The value of MR in differentiating subligamentous from supraligamentous lumbar disk herniations.

PURPOSE To determine the value of MR criteria in differentiating subligamentous from supraligamentous lumbar disk herniations. METHODS A retrospective review of surgical reports and MR images of 50 patients undergoing first-time lumbar surgery was performed. Three MR imaging criteria were assessed: the presence and integrity of a low-signal-intensity line posterior to the disk herniation, the size of the disk herniation in comparison with the size of the spinal canal, and the presence of disk fragments. Correlation was made with surgical findings to determine the value of these MR criteria in differentiating subligamentous from supraligamentous disk herniations. RESULTS For determining subligamentous disk herniations: the presence of a continuous low-signal-intensity line posterior to the disk herniation was 29% sensitive, 65% specific, and 42% accurate; disk herniation size less than 50% of the size of the spinal canal was 64% sensitive, 47% specific, and 58% accurate; and the absence of disk fragments was 88% sensitive, 12% specific, and 62% accurate. Combinations of individual MR criteria did not improve diagnostic accuracy. CONCLUSIONS For differentiating subligamentous from supraligamentous lumbar disk herniations, none of the MR imaging criteria assessed was reliable.