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. 2015 Jan 6;473(3):1011–1021. doi: 10.1007/s11999-014-4105-9

Metal-on-conventional Polyethylene Total Hip Arthroplasty Bearing Surfaces Have a Higher Risk of Revision Than Metal-on-highly Crosslinked Polyethylene: Results From a US Registry

Elizabeth W Paxton 1,✉, Maria C S Inacio 1, Robert S Namba 1,2, Rebecca Love 1, Steven M Kurtz 3
PMCID: PMC4317451  PMID: 25560957

Abstract

Background

Although studies have reported lower radiological wear in highly crosslinked polyethylene (HXLPE) versus conventional polyethylene in total hip arthroplasty (THA), there is limited clinical evidence on the risk of revision of these polyethylene THA bearing surfaces.

Questions/purposes

We asked: (1) Do primary THAs with a metal-on-conventional polyethylene bearing surface have a higher risk of revision (all-cause or aseptic) than metal-on-HXLPE? (2) Is the risk of revision (all-cause or aseptic) higher for conventional polyethylene versus HXLPE when the effect of femoral and acetabular components is controlled for in prosthesis-specific analyses?

Methods

The Kaiser Permanente’s Total Joint Replacement Registry was used to identify metal-on-conventional polyethylene and metal-on-HXLPE primary THAs (N = 26,823) performed between April 2001 and December 2011. The registry has 95% voluntary participation and 8% were lost to followup during the 10-year study period. Endpoints of interest were all-cause and aseptic revisions. Descriptive statistics and marginal Cox regression models with propensity score adjustments were applied to compare risk of revision for metal-on-conventional polyethylene versus metal-on-HXLPE THAs and to evaluate two specific manufacturers’ hip implant designs while controlling for femoral and acetabular components. Of the 26,823 THAs included in the study, 1815 (7%) were metal-on-conventional polyethylene and 25,008 (93%) were metal-on-HXLPE.

Results

At 7 years followup, the cumulative incidence of revision was 5.4% (95% confidence interval [CI], 4.4%–6.7%) for metal-on-conventional and 2.8% (95% CI, 2.6%–3.2%) for metal-on-HXLPE. There was a higher adjusted risk of all-cause (hazard ratio [HR], 1.75; 95% CI, 1.37–2.24; p < 0.001) and aseptic (HR, 1.91; 95% CI, 1.46–2.50; p < 0.001) revisions among metal-on-conventional polyethylene bearing surface hips compared with metal-on-HXLPE. Results were similar within manufacturer hip designs with the same femoral and acetabular components. Conclusions Metal-on-conventional polyethylene THA bearing surfaces have a higher risk of revision compared with metal-on-HXLPE bearing surfaces. Clinicians should consider the use of HXLPE when using a polyethylene bearing in THA.

Level of Evidence

Level II, cohort study.

Introduction

Ultrahigh-molecular-weight polyethylene wear and associated aseptic loosening and osteolysis are leading causes of long-term THA revision [26]. The rates of loosening and osteolysis in metal-on-conventional polyethylene THAs have been reported to range from 9% to 47% [3, 5, 6]. Highly crosslinked polyethylene (HXLPE) was introduced to reduce wear and THA revision rates; however, there is limited information about the reduced risk of revision associated with HXLPE compared with conventional polyethylene in THA.

Several hip simulator and randomized clinical trials (RCTs) have evaluated HXLPE versus conventional polyethylene wear. Simulator studies report decreased femoral penetration and wear in HXLPE compared with conventional polyethylene [13, 15]. Radiological evaluations of in vivo liner wear in RCTs have also found lower wear of HXLPE versus conventional polyethylene [2, 5, 12, 16, 27]. Meta-analyses and systematic reviews also suggest that HXLPE has lower femoral penetration and wear than conventional polyethylene [8, 10, 14]. Although these findings suggest decreased wear of HXLPE liners, these studies have not evaluated reduction in risk of THA revision rates.

Findings from studies that have examined THA revision rates in relationship to polyethylene formulation are conflicting. Although some studies report a reduction in risk of revision rate for metal-on-HXLPE versus metal-on-conventional [17], others did not find an increased risk or did not investigate reduction in risk of THA revision [3, 7]. These prior study findings are limited by the small sample sizes from single-center and academic institutions, loss to followup, and limited length of followup. Methodological differences and investigation of a variety of implant designs also limit the use of current findings.

Larger, registry-based studies have reported a higher risk of revision for conventional polyethylene versus HXLPE [1, 22]. These studies are important in that they provide large samples on a wide range of patients across multiple settings by surgeons with various experience levels. However, as a result of the limited availability of data from US registries, there is currently a reliance on information about THA bearing surface performance from other countries.

