Frictional torque and wear of retrieved hip prostheses: a comparison between alumina/PE and Co-Cr/PE prostheses.

Frictional torque and wear were measured in 67 retrieved cups with internal diameters of 32 mm. Thirty cups were combined with alumina heads and 37 were combined with metal heads. A lower prevalence of cup loosening was observed in alumina-polyethylene implants than in metal-polyethylene implants. No relation between frictional torque and stability of implants was detected for either type, whereas retrieved cups of alumina-polyethylene implants showed lower frictional torque than those of metal-polyethylene implants. Less wear was observed in the cups of alumina-polyethylene implants (0.13 mm/y) than in those of metal-polyethylene implants (0.19 mm/y). Less wear was observed in cups without loosening than in those with loosening in both types. These results suggest that wear has a greater influence on the stability of cups than frictional torque.