Multiple hereditary exostoses (familial osteochondromatosis)

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)

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

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 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.

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