Therefore, the purpose of this study was to compare risk of revision of metal-on-HXLPE compared with a metal-on-conventional polyethylene bearing surface in primary THAs using a large US registry. Specifically: (1) Do primary THAs with a metal-on-conventional polyethylene bearing surface have a higher risk of revision (all-cause or aseptic) than metal-on-HXLPE? (2) Is the risk of revision (all-cause or aseptic) higher for conventional polyethylene versus metal-on-HXLPE when the effects of femoral and acetabular components are controlled for in prosthesis-specific analyses?

Patients and Methods

A retrospective cohort study was conducted. Kaiser Permanente’s Total Joint Replacement Registry (TJRR) was used to identify cases during the study period. Data collection procedures, participation, and coverage of this TJRR have been published [19, 21]. In brief, the TJRR covers over 9 million members of an integrated healthcare system in seven geographical regions in the United States and enrolls over 20,000 joint arthroplasties a year. The registry has 95% voluntary participation and only 8% were lost to followup during the 10-year study period [20]. All elective nonbilateral primary THAs, in which patients were at least 18 years old at the time of their procedure and had metal-on-conventional polyethylene or metal-on-HXLPE bearing surfaces registered between April 1, 2001, and December 31, 2011, were included in the sample. Revision procedures, bilateral (same-day) primary procedures, and conversion procedures were not included. The overall study sample (N = 26,823) included all metal-on-conventional polyethylene and metal-on-HXLPE hips; cohorts for prosthesis-specific analysis to control for the femoral and acetabular components consisted of Duraloc (DePuy Inc, Warsaw, IN, USA) (N = 1146) and Reflection (Smith & Nephew Inc, Memphis, TN, USA) (N = 5202) THA cohorts. The cohort included cases from 51 medical centers and 333 surgeons were included.

The majority of the 26,823 primary THAs included in the study were women (n = 16,170 [60%]), white (n = 20,559 [77%]), had a body mass index < 30 kg/m2 (n = 16,233 [61%]), and had an American Society of Anesthesiologists (ASA) score of 1 or 2 (n = 15,374 [57%]) at the time of their surgery. The mean age of the total THA cohort was 70 years (SD = 10), and the prevalence of diabetes was 23% (n = 6239) (Table 1). Of the 26,823 THAs included in the study, 1815 (7%) had metal-on-conventional polyethylene bearing surfaces, and 25,008 (93.2%) had metal-on-HXLPE bearing surfaces. The median followup for this cohort was 2.9 years (interquartile range [IQR] 1.3–5.5 years). There were 1146 THAs in the Duraloc cohort, of which 382 (33%) had metal-on-conventional polyethylene and 764 (67%) had metal-on-HXLPE (Table 2). The median followup for this cohort was 8.2 years (IQR 5.8–9.2 years). There were 5202 THAs in the Reflection cohort, of which 753 (15%) had metal-on-conventional polyethylene and 4449 (86%) had metal-on-HXLPE (Table 3). The median followup for this cohort was 5.1 years (IQR 3.4–7.0 years). The conventional polyethylene cohorts included liners that were only gas-sterilized (uncrosslinked) or were gamma radiation-sterilized, corresponding to a dose of 25 to 40 kGy. The HXLPE cohorts included eight individual formulations with varying technical characteristics (Table 4).

Table 1.

Patient, surgeon, implant, and hospital characteristics for the total THA cohort, 2001–2011

