Characteristics

• Mechanism of injury usually involves massive force transmitted along the femoral shaft, e.g. a dashboard injury in a road-traffic accident.

• Posterior dislocation (80%) tends to occur with the hip flexed and adducted at time of impact. With abduction, anterior dislocation can occur.

• Central dislocation occurs with medial displacement of the femoral head through or partially through a fragmented acetabulum.

• Often associated with other injuries such as a patellar fracture, PCL injury or posterior acetabular-wall fracture.

Clinical features

• Posterior dislocations – leg is flexed, adducted and internally rotated – unless an associated femoral neck or shaft fracture mask the deformity.

• Pain tends to be excruciating.

• With an acetabular (e.g. posterior wall) fracture, spontaneous reduction possible.

• Sciatic nerve injuries are common – up to 20% – test preferentially the peroneal (test foot eversion) rather than tibial branch (foot plantar flexion) of the sciatic nerve.

Radiological features

• Abnormality usually obvious on the AP view. Lateral view recommended in all cases to aid in determining direction of dislocation and associated fractures.

• With posterior dislocations the femoral head appears smaller than the unaffected side on the AP view and conversely with anterior it appears larger.

• Look for the lesser trochanter – overlies the femoral shaft in posterior dislocation (due to internal rotation), whereas it is seen in profile in anterior (due to external rotation).

• Look for acetabular involvement as this affects likelihood of sciatic-nerve damage, stability and long-term functional outcome.

• Always assess the pelvic ring fully as associated fractures/disruption are common – see ‘Pelvic and acetabular fractures’.

Management

• ABCs, assess soft tissues, neurovascular status, reduce and immobilise.

• Early reduction is the definitive treatment. Complete muscle relaxation is desirable and thus reduction under general anaesthetic with screening is optimal to decrease femoral head trauma.

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