The effect of introducer gauge, design and bevel direction on the deflection of spinal needles

This study was performed to determine how the use of an introducer affects the extent to which a needle deflects during a spinal or combined spinal-epidural injection. A polystyrene block was used to simulate the paraspinal area of the back. A line was drawn perpendicular to the edge of the block to use as a guide and to measure the deflection. The use of an introducer needle decreased the deflection in all the bevelled needles (p < 0.001). Depending on the direction of both the bevels, the deflection decreased as the introducer bevel was changed from the same direction, to right-angles to bevel direction and then to a direction opposite to that of the spinal needle (p < 0.05). Deflection was decreased when a thick introducer was used (p < 0.001). The use of an introducer increased the deflection of the pencil-point needle only in the deflection direction of the introducer (p < 0.001). The 18-gauge Tuohy needle with a "backhole" deflected more than the corresponding needle without a backhole (p < 0.001), and the spinal needle inserted through the Tuohy needle with a backhole deflected more (p = 0.002). Besides the tip type and gauge, the deflection of a spinal needle depends upon the use of introducer, its gauge and bevel direction. The deflection of a Tuohy needle depends upon its design, gauge and the presence of a backhole.