Variables All Bearing surface
Conventional polyethylene HXLPE
Number Percent Number Percent Number Percent
26,823 1815 7 25,008 93
Sex Male 10,649 40 653 36 9996 40
Female 16,170 60 1160 64 15,010 60
Missing 4 <0.01 2 0.1 2 <0.01
ASA category 1 or 2 15,374 57 1028 57 14,346 57
≥ 3 10,704 40 706 39 9998 40
Unknown 745 3 81 5 664 3
BMI category (kg/m2) < 30 16,233 61 1088 60 15,145 61
≥ 30 and < 35 6263 23 370 20 5893 24
≥ 35 3857 14 214 12 3643 15
Unknown 470 2 143 8 327 1
Race Asian/Pacific Islander 886 3 94 5 792 3
Black 1882 7 61 3 1821 7
Native American 41 0.2 4 0.2 37 0.1
White 20,559 77 1232 68 19,327 77
Hispanic 1804 7 114 6 1690 7
Other 358 1 46 3 312 1
Unknown 1293 5 264 15 1029 4
Diabetes Yes 6239 23 468 26 5771 23
Surgeon fellowship Yes 10,893 41 718 40 10,175 41
Missing 45 0.2 0 <0.1 45 0.2
Surgeon average annual volume category < 10 1448 5 71 4 1377 6
10 to < 50 16,097 60 1145 63 14,952 60
≥ 50 9276 35 599 33 8677 35
Missing 2 <0.01 0 <0.1 2 <0.01
Site average annual volume category < 100 4436 17 395 22 4041 16
100 to < 200 13,703 51 889 49 12,814 51
≥ 200 8682 32 531 29 8151 33
Missing 2 <0.01 0 0.1 2 <0.01
Fixation Uncemented 20,991 78 727 40 20,264 81
Hybrid 3871 14 914 50 2957 12
Cemented 181 1 26 1 155 1
Missing 1780 7 148 8 1632 7
Head size ≤ 28 mm 1420 78 3773 15
32 mm 341 19 10,420 42
36 mm 54 3 9620 38
> 36 mm 0 0.2 1195 5
Cup Type Converge (Zimmer, Warsaw, IN, USA) 0 <0.1 463 2
Duraloc (DePuy Synthes, Warsaw, IN, USA) 382 21 764 3
Other 101 6 732 3
Pinnacle (DePuy Synthes) 9 1 12,877 52
Reflection (Smith & Nephew, Memphis, TN, USA) 753 42 4449 18
S-Rom (DePuy Synthes) 133 7 14 <0.01
Sector (DePuy Synthes) 49 3 523 2
Trabecular Metal (Zimmer) 1 <0.1 3160 13
Trident (Stryker, Kalamazoo, MI, USA) 369 21 353 1
Trilogy Ace (Zimmer) 1 0.1 1624 7
Mean SD Mean SD Mean SD
Membership termination
 Yes 2159 8 284 16 1875 8
Operative time (minutes) 93 34 98 33 93 34
Age (years) 70 10 72 10 69 10
BMI (kg/m2) 29 6 29 6 29 6
Days of followup 1286 973 2361 970 1208 926
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HXLPE = highly crosslinked polyethylene; ASA = American Society of Anesthesiologists; BMI = body mass index.

Table 2.

Patient, surgeon, implant, and hospital characteristics for the Duraloc cohort, 2001–2011

Variables Bearing surface
All Conventional polyethylene HXLPE
Number Percent Number Percent Number Percent
All 1146 382 33 764 68
Sex Male 472 41 130 34 342 45
Female 673 59 251 66 422 55
Missing 1 0.1 1 0.3 0 0
ASA category 1 or 2 713 62 248 65 465 61
≥ 3 384 34 99 26 285 37
Unknown 49 4 35 9 14 2
BMI category (kg/m2) < 30 617 54 220 58 397 52
≥ 30 and < 35 241 21 71 19 170 22
≥ 35 169 15 36 9 133 17
Unknown 119 10 55 14 64 8
Race Asian/Pacific Islander 24 2 12 3 12 2
Black 60 5 15 4 45 6
Native American 0 0 0 0 0 0
White 785 69 257 67 528 69
Hispanic 67 6 20 5 47 6
Other 12 1 4 1 8 1
Unknown 198 17 74 19 124 16
Diabetes Yes 257 22 83 22 174 23
Surgeon fellowship Yes 286 25 66 17 220 29
Missing 0 0 0 0 0 0
Surgeon average annual volume category < 10 64 6 14 4 50 7
10 to < 50 551 48 154 40 397 52
≥ 50 531 46 214 56 317 42
Missing 0 0 0 0 0 0
Site average annual volume category < 100 9 1 5 1 4 1
100 to < 200 437 38 94 25 343 45
≥ 200 700 61 283 74 417 55
Missing 0 0 0 0 0 0
Fixation Uncemented 527 46 59 15 468 61
Hybrid 471 41 268 70 203 27
Cemented 15 1 2 1 13 2
Missing 133 12 53 14 80 11
Head Size ≤ 28 mm 322 84 387 51
32 mm 60 16 344 45
36 mm 0 0 33 4
> 36 mm 0 0 0 0
Mean SD Mean SD Mean SD
Membership termination
 Yes 214 19 76 20 138 18
Operative time (minutes) 97 35 101 33 95 36 < 0.001 (NP)
Age (years) 68 12 70 11 66 12 < 0.001
BMI (kg/m2) 30 6 28 6 30 6 < 0.001
Days of followup 2582 1021 2697 1051 2525 1002 < 0.001 (NP)
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HXLPE = highly crosslinked polyethylene; ASA = American Society of Anesthesiologists; BMI = body mass index; NP = nonparametric.

Table 3.

