Early gait improvement but persistent knee extension deficit after medial unicompartmental knee arthroplasty assisted by 3D-printed patient-specific instrumentation: an observational self-controlled study

Medial unicompartmental knee arthroplasty (UKA) is an effective knee-preserving procedure for medial compartment knee osteoarthritis. Procedures assisted by 3D-printed patient-specific instrumentation (PSI) can improve the accuracy of prosthesis positioning. However, objective quantitative evidence regarding postoperative functional recovery remains insufficient, and few studies have included comparisons with normal populations. This study aims to evaluate the early postoperative recovery pattern of sagittal knee kinematics following PSI-assisted medial UKA via 3D gait analysis, and to identify specific manifestations of postoperative gait abnormalities.

An observational self-controlled (affected vs contralateral limb) study design was adopted. Twenty-four patients (25 knees) who underwent 3D-printed PSI-assisted medial UKA in Department of Trauma and Joint Surgery of First Affiliated Hospital of Army Medical University between November 2024 and May 2025 were enrolled as the observation group, and 15 asymptomatic contralateral knees were selected as the control group. The hip-knee-ankle (HKA) angle, Western Ontario and McMaster Universities Osteoarthritis Index (WOMAC) score, and sagittal knee kinematic parameters were collected preoperatively and at 3 months postoperatively. Intragroup comparisons (preoperative vs postoperative data within the observation group) and intergroup comparisons (preoperative and postoperative data of the observation group vs the control group) were performed.

The patients in the observation group were followed up for 2.90 to 6.73 (4.15±1.12) months. Regarding functional scores and radiographic outcomes: postoperative WOMAC score (0.92±1.44 vs 12.76±3.94), stiffness score (0.88± 1.09 vs 2.84±2.34), and function score (2.72±3.02 vs 47.80±12.08) in the observation group were significantly improved compared with preoperative values (all P<0.001). The postoperative HKA angle in the observation group was significantly larger than the preoperative value [(177.14±2.48) vs (171.44±4.21)°, P< 0.001], with obvious correction of varus deformity, but there was no significant difference in the angle between the postoperative HKA angle and the control group [(177.02±3.11)°]. In gait analysis: the peak knee flexion angle during swing phase was significantly increased postoperatively compared with preoperatively [(60.51± 5.20) vs (47.13±12.68)°, P<0.001], and the proportion of loading response phase was decreased significantly (10.36%±3.32% vs 15.84%±11.43%, P=0.029), both recovering to the levels of the control group. However, significant differences were observed between postoperative patients and the control group in term of the minimum knee flexion angle during stance phase [(10.63±5.85) vs (6.14±5.04)°, P=0.018], minimum knee flexion angle during swing phase [(7.62±5.28) vs (3.26±5.24)°, P=0.015], and knee flexion range during loading response [(6.84±3.03) vs (9.75±2.89)°, P=0.005], which suggested that knee extension function had not been fully restored. No statistically significant differences were found in the remaining gait parameters.

In the early postoperative period after 3D-printed PSI-assisted UKA, the patients present significant improvement in the loading response phase proportion and the maximum knee flexion angle during swing phase. However, incomplete knee extension persists during gait relative to the control group, as evidenced by increased minimum knee flexion angles during both stance and swing phases.