Abstract
Cauda equina nerve roots may become damaged during neuraxial anesthesia, and post-puncture headache may appear in the case of cerebrospinal fluid leakage if needle tips are deformed due to bone contact when several attempts are needed. Our aim was to verify the correlation between skin-transverse process distance (st) and skin-dural sac distance (d) for calculation of optimal angles in a free visual guide and as a reference for the maximal depth to be traversed by the needle. Randomly selected ex vivo samples (n = 10) were flexed to reproduce the position of the lumbosacral spine during spinal anesthesia. Spinal needles were inserted perpendicular to the skin either blindly or following the inferred paramedian angle corresponding to ultrasound-measured (d). After computed tomography and three-dimensional reconstruction, both (st) and (d) were measured, and the Pearson correlation index was calculated. A free 3D-PDF tool was used to illustrate the potential affectation of nerve cuffs by needles located lateral to the dural sac. Correlation between (d) and (st) was 0.84-0.93 at L4L5-L3L4 intervertebral levels, and most needle tips were located within the spinal canal, but some traversed the zone where nerve cuffs emerge. In conclusion, ultrasound may determine if a perpendicular needle insertion is viable at midline. If not, the optimal paramedian angle and maximal depth may be determined by measuring (st).