Effects of shoulder rotation and table tilt maneuvers on the lumbar and thoracic paramedian acoustic window in older adults: a quasi-experimental study

Background Shoulder rotation and table tilt have been reported to increase the acoustic window for spinal ultrasonography. However, evidence in older adults remains scarce. This study evaluated their anatomical effects on lumbar (L3–L4) and thoracic (T10–T11) paramedian acoustic windows, quantified by posterior longitudinal ligament (PLL) length, in older adults (≥ 65-years). Methods This quasi-experimental study enrolled 45 older adults scheduled for elective surgery at two tertiary hospitals in Brazil. The primary outcome was PLL ultrasonographic length (paramedian sagittal oblique plane) assessed at the L3‒L4 and T10‒T11 interspaces in three positions: F (standard spinal flexion), S (F plus 45° leftward shoulder rotation), and T (F plus 10° dorsal table tilt). Data were analyzed using a Linear Mixed-Effects Model with Bonferroni-adjusted post-hoc contrasts. Results The model revealed significant effects of position (p = 0.003) and spinal level (p < 0.001) on PLL length, with no Level × Position interaction (p = 0.693). PLL length was significantly greater at L3–L4 than at T10–T11. Table tilt produced a significant increase relative to the flexion position (difference = 2.2 mm; 95% CI 0.9–3.5; p = 0.002), whereas shoulder rotation did not (difference = 1.2 mm; 95% CI 0.0–2.5; p = 0.158), with homogeneous effects across both levels. Fixed effects explained 27% of total variance (R 2 m = 0.27; R 2 c = 0.55). Conclusion Table tilt produced a significant anatomical enlargement of the neuraxial acoustic window at both spinal levels studied, whereas shoulder rotation did not reach statistical significance. These findings establish an anatomical foundation for future trials evaluating their impact on neuraxial procedural outcomes in older adults. Trial registration https://ensaiosclinicos.gov.br/rg/RBR-3p6t9pm