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Symposium: 2014 Hip Society Proceedings

Highly Crosslinked Polyethylene Improves Wear But Not Surface Damage in Retrieved Acetabular Liners

Pang, Hee-Nee MD1; Naudie, Douglas D. R. MD1; McCalden, Richard W. MD1,a; MacDonald, Steven J. MD1; Teeter, Matthew G. PhD1

Author Information

1Division of Orthopaedic Surgery, London Health Sciences Centre, University Hospital, 339 Windemere Road, N6A 5A5, London, ON, Canada

ae-mail; [email protected]

One or more of the authors or the department with which they are affiliated have received institutional and/or research support from DePuy Orthopaedics (Warsaw, IN, USA; SJM); Smith & Nephew (Memphis, TN, USA; DDRN, RWM, MGT); and Stryker Orthopaedics (Mahwah, NJ, USA; DDRN).

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.

Clinical Orthopaedics and Related Research: February 2015 - Volume 473 - Issue 2 - p 463-468
doi: 10.1007/s11999-014-3858-5
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Abstract

Introduction

THA is a highly successful procedure, but its survivorship has been limited by aseptic loosening and osteolysis [8, 16]. This process is closely related to the polyethylene debris generated at metal-on-polyethylene articulations [10, 22]. Highly crosslinked polyethylene (XLPE) has been an important development, demonstrating superior wear properties to conventional polyethylene (CPE) in hip simulators [12] and clinical trials using radiographic evaluations [9, 18, 20, 23-25, 33]. It has been postulated that the reduction in wear debris will limit osteolysis and increase the longevity of total hip implants [7, 21, 26].

Despite this evidence for the superiority of XLPE, there have been concerns that crosslinking can increase material brittleness, leading to higher risks of rim fracture and damage. Recent retrieval studies have demonstrated no difference in damage and rim fractures between XLPE and CPE acetabular liners retrieved from patients [27, 30]. However, it is possible that the semiquantitative grading of damage resulting from wear may not correlate with the actual wear or the wear rate of the retrieved polyethylene liners. Micro-CT has been reported as a useful tool for quantifying wear in retrieval studies, including those of acetabular liners [28, 32].

The overall objective of this study was to compare wear and damage between matched retrieved XLPE and CPE acetabular liners. The specific purposes of this study were (1) to compare the visual damage scores and distribution of surface damage in these liners; and (2) to quantify depth of penetration of the liners using micro-CT as a proxy for wear.

Materials and Methods

Thirteen XLPE acetabular liners were retrieved under an institution review board-approved protocol between 1999 and 2011. These represented all the FDA-approved highly crosslinked liners of a specific design (Reflection XLPE; Smith & Nephew, Memphis, TN, USA) in the retrieval laboratory with a 26- or 28-mm head and implanted for at least 1 year. These retrieved liners were matched to a group of CPE liners of identical design (Reflection; Smith & Nephew) that were retrieved in the same time period. Fifty-four CPE liners were available for matching. These two groups were matched in terms of patient age, sex, liner dimensions, duration of implantation, and reason for revision (Table 1). If there were two or more matches, the liner belonging to the patient with the most identical body mass index measurement was chosen. Most of the liners were neutral liners with two 20° lipped liners in each group. The XLPE and CPE implants share a common design, polyethylene resin (GUR 105), and sterilization (ethylene oxide), with the only difference being the crosslinking of the polyethylene (10 Mrad gamma irradiation with remelting in the XLPE group). All operations were performed though a modified Hardinge approach. An uncemented proximal0coated stem (Synergy; Smith & Nephew) was used in each case. A cobalt-chrome femoral head was used in most cases with just one oxidized zirconium head used in each group.

T1-13
Table 1:
Patient demographics

Routine postoperative radiographs were obtained in the clinic. The abduction angle was determined from these digitized standard AP pelvic radiographs. The inclination of the acetabular cup was measured as the angle between the inter-teardrop line and the line bisecting the superior and inferior rims of the cup. This was done to ensure that any difference in damage and wear was not the result of cup malposition.

