Osteochondroma

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Tumor biology and incidence

  • Benign lesions originating from bone; grow outwards against adjacent tissue
  • Most common benign skeletal tumor; accounts for 40% of all benign bone tumors

Age

Affects persons 10-35 years of age

Gender

M:F = 2:1

Associated syndromes

Familial osteochondromatosis

Presentation

  • Non-painful mass or bump
  • Can be symptomatic if lesion:
    • Caused by impingement on surrounding structures (nerves, vessels, musculotendinous structures, bones)
    • Fractures (rare)
    • Associated with bursa formation in areas of soft tissue friction, such as near or beneath the iliotibial band
    • Undergoes sarcomatous transformation
  • Growth of the lesion usually ceases at maturity
  • Painful lesions or lesions that grow rapidly may be undergoing malignant transformation (occurs in less than 1% of cases), especially if pain and growth occur after skeletal maturity

Physical findings

  • Painless, firm mass fixed to underlying bone can often be palpated
  • May be able to elicit Tinel's sign if adjacent nerves are involved
  • Mechanical features – catching or snapping – can be reproduced if osteochondroma impinges on soft tissue.

Plain films

Site
  • Most commonly found at the knee, shoulder and hip
  • Generally metaphyseal
  • Arises from cortical surface, classified as a surface (parosteal) lesion
  • Typically grows perpendicular to cortex and away from epiphysis
  • Cortical surfaces and marrow cavity of lesion are contiguous with underlying bone
  • Always check for presence of other lesions (ie, familial osteochondromatosis)
Size
  • May be small or large lesions
  • Sessile lesions do not extend far from cortex; pedunculated lesions may have a long "stalk"
Tumor effect on bone

Lesion may expand, remodel entire shape of metaphyseal bone

Bone response to tumor

None

Matrix
  • Periphery of lesion is corticated by process of endochondral ossification
  • Cartilagenous matrix may be seen within cartilage cap prior to skeletal maturity
Cortex
  • Intact
  • Inderlying cortex is contiguous with cortex of lesion
Soft tissue mass
  • Overlying cartilage cap usually eliminated by process of endochondral ossification at termination of skeletal growth
  • Thick non-ossified cartilage cap (> 1-2 cm) in an adult suggests malignant transformation

Bone scan

  • Increased uptake may be seen if osteochondroma is still remodeling
    • Increased uptake in a previously cold lesion suggests recent injury or malignant transformation
  • Bone scan useful for the assessment of multiple lesions

MRI

  • MRI extremely useful in evaluation of symptomatic lesions
    • Used to determine thickness of cartilage cap in evaluation of possible sarcomatous degeneration
    • Can readily identify complications including bursa formation, fracture, and possible neurovascular impingement

Differential diagnosis

  • Juxtacortical myositis ossificans
  • Periosteal chondroma
  • Surface (parosteal) osteosarcoma (low or high grade)
  • Chondrosarcoma arising from osteochondroma
  • Soft tissue osteosarcoma
  • Bizarre parosteal osteochondromatous proliferation
  • Extraskeletal myxoid chondrosarcoma
  • Tumoral calcinosis

Natural history

  • Considered benign, active lesions (Enneking stage 2) during active growth
  • Become benign, latent (stage 1 lesions) with the cessation of growth
  • Isolated osteochondromas rarely undergo malignant transformation (<1%); however, clinical suspicion should be raised in the presence of atypical features

Pathology

  • Gross: Exophytic lesion with cartilage cap usually less than 2cm in thickness.
  • Microscopic: Cartilage cap matures into trabecular bone; at interface, cartilage cores may be seen in trabeculae
  • Intertrabecular spaces contain adipose tissue or hematopoietic marrow
  • Lesion merges with underlying marrow
  • Cartilage cap may be replaced by bone over time.

Diagnosis and treatment

  • Conventional radiographs usually so classic that no biopsy is needed
  • General rules:
    • Asymptomatic: Do not need treatment
    • Excise if locally painful, if there is neurovascular compromise (for example, in the popliteal fossa), or if there is a change in the cartilage cap
    • Pain is an acceptable indication for resection in skeletally immature patients
      • Surgery may lead to deformity, depending on proximity to the growth plate
      • Lesions may recur if not completely excised

Complications

  • Malignant transformation may occur, usually to a low-grade chondrosarcoma or osteosarcoma
    • Risk is probably <1% for solitary lesions, higher for multiple lesions
    • Greater risk of transformation with more proximal lesions and those that grow post maturity
  • Bursa formation
  • Fracture
  • Neurovascular compromise

Recommended reading

Enneking W. Musculoskeletal Tumor Surgery Churchill Livingstone, New York, NY 1983.

J Natl Cancer Inst. 2007 Mar 7;99(5):396-406. The role of EXT1 in nonhereditary osteochondroma: identification of homozygous deletions. Hameetman L, Szuhai K, Yavas A, Knijnenburg J, van Duin M, van Dekken H, Taminiau AH, Cleton-Jansen AM, Bovée JV, Hogendoorn PC.

Schmale GA, Conrad EU 3rd, Raskind WH. The natural history of hereditary multiple exostoses. J Bone Joint Surg Am. 1994 Jul;76(7):986-92.

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