Hip joint

Location: Deep in the upper thigh, below the middle of the inguinal ligament, where the head of the femur sits in the acetabulum of the hip bone.

Bones taking part

• Hip bone (acetabulum)
• Femur (head)

Articulating surfaces

• The almost spherical head of the femur, covered with articular cartilage except at the fovea for the ligament of the head.
• The horseshoe-shaped (lunate) articular surface of the acetabulum, covered with articular cartilage.
• The acetabular fossa (the floor of the socket) and the acetabular notch are non-articular; the fossa holds a fat pad and the ligament of the head of the femur.

Structural

Synovial joint.

Functional

Diarthrosis (freely movable).

Synovial subtype

Ball-and-socket (spheroidal).

Axes

Multiaxial: transverse (flexion–extension), anteroposterior (abduction–adduction) and vertical (medial–lateral rotation).

Capsule & cavity

Strong and dense. Proximally it attaches to the margin of the acetabulum outside the labrum and to the transverse acetabular ligament. Distally it attaches to the intertrochanteric line in front, but behind it reaches only part of the way down the femoral neck, ending above the intertrochanteric crest. Most of its fibres run longitudinally; deep circular fibres form the zona orbicularis around the neck. Some fibres reflect along the neck as retinacula carrying the retinacular arteries that supply the femoral head.

• Standing: transmits body weight from the pelvis to the femur; the iliofemoral ligament resists hyperextension, so upright standing needs little muscle work.
• Walking: flexion swings the limb forward, extension pushes the body over the planted foot.
• Balance: when one foot is off the ground, gluteus medius and minimus of the standing side keep the pelvis level.
• Locomotion: running, climbing and rising from sitting depend on powerful hip extension (gluteus maximus, hamstrings).

Stability vs mobility: Very stable, the opposite of the shoulder. The acetabulum is deep, the labrum grips the head beyond its widest part, the capsule is thick and three spiral ligaments tighten in extension, screwing the head into the socket. The shoulder has a shallow glenoid cavity and a loose capsule, so it moves more and dislocates more. The hip is least stable when flexed and adducted (for example, sitting with the knee struck from in front), which is when posterior dislocation can happen.

• Fracture of the femoral neck (common in older people with osteoporosis) can tear the retinacular arteries, leading to avascular necrosis of the femoral head.
• Posterior dislocation of the hip can injure the sciatic nerve.
• Developmental dysplasia (congenital dislocation) of the hip — the head is not held in a well-formed acetabulum; screened for in newborns.
• Weakness of gluteus medius and minimus (e.g. superior gluteal nerve injury) gives a positive Trendelenburg sign: the pelvis drops on the unsupported side.
• Hip disease may cause pain felt in the knee, because the same nerves supply both joints.

• Ball-and-socket, multiaxial synovial joint; femoral head with the lunate surface of the acetabulum.
• Labrum deepens the socket; transverse acetabular ligament bridges the acetabular notch.
• Iliofemoral (Y-shaped) ligament is the strongest and prevents hyperextension.
• Ischiofemoral is the weakest capsular ligament; pubofemoral limits over-abduction.
• Ligament of the head is weak; it carries a branch of the obturator artery; main blood supply is via retinacular arteries.
• Capsule reaches the intertrochanteric line in front but only part of the neck behind.
• Compare stability with the shoulder joint.

• Saying the ligament of the head is a major stabiliser — it is weak.
• Saying the ligament of the head carries the main blood supply of the adult femoral head — the retinacular arteries do.
• Saying the whole acetabulum is articular — only the lunate surface is.
• Saying the capsule attaches to the intertrochanteric crest behind — it stops short of it on the neck.
• Calling the ischiofemoral ligament the strongest — the iliofemoral is.