Causes / typical injury mechanism: Hip joints are known for their stability as there is quite a bit of bony constraint; thus, it takes significant force and trauma to dislocate the hip. Traumatic hip dislocation is most commonly seen in young adults after a high-energy trauma. Motor vehicle accidents are the number one cause. Other causes include sports-related injuries, pedestrian versus motor vehicle accidents, and falls. Overall, the incidence of hip dislocation is uncommon, even following a high-energy trauma, so it has been suggested that anatomical variants of the hip may make certain people more vulnerable to this injury, such as developmental dysplasia of the hip (congenital under-coverage of the hip making the joint less congruous or captured).

Classic history and presentation: Patients often present to the emergency department (ED) and require a full trauma evaluation due to the high likelihood of additional concomitant injuries. One study suggests that 67% of patients who incur a hip dislocation secondary to a motor vehicle accident will have other, nonorthopedic injuries, which can include head, thoracic, and abdominal injuries. Additionally, up to 95% of patients with a traumatic hip dislocation will require inpatient management. A traumatic hip dislocation is often associated with an acetabular fracture (typically the posterior wall) or femoral fracture (typically involving the femoral head), so these concomitant injuries must be evaluated.

Risk factors: Of note, hip dislocation is commonly seen in patients who have had a total hip arthroplasty (see total hip arthroplasty dislocation) or in children with developmental dysplasia of the hip. Other risk factors include traumatic injury to the hip.

Pathophysiology: Look for a traumatic injury in which the hip is typically driven posteriorly, resulting in dislocation from the joint. A common mechanism is a knee striking a dashboard during a motor vehicle accident, driving the femoral head posteriorly out of the acetabulum.

Classification system: Hip dislocations can be classified by their direction and further described by the head-acetabulum relationship and presence or absence of associated fractures.

Posterior hip dislocations (Thompson and Epstein classification)

• Type 1 – Simple dislocation with no or minor fracture
• Type 2 – Dislocation with a single large fracture of posterior acetabular rim
• Type 3 – Dislocation with a comminuted acetabular rim
• Type 4 – Dislocation with a fracture of the acetabular floor
• Type 5 – Dislocation with a fracture of the femoral head

Anterior hip dislocations (Epstein classification)

• Type 1 – Superior dislocations (pubic and subspinous)
  • Type 1A – No associated fractures
  • Type 1B – Associated fracture or impaction of femoral head
  • Type 1C – Associated fracture of acetabulum
• Type 2 – Inferior dislocations (obturator and perineal)
  • Type 2A – No associated fractures
  • Type 2B – Associated fracture or impaction of femoral head
  • Type 2C – Associated fracture of acetabulum