20 YEAR RESULTS OF THE MÜLLER STRAIGHT STEM A CLINICAL AND RADIOLOGICAL FOLLOW-UP OF 165 CONSECUTIVE STEMS

Abstract

Introduction: Since its introduction in 1977 the Müller straight stem and its various copies have become one of the most often used cemented stems worldwide (about 1.1 million stems, figure quoted by manufacturer). Though, there is still a lack of long-term follow-up data.

Material/Methods: A consecutive series of 165 primary hip replacements (161 patients) with the original forged Müller straight stem (CoNiCr) was operated between July 1984 and June 1987 and followed prospectively. Mean age at operation was 68.9 years (25.6 to 86.3 years). 70 stems were implanted in female patients. Operation was done in supine position through a transgluteal approach with no trochanteric osteotomy. All stems were cemented with a second-generation technique (distal plug, cement syringe). The head diameter was 32 mm diameter, 134 heads were out of metal, 31 out of ceramic. 151 hips had a cemented polyethylene cup (52 with armament screws). 13 were combined with an acetabular reinforcement ring (Müller ring) and one with an anti-protrusion cage (Burch-Schneider).

Clinical and radiological follow-up was planned at 4 months 1, 2, 5, 10, 15 and 20 years. Clinical follow-up included a standardised examination and the completion of an IDES form. Cumulative survival rates were calculated by Kaplan-Meier analysis. Radiographs were analysed for osteolysis according to Gruen et al. (zones 1–7) and radiolucent lines.

Results: 3 patients (3 stems) were lost to follow-up (two postoperative and one after 16.8 years), 103 patients (with 106 hips) had died without revision and 55 patients (56 hips) remained for follow-up. 15 stems were revised, 11 for aseptic loosening (9 in combination with the cup), two for infection and two for other reasons. 13 further patients had isolated cup revision. Survival with aseptic loosening of the stem as endpoint was 86% (95%-CI: 82 to 90%) at 20 years.

The median HHS at the last follow-up was 80 points (range 30–98 points).

31% of the non-revised 41 stems showed osteolysis, most of them in zone 7 (21.9%). Two stems with isolated cup revision had a continuous radiolucent line. Incomplete radiolucent lines appeared most frequently in zone 1. Cup revision was associated with increased radiological changes on the femoral side (p=0.094, Mann-Whitney U-test).

Conclusion: The Müller straight stem shows excellent clinical and radiological results in the long-term, comparable to those achieved with other well established cemented and non-cemented stems. From our data further clinical use of the Muller straight stem can highly be recommended.