Hip Resurfacing Arthroplasty: Moving Toward Sex-Specific Surgery?: Commentary on an article by Siôn Glyn-Jones, MA, DPhil, FRCS(Orth), et al.: “The In Vivo Linear and Volumetric Wear of Hip Resurfacing Implants Revised for Pseudotumor”

Commentary

Although pseudotumors have been reported infrequently following hip resurfacing arthroplasty, their potential for periarticular soft-tissue damage has heightened interest with regard to delineating their etiology. Glyn-Jones et al. report their observations on wear patterns from femoral and acetabular components retrieved from eighteen patients with a diagnosis of pseudotumor and a control group of eighteen patients who underwent revision for other modes of hip arthroplasty failure, including fracture, infection, instability, pain, and loosening. Their observations provide additive insight into the pathogenesis of pseudotumors that augments a growing body of literature assessing this clinical concern^1-3. The data presented in this study provide support to the association between increased wear (i.e., edge-loading of implants, linear wear, and volumetric wear) and the development of pseudotumors in patients with metal-on-metal hip resurfacing arthroplasties.

While mean acetabular component inclination and anteversion were similar between groups, significant variability in the inclination angle was reported among patients with pseudotumors, with individual acetabular component abduction as high as 80°. This is consistent with prior observations of increased serum metal ion levels reported in the setting of high cup-inclination angles⁴. These observations suggest that careful attention should be paid to acetabular component placement when metal-on-metal articulations are utilized. The finding of significant edge-loading for nearly all acetabular components among patients with pseudotumor in this study implicates an impingement process in increased wear. However, the absence of a linear relationship between wear rates and the development of pseudotumors suggest that soft-tissue reactions to hip resurfacing implants occur as the result of a multifactorial process that includes metal wear from mechanical conditions (e.g., impingement, microseparation, and edge-loading) coupled with a biological response that may stem from either direct cellular toxicity or an immunological response to wear debris⁵.

The detailed assessment of wear provided by the optical profilometry measurement technique and the substantiation of data on the differences in wear patterns between modes of failure are major strengths of this work. However, the limited number of revisions from a single institution for a considerable, but infrequent, complication and the predominance of the condition among female patients limit the generalizability of the results to a broader group of patients who experience pain following metal-on-metal hip resurfacing and invariably lead to speculation regarding the causes of increased soft-tissue inflammation among female patients. Hip impingement, associated with a reduction in femoral head size among female patients with lower preoperative head-neck ratios, has been implicated in the association of pseudotumors with sex². However, ongoing follow-up for patients who have had metal-on-metal resurfacing procedures will be important to fully characterize the mechanical and biological contributions on pseudotumor formation and to determine whether sex or implant-related factors, including component size, is more important in the pathogenesis of soft-tissue reaction to particulate metal wear.