Patient, surgeon, implant, and hospital characteristics for the Reflection cohort, 2001–2011

Variables Bearing surface
All Conventional polyethylene HXLPE
Number Percent Number Percent Number Percent
All 5202 753 15 4449 86
Sex Male 1899 37 273 36 1626 37
Female 3302 64 479 64 2823 64
Missing 1 0.1 1 0.1 0 0
ASA category 1 or 2 2937 57 376 50 2561 58
≥ 3 2188 42 358 48 1830 41
Unknown 77 2 19 3 58 1
BMI category (kg/m2) < 30 3161 61 455 60 2706 61
≥ 30 and < 35 1162 22 144 19 1018 23
≥ 35 734 14 95 13 639 14
Unknown 145 3 59 8 86 2
Race Asian/Pacific Islander 200 4 21 3 179 4
Black 392 8 34 5 358 8
Native American 9 0.2 3 0.4 6 0.1
White 3721 72 501 67 3220 72
Hispanic 449 9 73 10 376 9
Other 71 1 9 1 62 1
Unknown 360 7 112 15 248 6
Diabetes Yes 1307 25 195 26 1112 25
Surgeon fellowship Yes 1948 37 334 44 1614 36
Missing 0 0.1 0 1 0 0
Surgeon average annual volume category < 10 336 7 41 5 295 7
10 to < 50 3569 69 614 82 2955 66
≥ 50 1297 25 98 13 1199 27
Missing 0 0.2 0 0.1 0 0
Site average annual volume category < 100 756 15 54 7 702 16
100 to < 200 3592 69 570 76 3022 68
≥ 200 854 16 129 17 725 16
Missing 0 0.1 0 0.4 0 0
Fixation Uncemented 3315 64 256 34 3059 69
Hybrid 1549 30 446 59 1103 25
Cemented 56 1 6 1 50 1
Missing 282 5 45 6 237 5
Head size ≤ 28 mm 685 91 1719 39
32 mm 68 9 2065 46
36 mm 0 0.1 665 15
> 36 mm 0 0.4 0 0
Mean SD Mean SD Mean SD
Membership termination
 Yes 534 10 128 17 406 9
Operative time (minutes) 90 31 94 31 90 31
Age (years) 72 9 73 8 71 9
BMI (kg/m2) 29 6 29 6 29 6
Days of followup 1899 869 2412 1029 1812 808
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HXLPE = highly crosslinked polyethylene; ASA = American Society of Anesthesiologists; BMI = body mass index.

Table 4.

Summary of highly crosslinked polyethylene formulations in the present study and their technical characteristics

Number of patients undergoing THA HXLPE brand name Manufacturer (city, state) Total (cumulative) radiation crosslinking dose (kGy) HXLPE stabilization technology
11,510 Marathon DePuy Synthes (Warsaw, IN, USA) 50 Thermal treatment (remelting)
4,979 XLPE Smith & Nephew (Memphis, TN, USA) 100 Thermal treatment (remelting)
4,802 Longevity Zimmer (Warsaw, IN, USA) 100 Thermal treatment (remelting)
2,693 ALTRX DePuy Synthes 75 Thermal treatment (remelting)
466 Durasul Zimmer 95 Thermal treatment (remelting)
402 X3 Stryker Orthopaedics (Mahwah, NJ, USA) 90 Thermal treatment
(sequential annealing)
54 E1 Biomet (Warsaw, IN, USA) 130 Vitamin E
31 ArCom XL Biomet 50 Mechanical annealing
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HXLPE = highly crosslinked polyethylene.

Revision was the outcome of interest. All-cause revision included procedures for any reason in which removal and reimplantation of a component occurred at any time after the original index procedure. Aseptic revision was defined as a revision for which infection was not a reason performed any time after the original index procedure. The TJRR prospectively monitors all registered hips for subsequent revisions. After identification of a possible revision by the TJRR through electronic algorithms or surgeon reporting, the hip in question was reviewed by trained clinical research experts (see Acknowledgments), who adjudicated the event and confirmed the reason for revision.

Exposure and Covariates

The type of bearing surface used was the exposure of interest (metal-on-conventional polyethylene versus metal-on-HXLPE). Variables thought to be related to both bearing surface and revision-free survival time were included in a propensity score model to adjust for observed confounding. The variables included continuous covariates for age, operative time, body mass index, surgeon average annual volume, and hospital average annual volume as well as categorical covariates for gender, ASA score [18], diabetes diagnosis, race (six categories), and surgeon total joint arthroplasty fellowship training status.

Statistical Analysis

Frequencies, proportions, means and SDs as well as medians and IQRs were used to describe the total THA cohort and the Duraloc and Reflection cohorts within the two bearing surface groups. Cumulative incidence of revision was calculated. Crude cumulative incidence of all-cause and aseptic revision rate/100 years of observation (revision density) and reasons for revision were calculated for the total THA and the Duraloc and Reflection cohorts.

