AN ANALYSIS OF THE HEAD-NECK TAPER INTERFACE IN RETRIEVED HIP PROSTHESES

The purpose of this study was to examine the mating surfaces of femora l heads and stems for evidence of corrosion, and to evaluate the quali ty of the taper lock of modular hip prostheses. Forty-eight implants w ith three different designs were evaluated. Group I(26 implants) had a cobalt alloy head and cobalt alloy stem. Group II (ten implants) had a cobalt alloy head and a titanium stem. Group III (12 implants) had a cobalt alloy head with a titanium stem that was coupled in the factor y via a shrink fit. The implants were examined under light microscopy and scanning electron microscopy. In cases where the femoral head was still assembled to the stem, pull-off testing was performed using an M TS machine. The average failure load for the pull-off tests for each g roup of prostheses was as follows: Group I (nine implants) 3,003 N +/- 623 N; Group II (six implants) 4453 N +/- 570 N; and Group III (12 im plants) 6859 N +/- 3525 N. The Group III implants required a significa ntly greater pull-off force than those in Group I (p = 0.002). There w as no evidence of corrosion in Group I. In Group II, crevice corrosion was noted in one taper interface and there was fretting in two other tapers. No corrosion was noted in Group III. Improving the tolerances of the mating surfaces may be a key factor in preventing corrosion. Th is would not only increase the forces that are necessary to debond the matting surfaces, but also decrease corrosion by reducing micromotion and fluid at the taper interface.