Osteochondroma is a surface lesion that arises from the surface of the bone and continues with the medullary cavity. It arises from a trapped growth plate cartilage that herniates through the cortex and grows via the endochondral ossification beneath the periosteum. The cartilaginous cap produces the bony mass by progressive endochondral ossification. Osteochondroma is the most common benign bone tumor.
Presentation
Osteochondroma typically presents as a mass swelling, accompanied by pain or discomfort. The pain can originate from the bursa or due to a mechanical impingement. Pain may also occur due to a fracture of the stalk. Osteochondromas usually occur at the distal femur, proximal tibia, proximal femur, or proximal humerus; however, they can occur in any bone in which endochondral ossification occurs. The growth of the osteochondroma parallels the growth of the patient. The lesion will stop growing once the physis closes. Most patients have solitary lesions
Imaging
The cortex of the medullary cavity is continuous and the lesions may be either sessile (uncommon) or pedunculated (common). In sessile lesions, the base of the lesion is wider and this type will have a higher risk of malignancy. Pedunculated lesions present as a narrow stalk that usually grows away from the joint.
Osteochondroma often arises at the site of tendon insertion. The direction of the growth follows along the line of the tendon. An MRI will show the cartilage cap better than an x-ray. The cap is usually 2-3mm in thickness in adults and may be 1cm thick in a grown child. The thicker cap indicates growth, however this is not a reliable indication of malignant transformation.
The patient may have a bursa on top of the lesion and there may be calcified or ossified loose bodies within the bursa. The cartilaginous cap is made of hyaline cartilage and benign chondrocytes with a single nucleus. These are usually arranged in parallel clusters, similar to the physis. If the cartilage cap is thicker than 2cm in adults, the physician should rule out chondrosarcoma.
Multiple Hereditary Exostoses
Multiple Hereditary Exostoses is a disorder which is characterized by multiple osteochondromas. These lesions are similar in x-ray appearance and histologically to solitary osteochondroma. This condition may be characterized by metaphyseal widening and sessile lesions. This condition is autosomal dominant with an incomplete penetrance in females, making it more common in males. This genetic mutation occurs at EXT1, EXT2, and/or EXT3. If the mutation occurs at EXT1, there will be a more severe presentation, such as more limb malalignment and decreased range of motion at the knee and elbow. EXT1 mutations also have more exostoses and more malignancy than in EXT2. The malignant transformation in Solitary is less than 1% and multiple is about 10%. Proximal lesions tend to undergo more malignant transformations than distal lesions. If it transforms to malignant, then the lesion will be a low grade chondrosarcoma. This will usually occur at the pelvis. Multiple Hereditary Exostoses typically presents itself with a shortened stature (or shortened femur); skeletal deformities which are usually found at the knee, forearm, and ankle; and with Coxa Valga, which is a knee valgus with a short fibular and patellar dislocation. Ankle valgus may also occur due to a shortened fibula. In the upper extremities, there may be radial bowing with the ulnar shortened, the radial head dislocated, and ulnar deviation of the hand (treated by osteotomy and exostosis excision). This condition is similar to Madelung’s deformity.
A malignant transformation will be indicated by any new pain or sudden increase in the size of the lesion. This will signal a low grade chondrosarcoma that usually occurs in the scapula or the pelvis. The physician will want to assess the cartilage cap (An MRI will show the cap as a bright signal). The malignant transformation will occur more in sessile type lesions showing an area of lucency or destruction of the base of the osteochondroma or destruction of the adjacent bone. The physician may also find a calcified soft tissue mass. This usually occurs in older patients.
Differential diagnosis include Parosteal Osteosarcoma and Myositis Ossificans.
Treatment
If the patient is asymptomaic, treatment will consist of observation. If there is any pain, cosmetic deformity, or a loss of range of motion, an excision will be performed after skeletal maturity. The physician will need to be aware of any loss of pronation and supination in the forearm in these cases. A simple excision of the osteochondroma is usually the answer. Wide surgical excisions will be performed in cases of secondary chondrosarcoma.
