Abstract Background The study objective was to assess clinical outcomes and gait biomechanics in patients after 6 and 18 months after varus deformity (VD) surgical correction at knee joint (KJ). Method The study included 20 patients with medial osteoarthritis (OA) of the knee of grade 2–3 and a VD of >4°. A total of 21 surgeries were performed on the patients. Full length weight bearing (FLWB) X-ray and KJ assessments were done using the KSS, KOOS and VAS scoring systems were obtained from all the patients. Biomechanical gait parameters were captured using an inertial sensor system at timepoints before, and 6 and 18 months after surgery. Temporal and kinematic parameters of walking were analyzed. Results The radiological parameters showed a stable VD correction. According to the KOOS, KS and VAS scores, there was a moderate dynamic improvement in the operated knee function during the study. The biomechanical parameters remained virtually unchanged throughout the entire follow-up period. In the following year, there were some subjective improvements but without any significant changes in gait biomechanics or knee kinematics. Conclusions Thus, the main changes in the joint clinical condition and function occur in the first 6 months after surgery. According to the study data—assessments by VAS, KOOS, and KSS—there was a moderate clinical improvement during the long-term follow-up period, however, the biomechanical changes were minor.