BACKGROUND: This study was designed to determine the relationship between age and osteotomy gap filling rate, aiming to discern the specific time period during which age exerts its impact on this process. The osteotomy gap filling rate after medial opening wedge high tibial osteotomy (MOWHTO) was assessed by evaluating the absolute value of gap filling utilizing computed tomography (CT) scan obtained at multiple time points postoperatively. METHODS: The data of 30 patients who underwent MOWHTO between September 2019 and July 2021 were retrospectively analyzed. Computed tomography findings were used to measure osteotomy gap filling rate at 6, 12, and 24 months postoperatively. Patients were divided into two age groups (those aged ≤ 55 and >
55 years) for comparison. RESULTS: Patients aged ≤ 55 years showed significantly greater osteotomy gap filling rate at the 6-month follow-up examination compared to that of older patients (p = 0.034). Pearson's correlation analysis revealed a negative correlation between age and osteotomy gap filling rate at 6 and 12 months postoperatively (r = -0.426, p = 0.019
r = -0.368, p = 0.046, respectively). Multiple linear regression analysis identified age as the sole predictor of osteotomy gap filling rate at 6 months postoperatively (β = -0.407, p = 0.039). CONCLUSION: Age significantly influenced the osteotomy gap filling rate after MOWHTO. Patients aged ≤ 55 years demonstrated greater improvement in gap filling during the initial 6-month period compared to their older counterparts. These findings underscore the importance of age considerations when deciding on the indication for MOWHTO in cases of medial knee osteoarthritis in clinical practice. STUDY DESIGN: Case series, Level of Evidence IV.