Biomechanical effects of varus stem alignment on a cementless metadiaphyseal anchoring hip stem: a biomechanical investigation

Background Minimally invasive surgical approaches have become standard in cementless hip arthroplasty; however, the recent literature still lacks biomechanical evidence regarding the effects of stem malpositioning. This study aimed to biomechanically evaluate the stability of a cementless metadiaphyseal anchoring stem implanted with a varus malposition versus a neutral alignment. Methods Twenty paired human cadaveric femora were assigned pairwise for stem implantation featuring either 8° varus malposition (Group 1) or neutral orientation (Group 2). All specimens underwent quasi-static testing and progressively increasing cyclic loading to failure with monitoring via motion tracking. Results Axial stiffness in Group 1 was significantly lower than in Group 2, p = 0.023. Implant size in Group 1 was significantly smaller than that in Group 2, p = 0.002. Load at 0.15 mm stem subsidence and cycles to 0.15 mm subsidence in Group 1 were not significantly different compared to Group 2, p = 0.214. Load at 1 mm subsidence and cycles to 1 mm subsidence in Group 1 were significantly higher than those in Group 2, p = 0.022. Conclusion An 8° varus-aligned cementless metadiaphyseal anchoring hip stem demonstrates superior load-bearing capacity with higher loads and numbers of cycles until reaching defined subsidence thresholds under dynamic loading, as compared to neutral alignment. These results demonstrate the biomechanical tolerance of unintended intraoperative varus malalignment, but do not support or recommend intentional varus stem positioning.