Effect of the needle tip height on the puncture force in a simplified epidural anesthesia simulator

The purpose of the research is to create a safer needle used for epidural anesthesia. Medical doctors identify the location of the epidural space by feeling the drop of force in advancing the needle tip. A greater drop of force makes for a safer needle. The force pattern can be determined according to the tip shape. In this study, the effect of the height of the needle was examined. We fabricated two needles with 18-gauge diameters (1.2 mm). The shape of the needle tip was measured by X-ray computed tomography. One was 1.86 mm in height (A); the other was 1.4 mm (B). Both had tip angles that were the same: 27 degrees. A simplified simulator consisted of a motorized stage, a load cell, and a phantom as a substitute for the ligamentum flavum before the epidural space. The reaction force was measured while the needle punctured a silicone rubber (a hardness of 50 Hs in reference to ISO7619), and a porcine bone. The speed of insertion was set at 2, 4, and 8 mm/s. The results showed that the force increased up to the highest edge of the needle as it passed the phantom. The drop in the force was measured at all insertion speeds and in both the rubber and bone. The A needle observed twice the drop in force compared with the B needle. We found no obvious tendency regarding the effect of the insertion speed. The drop in the force in the rubber was one-fourth greater than that in the bone. Our conclusions are that 1) the higher the needle tip design, the greater the drop in force can be achieved; and that 2) silicone rubber with a hardness of 50 Hs is similar to the ligamentum flavum.