The secondary centers generally appear at the epiphysis. This usually involves only the lower extremities and on medial side of the epiphysis. Other important causes are infectious arthritis, osteomyelitis, and slipped capital femoral epiphysis in children. Pathologies of the epiphysis include avascular necrosis and osteochondritis dissecans (OCD). Cartilage is progressively eroded and replaced by hardened bone, extending towards the epiphysis. Here a cut into the bone will find the abnormal epiphysis. Stress on the hip causes the epiphysis to move posteriorly and medially. Many bone awls that retain an epiphysis, or rounded end of a bone. The upper epiphysis joins the body about the sixteenth, the lower about the twentieth year. The epiphysis is filled with red bone marrow, which produces erythrocytes (red blood cells).