A 26-year-old man with sickle cell disease (HbSS) presents to the clinic with a 3-month history of worsening right hip pain. He describes the pain as a deep ache that is exacerbated by weight-bearing and relieved by rest. He denies any recent trauma, fever, or history of alcohol use. Past medical history is otherwise unremarkable. Vital signs are within normal limits. On physical examination, there is a noticeable limp. Passive internal rotation and abduction of the right hip are significantly limited and painful. A plain radiograph of the pelvis is unremarkable.
Which of the following is the most appropriate next step in the evaluation of this patient?
The correct answer is:
D) Magnetic resonance imaging (MRI) of the hip
This patient is presenting with avascular necrosis (AVN) of the femoral head, a common complication of sickle cell disease. AVN, also known as osteonecrosis, occurs when microvascular occlusion by sickled red blood cells leads to bone ischemia and subsequent death of the bone marrow and osteocytes. This most frequently affects the femoral and humeral heads. In the early stages of AVN, plain radiographs often appear normal, as it takes time for the structural changes of the bone to become radiopaque. MRI is the most sensitive imaging modality for detecting early-stage avascular necrosis, showing characteristic changes such as bone marrow edema and the double-line sign before any collapse of the articular surface occurs.
Answer choice A: Aspiration of the hip joint for crystal analysis, is incorrect. Joint aspiration is primarily used to diagnose gout, pseudogout, or septic arthritis. While patients with sickle cell disease can develop septic arthritis or osteomyelitis (classically Salmonella), this patient’s chronic, activity-related pain and lack of systemic symptoms such as fever favor a mechanical or ischemic process like AVN.
Answer choice B: Bone scan with technetium-99m, is incorrect. While a bone scan can detect increased uptake in areas of bone turnover, it is less sensitive and specific than MRI for the early diagnosis and staging of AVN. It is rarely the preferred next step when MRI is available.
Answer choice C: Erythrocyte sedimentation rate and C-reactive protein, is incorrect. Inflammatory markers may be elevated during an acute vaso-occlusive crisis or in the setting of infection, but they are not diagnostic for the structural damage associated with chronic avascular necrosis.
Answer choice E: Repeat plain radiographs in 6 months, is incorrect. Delaying diagnosis is inappropriate. Early intervention, such as core decompression or physical therapy, may prevent or delay the collapse of the femoral head and the eventual need for total hip arthroplasty.
Key Learning Point
Avascular necrosis is a frequent complication in adults with sickle cell disease due to chronic micro-infarction of bone. It typically presents as insidious, weight-bearing joint pain with limited range of internal rotation. MRI is the diagnostic test of choice when clinical suspicion is high and plain radiographs are negative or inconclusive.
