Optimal Point of Insertion and Needle Angle in Neuraxial Blockade Using a Midline Approach: A Study in Computed Tomography Scans of Adult Patients

Background and Objectives Neuraxial blockade using a midline approach can be challenging. Part of this challenge lies in finding the optimal approach of the needle to its target. The present study aimed at finding (1) the optimal point of insertion of the needle between the tips of 2 adjacent spinous processes and (2) the optimal angle relative to the skin at which the needle should approach the epidural or subarachnoid space. Methods A computer algorithm systematically analyzed computed tomography scans of vertebral columns of a cohort of 52 patients. On midsagittal sections, the possible points of insertion of a virtual needle and the corresponding angles through which the epidural or subarachnoid space can be reached were calculated. Results The point chosen to introduce the needle between 2 adjacent spinous processes determines the range of angles through which the epidural or subarachnoid space can be reached. At the thoracic interspaces 1–2 through 3–4, thoracic interspaces 5–6 through 9–10, and at the lumbar vertebral interspaces 2–3 through 4–5, the optimal point of insertion is slightly inferior to the point halfway between the tips of the spinous processes. For thoracic interspace 4–5, the optimal point of insertion is slightly superior to the point halfway between the tips of the spinous processes. For the other interspaces, the optimal point of insertion is approximately halfway between the tips of the spinous processes. The optimal angle to direct the needle varies from 9 degrees at the thoracolumbar junction and at the lumbar interspaces 3–4 and 4–5, to 53 degrees at the thoracic interspace 7–8. Conclusions Our study has resulted in practical suggestions—based on accurate, reproducible measurements in patients—as to where to insert the needle and how to angulate the needle when performing neuraxial anesthesia using a midline approach.