In Vitro Estimation of Initial Stability of a Cementless Stem Made of Shape Memory Alloy

The success of total hip arthroplasty (THA) in improving joint function and reducing pain is regarded as one of the great achievements of modern medicine. For a cementless THA stem, however, lack of initial stability leads to thigh pain and eventual loosening of the stem because of a loss of bone integration on the stem surface or continuous bone resorption around the stem. Obtaining adequate initial stability is one of the important factors for good short‐term and long‐term clinical outcomes. While most of the cementless stems are made of either cobalt‐chrome‐molybdenum (CoCrMb) or titanium‐aluminum‐ vanadium (TiAlV) alloys, we have focused on an expandable cementless stem utilizing nickel‐titanium (Ni‐Ti) shape memory alloy (SMA). In this study, the expandable stem with SMA was evaluated in vitro, specifically testing to determine if the stem is able to achieve rigid initial stability in a medullary cavity under cyclic loadings. The proposed stem has Ni‐Ti SMA expandable blades on the stem surfaces. For shape memorization, the SMA was restrained at an expanded shape under appropriate thermo‐ mechanical treatments. At the time of implantation into bone, the SMA with a contracted shape was kept below the martensite transformation temperature. After implantation, the SMA blades undergo reverse transformation due to body temperature, accompanied by a change from the contracted shape to the expanded shapes. The shape change results in a residual pressure between the bone and the stem. In mechanical tests, the stem with SMA showed better initial stability, in terms of relative micromotions at the stem‐bone interface, under cyclic loadings. After 120 loadings, micromotion of 11 m was observed on the stem with SMA, while the micromotion on the stem without SMA was 122 m. Use of an expandable stem utilizing SMA is effective enough for rigid initial stability.