• Acetabulum

The acetabulum is formed at the meeting point of the ileum, ischium and pubic bones. In children, the triradiate cartilage refers to the three-pronged growth plate which fuses at 12-14 years old.

The acetabulum and the labrum form an incomplete cup with an inferior deficiency, known as the acetabular notch or incisura. This is bridged by the transverse acetabular ligament (TAL). The TAL gives rise to the ligamentum teres (also known as the ligamentum capitis femoris) which inserts into the fovea capitis of the femoral head. This is a remnant of the artery of ligamentum teres, which provides blood supply to the femoral head in children.

• Hip joint capsule and supporting structures

The hip capsule is reinforced by three major ligaments —one arising from each of the pelvic bones. Due to the natural femoral head anteversion, the anterior capsule is thicker and stronger compared to the posterior. There is additional reinforcement from a thickened ring of circumferential fibres within the subcapital joint capsule, known as the zona orbicularis.

Below are the three reinforcing capsular ligaments. Their fibres are orientated in a forward corkscrew manner, which tightens as the hip extends.

Iliofemoral ligament of Bigelow: This Y-shaped ligament —inserting at the greater and lesser trochanters— is the strongest ligament in the body. It runs downwards from the ASIS, across the femoral neck and inserts at the intertrochanteric crest. It is taut in extension, and is important in maintaining a standing posture without falling into hyperextension.

Pubofemoral ligament: Reinforces the anterior-inferior capsule as it descends from the superior pubic ramus.

Ischiofemoral ligament: Passes horizontally from the body of the ischium to the superior joint capsule.

• Short external rotators (SER) of the hip

The short external rotators each insert into the posterior aspect of the femoral greater trochanter with contraction externally rotating the hip. Apart from a couple of exceptions, they are each innervated by small individually-named nerves originating from the L5-S1 nerve roots.

Piriformis: The largest and most proximal SER muscle. It originates from the anterior surface of the sacrum (Denis zone 1) and passes out the greater sciatic foramen to insert at the apex of the greater trochanter. The sciatic nerve passes across the anterior surface of piriformis, before exiting the greater sciatic foramen in the interval between piriformis and superior gemellus.

Superior gemellus: At the greater trochanter, the tendons of obturator internus, superior and inferior gemelli blend, sometimes described as the triceps coxae. During a posterior approach to the hip, they are usually detached as one. Anatomically, the fibres of obturator internus are sandwiched by the gemelli above and below.

Obturator internus: Originates from the internal surface of the obturator membrane, and winds around the body of the ischium and through the lesser sciatic foramen to insert in the GT.

Inferior gemellus: Originates from the body of ischium alongside the superior gemellus.