[1] Univ Oxford, NIHR Musculoskeletal BRU, Oxford, England
[2] Univ Autonoma Barcelona, IDIAP Jordi Gol, Inst Catala Salut, E-08193 Barcelona, Spain
[3] Univ Calif San Francisco, Dept Epidemiol & Biostat, San Francisco, CA 94143 USA
[4] Univ Southampton, Southampton Gen Hosp, MRC Epidemiol Resource Ctr, Southampton, Hants, England