Fig. 2

Paramedian sagittal oblique and transverse median views of the mid-thoracic spine and corresponding computed tomography image illustrating patient stratification. Category I: AC is visible in the PMSO and TM views (a, b). Note that the adjacent laminae are not closely overlapping (c), and the interlaminar space is not obstructed by the skinny-based spinous process (d). Category II: AC is only visible in the PMSO view (e) but invisible in the TM view (d). Note that the adjacent laminae are not closely overlapping (g), and the medial part of the interlaminar space is covered by the wide-based spinous process, leaving only the lateral portion of the interlaminar space open (h). Category III: AC is invisible in both views (i, j). Note that the closely overlapping laminae narrowed the interlaminar space (k), and the wide-based spinous process obstructed the interlaminar space (l). The blue lines represent the caudal edge of the interlaminar space of the chosen level. The yellow arrows represent the skin-to-lamina depth at the determined needle puncture site. The yellow lines represent the midline of the spinous process. The red line encircled areas represent the base of the spinous process. (AC = anterior complex; CT = computed tomography; PMSO = paramedian sagittal oblique; TM = transverse median)