Pythagoras and Cosines: The skin–dural sac distance and optimal angles in paramedian spinal anesthesia

The classical recommendation for paramedian approaches is needle insertion 1–2 cm paramedian and an angle of 10°–15° medial–cephalad to the plane of the back, but contact with vertebrae is frequent. A mathematical approach to individualizing punctures is proposed on the basis of skin–dural sac distance (d): Optimal angle ∼ inverse cosine [d / ( 1 +d 2)  ] and the distance covered by the needle ∼1 +d 2for 1 cm paramedian punctures. The inferred angles were compared to optimal angles leading to the central dorsal part of the dural sac from 1 to 2 cm paramedian, measured by Magnetic Resonance Imaging (MRI) in seven cases and in a short stature volunteer (1.58 m, Body Mass Index (BMI) 23.2), to study supine and fetal positions using both closed MR and ultrasound. The average (d) decreased rostrally [6.8 cm (L4–L5)−4.3 cm (T11–T12)] while the mean optimal incidence angles increased [8.3°–16.5° (L4–L5) to 12.7°–24.1° (T11–T12) at 1–2 cm paramedian, respectively] and coincided with the estimated angles with a correlation coefficient = 0.98. In the volunteer, the optimal lateromedial angles increased from 14.4° to 26.7° (L3–L4) to 17.1°–30.3° (T11–T12) for a ( d ) = 3.7 cm (L3–L4)−3.1 cm (T11–T12) and increased ≤3.7° and ≤5.1° at 1 and 2 cm paramedian, respectively, in fetal positions in MR. Ultrasound yielded comparable figures. The range of possible angles for dural punctures is wider at 1 cm paramedian in lower approaches in lateral decubitus [from 3.6° at T12L1 (12.2°–15.8°) to 9° at L3L4 (8.8°–18.7°)]. The classically recommended angles of 10°–15° differ from the optimal angles, particularly in small patients, suggesting the need for ultrasound guidance or for inferring angles prior to spinal anesthesia. Clin. Anat. 29:1046–1052, 2016. © 2016 Wiley Periodicals, Inc.