The in vivo location of edge-wear in hip arthroplasties

Abstract

Aims Acetabular edge-loading was a cause of increased wear rates in metal-on-metal hip arthroplasties, ultimately contributing to their failure. Although such wear patterns have been regularly reported in retrieval analyses, this study aimed to determine their in vivo location and investigate their relationship with acetabular component positioning. Methods 3D CT imaging was combined with a recently validated method of mapping bearing surface wear in retrieved hip implants. The asymmetrical stabilizing fins of Birmingham hip replacements (BHRs) allowed the co-registration of their acetabular wear maps and their computational models, segmented from CT scans. The in vivo location of edge-wear was measured within a standardized coordinate system, defined using the anterior pelvic plane. Results Edge-wear was found predominantly along the superior acetabular edge in all cases, while its median location was 8° (interquartile range (IQR) -59° to 25°) within the anterosuperior quadrant. The deepest point of these scars had a median location of 16° (IQR -58° to 26°), which was statistically comparable to their centres (p = 0.496). Edge-wear was in closer proximity to the superior apex of the cups with greater angles of acetabular inclination, while a greater degree of anteversion influenced a more anteriorly centred scar. Conclusion The anterosuperior location of edge-wear was comparable to the degradation patterns observed in acetabular cartilage, supporting previous findings that hip joint forces are directed anteriorly during a greater portion of walking gait. The further application of this novel method could improve the current definition of optimal and safe acetabular component positioning. Cite this article: Bone Joint Res 2021;10(10):639–649.

Statistical Shape Modelling the In Vivo Location of Acetabular Wear in Retrieved Hip Implants

“…This is likely due to the differences in approach, as the method adopted in the present study allowed the additional inclusion of non-edge worn implants, and its mean surface output was influenced by linear wear depth. Nevertheless, in agreement with their study [8], the distribution of volumetric wear in the mean model was skewed towards the anterosuperior quadrant. This observation is consistent with previously recorded hip joint forces that were directed anteriorly during a greater portion of walking gait [26,27], while posteriorly directed hip joint forces were only recorded during the heel-strike portion of the gait cycle [28].…”

“…The subtly posterior location of the primary wear scar centre, measured from the mean acetabular model, was in contrast to the more anteriorly located edge-wear scars found in Bergiers et al's study [8]. This is likely due to the differences in approach, as the method adopted in the present study allowed the additional inclusion of non-edge worn implants, and its mean surface output was influenced by linear wear depth.…”

“…These include a reduced diametrical clearance (difference in diameter between the cup and head components), a small arc of coverage angle (degree of coverage provided by the cup over the head) and a greater degree of inclination [3,25]. Although the mechanism behind edge-wear is widely understood, prior to the study performed by Bergiers at al [8], the assumption that edgewear would develop at the superior acetabular edge (in vivo) had not been confirmed. Furthermore, no other method has allowed the investigation of the anterior-posterior distribution of edge-wear.…”

“…In order to align all 20 acetabular surfaces, while maintaining their in vivo orientation, a standardised coordinate system was defined using a plane parallel to the anterior pelvic plane (APP) that intersected the centre of the cup surface (Figure 3). As in a study by Bergiers et al [8], this plane was termed the Cup-APP (CAPP) and was representative of the vertical standing position due to its relationship to the APP. The CAPP and its normal plane at the cup centre formed the vertical and horizontal axis of a new coordinate system in the cup rim plane, with the cup vector defining the final axes.…”

“…Whilst it is known that acetabular edge wear occurs in vivo, its orientation within the acetabular cavity is less understood. A recent study combined retrieval metrology with postoperative 3D computed tomography (CT) to locate acetabular edge-wear in vivo [8]. This ability to orientate acetabular wear maps within the patient provides a unique opportunity to investigate the relationship between the location of wear and different surgeon, implant and patient (SIP) factors; however, a more encompassing approach is required to fully understand the most prominent locations of acetabular wear.…”