Osteochondroma and Osteochondromatosis (Hereditary Multiple Exostoses) Imaging

An osteochondroma is a cartilage-covered bony excrescence (exostosis) that arises from the surface of a bone. Osteochondromas, which are the most common bone tumors in children, may be solitary or multiple, and they may arise spontaneously or as a result of previous osseous trauma. An osteochondroma can affect any bone preformed in cartilage. ^{[1, 2]}  Osteochondromas account for 20-50% of benign bone tumors and 9% of all bone tumors. Most are asymptomatic and commonly diagnosed incidentally on radiographs obtained for other reasons. ^[3]

Hereditary multiple exostoses (HME), also known as osteochondromatosis, is the presence of multiple osteochondromas. ^{[4, 5, 6]}  Patients with HME may have anywhere from 2 osteochondromas to hundreds of them. Most solitary osteochondromas are discovered incidentally in children and adolescents. A painless skeletal swelling or a slowly growing mass is the usual mode of presentation. HME leads to abnormalities such as palpable bony masses and limb shortening in the first or second decade of life.

The most common site of origin for an osteochondroma is the metaphysis at bony sites of tendon and ligamentous attachments. Osteochondromas usually point away from its point of attachment toward the diaphysis. The metaphysis of the affected tubular bone may be widened. The long tubular bones are affected most frequently. In long bones, osteochondromas are typically located at the metaphysis. The sites of predilection include the distal femoral metaphysis, the proximal humeral metaphysis, the tibia, and the fibula.

The small bones of the hands and feet are affected in around 10% of patients. The innominate bone is involved in 5% of patients. The spine is less frequently involved (2%), but it can lead to cord compression. The scapula is affected in 1% of patients.

Osteochondromas arise less frequently from flat bones than from long bones. The spine, pelvis, ribs, and scapulae are the bones most commonly affected. A subungual osteochondroma is rare, but it is particularly prone to a painful bursa (not visible on plain radiographs) and fracture. An osteochondroma of the sesamoid bone of the hallux has been described, but it is extremely rare. Osteochondromas arising from the pelvis are commonly large and are typically associated with a soft-tissue mass that may grow outward or inward, displacing adjacent structures.

(See the images below.)

The true prevalence of solitary osteochondromas is not known, because many asymptomatic lesions go undiagnosed. Spontaneous regression of solitary lesions has been reported. ^{[7, 8]}  The majority of osteochondromas present as solitary nonhereditary lesions; 15% occur as multiple lesions associated with hereditary multiple osteochondromas (HME). ^[8]  

Hereditary multiple exostoses, also known as osteochondromatosis, is an inherited, autosomal dominant disorder in which multiple osteochondromas are seen throughout the skeleton. ^{[4, 5, 6]} John Hunter was the first to comment on HME and described a patient with the condition in his Lectures on the principles of surgery (1786). ^[9] The first description of a family with HME was published by Boyer, in 1814. ^[10] In 1825, a second family with HME was described. ^[11]

HME is associated with a loss-of-function type of mutation in the tumor suppressor genes exostosin-1 and -2 (EXT1 and EXT2), which are responsible for the synthesis of heparan sulfate proteoglycans (HSPG), resulting in HSPG deficiency and subsequent development of multiple osteochondromas. ^{[12, 13, 14, 15]}

Most osteochondromas, solitary or multiple, arise from tubular bones and are metaphyseal in location. Multiple epiphyseal dysplasia and dysplasia epiphysealis hemimelica (DEH), also known as Trevor disease, are autosomal dominant conditions in which the chondromas arise from the epiphysis and cause joint problems.

Complications of osteochondromas include fractures, bony deformities, neurologic and vascular injuries, bursa formation, and malignant transformation. ^[16] Advances have added to the understanding of the molecular and genetic bases of HME (see the images below).

