Femoral head modularity in total hip arthroplasty design emerged in the 1970s as an alternative to monolithic stem designs. Through the use of tapers, it has evolved to include the femoral neck and stem to provide off-the-shelf flexibility for accommodating difficult situations of femoral deformity and bone loss in both primary and revision situations. Although modular femoral stem designs have been successfully employed, they are not without clinical concerns, including structural compromise at taper interconnections because of cyclic microdisplacements. The ongoing laboratory-based structural fatigue characterization methodology presented in this paper was developed for the evaluation of composite materials, modular femoral stem designs, and systems incorporating proximal support.
Specifically, a single, contemporary modular, proximally supported design that utilizes a solid femoral stem with a metaphyseal sleeve is employed. Three assemblies (baseline, alternate forging process, and lateralized neck) of the S-ROM Modular Hip System (DePuy Synthes, Warsaw, IN/Joint Medical Products, Stamford, CT) were evaluated under dynamic loading conditions and the number of cycles and peak loads plotted on a semilog scale to create a structural fatigue curve for each. The resulting endurance limits were 545 kgf, 622 kgf, and 454 kgf, respectively, with safety factors of 2.1 for the alternate forging process and 1.5 for the lateralized neck design. All of the stem failures across the three designs occurred slightly proximal to the distal end of the sleeve, within the sleeve-stem taper, and initiated across a lateral fretting surface.
Although the examples presented are system specific, they demonstrate that the structural fatigue curve methodology can easily determine the influence of manufacturing and design variations once a baseline curve is established. This method has been employed continuously to determine the structural performance characteristics of other total hip systems since implementation. It suggests a proposed expansion of ASTM F2580-13 by providing testing parameters for researchers, medical device manufacturers, and regulatory agencies who are tasked with ensuring the safety and efficacy of total hip arthroplasty designs.