Diagnostic Radiology/Musculoskeletal Imaging/Tumors Basic/Osteochondroma

Osteochondromas are cartilage-covered, histologically normal bony projections (exostoses) on the external surface of a bone in the vicinity of growth plates. They are the most common benign bone lesions and present during late childhood and adolescence. Although any bone with enchondral ossification may be involved, the femur, the proximal tibia, and the proximal humerus account for approximately 65-85% of cases.

There is a hereditary skeletal dysplasia known as osteochondromatosis (multiple heritable exostoses) where multiple exostoses are distributed symmetrically on the skeleton. This is a autosomal-dominant disease and is one of the most common skeletal dysplasias. The condition is familial, with more severe manifestations in men.

Pathophysiology
Osteochondromas result from the growth of aberrant growth plate with cartilage on the bony surface. As the cartilage grows, it forms a cap over a bony mass that develops by progressive enchondral ossification. The bony portion contains mature, normal cortical and medullary bone with a marrow space contiguous with the parent bone. Deep lobules of cartilage may be present, often heavily ossified. Osteochondromas have their own growth plate and stop growing with skeletal maturity.

Epidemiology

 * Age: Present before the age of 20 in 80% of cases ( range of age 10–35 years)
 * Sex: There is a 2:1 male to female predominance for the development of solitary osteochondromas. In osteochondromatosis, men are more prone to the severe manifestations.

Clinical Findings
In general, osteochondromas are slow growing and painless. However, they can be associated with complications, such as:


 * Pressure on adjacent nerves and blood vessels
 * Pressure on adjacent bone leading to deformation, fractures
 * Overlying bursitis
 * Malignant transformation (in < 1%). Malignant transformation should be suspected if :
 * Pain in the absence of fracture, bursitis, or nerve compression
 * Growth of lesion after skeletal maturation

In the osteochonromatosis, the clinical complications also include severe growth abnormalities. In particular, deformities of the tubular bones are present and cause disproportionately short limbs. However, the degree of shortness appears to be unrelated to the number of exostoses. Malignant transformation is rare, but higher (1-10%) than in solitary osteochondromas. Malignant transformation is usually into chondrosarcoma.

Radiologic Findings
There are two characteristic types of osteochondromas, pedunculated and sessile. The pedunculated type has a slender pedicle directed away from the growth plate. The sessile are broad based. Below are other characteristic findings of osteochondromas:


 * Located at the metaphyses
 * Calcification in the chondrous portion of the cap can give the pedunculated lesions a cauliflower-like appearance
 * The lesions grow away from the joint
 * MR or CT imaging can demonstrate cortical and medullary continuity between the osteochondroma and the parent bone as a distinctive feature.
 * MR is the best imaging modality for visualizing the effect of the lesion on surrounding structures and for evaluating the hyaline cartilage cap. Mineralized areas in the cartilage cap remain low signal intensity with all MR pulse sequences, although as enchondral ossification proceeds, yellow marrow signal is ultimately apparent.

The lesions of osteochondromatosis are radiologically indistinguishable from solitary exostoses. They are typically sessile and metaphyseal in location. The skeleton is involved symmetrically, and the limbs are affected more than the spine. The number of exostoses varies. The knee, ankle, and shoulder are usually involved.

In both solitary osteochondromas and osteochondromatosis, the radiologic distinction between a benign exostosis and an exostotic chondrosarcoma is difficult unless growth and change in appearance can be demonstrated on serial films.



Fig. 1 22 year old male with solitary exostosis on the lateral aspect of distal femur. The axial T1 weighted image illustrates the continuity of the cortex of the exostosis with the parent bone.



Fig. 2 Osteochondroma on right, lesser tronchanter illustrating the peduculated cauliflower-like appearance.



Fig. 3 Osteochondromatosis in a 32 year old male with hip pain. Note the bilateral lesions in the pelvis and the additional lesion in the distal femur.

Treatment
No treatment is required if the diagnosis is not in doubt and if the patient is relatively asymptomatic. However, surgical resection is indicated for persistent irritation from bursitis or for neurovascular compromise. Surgical resection is also indicated for continued osteochondroma growth after skeletal maturity.

Definitive treatment includes marginal excision of an active exostosis including the cartilaginous cap and overlying perichondrium. The cartilaginous cap should not be traumatized during its removal. Less than 5% of osteochondromas return after removal.

For more information check the MHE Research Foundations website www.mheresearchfoundation.org