1. From the Departments of Epidemiology and Biostatistics (D.M.B., E.V.) and Orthopedic Surgery (E.J.G.), University of California, San Francisco, San Francisco, the Department of Research and Evaluation, Kaiser Permanente Southern California, Pasadena (D.M.B., B.H.L., D.S.R., A.L.A.), and the Department of Orthopedics, Kaiser Permanente Southern California, Downey (R.M.D.) — all in California; and the Department of Oncology and Metabolism, University of Sheffield, Sheffield, United Kingdom (R.E.).