Osteoarthritis is a hip problem that occurs predominantly in older children and adolescents. The average age is 12 years. The thigh is the top, or head, of the hip joint, which is a ball-shaped joint. The growth of the bony end of the leg (femur) is called the growth plate. This problem occurs when the growing herniated disc slides away from the femur. Most often the left hip is affected. It can occur in both boys and girls. Children with spondylolisthesis in one side have a 25% to 40% chance of having it in the other hip as well.