Ph Hernigou (Créteil)
After revision of alumina cups with a polyethylene socket without revision of the stem, there are 3 options for the femoral head: keeping the old alumina head with risk of metallic debris transfer and increased surface roughness; a new alumina head on the old taper (risk of fracture); or a new metal head on the old taper (risk of adherence of third-body particles on metal and increased surface roughness). 65 patients had an alumina cup retrieved (loosening, luxation, impingement, or fracture) and changed for a polyethylene cup without revision of the stem. Three different femoral heads were used. This study evaluated polyethylene wear after a mean follow-up of 17 years (15 to 25 years) .
Revisions were performed between 1982 and 1990. All the polyethylene cups were cemented and made by the same manufacturer. The old alumina femoral head was kept in 20 patients; 19 new alumina and 26 new metal heads were implanted on the original taper. All the femoral heads were 32 mm and made by the same manufacturer as the manufacturer of the old alumina hip. The mean age at revision was 63 years (48-74). The cohort was reviewed more than 15 years after surgery to determine wear. We measured polyethylene wear radiologically by determining the migration of the centre of the femoral head relative to the centre of the cup.
By comparing the co-ordinates of the two centres on the initial postoperative and most recent radiographs, the amount and the direction of penetration of the femoral head into the polyethylene were determined after correction for pelvic tilting and magnification. Although the migration of the femoral head thus determined may consist not only of true wear, but also creep deformation of polyethylene, we defined it as linear wear for the purpose of this study. These 65 hips were compared for wear with 65 matched primary arthroplasties (same follow-up) of the same manufacturer (35 alumina and 30 metal heads).
For the 65 matched primary hips the mean linear PE wear rate was 0.07 mm/year (range 0.05 to 0.09 mm/year) with the 25 alumina heads, and 0.12 mm/year (range 0.08 to 0.15 mm/year) with the 20 metal heads. For the 65 hips with retrieved alumina cups, the mean linear PE wear was 0.08 mm/year (range 0.06 to 0.10 mm/year) in the 19 hips with a new alumina head, 0.24 mm/year (range 0.17 to 0.28 mm/year) in the 20 hips with their old alumina head and 0.25 mm/year (range 0.16 to 0.29 mm/year) in the 26 hips with a new metal head.
After revision of an alumina cup with a PE cup , wear was significantly (p<0.05) increased with new metal heads (alumina third-body particles?) as compared with primary arthroplasty. Old alumina heads were not associated with reduced polyethylene wear as compared with metal heads probably because the surface quality was poor (first generation of alumina heads, metallic debris transfer). Wear was reduced after revision (as in primary arthroplasty) with new alumina heads with excellent surface quality. After revision of an alumina cup with a PE cup, wear was reduced after revision with a new alumina head as compared with the old alumina head or a new metal head.