Hip Dislocation in Emergency Medicine 

In general, hip dislocations can be classified into congenital and traumatic. The annual incidence of congenital hip dislocation is approximately 2-4 cases per 1000 births, and approximately 80-85% of the affected individuals are girls. Congenital hip dislocations are commonly the result of femoral head or acetabular dysplasia.

The focus of this article is on traumatic hip dislocations. High-energy blunt force trauma is the most common cause. Prosthetic hip joints may dislocate with much less force. Dislocation of the hip joint is an orthopedic emergency. Timely diagnosis and management may minimize the significant morbidity that may result.

The hip is a modified ball-socket joint. The femoral head is situated deep within the acetabular socket, which is further enhanced by a cartilaginous labrum. The hip is also bolstered by a fibrous joint capsule, the ischiofemoral ligament, and many strong muscles of the upper thigh and gluteal region. Because of this anatomic configuration, the hip is stable, as in the image below.

Subsequently, a large force is required to dislocate the joint. Because a high force mechanism is required, other life-threatening injuries and fractures are common.

Motor vehicle crashes (MVC) account for almost two thirds of traumatic hip dislocations. Falls from height and sports injuries are also common causes of hip dislocations.

The relationship of the femoral head to the acetabulum is used to classify hip dislocations. The 3 main patterns are posterior, anterior, and central.

Posterior dislocation

Posterior dislocations compromise approximately 80-90% of hip dislocations caused by MVCs. The femoral head is situated posterior to the acetabulum. During a MVC, force is transmitted to the flexed hip in one of two ways. During rapid deceleration, the knees strike the dashboard and transmit the force through the femur to the hip. If the leg is extended and the knee is locked, force can be transmitted from the floorboard though the entire lower and upper leg to the hip joint. A posterior dislocation is shown in the image below.

Anterior dislocation

The femoral head is situated anterior to the acetabulum. An anterior dislocation is most commonly caused by a hyperextension force against an abducted leg that levers the femoral head out of the acetabulum. Less commonly, an anterior force against the posterior femoral neck or head can produce this dislocation pattern.

Central dislocation

A central dislocation is a fracture-dislocation, shown in the image below, where the femoral head lies medial to a fractured acetabulum. This is caused by a lateral force against an adducted femur seen in side impact MVCs.

Frequency

United States

Posterior hip dislocations are more common than anterior ones and account for almost 90% of hip dislocations. The frequency has decreased with the increased use of belts and air bags. Anterior dislocations and central fracture-dislocations account for less than 10% of hip dislocations.

The incidence of congenital hip dislocations is approximately 1 case per 500 population, which is equivalent to 0.20% or 544,000 people. Extrapolated data suggest that the prevalence of congenital hip dislocation is approximately 587,310.

International

Good extrapolated data about the incidence of congenital hip dislocations are available for many countries. The reader is referred to the References.

Mortality/Morbidity

Hip dislocation is a marker for a high force mechanism. Most mortality is the result of associated injuries. Life-threatening injuries to the pelvis, abdomen, chest, and head should be specifically sought out.

Long-term disability after hip dislocations is a significant risk. Up to 50% of patients will have limited use or chronic pain as a result of hip dislocation. Prognosis becomes worse with delayed diagnosis and management. Complications include deep venous thrombosis (DVT), sciatic nerve injury, avascular necrosis (AVN), vascular injury, recurrent dislocation, arthritis, and chronic pain.

The local venous injury and prolonged immobilization associated with hip dislocations lead to a significant incidence of deep venous thrombosis (DVT) and potentially lethal pulmonary embolus in affected patients. If no contraindications exist, patients should receive DVT prophylaxis as part of the hospital and rehabilitation treatment.

Sciatic nerve injury is common, up to 19% in one study. The femoral head or bony fragments can stretch or tear the nerve as it passes posterior. The neurapraxia is generally transient or minor. A full recovery or recovery with only minor neurological findings can be expected for most patients. Performing and documenting a brief neurological examination before and after relocation is imperative.

Avascular necrosis of the femoral head occurs in 2-17% of patients. This can occur with pure dislocations but is more common with fracture-dislocations of the femoral head. Numerous studies suggest that the risk of AVN rises proportional to the time to relocation. The longer it takes to relocate a hip, the higher the risk of AVN. Early relocation of a hip can make the difference between a healthy joint and a chronically disabled joint.

Vascular compromise is a rare. With anterior dislocations, the femoral artery is at risk. Pulses and perfusion should be checked and documented before and after reduction. If a patient has vascular compromise, reducing the hip should not be delayed. If a patient has a persistent or new-onset perfusion deficit, an open reduction and consultation with a vascular surgeon may be indicated.

Recurrent hip dislocation is uncommon compared to recurrent shoulder dislocation. Risk factors for recurrent dislocation are large capsular defects, intra-articular fragments, or a prosthetic hip.

Posttraumatic arthritis is the most frequent long-term complication following hip dislocation. It occurs in up to 16% of affected individuals and is often associated with life-long gait disturbances and chronic pain. If an associated acetabular fracture is present, the incidence of traumatic arthritis is as high as 80%.

Race

Race is not a risk factor for hip dislocation.

Sex

Hip dislocations are more common in young males than in others because these injuries are associated with risk-taking behavior.

Age

Hip dislocations resulting from traumatic injuries (especially MVCs) are more common in those younger than 35 years than in older people. Hip dislocations resulting from falls are more common in those older than 65 years than in younger people.