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Multiple hereditary exostoses (familial osteochondromatosis)

Tumor biology and incidence

  • Autosomal-dominant disorder characterized by the formation of multiple osteochondromas or exostoses in the axial and appendicular skeleton
  • Estimated to occur in 1 of 50,000 people
  • Disease most commonly manifests itself in childhood
  • 96% penetrance with variable expressivity
  • Spontaneous mutation responsible for 10-20% of affected patients
  • Underlying genetic cause thought to be mutation in one of several tumor suppressor genes
    • Mutation in one of at least four genes has been identified at the time of writing (genes named Ext-1 to Ext-4)

Presentation

  • Long bones, the pelvis, ribs, scapulam and distal ends of the proximal and distal phalanges may be affected
  • Pain uncommon, but can result from nerve compression, reactive bursitis, fracture, or malignant degeneration
  • Restricted range of motion can occur as a result of soft-tissue tethering or mechanical obstruction
  • Skeletal abnormalities
    • Short stature
    • Unequal growth of the radius and ulna with radio-humeral subluxation, or Madelung deformity
    • Valgus knees and ankles caused by unequal tibia and fibula growth

Physical findings

  • Firm mass may be palpated
  • May elicit a Tinel's sign if adjacent nerves are involved
  • Mechanical features -- catching or snapping -- can be reproduced by patient if the osteochondroma is impinging on soft tissue

Radiographic studies

  • Radiographic hallmarks include multiple exostoses within a single site of osseous involvement (eg, long bones, hemipelvis, spinal column)
    • Exophytic projection of mature bone that demonstrates confluent trabeculae extending within the intramedullary canal in an "uninterrupted" fashion
    • Stalk can be narrow (pedunculated) or broad (sessile)
    • Involved metaphysis may appear flared from outward growth pattern
    • Reactive changes in surrounding bone, such as lysis or sclerosis, are uncommon; may indicate malignant conversion or fracture
  • Secondary imaging studies include CT and MRI, depending on whether osseous or soft tissue resolution is desired
    • Cartilage cap can be well visualized on MRI, and to some extent on plain films and CT
    • Usually less than 1 cm (maximal thickness); cap thickness > 2 cm is atypical, may indicate conversion to a low-grade chondrosarcoma
    • CT may clarify whether a lesion communicates with the intramedullary canal (pathognomonic for osteochondroma) or arises from periosteal surface or with adjacent soft tissues
  • Bone scan can be helpful to determine whether a lesion is active (increased uptake) or inactive (cold), and may help in predicting morbidity based on future growth  
    • Bone scan is often positive as ossification persists well into adulthood.
    • Negative bone scan implies that the mass is no longer active and future growth is unlikely.

Differential Diagnosis

  • Chondrosarcoma arising from osteochondroma
  • Surface (parosteal) osteosarcoma (low or high grade)
  • Myositis ossificans
  • Bizarre parosteal osteochondromatous proliferation (BPOP)
  • Heterotopic ossification
  • Extraskeletal myxoid chondrosarcoma
  • Tumoral calcinosis

Natural history

  • Considered benign, active lesions (Enneking stage 2) during active growth
  • Become benign, latent or stage 1 lesions with cessation of growth
  • Conversion to malignancy estimated to range from 0.5% to 8.3%
  • Chondrosarcoma most common sarcoma arising from an exostosis

Treatment

  • Deformity in lower extremities may require surgical correction
  • Monitor for malignant change.
  • Investigate or biopsy for:
    • Pain
    • Enlargement after skeletal maturity
    • New areas of lysis within pre-existing lesions
  • Recurrence after excision suspicious for malignancy

Recommended Reading

Legeai-Mallet L, Munnich A, Maroteaux P, Le Merrer M (1997). "Incomplete penetrance and expressivity skewing in hereditary multiple exostoses". Clin. Genet. 52(1):12-6.

Kivioja A, Ervasti H, Kinnunen J, Kaitila I, Wolf M, Böhling T (2000). "Chondrosarcoma in a family with multiple hereditary exostoses". J Bone Joint Surg Br 82(2):261-6.

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