The hip joint for surgeons

1. Bones of the hip

The femoral head sits deep in the acetabulum of the hip bone. The neck meets the shaft at about 130° and points forward 10–15° (anteversion). The greater trochanter is the bony landmark you feel on the side of the hip; the lesser trochanter sits behind and inside.

Surgeon’s corner: In a hip replacement you restore these numbers: cup at about 40° inclination and 15° anteversion, stem with its native version, and the leg length measured from the lesser trochanter.

2. Capsule and ligaments

The capsule is reinforced by three ligaments that spiral around the neck: iliofemoral (the Y-ligament in front, the strongest ligament of the body), pubofemoral (below) and ischiofemoral (behind).

Surgeon’s corner: Posterior dislocations happen when the hip is flexed and adducted (dashboard injury, or after a posterior-approach replacement). Repairing the posterior capsule and rotators reduces this risk.

3. Gluteal muscles

Gluteus maximus covers the back of the hip; it extends the hip. Under it, gluteus medius and minimus run from the outer ilium to the greater trochanter: the abductors that keep the pelvis level when you stand on one leg.

Surgeon’s corner: Weak or damaged abductors give a Trendelenburg gait. The superior gluteal nerve runs between medius and minimus about 5 cm above the tip of the trochanter: do not split gluteus medius higher than that.

4. Short external rotators

Remove gluteus maximus and you see a row of small muscles running horizontally from the pelvis to the back of the trochanter: piriformis, superior gemellus, obturator internus, inferior gemellus and quadratus femoris. They turn the thigh outwards.

Surgeon’s corner: In the posterior approach these tendons are released from the trochanter, kept as a flap and repaired at the end. Turning the leg inwards stretches them and moves them away from the sciatic nerve.

5. The sciatic nerve

The sciatic nerve leaves the pelvis through the greater sciatic foramen, usually just below piriformis, and runs down over the short rotators, midway between the ischial tuberosity and the greater trochanter.

Surgeon’s corner: Sciatic nerve palsy (mostly the common fibular division: foot drop) is the nerve injury of the posterior approach. Cut the rotators close to the bone and never retract blindly behind the posterior acetabulum.

6. Blood supply of the femoral head

Most of the blood to the femoral head comes from the medial circumflex femoral artery. Its deep branch runs behind, along obturator externus and under quadratus femoris, and sends small retinacular vessels up along the neck into the head.

Surgeon’s corner: A displaced intracapsular neck fracture tears the retinacular vessels: the head may die (avascular necrosis). That is why displaced neck fractures in the elderly are replaced, while trochanteric fractures (outside the capsule) are fixed.