BMI-adjusted low muscle mass under AWGS 2025 identifies functionally vulnerable patients missed by height-adjusted index in severe knee osteoarthritis

BackgroundIn severe knee osteoarthritis (KOA) patients, muscle mass is often evaluated using appendicular skeletal muscle mass adjusted for height (ASMI). However, with higher body mass index (BMI), muscle mass may appear normal relative to height but remain insufficient for body size and mechanical demands. The Asian Working Group for Sarcopenia (AWGS) 2025 criteria newly include BMI-adjusted appendicular skeletal muscle mass (ASM/BMI). This study examined discordance between ASMI- and ASM/BMI-based low muscle mass classifications in severe KOA patients undergoing total knee arthroplasty (TKA), and evaluated whether ASM/BMI identifies functional vulnerability.MethodsThis retrospective study analyzed 205 patients with severe KOA undergoing primary TKA. Appendicular skeletal muscle was measured using multi-frequency bioelectrical impedance analysis. Low muscle mass was classified using AWGS 2025 cut-offs for ASMI and ASM/BMI. Agreement and discordance were assessed overall, and discordance rates were compared between patients with BMI <25 and ≥25 kg/m². To determine whether ASM/BMI detects vulnerability missed by ASMI, patients with normal ASMI but low ASM/BMI were compared with those normal by both indices, and preoperative function and 12-month postoperative trajectories were analyzed using age- and sex-adjusted linear mixed-effects models.ResultsLow muscle mass was identified more frequently by ASM/BMI than by ASMI (32.7% vs 8.8%). Overall discordance was 26.9%, and was higher in patients with BMI ≥25 kg/m² than with BMI <25 kg/m² (41.0% vs 12.0%). Compared with the concordant normal group, the ASM/BMI-only group had weaker grip and hamstring strength and slower 40-m fast-paced walk and stair-climb performance preoperatively. Although recovery trajectories did not differ significantly, the ASM/BMI-only group showed poorer walking, stair-climbing, and quadriceps strength at 12 months.ConclusionsIn severe KOA, ASM/BMI identified patients whose muscle mass was insufficient relative to body size despite normal ASMI. ASM/BMI may help detect functional vulnerability, particularly in patients with BMI ≥25 kg/m².