The reported incidence of costal osteochondroma is very low [3
]. However, the actual incidence is likely to be underestimated because costal osteochondromas almost always arise at or near the costochondral junction and are usually asymptomatic [3
]. The type of osteochondroma described in this case, arising at the costovertebral junction with neural foraminal extension and spinal cord compression, is extremely rare. Only 7 such cases have been previously reported in the English-language literature (Table ) [3
]. All those 7 patients were younger than 23 years (range, 12-23 years). Three were known to have HMEs, another 3 patients had solitary lesions, and no definitive data were available for the 7th patient. All the patients showed clinical improvement during the immediate follow-up period after complete tumour excision.
Summary of the 7 previously reported cases of costal osteochondroma causing spinal cord compression
Osteochondroma is a disease of growing bone, and thus typically presents in younger patients [9
]. Tumour growth occurs early during childhood and usually arrests after puberty when the epiphysis closes [3
]. Osteochondroma has rarely been described in middle-aged and elderly patients [10
]. The pathophysiologic mechanisms underlying late-onset disease are not clearly understood. Some researchers believe that malignant transformation may abruptly increase the size of the lesion, thereby resulting in symptoms in older patients [12
]. In the present case, the patient was 58 years old and is the oldest reported patient with costal osteochondroma. Careful histopathologic examination indicated the absence of malignancy.
Although the location of the bony portion of the osteochondroma can be reliably determined using multiplanar CT reconstruction, the exact size of the tumour may be underestimated because the cartilage cap of the tumour is not detectable by CT [3
]. MR imaging is the best radiologic modality for evaluating the hyaline cartilage cap. The non-mineralized portions of the cartilage cap have high water content, resulting in intermediate-to-low signal intensity on T1-weighted images and high signal intensity on T2-weighted MR images. These features allow for accurate measurement of the thickness of the cartilage cap and distinction from overlying muscle on MR images [14
]. In this case, the unique curvilinear high-signalintensity region covering the tumour, seen on axial T2-weighted images, represents the cartilage cap, which led to accurate preoperative diagnosis.
Malignant transformation, usually into a chondrosarcoma, occurs in approximately 1% of solitary osteochondromas and 10% of HMEs [9
]. A sudden increase in lesion size or the development of new-onset pain suggests malignant transformation [12
]. Bess et al. [15
] emphasised that preoperative radiographic evaluation should consist of MR and CT imaging in order to provide optimal information about the lesion, which aids in surgical planning [3
]. Radiologic findings may show consistent growth of exostoses after closure of the growth plate, alterations in surface delineation in comparison with previous radiographic studies, internal lytic areas, erosion or destruction of adjacent bones, and the presence of soft tissue masses containing scattered or irregular calcifications [15
]. The size of the cartilaginous cap is the best indicator of malignancy [17
]. MR imaging results showing a cartilage cap thickness exceeding 2 cm in adults and 3 cm in children should raise the suspicion of malignancy [18
]. The use of gadolinium diethylenetriamine-pentacetate (Gd-DTPA)-enhanced MR imaging is an effective procedure for obtaining a differential diagnosis between malignant and benign lesions [19
]. Generally, osteochondromas do not show contrast enhancement, but mild enhancement may be observed within the marrow [12
]. In this case, Gd-DTPA-enhanced MR imaging was used, and the images did not show contrast enhancement. Because asymptomatic solitary osteochondromas have a low rate of malignant transformation, they can be followed up conservatively [20
]. When the tumour causes pain or neurologic complications because of compression, or when the diagnosis is uncertain, the tumour should be completely excised to avoid tumour recurrence [22
]. For any recurrence, the possibility of malignant transformation of the osteochondroma or of a low-grade chondrosarcoma that was initially poorly classified should be anticipated [24
]. Because of the age of the patient and the presence of erosion into adjacent bones, the clinical suspicion for malignancy was high in this case. A surgical strategy for complete tumour excision was carefully planned to prevent tumour recurrence. As osteochondromas may pass through the neural foramen and lead to cord compression, the surgical approach should include decompression surgery such as laminectomy and/or facetectomy at the corresponding level [3
]. If additional facetectomy is performed to remove the foramen and/or extraforaminal component, iatrogenic instability and kyphosis may occur during the follow-up period. In this case, hemilaminectomy of the right side of T12 and total facetectomy on the right side of T12-L1 provided wide exposure of the intraspinal and extraforaminal tumour originating from the rib. Additionally, because pressure erosions, suggestive of a slow-growing tumour, were noted in the adjacent bony structures, a total pediculectomy and partial vertebrectomy on the right side of T12 were performed, accompanied by the partial removal of the right proximal 12th rib. Therefore, it was essential to perform posterior thoracolumbar fixation and fusion for stability. However, fixation and fusion were performed only on the contralateral (left) side to avoid disturbing the radiologic follow-up required for monitoring tumour recurrence. Plain dynamic films showed that bone fusion was achieved 6 months after surgery. Although CT did not show any recurrence at the 1-year follow-up, further clinical and radiologic follow-up is required for monitoring tumour recurrence.