Hereditary multiple exostoses

Hereditary multiple exostoses (HME) is an inherited autosomal dominant disorder where multiple osteochondromas throughout the skeleton are present. The pathogenesis of HME is poorly understood, and theories range from isolated islets of cartilaginous tissues from the diaphyseal surface of growing cartilage, a defect in the anchoring of germinal cartilage cells to the physis, or physical-stress theory that focal accumulations of embryonic connective tissue at sites of tendon attachments are converted to hyaline cartilage. Patients with HME may present with short stature and asymmetric growth at the knees and ankles, which may lead to deformities. The osteochondromas are located close to the metaphyses, and they may be sessile or pedunculated. The cortex of the lesion is continuous with the cortex of the bone with a homogeneous continuation of the medulla which is a characteristic feature. Patients with HME may have 2 to 100s of osteochondromas. Most solitary osteochondromas are discovered incidentally in children and adolescents. Usually, the patients present with a painless skeletal swelling or a slowly growing mass. Fractures, bony deformities, bursa formation, neurologic and vascular injuries, and malignant transformation are complications of osteochondromas. The risk of malignant degeneration is 1-20%. The likelihood of malignant transformation is greater with HME than with other conditions. Most transformations are to a chondrosarcoma, but other sarcomata may complicate the disease. Most patients with this malignant transformation present with a painful mass. Malignant transformation occur only rarely in the first decade and after fifth decades of life. Malignancy should be suggested and must be assessed if an osteochondroma grows in a mature skeleton. Additionally, an osteochondroma with a cartilaginous cap greater than 1 cm in an adult should be carefully assessed because this finding has also been associated with an increased risk of malignancy. Radiographic features: Plain radiographs demonstrate pedunculated or sessile bony excrescence with well-defined margins. In adults, the cartilage cap often contains flecks of calcification. Osteochondromas arise from the surface of the bones contain spongiosa and cortex that appear continuous with the parent bone. Most common sites for osteochondromas are the metaphysis at bony sites of tendon and ligamentous attachments. Osteochondromas usually point away from its point of attachment towards the diaphysis with the metaphysis of the affected tubular bone often widened. Serial radiographs showing an enlarging osteochondroma with irregularity of its margin and accompanied by a soft tissue mass should alert the clinician to sarcomatous transformation, particularly when the finding is accompanied by pain. Bone erosions and irregularity or scattered calcification are further clues of malignant transformation. Often, there are associated defects of bone modeling and bony deformities, in particular bilateral coxa valga and widening of the proximal femoral metaphysis. Computer tomography: CT can provide excellent bone detail of osteochondromas developing in the spine, shoulder, or pelvis despite the complex nature of these bones. Magnetic resonance imaging: MRI is useful for assessing continuity of the cortical and medullary bone in an osteochondroma with the parent bone. Cartilage in the cap has high signal intensity on T2-weighted spin-echo MRI. This characteristic allows measurement of the cap, which is an important consideration in malignant transformation. MRI also provides information about inflammation in reactive bursa formation, impingement syndromes, arterial and venous compromise. This study is the method of choice for evaluating compression of the spinal cord, nerve roots, and peripheral nerves. MRIs contribute only to the diagnostic workup of cases in which malignant change is suspected because osteochondromas have a characteristic appearance on plain radiographs. With chondrosarcomas, the chondroid origin of tumors may be identified with the lobular high signal intensity. Short-tau inversion recovery (STIR) images show peritumoral soft tissue edema in 83% of chondrosarcomas. Muscle impingement should be considered in the differential diagnosis of pain in association with osteochondromatosis. On T2-weighted MRIs, muscle impingement is depicted as increased signal intensity within the muscle.  A known complication is the increasing size of osteochondromas due to bursitis, and a false-positive diagnosis of malignant transformation has been reported with both CT and MRI. Therefore, ultrasonographic evaluation is always recommended for the evaluation of enlarging solitary osteochondromas.