Revision rate/100 years of observation was compared using a Poisson regression. Because bearing surface material was not randomly assigned, we addressed observed confounding using a propensity score approach [4, 25]. The objective for using propensity scores was to remove or reduce confounding so that the magnitude of bias in the estimated treatment effect was negligible. Propensity score methods can minimize confounding by making the treatment groups equal (or approximately so) on a collection of measured variables. The fundamental theoretical property of propensity score methods is that hips with the same correctly estimated propensity scores will be comparable with respect to all covariates used to calculate the propensity scores so that it is only a matter of chance as to whether each actually receives one treatment or the other. In the specific approach used, the following steps were taken: (1) the propensity score was estimated in the conventional way by fitting a logistic regression model and estimating the conditional probability of treatment assignment for each record; (2) we checked that cases in one bearing group had comparable counterparts with respect to their covariate distribution in the other bearing group and those that did not were excluded based on a caliper width of 0.2 SD of the logit propensity score; (3) we stratified the sample into six strata based on the estimated logit propensity score; and finally (4) we calculated the weight for each record based on the number of units in a stratum multiplied by the proportion of units assigned to the treatment group of interest in the data and divided by the number of records assigned to the treatment group of interest in that particular stratum. Missing data were handled using multiple imputation. Ten imputed data sets were created and Rubin’s rules for aggregating parameter estimates and variances were used [23]. Logistic regression models were used to generate propensity scores.

Marginal multivariate Cox regression models accounting for surgeon clustering using robust variance estimation were fit with stratification (five strata) by propensity score for each imputed data set and results were subsequently aggregated across data sets [11]. Additionally, some of the analytic models also used regression adjustment for surgeon volume, site volume, and hybrid fixation to address imbalance remaining in these variables after stratification by propensity score. All analyses used metal-on-HXLPE as the reference group. Hazard ratios (HRs) with 95% confidence intervals (CIs) and Wald p values are provided. For the primary analysis models, individuals not experiencing a revision were treated as censored as of whichever date came first: the study end date (December 31, 2011), a membership termination date, or date of death. Data were analyzed using SAS (Version 9.2; SAS Institute, Cary, NC, USA) and p < 0.05 was used as the threshold for statistical significance. In this study, hypothesis testing was focused on the adjusted HR for the comparison of the bearings for three groups (total THA cohort, Duraloc cohort, Reflection cohort) for each of two outcomes (all-cause and aseptic revision), leading to six tests and an increased chance of committing a Type I error. Under a conservative approach of assuming these tests are independent, the Bonferroni-adjusted alpha is 0.0056.

Sensitivity Analysis

Based on the distribution of head size, the two bearings surfaces were not comparable for head size with metal-on-HXLPE containing head sizes > 36 mm, whereas conventional PE did not. Conventional PE also contained very few 36-mm heads. To address this issue we conducted a sensitivity analysis removing head sizes ≥ 36 mm and only included two categories: head size ≤28 mm and head size 32 mm. We included this head size variable in the propensity score model as well. We also examined whether the effect of the bearing was moderated by cup type. To do this we compared the bearing surface effect estimate for Duraloc versus Reflection for each of the outcomes using Wald chi square tests.

Results

Risk of Revision, All THA: Conventional Polyethylene versus HXLPE

The adjusted risks of all-cause (HR, 1.75; 95% CI, 1.37–2.24; p < 0.001) and aseptic (HR, 1.91; 95% CI, 1.46–2.50; p < 0.001) were higher in patients with metal-on-conventional polyethylene bearing surfaces compared with metal-on-HXLPE (Table 5). At 7 years followup, the cumulative incidence of revision was 5.4% (95% CI, 4.4%–6.7%) for metal-on-conventional and 2.8% (95% CI, 2.6%–3.2%) for metal-on-XLPE. The all-cause revision density for metal-on-conventional hips was 0.76 (95% CI, 0.68–0.84) revisions/100 years of followup and 0.60 (95% CI, 0.57–0.63) for metal-on-HXLPE hips (Table 6). The main reasons for revision in the metal-on-conventional polyethylene group were instability (49%), aseptic loosening (20%), infection (15%), and other (22%) (Table 6). The main reasons for revision in the metal-on-HXLPE group were instability (40%), infection (25%), periprosthetic fracture (13%), and other (14%). When accounting for differences in femoral head size distribution, the results were not substantively different from those previously reported for the overall effect (ie, without any cup restriction) (HR, 1.69; 95% CI, 1.19–2.40; p = 0.003 [all-cause]; HR, 1.73; 95% CI, 1.22–2.44; p = 0002 [aseptic]). Therefore, it appears that the size of the femoral head is not able to explain most of the differences observed in the performance of the bearings.

Table 5.

