Osteochondromas are developmental lesions rather than true neoplasms (
1). These lesions result from separation of a fragment of the epiphyseal growth plate cartilage, which subsequently herniates through the periosteal bone cuff that normally surrounds the growth plate ( 8). Trauma or genetic factors cause perichondral deficiency. A zone of metaplastic cartilage is produced and enlarged by enchondral ossification ( 9). They can be solitary (90% of cases) or multiple in the form of hereditary multiple exostosis (HME) (in 10% of cases), usually with autosomal dominant inheritance ( 2). The solitary and multiple forms are detected in males more frequently than in females with a ratio of 1.8:1 (multiple) to 3:1 (solitary) ( 10). Osteochondroma is a disease of the growing bone. Therefore, it often presents in young patients and its growth usually arrests after puberty with closure of the epiphysis ( 11, 12). Most of them are asymptomatic and are found incidentally. Symptomatic lesions usually occur in younger patients, with 75%-80% of such cases being discovered before the age of 20 years ( 5, 8). Patients with HME have at least two lesions that typically involve the proximal part of the humerus and the distal and proximal portions of the femur, tibia and fibula, but they may arise anywhere in the skeleton ( 1). There are often associated defects of bone modeling and bony deformities. HME leads to abnormalities such as palpable bony masses and limb shortening in the first or second decade of life ( 1, 8). The multiplicity of lesions and associated deformities lead to early radiologic evaluation and diagnosis of HME, in contrast to the relatively later diagnosis of solitary osteochondromas ( 13).
The HME cases are more likely complicated. Complications of exostoses include adjacent vascular and neural injury, fracture, osseous deformity, bursal formation and malignant degeneration (
1). Exostoses may also be radiation-induced ( 9). Malignant transformation to chondrosarcoma is the most serious complication with an incidence of 1-5% for the solitary type and 10-25% for HME ( 1, 9, 14). Incomplete excision can lead to recurrence, so complete "en bloc" resection in the first operation is recommended and often curative ( 1, 15).
Spinal involvement is more common in HME. In this condition, thoracic and lumbar vertebrae are more commonly affected, while the solitary type mostly affects the cervical spine particularly the atlantoaxial region (
1). Sacral involvement is rare ( 9). High prevalence in the cervical spine may be attributed to greater mobility and stress in this region. Lamina is the origin of most spinal osteochondromas. Extension into the spinal cord and cord compression, as occurred in our patient (Figures 2, 3 and 4), is a rare event. This usually occurs in the setting of HME ( 1). Our case is unusual in that a solitary vertebral exostosis has caused spinal cord compression. The most important clue in the diagnosis of osteochondroma is continuity between the cortex and medulla of the lesion and those of the underlying bone ( 2). In fact, this feature is pathognomonic for osteochondroma ( 1). As in our patient (Figure 3), this can be well-demonstrated by CT ( 6).
Diagnosis of spinal exostosis may be difficult on plain radiography owing to the complex anatomy (
1, 9). X-ray is normal in 15% of the patients ( 6). However, in our patient this lesion manifested as a large cauliflower-like exophytic mass at the level of T8, T9 and T10 vertebrae ( Figure 1). Lesions that protrude dorsally from the posterior vertebral elements (lamina or spinous process) are typically large and manifest at an earlier age with cosmetic deformity and a palpable mass, but lack neurologic symptoms. In contradistinction, osteochondromas that extend into the spinal canal are often small, but are associated with neurologic symptoms ( 1). Our case had both dorsal and ventral protrusions, causing a spinal mass with neurologic symptoms.
Malignant degeneration is best assessed by measuring the maximal cartilage cap thickness and MRI is more accurate than CT in this regard (
16). It is supposed that thin caps (less than 3 cm) and calcified caps are benign and thick caps (more than 3 cm) are malignant ( 9, 16). In the presented case, maximal cartilage cap thickness was 3 mm ( Figure 4), and pathology revealed no evidence of malignant transformation.
MRI often shows yellow marrow signal in the central aspect of the lesion. The cartilage cap is often thin and has low to intermediate signal on T1-weighted images and high signal on T2-weighted images (Figures
3 and 4).
Fat signal intensity within the medullary component of the spinal osteochondroma in MR imaging can occasionally be mistaken for a lipomatous neoplasm, particularly in small lesions projecting into the spinal canal (
1). Thin-section CT is the modality of choice to demonstrate the diagnostic appearance of marrow and cortical continuity in the rib head, skull base, and spinal osteochondromas that often have a very narrow stalk of attachment. Nevertheless, MR imaging is usually superior to CT in evaluating the relationship of the osteochondroma to the surrounding structures for presurgical assessment ( 1).
Our case highlights the value of MRI for early diagnosis of spinal osteochondroma. Obviously, early diagnosis and therefore early treatment would prevent future permanent neurologic deficits. In cases of spinal cord compression, in addition to more common causes like discopathy, trauma and metastasis, one should be also aware of rare entities such as osteochondroma and this is particularly important in patients with HME. Whether MRI of the spine should be used as a screening test in HME can be a subject for future studies.