Although fewer than 5% of solitary osteochondromas originate in the spine, the incidence of spinal osteochondromas in patients with HME has been reported in over 50% of cases. Spinal lesions have a predilection for the posterior elements of the cervical or thoracic spine. While most symptomatic spinal exostoses evolve slowly, neurologic deterioration can be rapid. ^[17] ​

Bari et al describe dorsal spine compression secondary to intrathecal exostosis in association with hereditary multiple exostoses. The patient presented with dorsal cord compression. Decompression was performed, and the complaints of myelopathy were improved. ^[18]

Preferred examination

Plain radiography remains the examination of choice in the evaluation of osteochondromas, and it may be the only imaging study required. The radiographic appearances of osteochondromas are usually characteristic.

El-Fiky et al assessed the anteroposterior radiographic features of 36 hips (18 patients aged 2-28 yr) with HME and found that osteochondromas were located most often in the femur and then the ilium. Of the 18 patients, 15 were asymptomatic and 3 had pain symptoms. None of the lesions were malignant. Coxa valga was present in 32 hips; an abnormal Reimer migration percentage in 26; an abnormal Sharp acetabular angle in 17; an abnormal center edge angle in 12; an abnormal femoral neck shaft angle in 32; and degenerative changes in 6. The authors noted that subluxated hips should undergo early operation, especially in children and in symptomatic adults. ^[19]

Computed tomography (CT) scanning is particularly useful in the assessment of osteochondromas in the pelvis, shoulder, or spine. With spiral and multisection CT scanning, excellent reconstructions can be formatted in various planes without exposing the patient to a further radiation burden.

In a study of 12 patients from with an osteocartilagenous lesion who underwent fluorodeoxyglucose (FDG) PET-CT study, Purandare et al found that whole body FDG PET-CT was helpful in identifying malignant transformation of osteochondromas. There was moderate to high FDG uptake in 7 patients who had histopathologic evidence of a sarcomatous transformation to grade II chondrosarcoma, and there was a focus of very intense FDG uptake in 1 patient with a dedifferentiated chondrosarcoma; low-grade FDG uptake occurred in 4 patients with diagnoses of benign osteocartilaginous lesions. In addition, FDG uptake was seen in an asymptomatic osteochondroma, with a grade II chondrosarcoma being identified on histopathology. ^[20]

Ultrasonography can be used in the evaluation of the cartilaginous cap and of complications associated with osteochondromas, such as arterial or venous thrombosis, aneurysm and pseudoaneurysm formation, and bursitis.

Magnetic resonance imaging (MRI) is useful for assessing continuity of the parent bone with the cortical and medullary bone in an osteochondroma. Cartilage in the cap has high signal intensity on T2-weighted, spin-echo MRI scans. 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, and arterial and venous compromise. This study is the method of choice for evaluating compression of the spinal cord, nerve roots, and peripheral nerves.

Arteriography remains the criterion standard for depicting vascular occlusion, as well as aneurysm and pseudoaneurysm formation. It is an essential part of vascular intervention. Vascular complications can occur as a result of osteochondromas, particularly when a bone lesion occurs around the knee. In this situation, arteriography is considered essential in planning surgical treatment. Effective diagnostic imaging is a key to the early operative removal of sarcomas. Angiography has also been used to assess malignant transformation; angiograms may depict neovascularity and the true extent of disease. False-negative angiograms are possible in true aneurysms and in false ones, because a laminated thrombus may lie along the wall and partially fill the aneurysm. In such cases, an ultrasonogram may prove invaluable.

Limitations of techniques

None of the imaging techniques described are reliable in differentiating a benign osteochondroma from a sarcomatous transformation.

Although plain radiographs are an excellent means of depicting osseous pathology, they do not provide reliable information about adjacent soft-tissue compromise (such as tendinous, vascular, or neurologic involvement) or about bursa inflammation. Plain radiographs may also not be sufficient to provide images of osteochondromas involving complex bones, such as the spinal column.

Ultrasonography can provide information on the cartilage cap but not on the underlying bone in an osteochondroma. Also, ultrasonography remains operator dependent.

With CT scanning, radiation burden in the young may be a disadvantage, particularly when several examinations may be required in the workup of hereditary multiple exostoses (HME) or sarcomatous transformation.

