Advanced Varus Deformity Associated with Medial Knee Osteoarthritis is a Potential Predictor of Anterior Cruciate Ligament Tear and Risk for Suitable Unicompartmental Knee Arthroplasty.

Background: Previous clinical studies suggest that preserving the anterior cruciate ligament (ACL) is crucial for stable knee motion and long-term longevity of the reconstructed knee. The ACL damage or loss often occurs in advanced medial osteoarthritis (OA). This study aimed to investigate the correlation between ACL damage and varus deformity progression as a risk factor for ACL tears in knee OA.

Methods: A retrospective study was conducted on medial knee OA patients who have various degrees of varus deformity treated with unicompartmental knee arthroplasty (UKA; n = 165), where ACLs had no or mild damage. Another group with ACL tears or loss and poor range of motion underwent total knee arthroplasty (TKA; n = 184). Surgeries were performed between November 2016 and March 2023. Preoperative varus angles were measured using the hip-knee-ankle angle (HKA) on standing radiographs. The ACL damage was graded using the Oxford system, and the correlation between preoperative HKA and ACL damage was analyzed.

Results: A significant correlation was found between preoperative varus angle and ACL damage. Severe damage was more frequent in knees with higher varus angles. The cut-off varus angle for a high risk of ACL tear was identified as 11.3º, with HKA above 15º indicating substantial ACL tears.

Conclusion: Medial knee OA with advanced varus deformity is associated with severe ACL tears. A UKA may be preferable for medial knee OA before the varus angle reaches more than 15º.