WHY CEMENTED MÜLLER TYPE STRAIGHT STEMS SHOULD BE POLISHED – A REGISTER BASED ANALYSIS OF 926 CONSECUTIVE CASES

Abstract

Introduction: The original Müller straight stem (MSS, Zimmer^®; Winterthur, Switzerland) is made out of CoNiCr and showed excellent 20 year results but later modifications of the stem proved to be inferior. Aim of this study was to analyse the effect of shape, material and surface roughness on aseptic loosening of cemented Müller type straight stems.

Materials and patients: Between 1984 and 1996 a total of 926 THR were operated with four different versions of cemented Müller type straight stems and followed prospectively in the in-house register at our institution. Two different shapes of cemented Müller type straight stems (MSS and SL), both made out of two different alloys (CoNiCr and TiAl), were included in this study. All four versions differed in surface roughness (MSS CoNiCr Ra 1.0μm (satin); SL CoNiCr Ra 1.2μm (satin); MSS TiAl Ra 2.0μm (rough); SL TiAl Ra > 2.0μm (rough)). Survival analysis was done using Kaplan-Meier curves with aseptic loosening as endpoint, risk factors were tested with regression analysis.

Results: The 4 groups did not differ in age, gender and diagnosis, the mean follow up was 11.4 (0.1 to 23.0) years. Survival with aseptic loosening as endpoint was 97.7% (MSS CoNiCr), 96.4% (SL CoNiCr), 82.5% (MSS TiAl) and 67.4% (SL TiAl), respectively, at ten years. At final follow up all four groups differed significantly (p=0.044 Log rank test). Increasing roughness increased the risk for aseptic loosening and the harder CoCr had better survival than TiAl. results were significantly worse for the combination of the soft TiAl with a rough surface (SL TiAL, Ra > 2.0μm). For both stem designs the MSS shape had better survival than the SL shape (p=0.001)

Discussion: Wear modalities (abrasion vs. fretting) in the cement-stem interface are directly correlated with surface roughness, the amount of the released particles correlates with loosening. All examined stems had a roughness exceeding the limit of Ra =0.4 μm, producing abrasive wear with a higher volume of wear particles as compared to established polished stems which show fretting wear. Stem roughness and hardness are more important for long-term survival than some modifications in the shape.

Conclusion: Cemented Müller type straight stems should have a polished surface (Ra < 0.4 μm) and be made out of a hard material (CoNiCr) to minimize surface wear. This might be true for all types of cemented stems.