Risk factors for aseptic loosening of Müller-type straight stems: a registry-based analysis of 828 consecutive cases with a minimum follow-up of 16 years

Abstract

Background and purpose Even small differences in design variables for the femoral stem may influence the outcome of a hip arthroplasty. We performed a risk factor analysis for aseptic loosening of 4 different versions of cemented Müller-type straight stems with special emphasis on design modifications (2 shapes, MSS or SL, and 2 materials, CoNiCrMo (Co) or Ti-6Al-7Nb (Ti)). Methods We investigated 828 total hip replacements, which were followed prospectively in our in-house register. All stems were operated in the same setup, using Sulfix-6 bone cement and a second-generation cementing technique. Demographic and design-specific risk factors were analyzed using an adjusted Cox regression model. Results The 4 versions showed marked differences in 15-year stem survival with aseptic loosening as the endpoint: 94% (95% CI: 89-99) for MSS Co, 83% (CI: 75-91) for SL Co, 81% (CI: 76-87) for MSS Ti and 63% (CI: 56-71) for SL Ti. Cox regression analysis showed a relative risk (RR) for aseptic loosening of 3 (CI: 2-5) for stems made of Ti and of 2 (CI: 1-2) for the SL design. The RR for aseptic stem loosening increased to 8 (CI: 4-15) when comparing the most and the least successful designs (MSS Co and SL Ti). Interpretation Cemented Müller-type straight stems should be MSS-shaped and made of a material with high flexural strength (e.g. cobalt-chrome). The surface finish should be polished (Ra < 0.4 µm). These technical aspects combined with modern cementing techniques would improve the survival of Müller-type straight stems. This may be true for all types of cemented stems.

Figure 1.

The 2 different designs of cemented Müller-type straight stems: MSS with standard offset and SL. Only the Co versions are shown. The inserts show cross sections of the proximal third.

Figure 2.

Kaplan-Meier charts for stem survival (bold lines) with 95% confidence intervals (thin lines). A. Revision for any reason of any component (stem and/or cup (including exchange of the liner)). B. Aseptic loosening of the stem and cases at risk at 0, 5, 10, 15, 20, and 25 years. C. Stem revision for any reason. D. Worst-case scenario for aseptic loosening, counting all stems lost to follow-up as aseptic loosening. E. Aseptic loosening of the cup and cases at risk at 0, 5, 10, 15, 20, and 25 years.