Increased prevalence and severity of femoral bone wear in Japanese patients with varus knee osteoarthritis.

Purpose: The calliper technique in kinematic alignment total knee arthroplasty assumes nearly identical medial and lateral femoral condylar radii and negligible subchondral bone wear. However, racial differences were not considered. This study aimed to assess the degree and severity of bone wear in Japanese patients with varus knee osteoarthritis.

Methods: This cross-sectional study included 155 knees from 130 patients who underwent total or unicompartmental knee arthroplasty for primary varus-type knee osteoarthritis between April 2020 and March 2024. Preoperative computed tomography scans were used to measure the femoral condylar radii via a circle-fitting technique. Bone wear was assessed at 0° and 90° by measuring the distance between the best-fit circle and the subchondral bone periphery, with additional measurements at the peak wear angle, if present.

Results: Among 155 knees, 16 (10.3%) exhibited bone wear. Bone wear >1 mm at 0° was observed in 3.2% of the cases. The peak wear angle was 43.1 ± 5.0°, with a mean depth of 2.0 ± 0.7 mm. The medial and lateral condylar radii were nearly identical (18.3 ± 1.2 mm vs. 18.2 ± 1.2 mm, p = 0.002), with a strong correlation (R² = 0.94, p < 0.001).

Conclusion: Bone wear exceeding 1 mm at 0° was observed in 3.2% of cases, while overall bone wear was present in 10.3% of knees. Although the difference in radii between the medial and lateral femoral condyles was statistically significant, it was clinically negligible. This finding supports the reliability of the cylindrical axis as a reference for surgical techniques. These results highlight the importance of recognizing potential racial differences in bone wear and underscore the need for accurate assessment to achieve anatomic restoration in kinematic alignment total knee arthroplasty.

Level of evidence: Level III.