[Dual articulation retentive acetabular liners and wear: surface analysis of 40 retrieved polyethylene implants]

The original concept of dual articulation retentive liners has been implemented in routine clinical practice for total hip arthroplasty since 1976. The retentive liner improves stability and the dual mobility favors preservation of joint motion. In this study, we analyzed wear of the two concave and convex surfaces in 40 retrieved polyethylene implants in order to search for factors favoring wear.

Forty polyethylene inserts were examined. These implants had been removed after mean implantation of eight years. Mean patient age at implantation was 48 years. The inserts were removed because of prosthesis infection or mechanical failure. Macroscopic examination was followed by a surface analysis with direct measurement of alterations of the curvature radii. Internal concavity was measured in the three dimensions using a 4-mm stylus (BNH 706). External convexity was measured by lateral projection. The estimated error was +/- 5 microm for both measurement methods. Manufacturers' tolerances for these implants were approximately 50 microm. Linear and volumetric wear was determined by comparing the measured dimensions with the theoretical dimensions of new inserts.

Macroscopically, all of the pieces studied had lost their initial striation on the convex surface; 40% of the pieces displayed visible wear of the retentive collar. Mean annual wear was 9 microm (SD 9microm) for the convex surface and 73 microm (SD 69 microm) for the concave surface. Mean total wear, taken as the sum of the wears on the convex and concave surfaces was 82 microm (SD 72 microm). Volumetric wear was 28.9 mm3/yr for the convex surface (SD 27.6) and 25.5 for the concave surface (SD 23.2) giving a mean annual total volumetric wear of 54.3 mm3/yr (SD 39.6).

Total wear observed for these 40 dual articulation liners which had functioned in vivo was to the same order as reported for the metal-polyethylene bearing with 22.2 mm femoral heads. Considering the wear on both the convex and concave surface, the dual articulation was not associated with increased wear compared with the conventional metal-polyethylene bearings, but with the advantages of retention and greater stability.

Use of a dual articulation acetabular liner is an attractive solution when a metal-polyethylene bearing is considered. The benefit in terms of joint stability does not increase wear.