An Easy and Effective Method for Evaluating the Position of Conus Medullaris: Counting the Number of Vertebral Ossification Center Below the End of Conus Medullaris

Objective: This study aimed to ascertain the conus medullaris position by counting the number of ossification centers in the vertebral bodies below the conus medullaris endpoint (N) and assess its utility in screening for closed spinal dysraphism and tethered cord syndrome. Methods: A total of 900 normal fetuses and 146 fetuses with closed spinal dysraphism or tethered cord syndrome were included in this study. The N values were tallied and compared along the spinal longitudinal plane. The receiver operating characteristic curve was utilized, and the cut-off value of N was analyzed. Results: The counting of N was successfully performed in 856 normal and 146 abnormal fetuses. In the normal group, an increase in N with gestational age was observed. Specifically, in the subgroup of 17-20 wk fetuses, N was >= 6 in 117 out of 131 cases. This figure increased to 211 out of 213 in 21-24 wk and 512 out of 512 in 25 -41 wk, respectively. Cases with N >= 7 accounted for 715 out of 856 fetuses in the 17-41 wk range. In the abnormal group, N was less than 7 in 152 out of 163 fetuses, showing statistical differences between the two groups. With a cut-off value of 6.5, specificity and sensitivity reached 93.3% and 83.5%. Conclusions: The counting of N was found to be a straightforward and efficient method for evaluating the position of the conus medullaris.