Uncontrolled manifold analysis of center-of-mass control during gait before and after total hip arthroplasty in patients with hip osteoarthritis: a secondary analysis of a prospective cohort study

Abstract Background Total hip arthroplasty (THA) is an effective treatment for hip osteoarthritis (HOA), and postoperative gait analysis is important for evaluating recovery and designing rehabilitation strategies. However, conventional gait assessment has focused primarily on the motion of individual joints and may not adequately capture the inter-joint coordination required to control the center of mass (CoM). Uncontrolled manifold (UCM) analysis quantifies this coordination by decomposing the trial-to-trial variability of segment angles into a component that does not affect the CoM (V UCM ) and a component that does (V ORT ), and by deriving a synergy index from these two components. In this study, we used UCM analysis to examine changes in CoM control during gait before and after THA in patients with HOA. Methods Using a publicly available gait dataset, we analyzed the gait of 53 patients with HOA (25 men and 28 women; mean age 65.2 ± 8.7 years) before and after THA. From motion-capture data, segment angles were defined as the elemental variables and the mediolateral position of the CoM as the performance variable. V UCM , V ORT , the synergy index, and the mean mediolateral CoM position and its trial-to-trial variability were computed and compared between the preoperative and postoperative time points. Before comparison, V UCM and V ORT were log-transformed and the synergy index was Fisher z-transformed. Results After surgery, V UCM decreased significantly in all phases, and V ORT decreased significantly in mid-stance and terminal stance ( p < 0.05). In contrast, the synergy index showed no significant change in any phase ( p > 0.05). The mean mediolateral CoM position and its trial-to-trial variability also showed no significant change between the preoperative and postoperative time points. Conclusions After THA, the trial-to-trial variability of the segment angles decreased, whereas the way it was organized toward stabilizing the CoM showed no significant change. Whether this reflects a control strategy preserved through the change in joint structure, or reorganization not yet complete at 6 months, cannot be determined from a single postoperative time point. Any implication for rehabilitation therefore remains a hypothesis for prospective intervention studies. Trial registration ClinicalTrials.gov, NCT01907503. Registered 17 July 2013. Retrospectively registered.