Effect of arching spine on deformation of the ligamentum flavum during epidural needle insertion.

When administering epidural anesthesia, anesthesiologists ask patients to arch their back. Arching the spine is thought to enlarge the gap between neighboring vertebral bones. The author hypothesized that tension inside the ligamentum flavum generated by arching the spine would reduce deformation of the ligamentum flavum during epidural needle insertion. Porcine spines from a slaughterhouse were cut at the vertebral body and separated into 10 pieces. Ligamentum flavum was painted black with liquid ink. A CCD camera recorded the deformation of the ligamentum flavum during needle insertion. To simulate the arching spine, the width of the ligamentum flavum was enlarged by a retractor. The thickness of the ligamentum flavum was measured from the stained images by the Elastica-van Gieson method. The maximum reaction force showed no significant difference between the natural and the enlarged ligamentum flavum. Average deformation was observed a statically significant decrease between the naturally deformed and retractor-enlarged ligamentum flavum. For the maximum reaction force the coefficient of variance decreased by dividing raw data with thickness of the ligamentum flavum, which meant that the maximum reaction force correlated with thickness of the ligamentum flavum. Less effect on deformation was observed. Hypothesis was correct in the porcine study, while the difference between the porcine and the patient's spine should be examined in the next research.