Retrieval Analysis of Failed Constrained Acetabular Liners

The Journal of Arthroplasty Vol. 24 No. 6 Suppl. 1 2009

Retrieval Analysis of Failed Constrained Acetabular Liners Steven N. Shah, MD,*y Rachel J. Kaye, BS,z Natalie H. Kelly, MS,z Edwin P. Su, MD,* Douglas E. Padgett, MD,* and Timothy M. Wright, PhDz

Abstract: Despite the large loads placed upon constrained acetabular liners, little is known of their mechanical performance. We analyzed retrieved liners to determine wear and other damage modes and assess associations between types and severity of damage and clinical, radiographic, and implant variables. Outer rim impingement frequency and severity were higher than that for the inner rim. The 20° elevation was most frequently affected by impingement. Inner rim impingement was more frequent with small heads. Outer bearing surface wear scores were higher than inner bearing scores. Liners removed for infection or stem failure had similar damage compared with other groups, demonstrating the complex relationship of impingement and wear with clinical performance. No association was found between liner damage and clinical and radiographic variables. Keywords: revision total hip arthroplasty, constrained acetabular liner, impingement, dislocation, retrieval analysis. © 2009 Published by Elsevier Inc.

Constrained acetabular liners are viable treatment options for selected unstable total hip arthroplasties. However, these components are subjected to high loads, resulting in mechanically related failure rates that average 10% but have been reported as high as 29% using locking ring designs [1-19]. One particular constrained liner, manufactured by Stryker (Mahwah, NJ), has had the best clinical results, with 96% success at 10 years [9,17]. However, we have experienced an increasing number of failures because follow-up times have increased. Several published studies have investigated the role of impingement as a mechanical cause of failure using retrieved conventional total hip components [2025]. Our objective was to perform a retrieval study on constrained acetabular liners, with the following specific goals: (1) assess the damage (impingement and wear) in a retrieved sample of failed constrained liners and (2) determine the effect of clinical, radiographic, and implant design variables on the damage incurred.

From the *Hip and Knee Service, Hospital for Special Surgery, New York, New York; ySlocum Center for Orthopedics and Sports Medicine, Eugene, Oregon; and zDepartment of Biomedical Mechanics and Materials, Hospital for Special Surgery, New York, New York. Submitted October 6, 2008; accepted May 5, 2009. The authors acknowledge the support of the Clark and Kirby Foundations. The implant retrieval system is maintained in part by the Zimmer-HSS research fund. Reprint requests: Steven N. Shah, MD, Slocum Center for Orthopedics and Sports Medicine, 55 Coburg Road, Eugene, OR 97401. © 2009 Published by Elsevier Inc. 0883-5403/09/2406-0012$36.00/0 doi:10.1016/j.arth.2009.05.002

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Methods Seventy Stryker Trident constrained liners, all with a 20° elevation, were retrieved at revision as part of a large, ongoing, institutional review board–approved implant retrieval system at our institution [26]. Three implants that had been severely damaged during removal were excluded. All retrieved implants, which had been received from the operating room via the pathology department, had previously been ultrasonically cleaned in a mild detergent bath, cataloged with an identification number (to insure patient anonymity), and stored as part of the standard retrieval system. The components were not autoclaved or otherwise sterilized between removal and examination. The Trident constrained liner consists of an inner and outer bearing surface and has thus been described as a “tripolar” design (Fig. 1). The retrieved liners were analyzed by sectioning the outer locking ring and dissociating the inner bearing. Evaluation of components was performed via visual and stereomicroscopic examination (at magnifications up to 31 times). The rims were graded for the location and severity of impingement and for the mode of wear. The bearing surfaces were graded for the location, mode, and severity of wear. Outer rim impingement location was defined by 4 quadrants, with quadrant 1 centered over the 20° elevated rim. Inner rim impingement location was not subdivided into quadrants. Outer and inner rim impingement severity was graded on a 4-point scale based upon depth of impingement into the rim [21]. A score of 1 denoted minimal impingement. Scores of 2, 3, and 4 were assigned for damage less than 1 mm, between 1 and 2

Constrained Liner Retrieval  Shah et al

Fig. 1. A photograph of a Stryker Trident constrained acetabular liner.