Angiography is invasive, and because of the iodinated contrast material used in this procedure, there is a risk of anaphylaxis and renal toxicity.

MRI is expensive, has limited availability, and cannot be performed in the claustrophobic patient and in patients with certain types of heart valves, surgical clips, or other ferromagnetic foreign bodies.

Radionuclide scanning has high sensitivity but low specificity. It is also expensive and has limited availability. Radionuclides are not reliable in differentiating between a benign osteochondroma and a chondrosarcoma.

Experience with 2-[fluorine-18]-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET) is limited. ^[21] In addition, the procedure is expensive and has limited availability.

The plain radiographic appearances of an osteochondroma are those of a pedunculated or sessile bony excrescence with well-defined margins. In adults, the cartilage cap often contains flecks of calcification. Osteochondromas arising from the surface of a bone contain spongiosa and cortex that appear continuous with the parent bone; this is particularly obvious in long bones. ^[22]

(See the radiographic images below.)

Radiologically differentiating a benign tumor from a sarcoma is problematic in the pelvis, particularly when the mass has a soft-tissue component. Planar tomography is still a cost-effective and useful procedure in depicting bone detail in complex skeletal areas. Typically, osteochondromas arising from the ribs are located at the costochondral junction, where they can cause a pneumothorax/hemothorax (rare) that may be evident on a plain radiograph. ^{[23, 24]} When the small bones of the hands and feet are affected, the appearances of the osteochondromas are identical to those found in the long bones.

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 that malignant transformation may have occurred.

Hereditary multiple exostoses (HME) is characterized by multiple osteochondromas that typically involve the proximal part of the humerus and the distal and proximal portions of the femur, tibia, and fibula. Often, there are associated defects of bone modeling and bony deformities—in particular, bilateral coxa valga and widening of the proximal femoral metaphysis. Bilateral, progressive changes in the forearm have been linked to the severity of the underlying disease.

Radial bowing may ensue as a result of disproportionate ulnar shortening with relative radial overgrowth. Radial-head subluxation or dislocation may be a sequel to the radial overgrowth, with a superficial resemblance to a Madelung anomaly, but the characteristic relative elongation or dorsal subluxation of the distal ulna seen in Madelung deformity is not present.

Plain radiographic findings of dysplasia epiphysealis hemimelica (DEH) include irregular ossification occurring to 1 side of the ossifying epiphysis or a carpal or tarsal bone. The adjacent metaphysis may be widened. With progression of disease, a lobulated bony mass protrudes from the epiphysis or the carpal or tarsal bone. Severe disease is associated with muscle wasting, growth disturbance, and joint deformities.

Degree of confidence

Plain radiography remains the primary modality for imaging osseous pathology. Experience with bone radiography extends over 100 years. The normal variants are well defined. The diagnosis of osteochondromas is straightforward, particularly at the common sites in long bones. Plain radiographs are particularly good for diagnosing complications related to osteochondromas, such as fractures, osseous deformity, and growth disturbances.

Plain radiography is inexpensive, effective, and universally available. With the advent of digital radiography, the radiation dose can be better regulated, and digital images have the advantage of better sensitivity, better image manipulation, and better storage. In addition, the images can be transmitted to distant facilities.

Osteochondromas arising from complex areas can be clarified by means of planar tomography.

The list of differential diagnoses for osteochondromas is extensive. Osteomas, osteophytes, enthesophytes, heterotopic ossification, and parosteal osteosarcomas can all mimic osteochondromas. The list of systemic disorders and developmental anomalies that are accompanied by osteochondromas or osteochondroma-like abnormalities is long, and these may cause confusion with solitary osteochondromas or with hereditary multiple exostoses (HME).

False-negative or false-positive diagnosis may occur with malignant transformation. However, problems may arise with resected tumors that appear radiologically aggressive, even with the histologic confirmation of malignancy.

CT scanning can provide excellent bone detail of osteochondromas developing in the spine, shoulder, or pelvis, despite the complex nature of these bones (see the image below). CT myelography is useful in evaluating the size and extent of spinal osteochondromas in patients presenting with compressive myelopathy.