The groups appeared well matched, confirming the efficacy of our matching protocol. There was no difference in the demographic data of the patients in both groups (Table 1). The mean age of the patients in the study was 63 ± SD 14 years old. The overall mean duration of implantation of the retrieved acetabular liners was 5 ± SD 4 years. Instability, infection, and aseptic loosening were the primary reasons for revision in both groups. There was also no difference in inclination angle between groups. The median abduction angle of the acetabular cups was 46° (range, 38°-52°) in the CPE group and 44° (range, 37°-51°) in the XLPE group.

The polyethylene liners underwent an identical sanitation protocol at our implant retrieval laboratory that included storage in 10% formalin solution and cleansing in 10% bleach solution. The articular surface and the rim were divided into eight areas, and the backside compartment was also inspected. These areas were evaluated with a validated surface damage scoring system [4], and the seven common damage modes were identified and scored. These included pitting, scratching, burnishing, abrasions, impingement, embedded debris, and delamination. Two blinded observers (H-NP, MGT) scored the damage according to the percentage of area affected and the severity of each damage mode.

Each liner was scanned using micro-CT and analyzed using a previously described approach [30, 32]. The surface geometry of each liner was reconstructed using a laboratory micro-CT scanner (eXplore Vision 120; GE Healthcare, London, Ontario, Canada). Scans were completed using an isotropic resolution of 50 μm. Three-dimensional micro-CT analysis software (MicroView Version 2.2; GE Healthcare) was used to generate three-dimensional surfaces from the scans. The retrieved liner geometries were coregistered to never-implanted scanned geometries of the same size, and the deviations between the two (resulting from wear and creep) were quantified. The maximum depth of penetration was measured for each retrieved liner, and the wear rate was calculated by dividing the penetration by the time in situ. The volumetric loss as quantified by micro-CT was also calculated (Figs. 1, 2).

F1-13
Fig. 1:
An example is shown of the reconstructed three-dimensional geometry acquired using micro-CT for a CPE liner with severe bedding-in deformation.
F2-13
Fig. 2:
Example deviation map is shown from micro-CT imaging revealing deviations (resulting from wear and creep) in a XLPE liner. Scale is in millimeters.

Statistical analysis was performed with SPSS statistical software (Version 11.0; SPSS, Chicago, IL, USA). Univariate analysis was performed with chi square or the Fisher's exact test for comparison of proportions between two categorical data. The Mann-Whitney U test was used to compare the nonparametric data between two independent samples. A p value < 0.05 was considered significant.

Results

There was no difference between groups (p = 0.32) for total damage score (Table 2) or for the bearing, rim, or backside surfaces damage scores. The most common damage modes in the XLPE group were abrasion, scratching, and cold flow. The most common damage modes in CPE were burnishing, abrasion, and cold flow. The mean abrasion damage score was higher (p = 0.03) in the XLPE group (9 ± SD 5) than the CPE group (3 ± SD 2), whereas the mean burnishing score was higher (p = 0.02) in the CPE group (8 ± SD 5) than the XLPE group (3 ± SD 2). The superior quadrants demonstrated greater mean damage scores (19 ± SD 9) than the inferior quadrants (16 ± SD 8) in both groups. No rim fractures were detected in either group.

T2-13
Table 2:
Damage scoring

There was three times greater penetration (odds ratio, 3.1; confidence interval, 2.3-5.1; p < 0.001) in the CPE group (mean 0.18 ± SD 0.09 mm/year) versus the XLPE group (mean 0.05 ± SD 0.07 mm/year) (Table 3). In the CPE group, maximum penetration ranged from 0.09 mm to 5 mm, whereas in the XLPE group, maximum penetration ranged from 0 mm to 2 mm. The amount of head penetration was less for the XLPE in all areas: the articular surface (mean 0.3 mm ± SD 0.5 mm, p = 0.042), rim (0.14 ± SD 0.18 mm, p = 0.009), and backside of the liner (0.08 ± SD 0.13 mm, p = 0.042). There was less volumetric loss in XLPE (82 ± SD 134 mm3) versus the CPE group (350 ± SD 342 mm3; p = 0.017). There was no correlation between the amount of articular surface damage and depth of penetration (r2 = 0.075, p = 0.22).