mm, and greater than 2 mm into the rim, respectively. Wear depth was measured with an electronic caliper by a single observer. Wear mode was described as one of the following: surface deformation, burnishing, pitting, embedded third-body debris, scratching, abrasion, and delamination [26]. Wear severity was graded on a 4point scale according to percentage of bearing surface area affected [26]. A score of 0 denoted no damage in a given quadrant. Scores of 1, 2, and 3 were assigned for damage on less than 10%, between 10% and 50%, and greater than 50% of a given quadrant, respectively. Total damage scores were recorded. A maximum score of 84 would describe a liner with level 3 damage in all 4 quadrants for all 7 damage modes. Patient medical records provided demographic data including age, sex, height, weight, and body mass index (BMI). Clinical data included indication for the constrained liner, failure mechanism (revision diagnosis), and length of implantation. Implant design variables included cup, liner, head, and neck size. Radiographs were examined to assess cup and/or liner abduction angle and offset. Statistical analyses (rate ratio, t test, χ2, and analysis of variance) were performed to examine for associations between clinical, radiographic, and implant variables and liner damage.

Results Medical records were available for 64 of the 70 retrieved liners; radiographs were available for 59.

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There were 32 males and 38 females with average age and BMI of 65.2 years and 26, respectively. Average length of implantation was 26.2 months (range, 0-125 months). Average cup abduction angle, liner abduction angle, and offset were 44.1° (range, 28°-64°), 42.3° (range, 20°-66°), and 37.6 mm (range, 25-53 mm), respectively. Indications for use of a constrained liner included recurrent instability in primary (18.8%) and revision (32.8%) arthroplasty, failure of a previous constrained liner (10.9%), prophylaxis in the revision setting without a previous history of instability (29.7%), alcohol abuse/ neuropathic condition (3.1%), and routine exchange during femoral revision (4.7%). The prophylaxis group included hips with inadequate posterior soft tissues and/or poor abductors. Noncemented cases included constrained liners snapped into a cementless Osteonics shell impacted during the same surgery [27], and constrained liners snapped into a previously implanted, well-fixed, Osteonics shell [3]. Cemented cases included liners cemented into a well-fixed shell from another manufacturer [22], liners cemented into a well-fixed reconstruction cage [12], and liners cemented directly into the bony acetabulum [6]. Failure mechanisms necessitating removal of the constrained liners included bearing surface failure (26.6%), liner dissociation (26.6%), cup/cage failure (15.6%), infection (18.8%), femoral stem failure (9.4%), and biomaterial failure (4.7%). Bearing surface failure refers to liners that failed because of single or multiple episodes of dislocation of the femoral head. Liner dissociation describes failure at the liner/cup, liner/ cage, or liner/bony pelvis interface. Cup/cage failure includes failures in which fixation of the hemispherical shell or reconstruction cage to the bony pelvis failed. Femoral stem failure indicates those modular liners that underwent routine exchange during a revision procedure performed for problems on the femoral side. Biomaterial failure describes those cases in which an intrinsic material failure (eg, a broken metallic constraining ring) in the liner created a clinically significant indication for revision surgery. The frequency of outer rim impingement (56 liners/ 80.0%) was significantly higher than that for the inner Table 1. Occurrence of Wear on the Outer Rim Compared With That Found on the Inner Rim Rim Wear Frequency (%) Wear Mode Scratching Burnishing Pitting Surface deformation Abrasion Delamination Embedded debris

Outer Rim 98.6 97.1 98.6 84.3 25.7 17.1 8.6

Inner Rim 97.1 95.7 100 20.0 10.0 8.6 11.4

P N.05 N.05 N.05 b.05 b.05 N.05 N.05

56 The Journal of Arthroplasty Vol. 24 No. 6 Suppl. 1 September 2009 Table 2. Occurrence of Wear on the Outer Bearing Surface Compared With That Found on the Inner Bearing Surface Bearing Surface Wear Frequency (%) Wear Mode

Outer

Inner

P

Scratching Burnishing Pitting Surface deformation Abrasion Delamination Embedded debris

