'Hard discs' associated with lumbar disc herniation: CT analysis

Thirty cases showing hard discs were collected from 206 consecutive cases of posterior disk herniation during the past one and a half years period. According to the shape of hard disk and the presence of marginal bone defect (MBD) of vertebral end plate, 30 cases could be categorized into 4 distinct groups. They were small hard disc with focal MBD (group 1), large hard disc with broad based MBD (group 2), hard disc separated from vertebral body without MBD (group 3) and hard disc fused with vertebral body without MBD (group 4). There were 14 patients in group 1. In all but one case, the hard disc and MBDs were located at the paramedial aspect of the posterior margin of the end plate. Ten (76.9%) of these had Schmorl's nodes and these were considered to represented an avulsion from the weakened bone margin as the disk herniated acutely. The posterior longitudinal ligament seemed to have prevented detachment of the bony fragment. Group 2 included 2 patients and CT findings as well as plain X-ray features were identical to a limbus vertebra. This was considered to represent passive displacement of bony fragment by the intravertebral herniation of the disk. Nine patients belonged to group 3 and represent dystrophic calcifications of herniated disk. Group 4 included 5 patients and was disclosed as a simple osteophyte. From our study, it was concluded; (1) that the hard disc seen in lumbar CT scan was probably the results of the detachment of a bone fragment from the vertebral end plate which has an underlying weak area due to the herniated disc as well as a calcification or osteophyte formation and (2) specific terms (avulsed fragment, limbus vertebra, calcification or spur formation) is more preferable to use than hard disc which is inaccurate