MRI of the left hip is the most appropriate diagnostic test to perform next in this patient with systemic lupus erythematosus (SLE) who has symptoms associated with osteonecrosis. Patients with SLE who have pain or limitation of motion of the large joints, especially the hips, should be evaluated for osteonecrosis. MRI is the best method for detecting early bone edema caused by osteonecrosis when plain radiographs are normal. MRI can also be prognostic: If more than 20% of the femoral volume demonstrates necrosis and edema on MRI, progressive disease (subchondral fracture and femoral head collapse) is the rule, whereas smaller infarcts rarely progress. Up to 37% of patients with SLE may have osteonecrosis by serial MRI monitoring, although less than 10% become symptomatic. Cushingoid features indicate a risk for osteonecrosis because enlargement of fat cells in the face is a marker for enlargement of fat cells in the ends of long bones. Increased adipose volume causes compression of small sinusoidal vessels that leads to interosseous hypertension and impairment of arterial inflow. Use of daily oral prednisone more than 20 mg/d for 4 to 6 weeks is also a risk factor, whereas use of intravenous glucocorticoids may not have the same risk. This patient has hip pain, cushingoid features, and recent use of high-dose prednisone, all indicators that she should be evaluated for osteonecrosis despite the normal hip radiograph.
MRI remains the gold standard to diagnose osteonecrosis. CT is less sensitive than MRI and exposes the patient to unnecessary radiation.
Plain radiography may not detect changes of osteonecrosis for several months following the onset of symptoms. Early radiographic findings include bone density changes, sclerosis, and, eventually, cyst formation. Subchondral radiolucency producing the “crescent sign” indicates subchondral collapse. End-stage disease is characterized by collapse of the femoral head, joint-space narrowing, and degenerative changes.
Ultrasonography can be used to evaluate for trochanteric bursitis as the cause of lateral hip pain but would not be useful to check for osteonecrosis. Trochanteric bursitis can be confirmed in patients in whom hip adduction intensifies the pain or in those in whom the examination reveals pain and tenderness over the bursa. Pain at night is present when the patient sleeps on the affected side.