Tumor biology and incidence
Osteochondromas, or exostoses, are benign lesions that originate from bone and grow outwards against adjacent tissue. The true incidence is unknown as most small osteochondromas are likely asymptomatic and therefore avoid clinical detection. The most common sites of involvement are (in decreasing order) distal femur, proximal humerus, proximal tibia.
Presentation
Commonly, patients present with a complaint of a mass or bump. Pain is an uncommon feature, but can be caused by impingement on surrounding structures. Bursae can develop in areas of soft tissue friction such as near or beneath the iliotibial band. Rarely, pedunculated masses can fracture. The most concerning aspect of a painful osteochondroma is possible malignant transformation.
Physical findings
A firm mass can often be palpated. If adjacent nerves are involved, it is possible to elicit a Tinel's sign. Mechanical features such as catching or snapping can be voluntarily reproduced by the patient if the osteochondroma is impinging on soft tissue.
Radiographic Characteristics
Defining characteristics of osteochondromas include an exophytic projection of mature bone that demonstrates confluent trabeculae extending within the intramedullary canal in an "uninterrupted" fashion. The stalk can be narrow (pedunculated) or broad (sessile). As a result of the outward growth pattern, the involved metaphysis may appear flared. Reactive changes in surrounding bone, such as lysis or sclerosis, are uncommon and may indicate malignant conversion or fracture.
Soft tissue masses are not a feature of osteochondroma. A cartilage cap is present that can be well visualized on MRI, and to some extent on plain films and CT, and is usually less than 1cm (maximal thickness). A cap thickness >2cm is atypical and may indicate conversion to a low grade chondrosarcoma. CT can often clarify whether a lesion communicates with the intramedullary canal (pathognomonic for osteochondroma) or arises from the periosteal surface or with adjacent soft tissues. Bone scan is often positive as ossification persists well into adulthood. A negative bone scan implies that the mass is no longer active and future growth is unlikely.
Differential Diagnosis
1. Chondrosarcoma arising from osteochondroma
2. Parosteal osteosarcoma
3. Surface osteosarcoma (low or high grade)
4. Myositis ossificans
5. Bizarre parosteal osteochondromatous proliferation (BPOP)
6. Heterotopic ossification
7. Extraskeletal myxoid chondrosarcoma
8. Tumoral calcinosis
Natural history
Osteochondromas are considered benign, active lesions (Enneking stage 2) during active growth. With the cessation of growth, they become benign, latent or stage 1 lesions. Isolated osteochondromas rarely undergo malignant transformation (<1%), but given their widespread prevalence within the population, clinical suspicion should be raised in the presence of atypical features.
Pathology
Pathologic features mimic radiographic findings insofar as mature cancellous bone and normal marrow elements are noted to extend within the intramedullary canal. The transition zone from mature bone to the cartilage cap resembles physeal growth. The cytological characteristics of the cartilage cap can overlap with some findings of low grade chondrosarcoma (increased cellularity, binucleate chondrocytes, high cell-matrix ratio, and bizarre nuclear forms). Therefore, the clinical presentation and imaging studies must be considered before determining the final pathologic diagnosis.
Diagnosis and treatment
Plain radiographs are often all that is needed to make the diagnosis of ostechondroma. If atypical features are present such as reactive changes in surrounding bone, soft-tissue mineralization, a history of pain or recent growth, both MRI and CT are useful. The mineralized portions are best visualized with CT, including soft-tissure mineralization. Soft-tissue masses can be characterized on a gadolinium-enhanced MRI study.
Cytogenetics
Osteochondromas can present in isolated fashion (85%) or as part of the syndrome, Multiple Hereditary Exostes (MHE). The vast majority of patients with MHE have a somatic mutation that affects a glycosyl transferase enzyme involved in the production of heparan sulfate (EXT1 or EXT2). Both EXT1 and EXT2 function as tumor suppressors and are believed to be responsible for the generation of multiple exostoses in a single patient, and possible explain the increased incidence of malignant degeneration of osteochondromas observed in MHE patients. A lifetime risk of malignant progression has been estimated as high as 25%, but more recent evidence suggests the actual risk falls between 0.9 and 2.8%.
Patients with isolated osteochondromas do not typically carry the somatic EXT1 or EXT2 mutation, but loss of heterozygosity and chromosome 8 (site of EXT gene) rearrangments have been identified locally within the lesions. One study of eight isolated osteochondromas found that homozygous EXT1 deletions were present only within the cartilage cap, therefore further implicating dysfunctional chondrocytes in the initiation and progression of isolated osteochondromas and potentially in MHE.
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.
|
