated a combination of unique features, some of which may have decreased wear rates whereas others increased wear, the net effect of these unquantified factors was not associated with perceptible differences in wear performance. When considering new designs, surgeons should be skeptical of modifications or innovations that are purported to improve wear performance if the design features have never been shown to significantly influence polyethylene wear rates based on clinical data. We should also note that this study examined polyethylene wear, not osteolysis.
Acknowledgment The authors gratefully acknowledge the general research funding provided by Inova Health Care Services to support this study.
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