Metal–Metal Reactivity: Houston, We Have a Problem!

Abstract

In this single-surgeon series, both resurfaced hips in 1 woman and a total hip arthroplasty in another were revised for symptomatic pseudotumor (3 of 588 hips; 0.51% overall incidence; 2.2% in women). All 3 hips had 50-mm acetabular components. There was no difference in mean lateral opening angle (mean 38.7° vs 42.8° for the others) but these 3 hips all had increased acetabular anteversion (mean 27.1° vs 16.4° for the others; P<.05). Increased combined anteversion is a mechanical common denominator in pseudotumor formation. Female sex and small component size are variables associated with congenital dysplasia, which typically has a small, shallow socket and high combined anteversion. Thus, native anatomy may predispose to the joint mechanics that lead to pseudotumor formation, and not sex or size. The aggregate results indicate that the determination of satisfactory component position includes (1) assessment of the acetabular component lateral opening, (2) acetabular component version, and (3) femoral version. A mechanical problem suggests a mechanical solution. To insure capture of the femoral head by the socket and the intended bearing tribology, acetabular lateral opening angles should be <50°, assuming a femoral neck-shaft angle of 130° to 135°. Combined anteversion should not exceed 40°. In resurfacing of dysplastic cases where the neck-shaft angle exceeds 140°, the acetabular lateral opening angle needs to be correspondingly lower to achieve equivalent head capture and bearing contact.