Osteoid Osteoma — Night Pain & RFA

Osteoid osteoma is a small, benign bone-forming tumour characterised by a central nidus of woven bone and osteoid surrounded by a zone of reactive sclerosis. It is notable for its pathognomonic presentation — severe nocturnal pain that is dramatically relieved by salicylates (aspirin) or NSAIDs. The pain is mediated by prostaglandins produced within the nidus, which is richly innervated. Osteoid osteoma is the most common benign bone tumour requiring treatment (excluding osteochondroma) and is most commonly encountered in young males in the first three decades of life.

• Epidemiology: peak incidence 10–25 years; male:female ratio approximately 2–3:1; accounts for approximately 10–12% of all benign bone tumours; most commonly affects the proximal femur (femoral neck — the most common location), tibia (posterior cortex), and spine (posterior elements — causes painful scoliosis); can affect any bone; the lesion does not undergo malignant transformation
• Pathophysiology of pain: the nidus contains abundant prostaglandins (PGE2 and PGI2) produced by the osteoblasts within the nidus; prostaglandins sensitise free nerve endings (which are present in abnormally high density within the nidus); aspirin and NSAIDs inhibit prostaglandin synthesis via COX inhibition, directly reducing the pain stimulus; this explains the dramatic, almost diagnostic, response to NSAIDs; the nocturnal exacerbation of pain is thought to relate to the vasodilatory effect of nocturnal prostaglandin release (increased blood flow to the nidus at night)
• The nidus: the nidus is the central active component of the lesion; it is typically <1.5–2 cm in diameter (a defining criterion — lesions >2 cm are classified as osteoblastoma, which has different behaviour and management); the nidus consists of a vascular fibrous stroma containing woven osteoid and trabeculae of woven bone; it is surrounded by dense reactive cortical sclerosis that can be dramatic and obliterate the nidus on plain X-ray

• Classic presentation: nocturnal pain (worse at night, waking the patient from sleep) that is dramatically and reliably relieved by aspirin or NSAIDs within 20–30 minutes; the pain is typically described as dull and aching; point tenderness may be present; local soft tissue swelling is absent (unlike malignant lesions); the duration of symptoms before diagnosis is often months to years (frequently misdiagnosed as `growing pains`, soft tissue injury, or arthritis); the response to NSAIDs is so characteristic that it should prompt imaging investigation
• Spinal osteoid osteoma: the posterior elements (pedicle, lamina, articular processes) are the most common spinal location; a classic presentation is painful scoliosis in a young patient — the scoliosis is caused by paravertebral muscle spasm on the side of the lesion causing the spine to curve AWAY from the lesion (the child leans away from the painful side); the scoliosis is non-structural and resolves after treatment; persistent scoliosis in a child should always prompt investigation to exclude an underlying spinal osteoid osteoma
• Femoral neck osteoid osteoma: can cause hip pain mimicking Perthes disease, synovitis, or stress fracture; the intra-articular location means the prostaglandins cause reactive joint synovitis, producing effusion; imaging is required to locate the nidus

• Plain radiographs: may show the nidus as a small radiolucent oval or round lesion with surrounding dense cortical sclerosis (the `target sign`); the nidus may be obscured by the surrounding sclerosis on plain X-ray (particularly for cortical lesions); the sclerosis itself is often visible as a focal area of cortical thickening; the `target sign` — a radiolucent central nidus with a rim of sclerosis — when visible on X-ray is highly characteristic; X-ray may be normal in early or intramedullary lesions
• CT scan: the gold standard for diagnosis and pre-treatment planning; CT shows the nidus with excellent clarity — a small round/oval radiolucent lesion (<2 cm) with central mineralisation (the central density represents the mineralised nidus) surrounded by the reactive sclerotic bone; CT is superior to MRI for demonstrating the nidus and is required for pre-procedural planning for radiofrequency ablation (to plan the needle trajectory and confirm safe margins from adjacent neurovascular structures)
• MRI: less specific than CT for the nidus (the nidus appears as a small round lesion with variable signal; the surrounding reactive oedema on T2 can be extensive and alarming, potentially mimicking a malignant lesion — the `misleading MRI` of osteoid osteoma is a recognised diagnostic pitfall); MRI is useful for assessing soft tissue involvement and for spinal lesions where neurological structures need to be assessed
• Bone scintigraphy (isotope bone scan): intense uptake at the nidus (`double density sign` — central hot spot within the reactive sclerosis); very sensitive; useful if X-ray is negative and CT is equivocal; now largely replaced by CT in most centres

