Non-ossifying fibroma

Overview

• Definition: A common non-neoplastic, developmental defect of bone characterized by proliferation of benign fibrous tissue and histiocytes.
• Pathophysiology: Originates at the insertion of tendons/ligaments into the perichondrium of the physis. As the bone lengthens, the lesion "migrates" into the metadiaphysis.
• Epidemiology:
  • Very common: Found in ~30–40% of all skeletally immature children.
  • Age: 5–20 years (rare after skeletal maturity).
  • Gender: M > F (2:1).
• Terminology:
  • Fibrous cortical defect (FCD): Small (< 2–3 cm), strictly intracortical.
  • Non-ossifying fibroma (NOF): Large (> 3 cm), expands into medullary cavity.
• Syndromic association:
  • Jaffe-Campanacci syndrome: Rare syndrome but exam favourite; considered in multiple NOFs with NF1-like features.

Anatomical distribution

• Location: Metaphysis $\to$ Metadiaphysis (migrates away from knee/joint with growth).
• Bones: Around the knee (80%).
  • Distal femur (most common).
  • Proximal tibia.
  • Distal tibia.
• Position: Eccentric and cortically based.

Imaging features

Plain radiograph (Diagnostic in 99% of cases)

• Appearance: Multiloculated, radiolucent lesion with a "bubbly" or "bunch of grapes" appearance.
• Margins: Well-defined with a sclerotic rim (scalloped).
• Orientation: Long axis parallels the long axis of the bone.
• Expansion: Mild cortical thinning/expansion, but the cortex remains intact (unless fractured).
• Evolution:
  1. Starts at physis.
  2. Migrates into diaphysis.
  3. Fills in with sclerosis (ossifies) in adulthood $\to$ becomes a "bone island" or disappears.

CT

• Usually unnecessary.
• Used to assess fracture risk (cortical occupancy).
• Shows lack of matrix mineralisation (unlike fibrous dysplasia or chondroid tumours).

MRI (Problem-solving)

• T1: Hypointense.
• T2: Variable (Key discriminator).
  • Immature: Hyperintense (cellular/hyperaemic).
  • Mature/Healing: Hypointense (collagenous/sclerotic).
• Enhancement: Variable. Can show enhancement (vascular fibrous tissue), which may confuse the diagnosis if X-ray correlation is not made.

Nuclear medicine

• Bone scan: Mild to moderate uptake (hyperaemic).
• Note: Intense uptake suggests a superimposed fracture or a different diagnosis.

Differential diagnosis

Management and prognosis

• Standard care: "Do not touch." No biopsy, no follow-up needed if classic.
• Pathological fracture risk:
  • Highest if lesion occupies > 50% of cortical diameter or exceeds 33 mm in length.
  • Management: Prophylactic curettage and bone grafting if high risk.
• Biopsy: Contraindicated unless atypical features (pain without fracture, soft tissue mass, rapid growth).

High-yield exam pearls