100 91.4 91.4 7.1 17.1 17.1 60.0

90.0 70.0 87.1 4.3 7.1 12.9 31.4

N.05 N.05 N.05 N.05 N.05 N.05 b.05

rim (11 liners/15.7%), as was the average impingement severity score (2.8 vs 1.4) (P b .05). The 20° elevation on the outer rim (quadrant 1) was involved in 43 (76.8%) liners with impingement; this frequency was significantly higher than that for quadrants 3 and 4 (P b .05), but not for quadrant 2. Inner rim impingement was more likely to occur with 22-mm heads (vs 26 or 28 mm) (P b .05). Outer and inner rim wear are summarized in Table 1. Of the 7 damage modes, the frequencies of surface deformation and abrasion were the only differences, being significantly higher in the outer rim (P b .05). Outer and inner bearing surface wear are summarized in Table 2. The frequency of embedded third-body debris was significantly higher in the outer bearing surface (P b .05). The average outer bearing surface wear score (27.2) was also statistically higher (P b .05) than that for the inner bearing surface (24.6). Outer and inner bearing surface wear scores were positively correlated with longer implantation (P b .05). No significant associations were found between liner damage and sex, BMI, surgical indication, failure mechanism, cup or liner abduction angle, offset, neck size, head-toneck ratio, inner to outer bearing surface diameter ratio, or cup size. Well-functioning liners removed for infection or stem failure had similar degrees of damage compared with all other groups.

Discussion

The indications for implantation of these 70 liners were consistent with published recommendations, as were the reasons for their removal [15-19]. For example, in a clinical review of 29 failed constrained liners, Yun et al [19] defined 4 failure modes that included failure of fixation to the pelvis, liner dissociation, biomaterial failure, and femoral head dislocation. In our group of liners, more than a fourth (28.2%) was clinically well functioning when retrieved for infection or femoral component failure. This group served as an internal control for comparison with the other group of liners that were removed for mechanical failure. Interestingly, damage incurred by these 2 groups was similar, demonstrating the complex relationship of impingement and wear with clinical performance. Similar observations were made in a recent study that examined retrieved

unconstrained liners, in which the prevalence of impingement in well-functioning liners removed in the setting of infection or femoral stem failure (55%) was similar to the group as a whole (59%) [21]. The frequency of outer rim impingement (80%) was higher than that reported for unconstrained liners in primary total hip arthroplasties (39%-59%), whereas involvement of the inner rim was relatively rare [21,23,27]. Impingement involving the elevated rim was significant, suggesting that the currently available liners without elevation should be used preferentially. Impingement of the inner rim was less common for larger heads, suggesting use of such heads when possible, although potentially at the expense of increased wear and deformation. The prevalence of wear on the 4 evaluated surfaces was high. Scratching, burnishing, and pitting were nearly ubiquitous. The high frequency of embedded third-body debris in both the outer (60.0%) and inner (31.4%) bearing surfaces demonstrates the ease with which debris can become entrapped in these closely conforming surfaces and the marked contribution to wear made by embedded debris [28]. Wear and subsequent osteolysis with component loosening is a potential late mechanism of failure for liners with longer implantation times. We found no significant associations between liner damage and implant design variables, with the exception of inner rim impingement being less common with larger heads. Although the range of motion before impingement (72°-84°) depends upon the inner to outer diameter ratio, we did not find an association between this ratio and impingement scores [29]. Most patients are likely exceeding 84° of motion on a frequent basis; therefore, optimizing this ratio may not be clinically critical. Our study has limitations. As with any study evaluating retrieved implants, conclusions that can be drawn are limited in that the implants were retrieved at reoperation and, thus, might not represent well-functioning hip arthroplasties. Another limitation was that the accuracy of our procedure for measuring wear depth has not been validated. Moreover, there was an inability to assess cup version because of inadequate radiographs. Finally, the in vivo orientation of the liner (eg, the location of the elevated rim) could not be determined, which limited our ability to examine fully relationships between orientation and wear. Despite these limitations, we believe this study presents valuable information about retrieved constrained liners that could potentially aid future implant use and design. This particular constrained liner has had excellent clinical results at 10-year follow-up, but our retrieval experience suggests that there is a high incidence of impingement and wear that may lead to implant failure.

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