A 22-year-old man has a painful lesion in the posterior cortex of the femoral diaphysis. X-ray shows a small radiolucent nidus (<1.5 cm) surrounded by dense cortical sclerosis. Night pain relieved by aspirin is the predominant symptom. What is the diagnosis and most important histological constituent of the nidus?
- A Osteoid osteoma; the nidus contains osteoid trabeculae, osteoblasts and vascular fibrous stroma with abundant prostaglandin E2 ✓
- B Osteoblastoma; the nidus contains woven bone trabeculae with vascular fibrous stroma
- C Stress fracture with callus formation; periosteal reaction and sclerosis
- D Brodie's abscess; the nidus contains purulent exudate and surrounding sclerosis
Correct answer: A. Osteoid osteoma; the nidus contains osteoid trabeculae, osteoblasts and vascular fibrous stroma with abundant prostaglandin E2
Explanation
Osteoid osteoma presents classically with a nidus <1.5 cm in cortical bone of long bones, nocturnal pain dramatically relieved by NSAIDs/aspirin (due to prostaglandin E2 and E3 production within the nidus), and a surrounding zone of reactive cortical sclerosis. The nidus consists of osteoid trabeculae lined by active osteoblasts in a vascular fibrous stroma. Osteoblastoma is histologically similar but exceeds 2 cm. Brodie's abscess causes a similar sclerotic X-ray appearance but is an infective lesion. Stress fractures lack a discrete nidus.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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