The os acetabuli form after 7 years and have completed growth and closed before 9 years. This cartilaginous cup is the structure that will form most of the mature acetabulum when growth is complete and is the location where the acetabular epiphyses form, all under the constantly shaping stimulus of the femoral head. Laterally, the triradiate cartilage gives rise to a circumferential lip composed of hyaline cartilage centrally and fibrocartilage peripherally. The triradiate cartilage is most responsible for the final depth of the acetabulum. This structure, the triradiate cartilage, is responsible for the formation of the anterior wall, posterior wall, and the dome of the acetabulum. Between the innominate bones lies a cartilaginous T-shaped structure ( Fig 2.1-1). Eventually formed as a coalescence of the three innominate bones of the pelvis, the hip is entirely cartilaginous at birth and remains so during much of the first year. The acetabulum and labrum develop much of their final morphological features during infancy and childhood. The hip enlarges during the remainder of fetal growth, but does not undergo other morphological changes until infancy. By the 16th week, the centers of ossification of the ilium, ischium, and pubis emerge, and the triradiate cartilage is formally created. By the end of the 8th week, the blood supply of the developing hip is fully established. The region of the future joint space forms via apoptosis of cells between the anlage and the femoral head. The femoral head develops centrally from a dense sphere of chondroblasts and is surrounded by a cartilaginous “anlage” with three disc-shaped masses which form the ilium, ischium, and pubis. The femoral head and acetabulum form from an interlocked structure composed of primitive chondroblasts. Early development of the hip begins with formation of the lower limb buds in the 4th week of embryological development, and development is practically complete by the 16th week.
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