Relevance of physical activity with decrease in minimal joint space of the nonoperative hip in female patients after unilateral THA

To clarify the relevance of physical activity (PA) with respect to a decrease in minimal joint space (MJS) of the nonoperative hip in female patients at 24 months post-contralateral total hip arthroplasty (THA). This prospective cohort study was conducted from six to 24 months post-THA. The subjects were 85 female patients with hip osteoarthritis (OA) who had undergone unilateral THA. The primary outcome was a change in MJS (ΔMJS). Daily step counts and moderate to vigorous physical activity were measured. To identify factors related to ΔMJS, a generalized linear model approach was used, with adjustment factors, Kellgren–Lawrence (KL) grade, PA, and interaction terms between PA and KL grade as explanatory variables. The interaction term between daily step count and KL grade was significant. The regression coefficient of the daily step count for ΔMJS was significant in the KL ≥ 1 group, but not in the KL = 0 group. A model with adjustment factors, an interaction term, and PA showed that the daily step count increased ΔMJS. PA was related to ΔMJS of the nonoperative hip in female patients with KL grade ≥1, but was not related to ΔMJS in patients without OA post-THA. Excessive daily step count may be a risk factor for a decrease in MJS of the nonoperative hip.