Hip Dislocation and Femoral Head Fractures

Introduction

Hip dislocations and femoral head fractures are usually the result of high-energy trauma. Associated injuries are common and include chest, abdominal, craniofacial, and other musculoskeletal trauma. Hip dislocations can be divided into anterior and posterior types, and femoral head fractures are classified according to the fracture location in the femoral head as well as the presence of an associated fracture (e.g., femoral neck or acetabular fracture).

Treatment principles for patients with either injury include (1) careful clinical evaluation to detect associated injuries; (2) emergent gentle closed or, if necessary, open reduction, followed by assessment of hip stability; (3) radiographic evaluation (including computed tomography [CT] scan) for congruency of reduction and any associated fractures; and (4) treatment of residual joint incongruity, removal of clinically significant intraarticular fragments, and establishment of hip stability.

Epidemiology and Risk Factors

Posterior hip dislocations are much more common than anterior dislocations. Anterior dislocations constitute 10% to 15% of traumatic dislocations of the hip and posterior dislocations account for the remainder. True shear-type fractures in the frontal plane of the femoral head occur in 10% of posterior dislocations and indentation lesions of the femoral head occur in 25% to 75% of anterior dislocations.

Hip dislocations and femoral head fractures are usually associated with other injuries, either systemic or musculoskeletal. Suraci reported that 95% of patients who sustained a hip dislocation after a motor vehicle accident had an associated injury requiring inpatient management. Ipsilateral knee injuries are particularly common. In a series of 187 patients who sustained a dislocation or fracture dislocation of the hip, Tabuenca and Truan reported that 25% sustained a major knee injury. In another series, 89% of patients who sustained a hip dislocation had visible evidence of soft tissue injury about the ipsilateral knee ; magnetic resonance imaging (MRI) revealed acute meniscal tear in 22% of patients, bone bruise in 33%, effusion in 37%, cruciate ligament injury in 25%, collateral ligament injury in 21%, and periarticular knee fracture in 15%.

Sciatic nerve injury occurs in 10% to 15% of hip dislocations. The peroneal division is affected more frequently than the tibial branch because it is tethered at the pelvis and fibular neck. Additionally, the fascicles of the peroneal division are fewer in number, larger, and less protected by connective tissue. Partial return of function in sciatic nerve palsy can be expected in more than 50% of affected patients.

Anatomy and Pathophysiology

The hip joint is inherently stable; more than 400 N of force is required to distract the hip. Hip stability is conferred by both osseous and ligamentous restraints as well as by femoral head congruity with the acetabulum. The labrum deepens the acetabulum and enhances joint stability. The hip joint capsule is formed by thick longitudinal fibers supplemented by much stronger ligamentous condensations (iliofemoral, pubofemoral, and ischiofemoral ligaments) that run in a spiral fashion, preventing excessive hip extension. Seventy percent of the femoral head articular surface is involved in load transfer; therefore, damage to this surface may lead to the development of posttraumatic arthritis.

The main vascular supply to the femoral head originates from the medial and lateral femoral circumflex arteries—branches of the profunda femoral artery. An extracapsular vascular ring is formed at the base of the femoral neck with ascending cervical branches that pierce the hip joint at the level of the capsular insertion. These branches ascend along the femoral neck and enter the bone just inferior to the cartilage of the femoral head. The artery of the ligamentum teres, a branch of the obturator artery, may contribute blood supply to the epiphyseal region of the femoral head.

The sciatic nerve exits the pelvis at the greater sciatic notch. A certain degree of variability exists in the relationship of the nerve to the piriformis muscle and short external rotators of the hip. Most frequently, the sciatic nerve exits the pelvis deep to the muscle belly of the piriformis.

Force transmission resulting in hip dislocation or femoral head fracture arises from one of three common sources : (1) the anterior surface of the flexed knee striking an object (e.g., dashboard injury); (2) the sole of the foot, with the ipsilateral knee extended (e.g., foot on brake pedal); or (3) the greater trochanter (e.g., lateral impact). Less frequently, the force resulting in hip dislocation may be applied to the posterior pelvis with the ipsilateral foot or knee acting as the counterforce. The direction of dislocation (anterior or posterior) is ultimately determined by the position of the lower extremity at injury and the direction of the pathologic force.

Posterior dislocations are far more common and usually result from a direct impact to the flexed knee with the hip in varying degrees of flexion. If the position of the hip is neutral or slightly adducted at the time of impact, a simple dislocation (no acetabular fracture) will likely occur. However, a femoral head fracture may occur as a result of avulsion by the ligamentum teres or impaction by the posterior acetabular rim. If the hip is in abduction, a posterior-superior rim of the acetabulum fracture usually results.

Anterior dislocations represent less than 10% of all hip dislocations. They typically result from a blow to an abducted and externally rotated hip. The amount of hip flexion determines whether one sustains a superior or inferior (obturator) type of anterior hip dislocation. Inferior dislocations are the result of simultaneous abduction, external rotation, and hip flexion; superior (iliac or pubic) dislocations are the result of abduction, external rotation, and hip extension. Anterior dislocations are frequently associated with femoral head impaction fractures secondary to impaction with the acetabular margin.

For the hip to dislocate, the ligamentum teres and a portion of the hip capsule must be disrupted. Tears of the acetabular labrum and associated muscle commonly occur. The capsule of the hip may be stripped off of the acetabulum or femur as a cuff secondary to rotational forces or may be split by direct pressure. In anterior dislocations, the hip capsule is usually disrupted anteriorly and inferiorly. In posterior dislocations, the capsule may be disrupted inferoposteriorly or directly posteriorly, depending on the position of the hip at impact.

Clinical Features and Diagnosis