Wear and performance of a tripolar total hip replacement.

Background: An unconstrained tripolar hip replacement matches a large capacity two-piece metal/polyethylene acetabular component with a bipolar prosthesis. This combination of accepted technology is different than the relatively new dual mobility prosthesis. The goal is to protect against dislocation and allow close to a normal range of motion (ROM). So far there has not been enough information about wear and performance of tripolar hip replacement to support its wide use.

Methods: Twenty-four tripolar prostheses were retrieved from 23 patients after a mean of 14 years (range, 5-21 years). All implants had been placed in high-demand patients who participated in adventure sports, had occupations where a dislocation would be dangerous, or in patients undergoing revision. The tripolar prosthesis has three important design features: (I) the acetabular component uses highly cross-linked polyethylene with an internal diameter of 41-54 mm, (II) the bipolar is titanium nitride-coated, and (III) the bipolar prosthesis has positive eccentricity. The retrievals were evaluated for wear, performance, and mechanical function.

Results: The total volumetric wear was 24 mm³/yr. compared to 54 mm³/yr. For a dual mobility prosthesis and 38 mm³/yr. for a 40 mm conventional hip replacement. The jump distance was 16 mm compared to 12 mm for a 36 mm hip replacement. There was no osteolysis. The combined flexion/extension was 145 compared to 119 for a conventional prosthesis. The mean UCLA score was 7.9. Radiographs showed continued shared movement between the inner and outer articulations. The prosthetic bipolar separation force was 2,180 N. High-demand activities generate approximately 340 N.

Conclusions: The wear of this tripolar hip prosthesis is low below the osteolytic threshold. The increased ROM and increased jump distance provide the most stable unconstrained hip replacement available. The wear and mechanical performance seen in this study suggest a lifetime of use is possible in even the highest demand patients. The absence of intraprosthetic dislocation, metal wear reaction, and limited acetabular stress shielding make this a safer technology compared to a dual mobility prosthesis.