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AN17.1-3 | Hip Joint — Part 2
CLINICAL PEARL
Why does AVN happen after intracapsular neck fractures?
The femoral head in adults depends almost entirely on the retinacular branches of the medial circumflex femoral artery, which run as fine vessels in the synovial folds (retinacula) along the femoral neck. In a displaced intracapsular fracture (Garden III/IV), these vessels are torn by the displacement → the femoral head loses its blood supply → AVN develops over weeks to months (the head becomes sclerotic and eventually collapses).
This is why the orthopaedic principle for displaced intracapsular fractures in the elderly is hemiarthroplasty (replacing the femoral head) rather than internal fixation — by the time the fracture heals (if it does), the head is likely already avascular.
In younger patients (<60 years), urgent reduction and internal fixation is attempted first, with hemiarthroplasty as a salvage. In younger patients, preservation of the native head is prioritised.
SELF-CHECK — Self-Check 1
The iliofemoral ligament is the strongest ligament in the body. Which movement does it primarily resist?
A. Hip flexion
B. Hip extension and lateral rotation
C. Hip adduction
D. Hip medial rotation
Reveal Answer
Answer: B. Hip extension and lateral rotation
In a displaced intracapsular fracture of the femoral neck (Garden III), which blood vessels are torn, causing AVN risk?
A. Obturator artery (foveal branch) within the ligamentum teres
B. Retinacular branches of the medial circumflex femoral artery
C. Lateral circumflex femoral artery
D. Inferior gluteal artery
Reveal Answer
Answer: B. Retinacular branches of the medial circumflex femoral artery
Hip Dislocation
Dislocation of the hip joint is uncommon due to the deep socket and strong ligaments, but when it occurs, it is usually due to high-energy trauma.
Posterior dislocation (most common, ~90%):
- Mechanism: dashboard injury in RTA — knee hits the dashboard with the hip in flexion → force drives the femoral head posteriorly out of the acetabulum
- Clinical picture: limb held in flexion, adduction, and internal rotation (the posterior capsule is lax in this position); limb appears shortened; femoral head not palpable in groin
- Complication: the sciatic nerve runs just posterior to the hip joint and is at risk → foot drop, loss of posterior thigh and below-knee sensation
- Treatment: urgent reduction under anaesthesia (within 6 hours to prevent AVN); check post-reduction for sciatic nerve function and acetabular fractures on CT
Anterior dislocation (rare, ~10%):
- Mechanism: forced external rotation in abduction (e.g., a fall straddling an object)
- Clinical picture: limb in flexion, abduction, and external rotation; femoral head palpable anteriorly in the femoral triangle
- Complication: femoral nerve and vessels at risk
Central fracture-dislocation: the femoral head is driven through the acetabular floor into the pelvis — requires CT and often open reduction.
Total Hip Replacement (Arthroplasty)
Total hip replacement (THR) replaces both the femoral head/neck (with a metal stem and ceramic/metal head) and the acetabular socket (with a metal/polyethylene cup). It is indicated for:
- Severe osteoarthritis (most common)
- Rheumatoid arthritis
- AVN of the femoral head
- Failed internal fixation of femoral neck fracture
Anatomical considerations for THR:
- Posterior approach (most common): detaches the short external rotators (piriformis, obturator internus, gemelli, quadratus femoris) to access the posterior capsule — these must be repaired to prevent posterior dislocation post-op; the sciatic nerve is protected
- Anterior approach (muscle-sparing): between sartorius/tensor fasciae latae (anterolateral) without detaching muscles; lower dislocation risk; steeper learning curve
Complications of THR:
- Dislocation — most common complication; more frequent with posterior approach; patients must avoid hip flexion >90° and internal rotation post-op
- DVT/PE — prophylaxis with anticoagulants is mandatory
- Infection — catastrophic; requires prosthesis removal
- Leg length discrepancy — improper implant sizing
- Aseptic loosening — long-term (>10 years); osteolysis from polyethylene wear particles
SELF-CHECK — Self-Check 2
A patient arrives in casualty after an RTA with the left hip held in flexion, adduction, and internal rotation. There is foot drop on the same side. What is the most likely diagnosis and associated nerve injury?
A. Anterior hip dislocation with femoral nerve injury
B. Posterior hip dislocation with sciatic nerve injury
C. Fracture neck of femur with femoral nerve injury
D. Central fracture-dislocation with obturator nerve injury
Reveal Answer
Answer: B. Posterior hip dislocation with sciatic nerve injury
In total hip replacement using the posterior approach, which muscle group is detached to access the posterior capsule?
A. Hip abductors (gluteus medius and minimus)
B. Short external rotators (piriformis, obturator internus, gemelli, quadratus femoris)
C. Hamstrings
D. Adductor magnus
Reveal Answer
Answer: B. Short external rotators (piriformis, obturator internus, gemelli, quadratus femoris)