Etiology and Pathophysiology | Managing Dislocations of the Hip, Knee, and Ankle

<< Managing Dislocations of the Hip, Knee, and Ankle in the Emergency Department (Trauma CME)

Etiology and Pathophysiology

Hip Dislocation

Hip dislocations are the most common lower-extremity dislocation. The emergency clinician should be able to diagnose the various types of dislocations and assess for associated injuries and complications. Good ED management depends on knowing when to reduce a hip dislocation and gauging the urgency of orthopedic consultation for intraoperative closed reduction or open reduction and internal fixation.

The hip is a ball-and-socket joint, with the femoral head situated within the acetabulum. The hip joint is extremely stable, due to the cartilaginous labrum, ligamentous hip capsule, ligamentum teres, and large muscle groups of the lower extremity. For this reason, most native hip dislocations are caused by a high-energy traumatic mechanism, with motor-vehicle crashes (MVCs) being the most common etiology.1 Over two-thirds of traumatic hip dislocations occur in MVC victims who were not wearing seatbelts.2 Traumatic dislocations are also seen after falls from height, and in athletes involved in contact sports.3

Traumatic hip dislocations may be classified as either simple or complex. Simple dislocations are isolated dislocations without an associated fracture, whereas complex dislocations have an associated fracture of the femoral head, femoral neck, or acetabulum.4 Approximately 90% of all native hip dislocations have an associated fracture.5 Acetabular fractures are the most common and are seen in 70% of cases.6 The more the hip is flexed at the time of dislocation, the lower the risk for associated fracture.7

Hip dislocations are further classified based on the direction of dislocation of the femoral head relative to the acetabulum, either posterior or anterior. Posterior dislocations are the most common, comprising 90% of all traumatic hip dislocations. These dislocations are the result of axial loading on the femur, with the hip flexed and adducted. This is also known as the classic “dashboard injury.”4 Anterior dislocations make up 10% of traumatic hip dislocations, and are subdivided into superior (10%) and inferior types (90%). The superior type of dislocation occurs when the hip is abducted, externally rotated, and extended, often resulting in a fractured femoral head. The inferior type of dislocation occurs when the hip is abducted, externally rotated, and flexed. The obturator can indent the femoral head, resulting in an indentation fracture.3 These typically occur secondary to MVCs where the victim’s leg is abducted, from falls from height, or a blow to the back while squatted.8

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