Avascular Necrosis (Osteonecrosis)

USMLE Step 1 trap: Confuses the mechanism of corticosteroid-induced AVN as thrombotic rather than fat emboli/lipid accumulation. Corticosteroids cause AVN primarily by promoting fat cell hypertrophy and lipid accumulation in bone marrow, increasing intraosseous pressure and impairing perfusion.

Avascular necrosis (AVN), or osteonecrosis, is bone death from interrupted blood supply, and USMLE Step 1 tests it from three angles with a consistent trap on early imaging: plain X-ray is essentially useless in early AVN. By the time the crescent sign or sclerosis appears on X-ray, the disease is already advanced. MRI is the gold standard for early detection — in any patient with risk factors (corticosteroids, sickle cell, alcohol, trauma, decompression sickness) and hip pain with a normal or equivocal X-ray, MRI is the correct next step. The corticosteroid mechanism is also commonly wrong — it's not thrombosis, it's fat cell hypertrophy raising intraosseous pressure, which chokes off microcirculation like compartment syndrome inside bone.

The high-yield risk factors are sickle cell disease (sickling obstructs vessels), corticosteroids (fat cell hypertrophy raises intraosseous pressure — not thrombosis), alcohol (similar fat accumulation mechanism), trauma (disrupts end-arterial supply), and decompression sickness (nitrogen bubbles embolize). On imaging, the progression goes: normal X-ray early → MRI shows marrow edema → subchondral lucency (crescent sign) → sclerosis and collapse on plain film. If a vignette gives you early hip pain with a normal X-ray, MRI is the next step and the expected finding.

The pediatric section is a classic USMLE Step 1 differential trap. Legg-Calvé-Perthes (LCP) is true AVN of the femoral head in a child ages 4–8, presenting with a limp and hip pain. Osgood-Schlatter is traction apophysitis at the tibial tubercle in an active adolescent — a completely different joint, mechanism, and age group. Mixing these two up is the single most common error on vignettes covering pediatric bone disorders.

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Common misconceptions

Common mistake

Wrong: Corticosteroids cause AVN by directly occluding blood vessels with thrombus.

Right: Corticosteroids cause AVN primarily by promoting fat cell hypertrophy and lipid accumulation in bone marrow, increasing intraosseous pressure and impairing perfusion.

Corticosteroids don't cause AVN by forming thrombus in vessels. Instead, they drive fat cell hypertrophy and lipid accumulation within the bone marrow, which physically raises intraosseous pressure and chokes off the microcirculation. Think of it like compartment syndrome happening inside the bone — the pressure rise impairs perfusion without any clot formation. This is the same general mechanism as alcohol-induced AVN.

Common mistake

Wrong: Osgood-Schlatter disease affects the femoral head like Legg-Calvé-Perthes.

Right: Legg-Calvé-Perthes is AVN of the femoral head in young children (4–8 years), while Osgood-Schlatter is traction apophysitis of the tibial tubercle in adolescent athletes.

Osgood-Schlatter has nothing to do with the femoral head — it's traction apophysitis where the patellar tendon repetitively pulls on the tibial tubercle in growing adolescents, causing pain and a visible bony bump just below the knee. Legg-Calvé-Perthes is true osteonecrosis of the femoral head in younger children (4–8 years). Different joint, different mechanism (vascular vs. traction), different age. The only thing they share is that both present with a limp.

Common mistake

Wrong: Plain X-ray is the most sensitive imaging modality for early AVN.

Right: MRI is the most sensitive modality for early AVN, detecting marrow edema and necrosis before any changes appear on plain X-ray.

Plain X-ray is essentially useless in early AVN — by the time you see the crescent sign or sclerosis, the disease is already advanced. MRI detects marrow edema and early necrosis well before any cortical or trabecular changes are visible on X-ray, making it the gold standard for early diagnosis. In any Step 1 vignette where a patient has risk factors for AVN and hip pain with a normal or equivocal X-ray, MRI is the correct next step.

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What the exam tests

Know the major risk factors for AVN and the specific mechanism by which each one interrupts blood supply to bone — especially corticosteroids and sickle cell disease.
Trace the imaging progression of femoral head AVN in order: early normal X-ray, MRI detection of marrow changes, crescent sign on plain film, then sclerosis and collapse — and know which modality detects disease earliest.
Distinguish Legg-Calvé-Perthes from Osgood-Schlatter by age group, anatomic site, and underlying mechanism — these two are routinely placed in direct contrast on pediatric musculoskeletal vignettes.

Can you avoid these mistakes?

A 35-year-old with SLE on long-term prednisone develops progressive right hip pain. X-ray is normal. What is the most likely diagnosis, what is the next best imaging step, and what is the mechanism by which his medication contributed to this condition?

A 6-year-old boy presents with a painless limp and limited hip abduction. X-ray shows flattening and increased density of the femoral head. What is the diagnosis, and how does it differ mechanistically and anatomically from Osgood-Schlatter disease?

You see the crescent sign on a plain X-ray of the hip. What does this finding represent pathologically, and at what stage of AVN does it appear relative to MRI findings?

A patient with sickle cell disease develops knee pain and a tender, prominent bump just below the patella after starting a new sport. What is the diagnosis, and what is the underlying mechanism — is this osteonecrosis?

Avascular Necrosis (Osteonecrosis)

Common misconceptions

What the exam tests

Can you avoid these mistakes?

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