T3-13
Table 3:
Micro-CT measurements

Discussion

Highly crosslinked polyethylene has been developed with superior wear properties to conventional polyethylene [12, 17, 25, 33]. It has been postulated that the reduction in wear debris will limit osteolysis and increase the longevity of the implants [26, 27, 30]. However, it is possible that the semiquantitative grading of damage resulting from wear may not correlate with actual wear or the wear rate of the retrieved liners. This fact is confirmed by our study, which demonstrated that although there was no major difference in surface damage scores between polyethylene types, there was a substantial decrease in head penetration (a surrogate for wear) with the use of XLPE.

Like with all retrieval studies, our study is limited by its retrospective nature and by examining failed implants, which may not be representative of well-functioning implants. The micro-CT measurements of penetration included the initial creep-dominated phase as well as the subsequent slower phase of polyethylene wear. In vitro studies have demonstrated that creep continues for 2 million loading cycles [11, 14], which may be achieved in 3 months to 2 years postsurgery [15]. The actual steady-state wear rate for these liners may be considerably less than the penetration rate presented here. However, this limitation affects both groups equally [14], and therefore the relative steady-state wear rates between them would be maintained or may even be proportionately more disparate. Finally, we relied on never-implanted reference components for the prewear geometry of the liners, which may vary from the true prewear state of each component as a result of manufacturing tolerances. Once again, this limitation affects both groups equally, and the tolerances are much lower than the amount of wear seen, especially in the CPE group. The relative level of penetration between the XLPE and CPE groups would thus be maintained.

Previous retrieval studies described the general condition and apparent wear of XLPE through damage scoring [4-6, 19, 29]. However, these studies did not include quantitative wear measures nor did they measure performance between XLPE and an identical (with the exception of crosslinking) contemporary CPE component. Schroder et al. [30] reported no difference in visual damage between retrieved XLPE and CPE liners as was the case in our study for overall damage scores. With respect to specific damage modes, XLPE demonstrated more abrasion and scratches than CPE in this study, whereas CPE had more burnishing. This is similar to the findings by Salineros et al. [29]. XLPE has a lower wear rate that prolongs the longevity of the articular surface from third-body debris, whereas CPE has a higher wear rate that tends to smooth out scratches through removal of surface material. Longer-term studies are required to investigate the effect of crosslinking on fatigue resistance [1, 2, 13].

With respect to the quantitative measurements of wear, our findings were similar to those of other studies comparing the wear rate of XLPE and CPE [14, 25, 33]. Micro-CT has been validated for assessing polyethylene damage in retrieval studies, and this technique can examine the surface geometry at high resolution without destroying the implant [3, 31, 32]. Although there was no difference in damage scores between the XLPE and CPE groups, the CPE group had a three times greater wear rate (odds ratio, 3.1; confidence interval, 2.3-5.1) (as measured by micro-CT). There was no correlation between the penetration rates and damage scores. Therefore, damage scoring may not correlate with wear, and one should be circumspect using damage scores as a surrogate marker for wear. However, damage scores do provide additional, useful data beyond the quantitative micro-CT measurements of wear, specifically relating to the damage modes that occurred while the implant was in situ.

In summary, XLPE demonstrated less wear than CPE based on quantitative measurements provided by micro-CT, which was not apparent using damage scoring alone. Longer-term studies are required to determine the effect on implant longevity, but the superior wear properties of XLPE found in this study support its clinical use.

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