Musculoskeletal · Trauma
The facts most likely to be tested
The most common mechanism for a posterior hip dislocation is a high-energy trauma such as a dashboard injury in a motor vehicle accident.
Patients with a posterior hip dislocation classically present with the affected limb in shortening, adduction, internal rotation, and flexion.
An anterior hip dislocation presents with the limb in abduction and external rotation.
The most serious and common neurovascular complication of a posterior hip dislocation is injury to the sciatic nerve.
Avascular necrosis (AVN) of the femoral head is the most feared long-term complication due to disruption of the medial circumflex femoral artery.
Urgent closed reduction under sedation is the definitive treatment and must be performed within six hours to minimize the risk of osteonecrosis.
A post-reduction CT scan is mandatory to evaluate for intra-articular loose bodies or acetabular fractures.
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A 28-year-old male is brought to the emergency department after a high-speed motor vehicle collision where his vehicle struck a tree. He was the driver and his knee struck the dashboard upon impact. On physical examination, his right lower extremity is shortened, adducted, and internally rotated. He has decreased sensation in the dorsal aspect of his foot and weakness in ankle dorsiflexion.
What is the most likely diagnosis and the most likely nerve injury?
Posterior hip dislocation with sciatic nerve injury.
The patient's classic 'shortened, adducted, and internally rotated' position is pathognomonic for posterior hip dislocation, and the associated foot drop/sensory loss indicates injury to the sciatic nerve.
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Etiology / Epidemiology
High-energy trauma (MVA) is the primary cause. Posterior dislocation accounts for 90% of cases.
Clinical Manifestations
Limb is shortened, adducted, and internally rotated. Dashboard injury is the classic mechanism.
Diagnosis
CT scan is the gold standard for evaluating associated fractures and joint congruency.
Treatment
Urgent closed reduction under conscious sedation is the first-line intervention.
Prognosis
Avascular necrosis (AVN) risk increases significantly if reduction is delayed beyond 6 hours.
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Epidemiology & Etiology
Most common in young adults following high-energy trauma such as motor vehicle accidents. Pedestrians struck by vehicles are also at high risk. Posterior dislocation is the most frequent type, typically occurring when the hip is flexed and adducted.
Pertinent Anatomy
The hip joint is a stable ball-and-socket joint reinforced by the iliofemoral ligament. The sciatic nerve is anatomically vulnerable during posterior displacement, requiring immediate neurovascular assessment.
Pathophysiology
Force applied to a flexed knee transmits energy through the femur to the hip joint. The femoral head is forced out of the acetabulum, often tearing the posterior capsule. Failure to reduce the joint promptly leads to ischemia of the femoral head due to disruption of the medial circumflex femoral artery.
Clinical Manifestations
Patients present with severe pain and inability to move the hip. Posterior dislocation presents with the leg shortened, adducted, and internally rotated. Anterior dislocation presents with the leg abducted and externally rotated. Sciatic nerve palsy (foot drop) is a critical red flag requiring immediate documentation.
Diagnosis
Initial evaluation requires AP pelvis radiograph to confirm dislocation and rule out associated fractures. CT scan is the gold standard to assess for intra-articular fragments or acetabular fractures post-reduction. MRI may be indicated later to evaluate for labral tears or occult fractures.
Treatment
Urgent closed reduction should be performed within 6 hours to minimize the risk of AVN. Procedural sedation with propofol or benzodiazepines is standard. Open reduction is indicated if the joint is irreducible or if there are significant intra-articular fragments.
Prognosis
The primary long-term concern is avascular necrosis (AVN), which correlates directly with the time to reduction. Post-traumatic arthritis is a common late complication. Patients require serial imaging to monitor for femoral head collapse.
Differential Diagnosis
Femoral neck fracture: leg is externally rotated and abducted
Acetabular fracture: often associated with dislocation, requires CT
Hip pointer: localized contusion without deformity
Septic arthritis: systemic symptoms, no trauma history
Slipped Capital Femoral Epiphysis: adolescent patient, insidious onset