Reduced articular surface of one-piece acetabular cups increases risk of early failure

Griffin WL. Clin Orthop Relat Res. doi:10.1007/s11999-010-1383-8.

Acetabular cup designs with a decreased articular surface arc are at a higher risk for failure, according to the authors of this study.

William L. Griffin, MD, and colleagues defined the functional articular arc as the effective coverage of the head in the shell. They calculated the functional articular arc in 33 one-piece metal cups produced by several manufacturers using measurements of cup depth and internal cup radius. These measurements were compared with the 180° arc of conventional hip arthroplasty acetabular components.

The authors reported that articular surface arc varied among manufacturers and generally decreased with decreasing shell diameter. The mean functional articular arc was 160.5° +/- 3.6° (range, 151.8°–165.8°), which was less than the 180° arc of a conventional acetabular component.

These data, along with analysis of abduction angles, support the recent findings of bearing failure with vertically placed implants.

“Care must be taken when implanting these shells to ensure they are placed in less abduction to avoid edge loading and the potential for early bearing failure,” the authors concluded in their abstract.

This is an important study that highlights how small changes in implant design can make large clinical differences, sometimes leading to poorer results. In particular, this study expands upon the concept of “arc of cover” or “coverage angle” of the acetabular component, first described by De Haan and colleagues in 2008.

As described in the paper, one-piece metal acetabular components, frequently used in hip resurfacing and large diameter metal head THR, have a smaller functional articular arc than the 180º offered by a traditional THR component. Therefore, the effective coverage by the component is less than what a hip surgeon may be accustomed to with a conventional implant. As such, it is essential to place a one-piece metal acetabular component precisely in order to avoid edge-loading; in particular, it is critical to place the components in less abduction to reduce wear and ensure good long-term function of the joint.

Griffin and colleagues have done an excellent job describing how this functional articular arc varies with the size of the component, decreasing further with diminishing size. Additionally, designs vary by manufacturer, and thus certain implants are more subject to this phenomenon.

Overall, this paper is an important read for users of one-piece metal acetabular components, calling attention to the variations in coverage angles that may affect clinical results. It also stresses the importance of a surgeon knowing details of the implant that he or she is using.

– Edwin P. Su, MD

Assistant Attending Orthopaedic Surgeon

Hospital for Special Surgery

Reference:

De Haan R, Pattyn C, Gill HS, et al. Correlation between inclination of the acetabular component and metal ion levels in metal-on-metal hip resurfacing replacement. J Bone Joint Surg Br. 2008;90(10):1291-1297.