Patient-specific three-dimensional evaluation of interface micromotion in two different short stem designs in cementless total hip arthroplasty: a finite element analysis

BACKGROUND: Evaluation of micromotion in various activities in daily life is essential to the assessment of the initial fixation of cementless short stems in total hip arthroplasty. This study sought to evaluate three-dimensionally the micromotion of two types of cementless short stems. METHODS: Two types of stems were used: the Fitmore stem with a rectangular cross-section (rectangular stem) and the octagonal-oval GTS stem with fins (finned stem). Finite element analysis was used to calculate the micromotion of two activities that place a heavy load on the stem (single-leg stance and stair climbing). Three values were measured: the magnitude of micromotion (mean and 95th percentile), the location of micromotion above the 95th percentile value, and the directions of the micromotion vector. RESULTS: 1. There was no significant difference in the magnitude of the micromotion between the rectangular stem and finned stem groups for single-leg stance or stair climbing. 2. In both groups, the micromotion was greatest at the proximal and distal ends. 3. The direction of the micromotion was similar in both groups; internal rotation occurred from the distal to the middle of the stem during stair climbing. CONCLUSIONS: The rectangular stem had comparable initial fixation to that of the finned stem. In both models, the micromotion was greater at the proximal and distal ends. The direction of the micromotion was not dependent on the stem shape but on the direction of the load on the artificial femoral head. These results will be important for stem selection and future stem development. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1186/s13018-022-03329-5.