A 64-year-old man is brought to the emergency department after a high-speed motor vehicle collision. The patient was a restrained passenger in a truck involved in a head-on collision with another vehicle. On examination, he is alert and oriented with severe pain and deformity of the right lower extremity. The patient is lying on his left side with a shortened and internally rotated right leg. The right hip is flexed and adducted, the right knee is flexed, and he is unable to dorsiflex or to straighten the right leg, even with assistance. Which of the following is the most likely diagnosis?
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This patient has a posterior hip dislocation following high-velocity trauma. Posterior hip dislocation (ie, femoral head displaced posteriorly with respect to the acetabulum) generally occurs in head-on motor vehicle collisions in which the knee strikes the dashboard while the hip is flexed and abducted. On examination the leg appears shortened and internally rotated, often with the hip held in flexion and adduction. Range of motion is limited and can be very painful.
The dislocation itself can leave the leg in a fixed position. In addition, because it runs close to the posterior aspect of the hip joint, the sciatic nerve, which branches into the common fibular (peroneal) and tibial nerves and participates in the innervation of most of the lower leg musculature, is vulnerable to injury. Weakness of knee flexion (sciatic nerve) and ankle dorsiflexion (common fibular nerve), decreased ankle reflex (tibial nerve), and decreased sensation in the distribution of the nerve can be seen.
Other potential complications of posterior hip dislocation include tear of the labrum, fracture of the femoral head or acetabulum, and arterial injury with avascular necrosis of the femoral head.
(Choices A, B, and E) Anterior hip dislocation (ie, femoral head displaced anteriorly with respect to the acetabulum), which is much less common than posterior hip dislocation, can occur in violent trauma in which a flexion force is applied with the hip abducted and externally rotated. Femoral neck and intertrochanteric fractures are common injuries that can result from a variety of traumatic insults. In these conditions, the leg typically appears shortened and externally, rather than internally, rotated (eg, due to contraction of the psoas and iliacus without the normal acetabular counterforce).
(Choice C) Like posterior hip dislocation, knee (tibiofemoral) dislocation can occur due to dashboard injury in a motor vehicle collision. However, the knee appears swollen and grossly deformed; this patient's leg shortening and internal rotation are more consistent with hip dislocation.
Educational objective:
Posterior hip dislocation commonly occurs in head-on motor vehicle collisions in which the knee strikes the dashboard. The leg appears shortened and internally rotated. Complications include sciatic nerve injury (eg, impaired dorsiflexion) and arterial injury with avascular necrosis of the femoral head.