Varus deformity in medial knee osteoarthritis correlates with fatty degeneration of lower limb muscles: Artificial intelligence-based computed tomography analysis.

Purpose: As knee osteoarthritis (KOA) progresses, lower limb alignment deteriorates, severely impairing daily activities. However, the impact of lower limb malalignment on muscle atrophy and degeneration from the trunk to the foot remains unclear. This study aimed to elucidate the effects of varus deformity on muscle volume and fatty degeneration from the trunk to the foot in medial KOA. Methods: This study included 79 patients with end-stage medial KOA (16 males and 63 females). The hip-knee-ankle (HKA) angle, measured on standing leg radiographs, served as an index of varus deformity. Ten muscle groups were segmented on computed tomography (CT) images using an artificial intelligence-based method. Muscle volume was standardised by dividing by height squared, and the mean CT value for each muscle group was calculated as an index of fatty degeneration. Multiple linear regression analysis was used to assess the correlation between muscle volume or CT value and HKA, adjusting for age, sex and body mass index. Statistical significance was set at p < 0.05. Results: The mean HKA was 10.2° (standard deviation 5.9°, range 0-30°). Multiple regression analysis showed a significant association between HKA and the mean CT values of the gluteus maximus, gluteus medius and minimus, adductor muscles, quadriceps and deep and superficial posterior compartments of the lower leg (β = -0.50, p < 0.001; β = -0.25, p = 0.048; β = -0.32, p = 0.007; β = -0.39, p < 0.001; β = -0.38, p = 0.001; and β = -0.42, p < 0.001, respectively). Conclusion: Varus deformity in medial KOA correlates with fatty degeneration of lower limb muscles rather than muscle volume. KOA progression affects not only the muscles around the knee but also those in the trunk, hip and lower leg. Level of Evidence: Level III.