Tumur template based on A Clinical Guide to Primary Bone Tumors. Levesque et al.

Tumor biology and incidence

Multiple Hereditary Exostoses (MHE) is an inherited autosomal dominant disorder that is characterized by the formation of multiple osteochondromas or exostoses in the axial and appendicular skeleton. It is estimated to occur in 1 of 50,000 people. The disease most commonly manifests itself in childhood.  It has a 96% penetrance with variable expressivity.  Spontaneous mutation is responsible for 10-20% of affected patients.

Presentation

Historical features can vary depending on phenotype severity. In the most severe cases, bowing of long bones can occur resulting in significant deformity and short stature. Often patients present with symptoms caused by mass effect and local compression of nerves, muscle, tendon and adjacent bone. Pain is 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.

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 Studies

Radiographic hallmarks include multiple exostoses within a single site of osseous involvement (e.g. long bones, hemipelvis, spinal column). Secondary imaging studies include CT and MRI, depending on whether osseous or soft tissue resolution is desired.  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.   

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

Isolated osteochondromas in the absence of MHE are considered benign, active lesions (Enneking stage 2) during active growth. With the cessation of growth, they become benign, latent or stage 1 lesions.  They have a low rate of malignant transformation (<1%).  Osteochondromas in the setting of MHE are considered similarly in terms of staging, however the conversion to malignancy is more common.  2.7% is an often cited rate of malignant transformation, but estimates range from 0.5% to 8.3%.  The most common sarcoma arising from an exostosis is chondrosarcoma. 

Pathology

Treatment

Treatment of MHE can be challenging.  Conservative measures include PT and NSAID's.  Surgical treatment is indicated for 1) local mass effect causing pain/diminished ROM/neurovascular compromise 2) radiographic or historical characteristics concerning for malignancy.  In the case of the latter, referral to an orthopaedic oncologist for consideration of biopsy and potential wide excision may be required.

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.