Hip Joint
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Direct answer
Everything the shoulder gives up for range, the hip keeps for stability. It is a synovial multiaxial ball-and-socket joint between the femoral head and the deep acetabulum, which the fibrocartilaginous labrum and transverse acetabular ligament complete into a socket gripping almost the whole head. The capsule is reinforced by the strongest ligament in the body — the inverted-Y iliofemoral ligament of Bigelow — and the head's blood supply, riding on retinacular vessels along the neck, is the joint's clinical weak point: a femoral neck fracture threatens avascular necrosis with every displacement.
What you must remember
- Type and fit: ball-and-socket; the acetabulum, deepened by the labrum, encloses the head so completely that dislocation needs enormous force.
- Capsule: attached to the intertrochanteric line in front and about a centimetre medial to the intertrochanteric crest behind — most of the neck is intracapsular, the trochanters outside.
- Ligaments: iliofemoral (Bigelow) — strongest in the body, inverted Y in front, taut in extension, resisting hyperextension; pubofemoral — below and in front, limiting abduction; ischiofemoral — weakest, spiralling from behind, checking medial rotation and extension.
- Ligamentum teres: from the transverse acetabular ligament to the fovea, carrying the artery to the head from the obturator artery — a genuine supply in children, a minor one in adults.
- Blood supply of the head: chiefly the retinacular branches of the medial circumflex femoral artery ascending the neck under the synovial reflection, reinforced by lateral circumflex femoral branches and the ligamentum teres artery; the retinacular vessels are torn in intracapsular neck fractures.
- Nerve supply: Hilton's law — femoral (via nerve to rectus femoris), obturator, superior gluteal and sciatic (nerve to quadratus femoris) nerves.
- Movements and movers: flexion — iliopsoas; extension — gluteus maximus and hamstrings; abduction — gluteus medius and minimus; adduction — adductors; medial rotation — tensor fasciae latae.
- Clinical anchors: posterior dislocation of the dashboard injury — short, adducted, medially rotated limb, sciatic nerve at risk; intracapsular neck fracture — avascular necrosis and non-union; gluteus medius weakness — positive Trendelenburg sign, waddling Duchenne lurch when bilateral.
Two patients, one artery at risk
An elderly woman slips in the bathroom and cannot stand; the right leg lies short and externally rotated. The retinacular vessels are the story: an intracapsular subcapital or transcervical fracture leaves the head dependent on the medial circumflex femoral retinacular branches ascending the neck under the synovial reflection, and displacement tears them — the head dies, avascular necrosis and non-union follow. That is why a displaced fracture in the elderly is usually treated with hemiarthroplasty, while an undisplaced impacted fracture, whose retinacular supply survives, can be fixed.
Now the dashboard injury: a young man's flexed knee strikes the dashboard, driving the femoral head backwards out of the acetabulum, sometimes fracturing the posterior wall. The limb lies short, adducted and medially rotated, and the sciatic nerve lies directly behind the joint, so dorsiflexion and sensation are documented before reduction. One more bedside link: standing on one leg, watch the pelvis — a weak gluteus medius and minimus (superior gluteal nerve, L5) on the stance side lets it sag, the Trendelenburg sign; bilateral weakness gives the waddling gait of congenital dysplasia and muscular dystrophy.
Where students slip
The ligaments' strengths are shuffled: iliofemoral is anterior and strongest, ischiofemoral posterior and weakest — extracted with "which resists hyperextension?" (iliofemoral) and "which limits abduction?" (pubofemoral). The ligamentum teres is credited with the adult head's supply; in the adult it feeds only a small territory near the fovea, the medial circumflex retinacular branches doing the work — why Perthes disease and neck fractures both kill the head when that supply fails. The capsule's posterior attachment is placed on the intertrochanteric crest itself; it attaches a centimetre medial to it, leaving a bare extracapsular strip of neck — the detail that decides which fractures are intracapsular. In the shoulder comparison, say it numerically: the acetabulum covers nearly half the head where the glenoid covers a third, and hip dislocation is a high-energy injury while the shoulder dislocates reaching for a switchboard.
Frequently asked questions
Which is the strongest ligament of the hip, and what does it do?
The iliofemoral ligament (of Bigelow), an inverted Y anterior to the joint — taut in extension, checking hyperextension so the erect posture is held with minimal muscle effort.
Why does fracture of the neck of the femur cause avascular necrosis?
Most retinacular vessels run along the neck within the capsule; an intracapsular fracture tears them, leaving the head with little supply, so it necroses and the fracture fails to unite.
What is the Trendelenburg sign?
Standing on one leg, the pelvis drops on the opposite side when the stance-side abductors (gluteus medius and minimus, superior gluteal nerve, L5) are weak — seen in hip dysplasia, poliomyelitis, nerve injury and hip pain.
Which nerve is injured in posterior dislocation of the hip?
The sciatic nerve, directly behind the joint; the dashboard injury produces a short, adducted, medially rotated limb, and foot drop must be sought before reduction.