Clinical Science (39)

How much work is required to puncture dura with Tuohy needles? (Veterans Affairs Medical Center/Jackson Memorial Medical Center, University of Miami, Miami, FL) BR J Anaesth 2000;85:238–241. This study was designed to evaluate whether bevel orientation or patient position alters the force necessary to produce dural puncture. A constant hydrostatic pressure was applied to the subdural surface, either high or low, simulating the sitting and lateral positions. A 17‐gauge Tuohy needle was advanced through the dura with the bevel oriented parallel or perpendicular to dural fibres. Travel distance and peak force at which dural penetration occurred was measured under both pressure conditions. The work required to produce dural penetration was calculated. Greater force and work were required to penetrate dura in the perpendicular orientation ( P < 0.05), regardless of the subdural pressure exterted. Dural displacement was similar under both pressure conditions. Comment by Hemmo A. Bosscher, MD. Work is the product of displacement and force (the area under the curve). The same amount of work is required whether the dura is under pressure (sitting position) or not (lying down). This is surprising. Absolute force is, therefore, not the most important factor. Penetration of the dura probably depends more on local pressure, the area of contact between needle and dura, and the structure of the dura (eg, fiber direction). A sharp needle will obviously penetrate much easier than a blunt one. Considering the shape of a Tuohy needle you would expect that the area of contact not only depends on orientation of the needle, but also strongly on the angle at which the dura is approached. Since dural puncture with a Tuohy needle in a young person almost invariably results in a “spinal headache” most benefit will be obtained from reducing the change of dural puncture. This study supports the concept of putting the bevel of the needle perpendicular to the fiber direction to reduce this change.