Propensity score-weighted regression results for risk of all-cause and aseptic revision in conventional bearings compared with HXLPE bearings for the overall THA, Duraloc, and Reflection cohorts

All (N = 26,823) Duraloc cohort (N = 1146) Reflection cohort (N = 5202)
Crude
HR (95% CI)		 Adjusted*
HR (95% CI)		 p value
(adjusted model)		 Crude
HR (95% CI)		 Adjusted*
HR (95% CI)		 p value
(adjusted model)		 Crude
HR (95% CI)		 Adjusted*
HR (95% CI)		 p value
(adjusted model)
All-cause 1.73 (1.38–2.18) 1.75 (1.37–2.24) < 0.001 2.67 (1.54–4.64) 3.15 (1.65–6.02) < 0.001 1.83 (1.21–2.77) 1.93 (1.23–3.01) 0.004
Aseptic 1.91 (1.48–2.45) 1.91 (1.46–2.50) < 0.001 2.35 (1.30–4.25) 2.87 (1.43–5.78) 0.003 2.14 (1.36–3.37) 2.44 (1.49–3.48) < 0.001
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* Cox regression stratified by propensity score and also using surgeon volume, site volume, and hybrid fixation versus other as covariates; propensity score model includes body mass index (continuous), age (continuous), gender, race, diabetes, diagnosis, surgeon average volume, fellowship status, site average volume, American Society of Anesthesiologists, and operative time. HXLPE = highly crosslinked polyethylene; HR = hazard ratio; CI = confidence interval.

Table 6.

Crude all-cause and aseptic cumulative incidence of revision, revision rate per 100 years of followup, and reasons for revision for the overall THA, Duraloc, and Reflection cohorts

Revision type and reason All Duraloc Reflection
Conventional polyethylene HXLPE Conventional polyethylene HXLPE Conventional polyethylene HXLPE
Number 1815 25008 382 764 753 4449
All-cause Number of revisions (%) 89 (4.9) 495 (2.0) 30 (7.9) 22 (2.9) 31 (4.1) 87 (2.0)
Rate/100 years observation time (95% CI) 0.8 (0.7–0.8) 0.6 (0.6–0.6) 1.1 (0.9–1.3) 0.4 (0.3–0.5) 0.6 (0.5–0.7) 0.4 (0.4–0.4)
Aseptic Number of revisions (%) 76 (4.2) 371 (1.5) 24 (6.3) 20 (2.6) 27 (3.6) 64 (1.4)
Rate/100 years observation time (95% CI) 0.7 (0.6–0.7) 0.5 (0.4–0.6) 0.9 (0.7–1.0) 0.4 (0.3–0.5) 0.5 (0.4–0.7) 0.3 (0.3–0.3)
Reasons for revisiona Instability 44 (49.4) 200 (40.4) 13 (43.3) 15 (68.2) 20 (64.5) 35 (40.2)
Infection 13 (14.6) 124 (25.1) 6 (20.0) 2 (9.1) 4 (12.9) 23 (26.4)
Periprosthetic fracture 7 (7.9) 62 (12.5) 0 (0.0) 2 (9.1) 3 (9.7) 10 (11.5)
Aseptic loosening 18 (20.2) 51 (10.3) 8 (26.7) 3 (13.6) 3 (9.7) 7 (8.0)
Pain 8 (9.0) 43 (8.7) 3 (10.0) 3 (13.6) 2 (6.5) 6 (6.9)
Femoral fracture 3 (3.4) 23 (4.6) 0 (0.0) 1 (4.5) 1 (3.2) 6 (6.9)
Hematoma/seroma 1 (1.1) 19 (3.8) 0 (0.0) 0 (0.0) 1 (3.2) 0 (0.0)
Hematoma 1 (1.1) 14 (2.8) 0 (0.0) 0 (0.0) 1 (3.2) 0 (0.0)
Linear wear 5 (5.6) 13 (2.6) 3 (10.0) 0 (0.0) 2 (6.5) 3 (3.4)
Wound drain 1 (1.1) 12 (2.4) 0 (0.0) 0 (0.0) 1 (3.2) 0 (0.0)
Compound fracture 1 (1.1) 5 (1.0) 0 (0.0) 0 (0.0) 0 (0.0) 1 (1.1)
Failed ORIF 1 (1.1) 2 (0.4) 0 (0.0) 0 (0.0) 1 (3.2) 0 (0.0)
Rheumatoid arthritis 0 (0.0) 1 (0.2) 0 (0.0) 1 (4.5) 0 (0.0) 0 (0.0)
Other 19 (21.4) 67 (13.5) 10 (33.4) 2 (9.0) 8 (25.9) 15 (17.2)
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aNumber (%), percents are based on the total number of revisions in the specific cohort

HXLPE = highly crosslinked polyethylene; CI = confidence interval; ORIF = open reduction and internal fixation.

