Exploring the association between Frailty Index and Knee osteoarthritis in middle-aged and older Chinese adults: A cross-sectional analysis of data from the China Health and Retirement Longitudinal Study - Na Zhu; Hongming Lin; Jia Yu; Rong Yang; Danhao Tao; Guoxin Huang; Da Qian; Yu Liu; Bingqian Chen (PLoS ONE, 2026)