Medullary Cone Movement in Subjects with a Normal Spinal Cord and in Patients with a Tethered Spinal Cord

PURPOSE: To compare movement of the normal medullary cone when the patient has changed from a supine to prone position with that in patients with known or suspected tethered spinal cord syndrome. MATERIALS AND METHODS: Fifty-six individuals divided into three groups were examined with lumbar spine magnetic resonance (MR) imaging performed with the patient in the prone and supine positions. Group 1 consisted of 15 healthy volunteers and six patients with a herniated disk; group 2, 25 patients clinically suspected of having a tethered cord; and group 3, 10 patients who previously had undergone tethered cord surgery. RESULTS: All group 1 subjects showed distinct and statistically significant medullary cone movement (range, 21%–41%); no patient in group 3 showed movement (Wilcoxon rank sum test, P < .001). In group 2, the 20 patients in whom a definite diagnosis of tethered cord syndrome was made on the basis of initial supine MR image findings showed no movement, whereas two of five patients with normal supine MR images had abnormal and decreased cone movement at prone imaging. CONCLUSION: Prone MR imaging has no additional value when the supine MR image has clearly shown the cause of tethering or in patients who have undergone tethered cord surgery, but it can provide additional information in patients clinically suspected of having a tethered cord and in whom supine MR imaging depicted no abnormalities.