Mechanical causes of osteoarthritis in young adults.

Femoroacetabular impingement is a very common cause of secondary osteoarthritis (OA) in the young adult. It is an important co-factor in the better recognized prearthritic deformities such as residual hip dysplasia (RHD), Perthes disease and slipped capital femoral epiphysis (SCFE). Another subgroup of patients has isolated malrotation of the hip joint and/or reduced femoral head-neck offset causing femoroacetabular impingement and chronic hip joint pain. Special clinical tests and imaging modalities can identify these patients at an early stage when they have little or no OA. The common biomechanical pathway for deformities causing chronic femoroacetabular impingement is local damage of the capsular-labrum complex and the cartilage. Understanding the anatomy, biomechanics and pathophysiology of these conditions of the hip joint is a prerequisite for planning treatment.