• Watchful waiting + NSAIDs: osteoid osteomas are known to spontaneously involute (the lesion heals itself) over a period of approximately 3–7 years; for patients whose symptoms are adequately controlled with NSAIDs and who are willing to wait, observation is a legitimate option; approximately 30–50% of patients will have complete symptom resolution over 3–5 years without intervention; the risk of long-term NSAID use (GI and renal effects) must be balanced against this; NSAIDs are first-line symptomatic management regardless of whether definitive treatment is planned
• CT-guided radiofrequency ablation (RFA) — the treatment of choice: a radiofrequency electrode is placed into the nidus under CT guidance; the electrode is heated to 80–90°C for 4–6 minutes; the heat destroys the nidus tissue; performed under general or spinal anaesthesia; day-case or overnight stay procedure; success rate >90% for primary treatment; second treatment (re-ablation) achieves >95% cumulative success for lesions not responding to the first RFA; RFA is performed in a radiology or interventional suite; complications are rare — mainly local skin burns, nerve injury (if the lesion is adjacent to a nerve), and infection
• Safety margins for RFA: the critical consideration is proximity of the nidus to neurovascular structures (spinal cord, nerve roots, major vessels); lesions within 1 cm of a nerve root, the spinal cord, or major vessels are relative contraindications for standard RFA; protective measures include cold saline irrigation, carbon dioxide injection to create a thermal barrier, and modified electrode placement; spinal osteoid osteomas near the spinal canal may require open surgical excision rather than RFA
• Surgical excision: historically the standard treatment; now reserved for lesions not suitable for RFA (proximity to neural structures, failed RFA, technically inaccessible) or when a tissue diagnosis is required; en bloc excision of the nidus through an open or arthroscopic approach; the reactive sclerotic bone does not need to be excised — only the nidus; confirmation of complete nidus excision can be performed with intraoperative X-ray or fluoroscopy; incomplete excision results in recurrence
• Other ablation modalities: microwave ablation, cryoablation, laser ablation, and MRI-guided high-intensity focused ultrasound (HIFU) — all achieving thermal destruction of the nidus; these are used in centres without RFA capability or for lesions near structures not suitable for RFA heating (HIFU is useful for superficial lesions)

• Osteoid osteoma vs osteoblastoma: osteoblastoma is the `big brother` — nidus >2 cm; shares the same histology (osteoid and woven bone in vascular stroma) but differs clinically; osteoblastoma is less reliably relieved by NSAIDs; may expand (locally aggressive behaviour); can undergo malignant transformation to osteosarcoma (rare — unlike osteoid osteoma which never transforms); osteoblastoma most commonly affects the spine posterior elements; treatment is wide resection (not intralesional curettage); the >2 cm size threshold is the key distinguishing feature
• Delayed diagnosis and misdiagnosis: osteoid osteoma is frequently misdiagnosed for months to years; the differential diagnosis of nocturnal pain in a young person includes stress fracture (no nidus, history of overuse), Brodie abscess (chronic bone abscess — may mimic osteoid osteoma radiologically; distinguish by clinical features and MRI — abscess will show a surrounding T2 high signal penumbra), and Ewing sarcoma or osteomyelitis in the early aggressive phase; the response to NSAIDs should trigger early CT; a lesion not responding to NSAIDs should be reassessed for alternative diagnosis
• Intra-articular osteoid osteoma and synovitis: when the nidus is intra-articular (femoral neck, posterior elbow, talar neck), the prostaglandins diffuse into the joint, causing reactive synovitis and effusion; the effusion can be the presenting feature before the nidus is identified; intra-articular osteoid osteoma causes joint stiffness, limping, and secondary OA-like changes if untreated over years; children with unexplained hip or ankle effusions and nocturnal pain must be investigated for osteoid osteoma; treatment of the nidus resolves the synovitis

• Osteoid osteoma: benign; nidus <2 cm; young males 10–25 years; NOCTURNAL PAIN relieved by NSAIDs (aspirin) within 20–30 minutes — pathognomonic; proximal femur most common site
• Pain mechanism: prostaglandins (PGE2) produced in the nidus; sensitise free nerve endings within nidus; NSAIDs block COX → inhibit prostaglandins → pain relief
• CT scan: gold standard for diagnosis + RFA planning; small (<2 cm) radiolucent nidus ± central mineralisation surrounded by reactive sclerosis; target sign
• MRI: can be misleading — extensive T2 reactive oedema around the nidus mimics aggressive/malignant lesion; CT is superior for nidus characterisation
• CT-guided RFA: treatment of choice; 80–90°C for 4–6 minutes; >90% success primary; cumulative >95% with re-ablation; day case; avoid if nidus within 1 cm of nerve/spinal cord
• Spinal osteoid osteoma: posterior elements (pedicle/lamina); painful scoliosis — spine curves AWAY from the lesion due to muscle spasm; scoliosis resolves after treatment
• Spontaneous involution: osteoid osteoma involutes over 3–7 years; NSAIDs alone are legitimate management; 30–50% resolution without intervention
• Osteoid osteoma vs osteoblastoma: osteoblastoma nidus >2 cm; less reliable NSAID response; locally aggressive; rare malignant transformation; wide resection required; spinal most common site
• Intra-articular nidus: prostaglandins → reactive joint effusion + synovitis; children with unexplained hip/ankle effusion + nocturnal pain → investigate for osteoid osteoma
• No malignant transformation: osteoid osteoma never undergoes malignant transformation — reassure patients; biopsy is NOT required if CT is diagnostic