JointLower limb

Hip joint

articulatio coxae

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Study entry

Classification

Multiaxial synovial ball-and-socket joint, strong and stable. With the shoulder, it is the most mobile in the body.

Surfaces

The head of the femur (about two thirds of a sphere, covered with cartilage except at the fovea) with the acetabulum, the cup-shaped cavity of the hip bone, which forms three quarters of a circle at the lunate surface and is completed by the transverse acetabular ligament across the acetabular notch.

Capsule

A strong capsule, with a loose fibrous membrane outside and a synovial one inside. It attaches to the acetabulum next to the acetabular labrum and the transverse ligament, and, distally, to the neck of the femur in front (intertrochanteric line) and behind next to the intertrochanteric crest, without covering it. The fibers run in a spiral: extension winds and tightens them (limiting it to 10 to 20° beyond the vertical), and flexion unwinds them. The ligaments are thickenings of it: the Y-shaped iliofemoral, in front and above, the strongest in the body, which prevents hyperextension; the pubofemoral, in front and below, which prevents excessive abduction; the ischiofemoral, behind, the weakest. The synovial membrane is reflected along the neck, forming retinacula. The ligament of the head of the femur is a synovial fold that contains a small artery, of little importance for stability; a fat pad fills the rest of the acetabular fossa.

Movements

Ball-and-socket, with movement about all three axes (flexion, extension, abduction, adduction, medial and lateral rotation and circumduction). Flexion is greater than extension because the ligaments unwind. The lateral and medial rotators and the ligaments act together, pulling the head into the acetabulum.

Blood supply

Subsynovial retinacular branches, mainly from the medial circumflex femoral artery and a few from the lateral, which supply the head and neck of the femur; artery of the head of the femur, in the ligament.

Innervation

By Hilton's law: branches of the nerves that supply the muscles that cross the joint. The anterior surface, by the femoral nerve; the inferior, by the obturator; the posterior, by branches of the nerve to the quadratus femoris; the superior, by the superior gluteal nerve.

Clinical

Pain can be referred from the lumbar spine.

Source: cap. 7, «Articulaciones del miembro inferior»: articulação coxal

Clinical correlation

Coxa vara and coxa valga

The angle of inclination between the neck and the shaft of the femur changes with age, sex and the development of the bone, and also with diseases that weaken the neck (rickets, for example). A smaller angle is coxa vara; a larger angle is coxa valga. «Varus» means that the distal part deviates toward the midline; «valgus», that it moves away from it. Coxa vara shortens the limb a little and limits passive abduction of the hip.

Slipped capital femoral epiphysis

Between 10 and 17 years of age, the epiphysis of the femoral head can slip on the neck because the epiphysial plate is the weak point. The cause is acute trauma or repeated microtrauma that shears the epiphysis, especially in abduction and lateral rotation of the thigh. The slip is usually slow and produces a progressive coxa vara. The first symptom is often discomfort in the hip that may be felt in the knee. A radiograph of the upper end of the femur confirms it.

Upright posture and the fit of the hip

The human acetabulum faces downward, outward and forward. The iliac part of its rim covers the femoral head and passes the weight to it in standing, still or walking; for this reason the hip is more stable under load, as when lifting something heavy. If the ilium covers the head less (which the radiograph measures by the Wiberg angle), the joint can be unstable. The axis of the acetabulum points forward and that of the neck of the femur backward, forming between them 30° to 40°: in standing, the anterior part of the head is «exposed», against the capsule. Rarely does more than 40% of the head touch the acetabulum in any position.

Fracture of the neck of the femur and necrosis of the head

Calling it a «hip fracture» is improper, because it suggests the hip bone. It is rare in sport: before age 40 the neck is strong and only breaks with a lot of energy, with the limb extended (foot firm on the car floor, knee hitting the dashboard), and the force rises to the hip. After 60 it is very common, especially in women, because osteoporosis leaves the neck weak. It is usually intracapsular and needs internal fixation. The limb is rotated outward. Most of the blood of the head and neck comes from the medial circumflex femoral, through the retinacular arteries, which tear in the fracture of the neck and in dislocation of the hip.

Related clinical cases

Coxa vara and coxa valga
Slipped capital femoral epiphysis
Upright posture and the fit of the hip
Fracture of the neck of the femur and necrosis of the head
Hip and knee arthroplasty