Risk of Revision, THA Design-specific: Conventional Polyethylene versus HXLPE

Within the Duraloc cohort, the adjusted risks of all-cause (HR, 3.15; 95% CI, 1.65–6.02; p < 0.001) and aseptic (HR, 2.87; 95% CI, 1.43–5.78; p = 0.003) revision were higher in patients with metal-on-conventional polyethylene compared with those with metal-on-HXLPE bearing surfaces (Table 5). The 7-year cumulative incidence of revision was 8.3% (95% CI, 5.8%–11%) for metal-on-conventional polyethylene versus 2.6% (95% CI, 1.7%–4.2%) for metal-on-HXLPE polyethylene (Table 7). The all-cause revision density for metal-on-conventional polyethylene hips was 1.06 (95% CI, 0.87–1.26) revisions/100 years of followup and 0.42 (95% CI, 0.33–0.51) for metal-on-HXLPE hips (Table 6). The main reasons for revision in the metal-on-conventional polyethylene group were instability (43%), aseptic loosening (27%), infection (20%), and other (33% each). The main reasons for revision in the metal-on-HXLPE group were instability (68%), aseptic loosening (14%), pain (14%), infection (9%), and periprosthetic fracture (9%).

Table 7.

Yearly number of procedures at risk and cumulative incidence of revision by bearing surface for the overall THA, Duraloc, and Reflection cohorts

Bearing surface Number at risk, Year 0 Cumulative incidence of revision, Year 1 (%, CI) Number at risk, Year 1 Cumulative incidence of revision, Year 5 (%, CI) Number at risk, Year 5 Cumulative incidence of revision, Year 7 (%, CI) Number at risk, Year 7 Cumulative incidence of revision, Year 10 (%, CI) Number at risk, Year 10
HXLPE 25,008 1.4 (1.3–1.6) 19,361 2.4 (2.2–2.7) 6289 2.8 (2.6–3.2) 2672 3.9 (2.9–5.2) 62
Conventional polyethylene 1815 2 (1.5–2.8) 1706 4 (3.2–5.1) 1289 5.4 (4.4–6.7) 816 6.4 (5.1–7.9) 74
Duraloc HXLPE 764 1.3 (0.7–2.4) 725 2.1 (1.3–3.5) 583 2.6 (1.7–4.2) 480 4.9 (2.5–9.5) 19
Duraloc conventional polyethylene 382 2.4 (1.3–4.6) 357 6.9 (4.7–10.1) 283 8.3 (5.8–11.8) 245 9.1 (6.4–12.8) 37
Reflection HXLPE 4449 1.2 (0.9–1.6) 4244 2 (1.6–2.5) 2086 2.2 (1.7–2.7) 844 2.7 (2–3.7) 16
Reflection conventional polyethylene 753 2.2 (1.3–3.5) 703 3.1 (2–4.6) 530 4.6 (3.2–6.6) 385 5.2 (3.6–7.4) 33
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CI = confidence interval; HXLPE = highly crosslinked polyethylene.

Within the Reflection cohort, the adjusted risks of all-cause (HR, 1.93; 95% CI, 1.23–3.01; p = 0.004) and aseptic (HR, 2.44; 95% CI, 1.49–3.48; p < 0.001) were higher in patients with metal-on-conventional polyethylene compared with those with metal-on-HXLPE bearing surfaces (Table 5). The 7-year cumulative incidence of revision was 4.6% (95% CI, 3.2%–6.6%) for metal-on- conventional polyethylene versus 2.2% (95% CI, 1.7%–2.7%) for metal-on-HXLPE (Table 7). The all-cause revision density for metal-on-conventional polyethylene hips was 0.63 (95% CI, 0.51–0.74) revisions/100 years of followup and 0.39 (95% CI, 0.35–0.44) for metal-on-HXLPE (Table 6). The main reasons for revision in the metal-on-conventional polyethylene group were instability (65%), other (26%), infection (13%), periprosthetic fracture (10%), and aseptic loosening (10%). The main reasons for revision in the metal-on-HXLPE group were instability (40%), infection (26%), other (17%), and periprosthetic fracture (12%).

The hypothesis testing, assuming tests for the outcomes by cohort are independent, found that all tests that would be significant under an alpha of 0.05 would still be significant with this stricter threshold (Table 5). Despite apparent differences in the magnitude of the HR when examining the effect of the bearing moderated by cup type, none of these tests achieved statistical significance: chi square (1) = 1.48, p = 0.223 (all-cause), chi square (1) = 0.14, p = 0.709 (aseptic).

