Osteochondroma

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.