Osteoid Osteoma

• Usually young males, pain worsens at night, relieved by NSAIDs (prostaglandin E2 related)
• Small, round, radiolucent nidus surrounded by a zone of reactive sclerosis. Calcified central sequestrum. Targetoid appearance.
• Oedema out of proportion to size

Description

Osteoid osteoma is a benign bone tumour, characterised by its small size and the presence of a radiolucent nidus surrounded by sclerotic bone. It predominantly affects young people and has a predilection for long bones, particularly the femur and tibia.

Not to be confused with osteoma which are generally asymptomatic, larger, lack a nidus, and are commonly found in the skull and facial bones.

Pathogenesis

The exact pathogenesis of osteoid osteoma is not completely understood. It is believed to arise from osteoblasts, the bone-forming cells, leading to the formation of a highly vascular, osteoid and woven bone nidus.

Osteoid osteomas produce high levels of prostaglandins, particularly PGE2 and PGI2, within the nidus. This has two key effects:

1. Pain mechanism: Prostaglandins sensitise nociceptors, causing intense nocturnal pain that is dramatically relieved by NSAIDs, which inhibit prostaglandin synthesis via COX enzymes.
2. Vascular effects: Prostaglandins contribute to the lesion’s surrounding reactive sclerosis and vascularity.

Hence, the pain pattern and NSAID responsiveness are directly due to prostaglandin overproduction.

Epidemiology, Risk Factors & Associations

• Osteoid osteoma accounts for about 10-12% of all benign bone tumours.
• It primarily affects children and young adults, with a peak incidence between the ages of 10-25 years.
• There is a male predominance, with a male-to-female ratio of approximately 2-3:1.
• It is considered rare in Black patients
• There are no known associations with familial syndromes or environmental risk factors.

Clinical Features

• The classic presentation includes nocturnal pain that is disproportionate to the size of the lesion, typically relieved by NSAIDs.
• Physical examination may reveal local tenderness and swelling.
• If the lesion is located near a growth plate in a growing child, it may cause limb length discrepancy or angular deformity.
• Can cause painful scoliosis, with deviations away from the side of pain, resulting in non-structural (functional) scoliosis.

Complications

• Pathological fracture, although this is rare due to the small size of the lesion.
• Growth disturbances in children if the lesion is near the growth plate.
• Malignant transformation is rare.
• Does not metastasise.

Pathological Features

Morphology

• Gross: Small round-oval masses of hemorrhagic, gritty, tan tissue lesion, typically less than 1.5 cm in diameter.
• Microscopic:
  • Tumours are well-circumscribed nodules of radiologically translucent woven bone (nidus) rimmed by osteoblasts
  • Surrounded by highly vascular, loose connective tissue enclosed by radiologically dense reactive sclerotic bone.
• Prostaglandin E2 is markedly elevated (100-1000 times) within nidus – may be cause of pain & vasodilation and explain relief provided by NSAIDs

Radiological Features

General Features

• Can arise in any bone but have a predilection for the appendicular skeleton
• Typically cortically based in a long bone diaphysis or metaphysis (in up to 80% of cases). 50% of cases involve the femur or tibia.
• The less common epiphyseal location is typically intramedullary.
• The spine is uncommonly involved (less than 10% of cases) and will usually involve posterior elements (90% of cases).
• May be associated with short-segment non-rotational scoliosis: Typically involves only a few vertebral levels, with convexity away from the lesion, due to paraspinal muscle spasm on the side of the osteoid osteoma. Lesion forms at the concave margin of the apex.
• Typically appears as fusiform sclerotic thickening in the shaft of a long bone
• The lucent nidus represents the actual tumour with the adjacent sclerosis being reactive.
• There is often adjacent inflammatory change with marrow oedema, periosteal reaction, synovitis, and joint effusion noted.
• When the tumour is greater than 15 mm in size it is referred to as an osteoblastoma (which occurs with greater frequency in the axial skeleton).

XR

• Typically appear as a small radiolucent nidus (<1.5 cm in diameter) surrounded by dense sclerotic bone. The nidus may contain central calcification or ossification (the so-called “target sign”).
• Cortical thickening may be so dense that the nidus may not be appreciated on XR
• Sclerosis of the pedicle may be seen

CT

• CT is the modality of choice for imaging and treatment.
• It shows the nidus more clearly than plain radiographs and can accurately demonstrate its size and location.
• The hypervascular nidus often shows enhancement after contrast administration.

MRI

• T1: Low to intermediate signal intensity nidus
• T2 FS: Lesion with target-like appearance with a round low-signal intensity center (likely corresponding to a central calcification), high T2-signal intensity middle layer, and outer low signal intensity rim. There is pronounced perilesional oedema disproportionate to the size of the nidus.
• Dynamic T1 FS Gd+: Early arterial enhancement of nidus with partial washout. Slower enhancement of adjacent marrow oedema. Reactive synovitis may be seen.
• Post-ablation: Decreased volume of nidus. T2 hypointense centre with hyperintense band. Increased ADC values of nidus. No dynamic enhancement.

Nuclear Medicine

• SPECT-CT: Bone scintigraphy shows intense radionuclide uptake (“hot spot”) due to increased osteoblastic activity.
• 18F-FDG PET: Variable avidity – not reliable for detecting the nidus based on metabolic activity.
• F-18 NaF: High avidity

Grading and Staging

Osteoid osteoma does not have a specific grading or staging system.

Differential Diagnosis

• Osteoblastoma – The main differential diagnosis is osteoblastoma, which is a benign bone-forming tumour with identical histological features but is larger in size (>2 cm in diameter) and typically found in the posterior elements of the vertebral column. It also typically presents with pain which does not show the same dramatic relief with NSAIDs.
• Intracortical abscess – Difficult to distinguish on plain radiography as both appear as focal cortical thickening with a lucent lesion. However, on CT demonstrates intracortical abscess demonstrates irregular inner margin, eccentric/irregular sequestrum, osseous tunnelling, possible adjacent soft-tissue abscess, whereas osteoid osteoma demonstrates smooth inner nidus, central round calcification, unmineralised nidus enhances strongly on MR. Abscess centre does not enhance.
• Stress fracture – A common cause for focal cortical thickening or uninterrupted periosteal reaction, which occurs intially in attempt to buttress the weakened cortex. CT demonstrates subtle fracture lines (potentially longitudinal) and comparatively less sclerosis than osteoid ostoema. Interval size reduction favours stress fracture.Bone scintigraphy demonstrates intense, linear uptake, whereas osteoid osteoma displays the “double density” sign, in which intense central uptake is seen at the nidus and moderate uptake is seen in the surrounding area

Management

• Pain management with NSAIDs is often the first step in treatment.
• The definitive treatment is usually surgical, involving either excision of the nidus or ablation using techniques such as CT-guided radiofrequency ablation.
• CT-guided radiofrequency ablation (RFA), a minimally invasive procedure with a high success and low complication rate, is the first-line treatment for most osteoid osteomas. It involves thermal ablation of the nidus using an RFA probe introduced under CT guidance.
• Referral to an orthopaedic surgeon or interventional radiologist is usually the next step after diagnosis.