Validation of volumetric and linear wear-measurement in UHMWPE cups—a hip simulator analysis

Wear in retrieved cups can be assessed by linear measurements of cup thickness, 3-D tracings of cup geometry and by fluid-displacement techniques. We used a hip simulator model to produce both creep and wear in 22.25 and 28 mm diameter hip implants and assessed wear by gravimetric methods (no creep) for comparison to linear and volumetric methods. With the oil-displacement method, wear was overestimated by a ratio 1.07. With the linear method and wear expression “A” (dV=πR12p), the ratio was 1.20. With the geometric wear expression “B” (dV=πR12p/[1+(R2−R1)/p]), the ratio was reduced to 1.15. The oil method demonstrated an accuracy better than 15% (upper 95% CL) when wear exceeded 200 mm3 (4.5–8 million). Accuracy in method “B” was >20%. We would not recommend using method “A”, with its accuracy not >40%.