We selected from archived M-M hip retrievals 32 specimens that were submitted with an unusual soft tissue reaction described by the revising surgeons as an aseptic “soft tissue mass,” “enlarged bursa,” or a “cyst” which could be considered as “pseudotumor-like.” Twenty-seven of the 32 cases were hip resurfacings (four articular surface replacements (ASR, DePuy International, Leeds, UK), 20 Birmingham Hip Resurfacings (BHR, Smith and Nephew, Memphis, TN), two Conserve Plus hip resurfacings (Wright Medical Technology, Memphis, TN), and one McMinn resurfacing (Corin, Cirencester, UK). The remaining five cases were conventional total hip arthroplasties (one Biomet M2 THA, Biomet, Warsaw, IN), one big femoral head THA (Wright Medical Technology), and three Metasul bearing total hips (Zimmer, Warsaw, IN). There were 23 females and nine males with average ages of 54 years (range, 18–68 years) and 62 years (range, 48–82 years), respectively. As documented by the revising surgeons, the reasons for revision were acetabular malposition (steep abduction angle, excessive or insufficient anteversion, n = 15), unexplained pain (i.e., in the absence of infection, radiographic loosening or malposition, and where metal sensitivity was suspected, n = 9), and aseptic loosening (n = 5).
We calculated the wear depth of 24 of the 32 explanted components including HRAs that had not been sectioned and were still intact by digitizing 300 to 400 points on the bearing surface with a coordinate measurement machine (BMT 504; Mitotoyo, Aurora, IL). The remainder had been sectioned without prior wear measurements for a separate study. The resolution of this equipment was approximately 4 μm, so wear depths at or below this level were considered “undetectable.” Acetabular cup abduction angles were measured by the revising surgeons on AP radiographs in 28 of the 32 cases using standard radiographic techniques [33
] (four cases had poor-quality radiographs deemed unsuitable for this analysis). This involved measuring the angle between a line connecting the ischial spines and another line drawn tangent to the opening of the cup, representing the large diameter of the ellipse. This method is widely used in clinical practice for postoperative measurement of cup position [35
Twenty-two of the tissue samples submitted were large, smooth-walled sacs, and some of the tissues were clearly metal-stained (Fig. ). All of the tissues were fixed in 10% formalin immediately after removal. They were weighed, measured, photographed, and their gross appearance was noted. From two to five tissue samples from several sites, especially if there were variations in color or texture of the specimen, were embedded in paraffin blocks for routine sectioning and staining with hematoxylin and eosin. Three of us (PC, SN, KT) examined at least six tissue specimens per case semiquantitatively for lymphocytes, macrophages, plasma cells, giant cells, necrosis and metal wear particles using the method of Doorn et al. [11
], i.e., where a zero to 3 plus score is given as features of interest become more numerous in a high power 40× microscopic field of view. This type of method reportedly has an interobserver agreement level of 0.91 [3
Fig. 1 An enlarged fluid-filled bursa excised from the hip of a male patient during revision surgery for acetabular malpositioning 13 months after metal-on-metal hip resurfacing arthroplasty is shown. There is light gray discoloration and the wear measurement (more ...)
Each case was also given an ALVAL (aseptic lymphocytic vasculitis-associated lesion) score of 1 to 10 (Table ); ALVAL has been applied to a unique, lymphocyte-dominated reaction in M-M periprosthetic tissues [37
]. To check the reproducibility of scoring, two of us (PC, KT) performed the scoring in a blinded fashion on two separate occasions. The kappa coefficient for interobserver variability showed a correlation between the two observers of 0.71 and between the two separate measurements of each observer of 0.68. Using only the histologic features and the ALVAL scores, each observer predicted whether each case failed in association with high-wear, suspected metal hypersensitivity, or some other cause. The kappa coefficient for interobserver variability was used to determine the validity of these predictions against the actual wear measurement from the retrievals. The kappa coefficient for the agreement between the first observer’s prediction and the retrieval findings was between 0.69 and 0.81 and the second observer’s between 0.43 and 0.73. The kappa coefficient for the prediction regarding the association of a case with high wear was higher for the observer with many years of experience with histologic analysis of retrieval tissues (PC) compared with the second, less experienced observer.
Histologic scoring criteria for ALVAL score
The independent variable considered in this study was whether the patients were revised with suspected high wear or with suspected metal hypersensitivity. The dependent variables were the histologic features related to the intensity of the inflammatory reaction: ALVAL score, lymphocytes, macrophages, plasma cells, giant cells, necrosis, and metal particles. Univariate analysis was used to determine the mean, median, SD, and distribution for each variable as necessary. The Mann-Whitney test was used to compare the femoral wear and wear rate of patients with suspected high wear with those suspected to have metal hypersensitivity. Likewise, histologic ratings for ALVAL were compared in these two groups. The Mann-Whitney test was determined to be appropriate because the dependent variables were not normally distributed.