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Background and Aims When conducting spinal anesthesia, pregnant patients and patients with lower limb injuries may have difficulties taking the optimal position. This study investigates whether the posture of the trunk, flexing of the lower limbs and tilting of the head affects the interspinous distance.

Methods This cross-sectional study was conducted on 25 healthy adult volunteers with their consent and approval by the Institutional Ethics Committee of Shizuoka general hospital (No. SGHIRB#2023051). We performed lumbar ultrasonography in the left lateral position to measure the interspinous distance at the L2/3, L3/4 and L4/5 interspace, in seven different positions: P1: straight-back with bilateral lower limb extension, P2: straight-back with unilateral lower limb flexion, P3: straight-back with bilateral lower limb flexion, P4: P3 with head tilted forward, P5: forward bending with bilateral lower limb flexion, P6: forward bending with unilateral lower limb flexion, P7: forward bending with bilateral lower limb extension. Using P1 as the reference, each position was compared using linear regression analysis with statistical significance set at p<0.0071 after Bonferroni adjustment for multiple comparisons.

Results Positions that significantly affected the widening of the interspinous distance were P5 at the L2/3, L3/4 and L4/5 interspace (P<0.001, P<0.001, and P<0.001, respectively) and P6 at the L2/3 and L4/5 interspace (P<0.003, and P<0.003, respectively). Whereas the tilting position of the head did not affect the interspinous space.

Conclusions In healthy adults, forward bending with even unilateral lower limb flexion affects widening of the interspinous space, while tilting of the head does not have an impact.