Effect of orifice-area reduction on flow characteristics during injection through spinal needles.

A reduction in hole size for certain side-port spinal needles has been advocated in recent reports. While the influence of orifice-area reduction on the aspiration capability of the needle has been studied, the influence on the anaesthetic delivery properties is relatively unknown. As a first step in understanding the effects of hole-size reduction on anaesthetic distribution within the subarachnoid space, we studied flows emanating from isolated needles using computer simulations. Following validation of the numerical model using experimental particle visualisation, trajectories of anaesthetic particles injected through 25 G Whitacre needles of various orifice areas were computed and used to determine the orientation and rate of spread of the anaesthetic jet exiting the needle. Two factors impacting the concentration distribution were observed: the rate of spread of the anaesthetic jet increases markedly with decreasing orifice area and the jet alignment shifts toward perpendicular to the needle axis.