In summary, the relationship between risk factors and OA may differ across joints. For knees, obesity and knee injury either due to acute events or to repetitive impact loading may be the most important preventable causes of disease. Hand OA is probably also caused by repetitive use, but its association with other risk factors is unknown. Hip OA is probably different from OA in other joints, in that many cases are due to congenital and developmental abnormalities. Current inquiry focuses on generalized OA and its predictors. Future studies will look at the long-term course of OA and will focus especially on as-yet unidentified risk factors.