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Multiple osteochondromas (MO) prevalence is estimated at 1:50,000, and seems to be higher in males (male-to-female ratio 1.5:1).

Osteochondromas develop and increase in size in the first decade of life, and usually cease to grow when the growth plates close at puberty. They are pedunculated or sessile (broad base) and can vary widely in size. The number of osteochondromas may vary significantly within and between families, the mean number of locations is 15-18. The majority are asymptomatic and located in bones that develop from cartilage, especially the long bones of the extremities, predominantly around the knee. The facial bones are not affected. Bone lesions on flat bones, vertebrae, and the ribs are less common and the skull is usually not involved. Osteochondromas may cause pain, functional problems and deformities (especially of the forearm), which may provide reason for surgical removal. The most important complication is malignant transformation of osteochondroma towards secondary peripheral atypical cartilaginous tumor or chondrosarcoma, which is estimated to occur in 0.5-5% of cases.

Germline mutations in the tumor suppressor genes, EXT1 (8q24.11) or EXT2 (11p11.2), are found in almost 90% of MO patients. The EXT genes encode glycosyltransferases, catalyzing heparan sulphate polymerization.

The diagnosis is based on radiological and clinical documentation, supplemented with, if available, histological evaluation of the osteochondromas. Diagnostic criteria (WHO 2020) include radiological findings of at least two osteochondromas of the juxta-epiphyseal region of long bones, and a positive family history and/or a proven germline mutation in one of the EXT genes.

MO should be distinguished from metachondromatosis, dysplasia epiphysealis hemimelica and Ollier disease.

If the exact mutation is known antenatal diagnosis is technically possible.

MO is an autosomal dominant disorder and is genetically heterogeneous. Genetic counseling should be proposed to individuals having the disease-causing mutation informing them that there is 50% risk of passing the mutation to offspring.

Management includes removal of osteochondromas when they are the cause of complaints. Removed osteochondromas should be examined for malignant transformation towards secondary peripheral atypical cartilaginous tumor or chondrosarcoma. Patients should be well instructed and regular follow-up for early detection of malignancy seems justified. For secondary peripheral chondrosarcoma, en-bloc resection of the lesion and its pseudocapsule with tumor-free margins should be performed, preferably in a bone tumor referral centre.

Osteochondromas are benign lesions and do not affect life expectancy.