Vertebral level in pregnancy according to the posterior superior iliac spine: an observational study

Background: While the line joining the posterior superior iliac spine (PSIS) intersects a relatively stable sacral vertebra, it does not directly facilitate the localization of lumbar interspace or assist in the positioning for neuraxial anesthesia. Our study aimed to explore the potential of the PSIS line as a reference point and to determine its practical applicability in clinical settings.

Methods: We consecutively enrolled pregnant women with gestational ages ranging from 24 to 38 weeks scheduled for magnetic resonance imaging (MRI) examination. Two anesthesiologists used ink to mark the PSIS line through visual localization or palpation. Subsequently, an equilateral triangle was constructed with the PSIS line as the baseline, with the vertex intersected the lumbar spine towards the head. Marker capsules were then affixed to the vertex of PSIS-line-based triangle and PSIS line, and MRI was performed with participants in the supine position. The vertebral body or interspinous space intersected by the PSIS line and vertex was recorded.

Results: One hundred parturients were analyzed. The PSIS line intersected the L5 vertebral body, L5-S1 interspinous space, S1 vertebral body, S2 vertebral body, and S3 vertebral body in 11, 10, 62, 15, and 2 cases, respectively. Furthermore, the vertex of PSIS-line-based triangle intersected the L2 vertebral body, L2-3 interspinous space, L3 vertebral body, L3-4 interspinous space, and L4 vertebral body in 4, 7, 53, 20, and 16 cases, respectively. No significant correlation was detected between the vertebral levels intersected of the PSIS line and the parameters of height, weight, and gestational age.

Conclusions: The PSIS line commonly intersects S1 vertebral body, and vertebral levels intersected of the PSIS-line-based vertex rang from L2 to L4 vertebral body in pregnant women. The PSIS-line-based triangle has potential to aid identification of vertebral levels, which could be applied to neuraxial anesthesia.

Trial registration: The study was registered in Chinese Clinical Trials Registry(Ref. ID: ChiCTR-DDD-16010067).