[Failure of the polyethylene cups directly implanted into bone. A study of 32 Freeman cups after 9 years of followup (6-12 years)].

PURPOSE OF THE STUDY: A clinical and radiological assessment was performed on 32 Freeman polyethylene uncemented acetabular cups, at an average of 9 years follow up. This study was dedicated to analyze polyethylene bone direct fixation.

MATERIAL: Between 1984 and 1988, 30 patients were operated for 32 total hip prostheses. Mean age of the population at the time of surgery was 66 years. In all cases the prosthesis was a cemented Müller femoral stem with a Freeman uncemented cup. The acetabular component was made with polyethylene with three pegs fitted with an offset hemispheric cup.

METHOD: All patients were reviewed at 1, 3, 5 years and maximal follow-up. Their functional evaluation was performed using Merle d'Aubigné's hip rating scale: pain score, motion score and walking score. Radiological assessment was performed to identify acetabular cup migration, bone lysis, and radio lucency. Revision for acetabular cup loosening was compiled.

RESULTS: Five patients died, 27 THA were evaluated at maximum follow-up. Seven revisions were performed after the fifth years. Merle d'Aubigné's score was 14 at the time of revision. X-rays evidenced more than 5 mm migration of the cup, with massive bone lysis. Merle d'Aubigné's score was 16 for the 20 remaining THA. Radiologically, 10 cups showed migration less than 3 mm, five of which with lysis. Among the 10 cups without migration, 5 showed sclerosis near the acetabulum. The study evidenced that cases of migration exceeding 5 mm induced a revision during the next 3 years.

DISCUSSION: Our study confirms failure of bone polyethylene direct fixation, with 25 per cent of revision after a five years period. This seems to be the result of micro movement, wear and bone lysis. A five years follow up is not sufficient to evaluate a component, since all revisions were performed after five years. For many authors, radiological evaluation of acetabular component migration is an important criterion for uncemented prosthesis. The EBRA computerized method is an accurate method for determining early migration, and can be used for pro and retrospective studies.

CONCLUSION: Our study confirms a high rate of failures of the Freeman uncemented cup at an average of nine-year period, as previously reported in literature.