Osteoid Osteoma

Osteoid osteoma is a benign, osteoblastic bone tumour made of a small central nidus (under 1.5 to 2 cm) of vascular osteoid and woven bone, surrounded by reactive sclerosis. It causes characteristic night pain that is relieved promptly by aspirin or other NSAIDs.

It accounts for about 10% of benign bone tumours and 3% of all primary bone tumours. It peaks between 5 and 25 years of age, and is 2 to 3 times more common in males. About half arise in the femur and tibia (the proximal femur, neck and intertrochanteric region, is the commonest site), 10% in the spine (posterior elements, often the lumbar spine), and the remainder in the hands, feet, humerus and talus.

Lesions are described by position: cortical (most common, with dense fusiform sclerosis), cancellous or medullary (little sclerosis, often intra-articular), and subperiosteal (often in the neck of the talus or femur). Intra-articular and spinal lesions are more often missed.

Complications:

• chronic synovitis with effusion and joint contracture
• limb length discrepancy or angular deformity in a growing child
• painful scoliosis (the nidus lies on the concave side of the curve)
• muscle atrophy
• recurrence after incomplete removal

Malignant change does not occur.

Pathophysiology.

The nidus contains high concentrations of prostaglandin E2, with strong expression of cyclo oxygenase 1 and 2, and a dense network of unmyelinated nerve fibres. Prostaglandins explain the pain and the dramatic response to NSAIDs, and the vasodilation they produce drives the surrounding sclerosis and marrow oedema. Rearrangements of the FOS gene are found in most cases.

The natural history is slow spontaneous resolution of the nidus over 3 to 7 years in many patients, which is why prolonged NSAID treatment is an accepted alternative to ablation.

Deep, aching pain that is worse at night and wakes the patient, relieved within 20 to 30 minutes by an NSAID. Pain is localised at first and may be referred to the knee when the lesion is in the hip. Local tenderness, swelling and warmth are common in superficial bones.

Spinal lesions cause painful scoliosis with muscle spasm, stiffness and, rarely, radicular signs. Intra-articular lesions cause effusion, stiffness and a limp, and in the hip may mimic inflammatory arthritis. In the growing child, look for leg length difference and angular deformity.

Examine gait, local tenderness, joint range, muscle bulk, leg length, spinal alignment and the neurological state.

• Radiographs show a lucent nidus, sometimes with central calcification, inside dense cortical sclerosis. In the spine, cancellous bone or the neck of the femur, the nidus may not be visible on plain films.
• Thin section CT (1 mm or less) is the best test: a round or oval lucent nidus with or without central mineralisation, and surrounding sclerosis. It is also used to plan ablation.
• MRI shows intense bone marrow and soft tissue oedema that can hide the nidus and mimic infection or a malignant tumour; use it for intra-articular and spinal lesions after CT.
• Bone scan shows a double density sign (intense central uptake within a larger area of moderate uptake), a useful test when the site is unknown.
• Histology (when tissue is obtained): interlacing trabeculae of osteoid and woven bone rimmed by osteoblasts in a vascular fibrous stroma. Biopsy is not needed when the clinical picture and CT are typical.
• Blood tests: normal; inflammatory markers are normal or only slightly raised.

Differential diagnosis.

• Brodie abscess and stress fracture.
• Osteoblastoma (nidus over 2 cm) and osteomyelitis.
• Intra-articular osteoid osteoma versus inflammatory arthritis, synovitis or Perthes disease.
• Osteosarcoma, Ewing sarcoma and Langerhans cell histiocytosis.

Treatment relieves pain and prevents deformity. The options are medical (NSAIDs until the lesion burns out) or removal of the nidus.

Medical treatment (patients who accept long treatment, or when ablation is unsafe).

• Ibuprofen 400 mg orally every 8 hours with food (maximum 1200 mg daily; avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use). Naproxen 500 mg orally twice daily with food (maximum 1000 mg daily; avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use, and in the third trimester of pregnancy) is an alternative when a longer dosing interval is preferred. Review the need every 3 months, and stop when pain resolves, with a gastric protective agent if treatment continues beyond 3 months. Reduce the dose and watch kidney function in older patients.
• Children: ibuprofen 10 mg/kg orally every 6 to 8 hours with food (maximum 40 mg/kg daily and not exceeding 2400 mg daily). Naproxen 5 mg/kg orally twice daily with food (maximum 500 mg a dose and 1000 mg daily).
• Treatment is continued for 2 to 3 years in many patients before the nidus resolves; about 60 to 70% become pain free. A loss of response means the diagnosis or the lesion needs review.

Percutaneous ablation (first line for accessible lesions).

• CT-directed radiofrequency ablation: the electrode is placed in the nidus and heated to 90 degrees Celsius for 4 to 6 minutes. Technical success is about 90 to 95% at the first attempt and over 95% after repeat treatment. It is done as a day case under general or regional anaesthesia.
• Alternatives: laser photocoagulation, cryoablation, microwave ablation, high intensity focused ultrasound under MRI, and CT-directed trephine excision (which provides tissue).
• Keep a safe distance (at least 1 cm) from the spinal cord, nerve roots and skin. Hydrodissection, carbon dioxide insulation, thermocouple monitoring and electrode cooling reduce the risk of nerve injury and burns. Spinal lesions close to the canal may need open resection.
• Complications are skin burns, nerve injury, infection, bone fracture (rare) and recurrence in 5 to 10%.

Open surgery. En bloc resection or burr down with curettage of the nidus is used for lesions near the spinal cord, nerve or physis, failure of ablation, or diagnostic uncertainty. Bone graft or fixation may be needed to prevent a pathological fracture of the femoral neck after large resections. Confirm removal by intraoperative radiography or a nuclear scan.

Operative antibiotic prophylaxis (open surgery). Cefazolin 2 g intravenously (3 g when weight is 120 kg or over; 30 mg/kg to a maximum of 2 g in children) within 60 minutes before incision as a single dose; with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Analgesia (after ablation or surgery).

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use.
• Children: paracetamol 15 mg/kg orally every 4 to 6 hours (maximum 75 mg/kg daily and not exceeding 4 g daily).

Monitoring. Pain relief is usually immediate or within days after ablation, and is reviewed at 2 weeks and 3 months. Recurrent pain after a symptom free interval, usually within 1 year, means residual nidus; repeat CT and re-ablate. Leg length and spinal alignment in children until skeletal maturity. Scoliosis often corrects within 1 to 2 years if treated early.

Referral and escalation. Refer every suspected osteoid osteoma to orthopaedics or a bone tumour service for CT and treatment. Urgent referral for night pain with fever, a progressive neurological deficit, a lesion of uncertain nature, or pain not relieved by NSAIDs.

Special groups.

• Children: protect the physis; ablation is usually possible but the margin to the growth plate must be at least 1 cm.
• Spinal lesions: early treatment prevents fixed scoliosis; use surgical resection when a nerve or the canal is within the thermal zone.
• Kidney impairment, peptic ulcer or anticoagulant use: avoid NSAIDs and favour early ablation.
• Pregnancy: avoid NSAIDs in the third trimester; defer ablation if possible, and use paracetamol.