Discussion

Osteolysis associated with polyethylene wear is a long established cause of THA revision of surgery [3, 9, 24, 26]. A reduction in polyethylene liner wear therefore should reduce THA revision. Although prior studies suggest differences in radiologically measured wear in metal-on-HXLPE versus metal-on-conventional polyethylene bearing surfaces, findings regarding reduction in risk of revision are conflicting [7, 17] and limited based on sample sizes, single-center and academic studies, limited length of followup, methodological differences, and investigation of a variety of implant designs. Larger population-based registry studies have primarily focused on countries outside of the United States [11]. Our study provides the risk of THA revision associated with conventional versus HXLPE-polyethylene in a large US representative sample. The strengths of our study include the large, representative US sample, the ability to evaluate different implant designs with different HXLPE formulations, and the inclusion of revision as the study endpoint, which has been reviewed and adjudicated by trained clinical content experts. In our study, the risk of all-cause and aseptic revision in primary THA was higher for metal-on-conventional polyethylene bearings compared with metal-on-HXLPE bearing surfaces.

This study had a number of limitations. First, this study is observational and it is possible that we did not address every potential confounding variable in our analyses. In our study, we addressed confounding using propensity score-matching techniques to address differences in the conventional and HXLPE groups. Second, to control for femoral and acetabular components, we included only two cohorts (Duraloc and Reflection) with sufficient samples in the subgroup prosthesis-specific analyses. Third, lack of radiological, functional, or patient-reported outcomes may be perceived as limitations. However, revision is the definitive endpoint of wear, which HXLPE was designed to address. Finally, followup for greater than 10 years is necessary to evaluate longer-term results. Despite this limitation, the benefits of HXLPE are already observed within our study.

Our study findings confirm the results of in vitro hip simulator and other clinical studies that compared HXLPE with conventional polyethylene liners. Similar to Nakashima et al [17], we found an increased risk of revision for conventional polyethylene versus HXLPE. Our results differ from Howard et al’s [7] study, which did not report a higher risk of revision in conventional polyethylene versus HXLPE. Most likely this difference is related to limitation in statistical power associated with sample size because rates were similar to our study but did not reach statistical significance. The higher risk of revision in metal-on-conventional polyethylene bearing surfaces in our US sample is consistent with results reported by the Australian Orthopedic Association National Joint Replacement Registry [1]. Similar to the Australian Registry results, the difference between HXLPE and conventional polyethylene is evident in less than 10 years followup. These findings suggest that metal-on-conventional bearing surfaces have a higher risk of revision in both populations.

Within the Duraloc and Reflection cohorts, metal-on-conventional polyethylene also had a higher risk of revision than metal-on-HXLPE bearings. This finding confirms findings from registries from other countries [1] in a US sample of THA and emphasizes that higher risk of revision for conventional polyethylene is consistent when controlling for femoral and acetabular components.

In conclusion, in a large US population-based study, metal-on-conventional polyethylene THA bearing surfaces had a higher risk of revision compared with metal-on-HXLPE bearing surfaces. Clinicians should consider the use of HXLPE when using a polyethylene bearing in THA.

Acknowledgments

We thank all the Kaiser Permanente orthopaedic surgeons who contribute to the TJRR and the Surgical Outcomes and Analysis Department, which coordinates Registry operations.

Footnotes

One of the authors (SMK) is an employee and shareholder of Exponent, Inc (Menlo Park, CA, USA) and received institutional support as a Principal Investigator from Smith & Nephew (Memphis, TN, USA), Stryker (Mahwah, NJ, USA), Zimmer (Warsaw, IN, USA), Biomet (Warsaw, IN, USA), DePuy Synthes (Warsaw, IN, USA), Medtronic (Memphis, TN, USA), Invibio (Thornton Cleveleys, UK), Stelkast (McMurray, PA, USA), Formae (Paoli, PA, USA), Kyocera Medical (Kyoto, Japan), Wright Medical Technology (Memphis, TN, USA), Ceramtec (Plochingen, Germany), DJO (Vista, CA, USA), Celanese (Florence, KY, USA), Aesculap (Tuttlingen, Germany), Spinal Motion (Mountain View, CA, USA), and Active Implants (Memphis, TN, USA), outside the submitted work.

All ICMJE Conflict of Interest Forms for authors and Clinical Orthopaedics and Related Research ® editors and board members are on file with the publication and can be viewed on request.

Clinical Orthopaedics and Related Research ® neither advocates nor endorses the use of any treatment, drug, or device. Readers are encouraged to always seek additional information, including FDA-approval status, of any drug or device prior to clinical use.

Each author certifies that his or her institution approved the human protocol for this investigation, that all investigations were conducted in conformity with ethical principles of research, and that informed consent for participation in the study was obtained.

This work was performed at Kaiser Permanente, San Diego, CA, USA.

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