PURPOSE OF REVIEW:This review presents recent data on joint injury as the cause of osteoarthritis development in young adults and proposes and discusses a model that highlights how joint injury and other risk factors may contribute to osteoarthritis development through two different pathways: joint-related mechanisms and deconditioning of the musculoskeletal system. RECENT FINDINGS:Knee osteoarthritis in young adults is common after knee injury. In women who sustained an anterior cruciate ligament injury in soccer, 51% (mean age 31) had radiographic changes after 12 years. Men, 41% (mean age 36) had osteoarthritis after 14 years. An injury to the menisci during middle age resulting in a horizontal tear is more probably the first signal of an already ongoing osteoarthritis process of the knee. Known risk factors seem to contribute to osteoarthritis through pathways related to (1) mechanical aspects of the joint and (2) the musculoskeletal system. SUMMARY:Osteoarthritis in young adults is most commonly a result of a previous injury to the knee. Deconditioning of the musculoskeletal system is seen with joint injury, obesity, and aging, all known risk factors for osteoarthritis. Muscular deconditioning contributes to increased joint loads and knee osteoarthritis development and progression. So far, it is unclear whether surgery reduces the development or progression of osteoarthritis. The risk for osteoarthritis development seems to increase as the amount of meniscal tissue removed increases. Knee injury prevention, rehabilitation after knee injuries, regular exercise, maintaining body weight, and a changed locomotion pattern may prevent osteoarthritis initiation and progression in young adults.