Introduction: The Müller straight-stem prosthesis (GS-77), in contrast to the Müller SL prosthesis (SL-88), is equipped with a cement collar and longitudinal grooves. Comparative observation of these prostheses suggests that cement collars protect against rapid subsidence and that longitudinal grooves prevent premature formation of a varus tilting deformity.
Method: Because loosening is often observed in the titanium-alloy models of these implants, this retrospective study focused on these models. Differences based on other parameters known to affect mechanical performance were minimized by careful selection: the use of lateralizing implants, patient population characteristics (gender distribution, age, BMI, activity level), and consistency of prosthetic size (10). The comparability of the bending stiffness of both implants in corresponding sizes was verified using finite element method calculations.
Results: Up to the time of revision, 17 GS-77 and 33 SL-88 implants were compared for the amounts of subsidence and prosthetic tilting. Standardized follow-up films were contrasted and radiographically measured. At the time of revision, the GS-77 implants showed subsidence of 4.8 (+/-3.7) mm and the SL-88 implants 8.2 (+/-5.1) mm (p = 0.012). Leaving apart 3 cases with an extreme situation tilting of the GS-77 implants, with 1.2 (+/-0.9) degrees , averaged only two-thirds that of the SL-88 implants, with 1.9 (+/-1.1(p = 0.032). These results support the hypothesis that the cement collar of the Müller straight-stem prosthesis protects against rapid subsidence occurring with aseptic loosening of the implant. The longitudinal grooves appear to counter the development of varus tilting.