Exostosis

Histology of an osteochondroma showing the cartilage cap over cancellous bone
Histology of an osteochondroma showing the cartilage cap over cancellous bone. Image by Sarahkayb, Wikimedia Commons, CC BY-SA 4.0.

Definition

  • Cartilage capped bony projection
  • Cortex and medulla continuous with the host bone
  • Commonest benign bone tumour
  • Solitary, or multiple in hereditary multiple exostoses (HME)

Aetiology

  • Displaced physeal cartilage growing at an angle to the bone
  • Grows by enchondral ossification, stopping at skeletal maturity
  • HME is autosomal dominant, from EXT1 or EXT2 mutations affecting heparan sulphate synthesis
  • EXT1 mutations cause more severe disease
  • Radiation induced lesions follow childhood radiotherapy

Clinical Features

  • Painless hard lump near a joint in childhood or adolescence
  • Distal femur, proximal tibia and proximal humerus metaphyses
  • Points away from the adjacent joint
  • Pedunculated or sessile
  • Mechanical irritation with an overlying bursa
  • Common peroneal nerve compression at the fibular neck
  • Popliteal artery pseudoaneurysm
  • Fracture through the stalk or restricted joint movement
  • HME causes short stature, ulnar shortening, radial bowing and radial head dislocation
  • HME also causes genu valgum and ankle valgus

Investigations

Exostosis, radiograph of an osteochondroma
Radiograph of an osteochondroma. Image by Michael R Carmont, Sian Davies, Daniel Gey van Pittius and Robin Rees, Wikimedia Commons, CC BY 2.0.
  • Radiographs show corticomedullary continuity and are usually diagnostic
  • MRI or CT for atypical, pelvic or scapular lesions and cap thickness
  • Ultrasound measures the cap in superficial lesions

Complications

  • Secondary peripheral chondrosarcoma, usually low grade
  • Risk around 1% or less for solitary lesions, a few percent lifetime in HME
  • Pelvis, shoulder girdle and proximal femur carry greater risk
  • Warning signs are growth after maturity and new non-mechanical pain
  • Cap thickness over 2 cm in an adult
  • Irregular cap margin, scattered calcification or soft tissue mass

Management

  • Observe asymptomatic lesions
  • Excise at the base with the entire cap and perichondrium
  • Delay excision until near skeletal maturity to reduce recurrence
  • Protect the physis and neurovascular structures
  • HME may need guided growth, ulnar lengthening, osteotomy or radial head excision
  • Suspected chondrosarcoma goes to a sarcoma centre for wide resection, often without prior biopsy
  • Adults with HME report new pain or growth

Reviewed by Professor Phong Tran, Head of Orthopaedic Surgery, Western Health. Last updated 10 October 2026.