Osteochondroma

Osteochondroma
Lateral radiograph of the knee demonstrating ossification in the peritendinous tissues in a patient with osteochondroma.
Classification and external resources
Specialty	oncology
ICD-10	D16
ICD-O	9210/0
DiseasesDB	34033
eMedicine	article/1256477
MeSH	D015831
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Osteochondromas or osteocartilaginous exostoses are the most common benign tumors of the bones. The tumors take the form of cartilage-capped bony projections or outgrowth on the surface of bones (exostoses). It is characterized as a type of overgrowth that can occur in any bone where cartilage forms bone. Tumors most commonly affect long bones in the leg, pelvis, or scapula (shoulder blade). Development of osteochondromas take place during skeletal growth between the ages of 13 and 15 and ceases when the growth plate fuses at puberty. They arise within the first three decades of life affecting children and adolescents. Osteochondromas occur in 3% of the general population and represent 35% of all benign tumors and 8% of all bone tumors. Majority of these tumors are solitary non-hereditary lesions and approximately 15% of osteochondromas occur as hereditary multiple osteochondromas (HMOs). They can occur as a solitary lesion (solitary osteochondroma) or multiple lesions within the context of the same bone (Multiple Osteochondroma). Osteochondromas do not result from injury and the exact cause remains unknown. Recent research has indicated that multiple osteochondromas is an autosomal dominant inherited disease. Germ line Mutations in EXT1 and EXT2 genes located on chromosomes 8 and 11 have been associated with the cause of the disease. The treatment choice for osteochondroma is surgical removal of solitary lesion or partial excision of the outgrowth, when symptoms cause motion limitations or nerve and blood vessel impingements.

Osteochondromas are long and slender, pedunculated on a stalk often taking the shape of a cauliflower. The cartilage cap is covered by fibrous perichondrium and continues with the periosteum of the underlying bone. The cartilage cap is less than 2 cm thick and the thickness decreases with age. A cap more than 2 cm thick, indicates malignant transformation of a tumor. The cartilage cap merges with the epiphyseal area of the long bones called spongiosa. In the spongiosa, the chondrocytes are arranged in accordance with the epiphyseal growth plate. The spongiosa of the stalk continues with the underlying cancellous bone. Fractures within the stalk causes fibroblastic proliferation and formation of a new bone. Development of bursa takes place over the osteochondroma, which is attached to the perichondrium of the cap. Inflammation of the bone is indicated by the bursal wall lined by the synovium. As a result, patients may have swelling for years related to the location and site of the lesion indicative of mechanical obstruction, nerve impingement, pseudoaneurysm of the overlying vessel, fracture at the stalk of the lesion, or formation of bursa over the osteochondroma. Heparan sulphate (HS) are glycosaminoglycans which are involved in the formation of proteoglycans. The biosynthesis of HS takes place in the Golgi apparatus and Endoplasmic Reticulum, where glycosaminoglycans chains are maintained by type II glycosyltransferases encoded by EXOSTOSIN genes EXT1 and EXT2. Decreased levels of HS leads to mutations in EXT1 or EXT2 causing skeletal abnormality. The underlying mechanism for solitary and multiple osteochondromas have been associated with genetic alterations in EXT1 or EXT2 genes located on chromosomes 8 and 11. Approximately 65% of osteochondromas arise in the EXT1 gene loci on chromosome 8 and 35% arise in EXT2 gene loci on chromosome 11. About 70–75% of multiple osteochondromas are caused by point mutations, often involving deletion of single or multiple axons as found in 10% of all hereditary cases. In about 10–15% of all cases no genomic alterations are detected. The mechanism behind the formation of multiple osteochondroma is large genomic deletions of EXT1 and EXT2 genes. The identified mechanism behind solitary osteochondromas is the homozygous deletions of the EXT1 gene. However, the exact cause of osteochondroma is unknown. Additionally, the molecular basis of genetics and clinical variability of multiple osteochondroma as well as the underlying causes for the malignant transformation and the onset of osteochondroma in EXT negative patients is also currently unknown.

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