The cup-shaped acetabulum is formed by the innominate bone with contributions from the ilium (approximately 40% of the acetabulum), ischium (40%) and the pubis (20%) [6]. In the skeletally immature these three bones are separated by
the triradiate cartilage – fusion of this starts to occur around the age of 14 – 16 years and is complete usually by the age
of 23 [7]. The actual articular surface appears a lunate shaped when viewed looking into the acetabulum. Within the lunate, or horseshoe shaped articular cartilage is a central area – the central inferior acetabular fossa. This fat filled space houses a synovial covered fat pad and also contains the acetabular attachment of the ligamentum teres. Inferior to this, the socket of the hip is completed by the inferior
transverse ligament.