Purpose or Case Report: To assess the level of the conus medullaris using spinal ultrasonography, bony landmarks such as the lowest ribs, the most caudal ossification center (MCOC) of the vertebral body, and angulation in the lumbosacral region are used. However, determining the level of the conus medullaris is sometimes difficult because of the variations of these landmarks.
Purpose of this study was to analyze the variation of these landmarks. However, it is difficult to evaluate these variations using ultrasonography, because this method allows a narrow field of view. To supplement this limitation, we tried to evaluate these landmarks retrospectively using (1) whole spine CT or (2) lumbosacral CT in combination with chest radiography.
Methods & Materials: Patients aged less than 4 months who underwent spine CT for at least the lumbosacral region at our institution were included. Patients with vertebral segmentaion anomalies, severe vertebral deformity, known bone dysplasias, and sacrococcygeal teratomas were excluded.
In patients who underwent whole spine CT, MCOC were counted down from the C2 on sagittal reformatted CT images. In patients who did not undergo whole spine CT, chest radiograph was referred and MCOC were counted down from the 12^th thoracic spine or equivalent vertebra. Angulation of the spine in the lumbosacral region was determined similarly. We also correlated the level of the landmarks using full spine MRI. The number of ribs was counted using 3D reconstructed imaging and chest radiography.
Results: A total of 155 CT studies (99 patients) were included in this study. Whole spine CT was performed in 31 studies, and only caudal to the lower thoracic vertebrae were scanned in 124 studies. About 10% of the patients did not have 12 pairs of ribs. MCOC was located at the S5 (or S5 equivalent) in 42% of patients aged less than 1 month. MCOC tends to be more caudally located with age. The angulation in the lumbosacral region was formed by the 24^th and 25^th vertebra from the C1 in 97% of patients.
Conclusions: It might be difficult to determine the level of the conus medullaris using a single anatomical landmark, and a combination of the multiple landmarks might result in more precise level determination.
Knowledge about the variations in the lumbosacral spine and ribs might be useful for evaluating the level of the conus medullaris in a spinal ultrasonography.