LONG-TERM OUTCOME OF SCREW FIXATION OF METAL-BACKED ALUMINA CUPS: A CONSECUTIVE SERIES OF 117 CASES

Abstract

Purpose of the study: The limitations of cemented fixation of alumina cups was demonstrated in 1983. At that time, a new metal-backed cup with a titanium ring for screw fixation and a massive alumina insert was introduced. Since the 1990s, the high rat of mobilization of screwed cups, confirmed by midterm studies, has led to the use of more stable cups. The purpose of this study was to follow the clinical and radiological course of screwed cups implanted for more than 20 years, evaluating the real need for surgical revision.

Material and methods: We reviewed independently a consecutive series of 117 prostheses implanted in 105 patients in 1984–1986. All patients had a screwed cup with an alumina insert, a cemented stem, and a 32 mm alumina head. We retained for study patients aged less than 65 years and excluded revision procedures or patients with prior infection. The clinical assessment was made with the Postel-Merle-d’Aubigné (PMA) score: for eight patients, all data were collected during a phone interview. Radiologically, cups were considered to be mobilized if the change in inclination was greater than 6° or the protrusion greater than 5 mm. Actuarial survival was calculated with the Kaplan-Meier method.

Results: Four prostheses in three patients were lost to follow-up before one year. Mean follow-up was 12.62±6.3 years (maximum 21 years). Fourteen patients had died, twelve had had revision and fifteen were lost to follow-up at more than one year. The mean PMA score improved from 11 preoperatively to 17 at last follow-up. Radiographically, 62% of the cups did not present any sign of mobilization. Four percent exhibited a periprosthetic lucent line around the stem. Considering all results together, four outcome categories could be distinguished: A: good clinical result and no radiological change (58.4%); B: good clinical result and presence of a radiological problem (15%); C: poor clinical result and no radiological unchanged; D: poor clinical result and radiological problem (n21.8%) (including revisions). For category B, the cups showed a 20° mobilization. For these patients, revision was not planned because of very satisfactory quality-of-life for age and activity level. All patients in category C had co-morbid conditions explaining the poor clinical result. Category D included revision procedures which had already been performed (73%) and symptomatic cup mobilizations (27%) measured at 13–31°: revision was planned for most of these patients. The 10-year survival was 82%, all revisions considered; at 15 years, the survival was 66.6%, warranting the change in acetabular fixation made in 1989. The decrease in survival from 82.5% at 10 to 66.6% at 15 years shows that this change was indeed necessity.

Conclusion: Good quality-of-life was achieved for 74% of patients at mean follow-up of 13 years. It would be useful to continue following the patients in category B to check whether cup mobilization has been arrested with a certain degree of adaptation. Furthermore, patients who had a revision were free of osteolysiss and the second operation was generally quite uneventful, usually with preservation of the stem and sometimes even the alumina head which was not deteriorated.