The hip is a ball-and-socket joint. The 3 main types of hip dislocation are (1) traumatic dislocation of a previously normal hip, (2) dislocation of a prosthetic hip, and (3) developmental dysplasia of the hip resulting in spontaneous and often chronic dislocation. Traumatic dislocations can be described as being anterior, posterior, or central.

Anterior dislocation of the hip occurs from a direct blow to the posterior aspect of the hip or, more commonly, from a force applied to an abducted leg that levers the hip anteriorly out of the acetabulum. The hip is forced into abduction and the force pushes the femur medially. Abduction causes the femoral neck or greater trochanter to jam against the superior segment of the acetabulum. The greater trochanter or femoral neck then acts like a lever, lifting the femoral head out of the acetabulum. A medially directed force then pushes the femoral head through the anterior acetabular capsule.

Posterior dislocations account of more than 90% of dislocations and occur when the knee and hip are flexed and a posterior force is applied at the knee. Posterior hip dislocations occur typically during MVAs, especially head-on collisions, when the knees of the front-seat occupant strike the dashboard. Energy is transmitted along the femoral shaft to the hip joint. If the leg is struck while in an adducted position, a posterior dislocation may result. If the leg is in neutral or an abducted position when struck, an anterior dislocation or fracture/dislocation may occur. In the latter case, the posterior wall of the acetabulum is fractured, making subsequent reduction less stable.

The third type of hip dislocation is a central dislocation in which a direct impact to the lateral aspect of the hip forces the hip centrally through the acetabulum into the pelvis. This is a fracture-dislocation.

A hip dislocation requires immediate pain management, full medical screening examination, and reduction of the dislocation within 6 hours. The incidence of subsequent avascular necrosis (AVN) of the femoral head is a time- dependent phenomenon, one most likely to occur if relocation is delayed beyond 6 hours.

Many patients with hip dislocation have multiple injuries that may take  precedence in the resuscitation sequence. Conversely, the physical findings of a hip dislocation may be overlooked on initial resuscitation of a patient with trauma, especially an unconscious one. The secondary trauma survey should include an assessment of the hips and other large joints. In all instances neurovascular examination is imperative before any reduction is attempted.

Posterior hip dislocations:

• The affected limb is shortened, adducted, and internally rotated, with the hip and knee held in slight flexion.
  
  
• Patient may be unable to walk or adduct the leg.
  
• Signs of vascular or sciatic nerve injury may be present.
  

Anterior hip dislocation:

• The leg is externally rotated, abducted, and extended at the hip.
  
• The femoral head may be palpated anterior to the pelvis.
  
• Signs of injury to the femoral nerve or artery may be present.
  

Central dislocation:

• The leg is shortened, abducted or adducted, and internally or externally rotated, depending on the type and extent of penetration into the pelvis.
  
• The typical posture of the leg with anterior or posterior hip dislocation may not be seen if an associated femoral shaft fracture is present.
  
• The leg distal to the fracture assumes a neutral position, masking the usual rotation seen with a dislocation.
  
• The incidence of missed hip dislocation is increased in the presence of a femoral shaft fracture.

Causes:

The most common cause of a hip dislocation is a MVA, in which a front seat occupant strikes a flexed knee against the dashboard during a head-on impact. Transmitted forces displace the hip posteriorly out of the acetabulum. Patients with hip prostheses may undergo hip dislocation with relatively little trauma, as the ligaments supporting the joint are no longer functioning normally.

Imaging Studies:

The appearance of a hip dislocation may be subtle on a single anteroposterior (AP) pelvis view because the femoral head may lie in an apparently normal position even though it is dislocated. It is necessary to obtain a lateral view to confirm the dislocation. Both CTs and MRIs are quite accurate in delineating the exact injury. MRI is also useful for detecting AVN of the hip as well as nondisplaced stress fractures of the femoral neck.

Treatment:

There is a 6 hour window for doing the reduction. If a neurovascular deficit is present the reduction should be done sooner. Closed reductions should initially be attempted. It is first necessary to give the patient conscious sedation which consists of an IV pain medication and muscle relaxant. Several techniques can be tried. For posterior hip dislocations there is the Allis maneuver and the Stimson maneuver. There are other techniques which can be tried for both posterior and anterior dislocations. These include the reverse Bigelow maneuver, the leg-crossing maneuver, longitudinal traction, and the Whistler maneuver.

If relocation is successful, the legs should be immobilized in slight abduction by using a pad between the legs to prevent adduction until skeletal traction can be instituted. The duration of traction and non-weight bearing is controversial since early weight bearing may increase the severity of aseptic necrosis of the joint. Early weight bearing, on the other hand, decreases the incidence of other complications like deep venous thrombosis and skin ulcers. The usual duration of traction is about 2 weeks with non-weight bearing from 2 weeks to 3 months.

If closed reduction fails, then the patient must be taken to surgery for open reduction. This occurs about 10% of the time.

Complications:

These include AVN of the hip which occurs about 8- 13% of the time. The incidence of AVN is increased with delayed reduction, repeated attempts at reduction, and open reduction. Other complications include osteoarthritis, heterotopic calcification, recurrent dislocations, complications of immobilization (DVT, pulmonary embolus, decubiti, pneumonia), sciatic nerve injury (from posterior dislocations in 10- 14% of cases), and femoral nerve injury associated with femoral nerve injury.

Prognosis:

The prognosis of the patient with a hip dislocation varies with the type of dislocation, with the associated fractures of the femoral head or acetabulum, and the presence of other injuries. Overall, good results are obtained in 70-90% of patients. When things don't go well, inevitably it is due to AVN, osteoarthritis or nerve injury. Recurrent dislocation is a common complication because supporting ligaments have been disrupted.

Failure to diagnose and reduce hip dislocations in the first 6 hours is associated with a very poor prognosis, and can be the basis for medical liability. Clinical pitfalls in the management of hip dislocation include the following: failure to diagnose hip dislocation in the presence of associated fracture is a known risk; reliance on a single AP view to rule in or out the diagnosis is a mistake; failure to test femoral and sciatic nerve function and distal perfusion before and after attempts at closed reduction.