The Association Between Phase Angle Decline and Functional Recovery Following Periacetabular Osteotomy: A One-Year Prospective Evaluation

<b>Background/Objectives</b>: Curved periacetabular osteotomy (CPO) is an effective joint-preserving procedure for osteoarthritis of the hip; however, postoperative weight-bearing restrictions may influence muscle quality and functional recovery. This study aimed to examine longitudinal changes in muscle mass, muscle quality assessed by phase angle (PhA), and physical function after CPO and explored their postoperative interrelationships. <b>Methods</b>: This prospective longitudinal study included 35 female patients (mean age 34.9 ± 13.4 years) undergoing CPO. Assessments were conducted preoperatively, at full weight-bearing (FWB), and 12 months postoperatively. Lower-limb muscle mass and PhA were measured using multifrequency bioelectrical impedance analysis. Physical function was evaluated using Timed Up and Go (TUG), body weight-normalized ground reaction force during sit-to-stand (F/w), and operated-side weight-bearing capacity. <b>Results</b>: Operated-side muscle mass decreased at FWB and partially recovered by 12 months. In contrast, PhA declined markedly at FWB on the operated side (5.21 ± 0.69° to 4.15 ± 0.67°, <i>p</i> < 0.001) and remained significantly lower than baseline at 12 months. Functional measures declined during restricted loading and recovered to levels comparable to baseline by 12 months. At FWB, PhA was independently associated with TUG, F/w, and power generation indices. The reduction in PhA was greater than that observed for muscle mass, and contralateral PhA also declined. <b>Conclusions</b>: CPO is associated with a transient decline in muscle quality and functional performance during postoperative loading restriction. Although functional measures recover within 12 months, muscle quality remains partially impaired. Early strategies aimed at preserving muscle quality may support postoperative recovery.