Articular surfaces and ligaments
The femoral head forms about two-thirds of a sphere and is covered with cartilage except at the fovea, a small pit for the ligament of the head. The acetabulum is a deep cup formed by the ilium, ischium and pubis; only its horseshoe-shaped lunate surface is articular. The central acetabular fossa holds fat and the ligament of the head. The fibrocartilage acetabular labrum deepens the rim, and the transverse acetabular ligament bridges the notch at its lower edge.
Capsule
The fibrous capsule attaches to the acetabular rim and labrum and, on the femur, to the intertrochanteric line in front. Behind, it attaches to the neck above the intertrochanteric crest, so the back of the neck is partly outside the joint. Some capsular fibres turn back along the neck as retinacula, carrying the vessels to the head.
Ligaments
| Ligament | Attachments | Limits |
|---|---|---|
| Iliofemoral | Anterior inferior iliac spine and acetabular rim to the intertrochanteric line, as an inverted Y | Extension; the strongest ligament in the body |
| Pubofemoral | Pubic part of the acetabular rim and superior pubic ramus, blending with the capsule | Abduction and extension |
| Ischiofemoral | Ischium behind the acetabulum, spiralling up to the neck near the greater trochanter | Medial rotation and extension |
| Ligament of the head (ligamentum teres) | Acetabular notch and transverse ligament to the fovea | Little mechanical role; carries a small artery |
The three capsular ligaments spiral around the neck and all tighten in extension. Standing therefore needs little muscle effort: body weight falls behind the joint and the iliofemoral ligament stops the pelvis tipping backwards.
Relations and muscles acting on the hip
The hip is surrounded by muscle on every side, and each group produces one movement.
| Movement | Main muscles |
|---|---|
| Flexion | Iliopsoas, helped by rectus femoris, sartorius, pectineus, tensor fasciae latae |
| Extension | Gluteus maximus and the hamstrings |
| Abduction | Gluteus medius and minimus, tensor fasciae latae |
| Adduction | Adductors longus, brevis and magnus, gracilis, pectineus |
| Lateral rotation | Piriformis, obturator internus with the gemelli, quadratus femoris, obturator externus, gluteus maximus |
| Medial rotation | Anterior fibres of gluteus medius and minimus, tensor fasciae latae |
In front of the capsule lie iliopsoas, with a bursa between its tendon and the capsule, and pectineus; the femoral nerve, artery and vein lie on these muscles. Above lie gluteus minimus and the reflected head of rectus femoris. Behind lie the short rotators, with the sciatic nerve on them. Below lies obturator externus, winding under the neck. Vastus medialis arises from the lower intertrochanteric line just below the capsular attachment.
Blood supply and innervation
The femoral head is supplied mainly by retinacular arteries that run up the neck under the synovium. Most arise from the medial circumflex femoral artery, whose deep branch passes behind the neck along the upper border of quadratus femoris; the lateral circumflex femoral artery supplies the front. Both circumflex arteries usually come from the profunda femoris and form a ring around the base of the neck. A small artery in the ligament of the head, from the obturator artery, adds little in adults. The gluteal arteries supply the capsule.
By Hilton's law the joint is supplied by nerves to muscles that cross it: the femoral nerve (through the nerve to rectus femoris), the obturator nerve, the superior gluteal nerve and the nerve to quadratus femoris. Because the femoral and obturator nerves also supply the knee, hip disease often presents as knee pain.
Clinical relevance
The precarious supply of the femoral head and the strength of the capsule shape most hip pathology.
- Intracapsular femoral neck fracture: tears the retinacular vessels, so the head may undergo avascular necrosis and the fracture may not unite. Displaced fractures in older patients are usually treated by replacing the head rather than fixing it.
- Posterior dislocation: from a knee striking a dashboard with the hip flexed. The leg is shortened, flexed, adducted and medially rotated, and the sciatic nerve is at risk.
- Anterior dislocation: rarer; the leg lies abducted and laterally rotated, and the femoral vessels and nerve are at risk.
- Developmental dysplasia of the hip: a shallow acetabulum lets the head sublux; screening uses the Barlow and Ortolani tests and ultrasound.
- Perthes disease: idiopathic avascular necrosis of the femoral head in young children.
- Femoroacetabular impingement and labral tears: abnormal contact between neck and rim damages the labrum, causing groin pain in young adults.
On the specimen
The station shows the hip from the front with the capsule exposed, and the joint opened to show the femoral head. The iliofemoral ligament is the thick inverted Y on the front of the capsule, from the anterior inferior iliac spine to the intertrochanteric line. The ligament of the head is the flattened band running from the acetabular fossa to the pit on the femoral head, seen only when the head is lifted out.
Around the joint, gluteus minimus covers the top of the capsule and inserts on the front of the greater trochanter; gluteus medius lies over it, inserting further back on the lateral surface. Piriformis reaches the upper border of the trochanter, and obturator internus with the gemelli sits just below it. The muscle arising from the lower intertrochanteric line and spiral line on the front of the femur is vastus medialis.