Introduction:High tibial osteotomy is a reliable procedure for the correction of knee varus deformity. An opening-wedge high tibial osteotomy (OW-HTO) is the most popular technique. The results of the bone defect after opening the wedge needed special treatment to ensure bone healing. This study aims to evaluate the use of bovine-derived hydroxyapatite graft for defect filling after OW-HTO.Methods:A retrospective study was performed on all patients who received OW-HTO at Prof. Dr R. Soeharso Orthopaedic Hospital from November 2019 to December 2022. A total of 21 patients (24 knees) were included in this study. Clinical dan radiological evaluation was performed on all patients preoperative and postoperatively. The mean of the follow-up period was 12.6 months with a minimum of 4 months follow-up.Results:Primary medial uni-compartment knee osteoarthritis was the most common diagnosis, with 17 of 24 cases (70.8%). Mechanical axis deviation was changed from 31 mm medial deviation (range: 8–52 mm) to 0.45 mm medial deviation (range: 13 - (-8) mm). The anatomic tibiofemoral angle was corrected from a preoperative mean of 4.7°of varus to a mean of 5.8°of valgus postoperatively. Bone defect height was mean 15.9 mm with a range of 10–23 mm. Bone defect width was mean 46.7 mm (range: 34–60 mm). Hydroxyapatite graft integration with the host bone was found in all patients during the final follow-up period.Conclusions:Bovine-derived hydroxyapatite graft is a safe and effective material for bone defect filling in OW-HTO procedures with a high bone union rate.