CT scanning is an excellent modality for depicting bone detail in skeletal lesions and calcification within surrounding cartilage and soft tissue. One major disadvantage of CT scanning, however, is that it provides no information about the metabolic activity of bone lesions.

An increase in the size of osteochondromas due to bursitis is a known complication, and a false-positive diagnosis of malignant transformation has been reported with CT scanning and MRI. Therefore, ultrasonographic evaluation is always recommended for the evaluation of enlarging solitary osteochondromas.

 

MRI is useful for assessing the continuity of the parent bone with the cortical and medullary bone in an osteochondroma. This is an important prerequisite in differentiating osteochondromas from other surface bone lesions. Cartilage in the cap has high signal intensity on T2-weighted, spin-echo MRI scans. This characteristic allows measurement of the cap, which is an important consideration in malignant transformation.

MRI allows the distinction of muscle impingement, which may be radiographically occult and can be clinically confused with other complications, such as a fracture, bursitis, or malignant degeneration. MRI also improves accuracy in diagnosing low-grade chondrosarcomas. MRI contributes only in cases in which a malignant transformation is suspected.

MRI provides information about inflammation in reactive bursa formation, impingement syndromes, and arterial and venous compromise. This study is the method of choice for evaluating compression of the spinal cord, nerve roots, and peripheral nerves. ^{[25, 26, 27, 28, 29, 30]}

De Beuckleer and associates showed that MRI improves accuracy in the diagnosis of low-grade chondrosarcomas. ^[31] MRI scans 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 MRI scans, muscle impingement is depicted as increased signal intensity within the muscle.

Ultrasonography can be applied to analyze the cartilaginous cap of an osteochondroma. ^[32] The cap appears as a hypoechoic layer covering a hyperechoic underlying bone. Malghem and associates compared ultrasonographic measurements of cap thickness with measurement performed on pathologic specimens in 22 resected exostoses and 2 exostotic chondrosarcomas. ^[33] The ultrasonographic measurements proved accurate, with a mean measurement error of less than 2 mm for cartilaginous caps thinner than 2 cm.

Ultrasonography is also valuable in the diagnosis of bursitis and other complications associated with osteochondromas, such as arterial or venous thrombosis, as well as aneurysm and pseudoaneurysm formation.

The detection rate and measurement accuracy of ultrasonography in the search for and evaluation of cartilaginous caps are comparable to those of MRI and are higher than those of CT. The high sensitivity and specificity of ultrasonography in peripheral vascular pathology is well established.

Ultrasonography remains operator dependent and can be labor intensive. Ultrasonograms cannot depict the cartilage cap when it is inwardly orientated; however, this is relatively uncommon. False-positive and false-negative results can occur, particularly in cases of deep vein thrombosis in the lower calf.

Scintigraphy with bone-seeking isotopes is an effective method of imaging osteochondromas that are metabolically active. Thallium-201 (²⁰¹Tl) scintigraphy is useful in differentiating malignant transformation from benign osteochondroma in hereditary multiple exostoses (HME). ^[34]  Bone scintigraphy also makes it possible to obtain whole-body images in a single examination, which can be very useful in detecting the presence of new bone lesions. ^[35]

Aoki and associates showed that fluorodeoxyglucose positron emission tomography (FDG-PET) could be an objective and quantitative adjunct in the differential diagnosis and grading of chondrosarcomas. ^[21]

A normal isotopic bone scan virtually excludes the diagnosis of malignant transformation of an osteochondroma. A positive isotopic bone scan does not allow differentiation of the endochondral ossification occurring in a benign osteochondroma from the hyperemia and osteoblastic reaction occurring in a chondrosarcoma. Negative findings of ²⁰¹Tl scintigraphy may not exclude the possibility of chondrosarcomas, and the utility of this method may be limited.

In their article about the role of radionuclide scintigraphy, Hendel and associates concluded that single, standing, planar bone scintigraphy has no value in distinguishing benign osteochondromas from malignant chondrosarcomas. ^[36]

(See the scintigram below displaying an osteochondroma with malignant degeneration.)