What is the optimum length of the introducer for spinal anaesthesia?

Introduction: The 27g BD needle has previously been shown to possess many of the characteristics of the ideal spinal needle (1). However, we noticed it was the needle of choice in only 720 out of 4,000 spinal anaesthetics in our hospital in the previous year. The commonest criticism was that there was too much buckling of the shaft when used with a 36 mm introducer. Concurrent research measuring in vivo forces during spinal anaesthesia needle using a force transducer attached to the hub of the spinal needle has shown that the mean force required to penetrate ligament is 4.34 Newtons (N) though can be as high as ION. We performed an in vitro study to assess how the bending force of the proximal shaft varied with introducer length. Methods: Using a specifically built test rig, we looked at the forces applied down the needle if the introducer was kept 1cm from the Whitacre tip. We also examined the effect of longer introducers which gave the proximal shaft more support. The mean peak force was measured for 3 needles at each length of introducer tested. Results: The 36mm introducer allowed a mean force of 3.4 Newtons to be applied before proximal shaft bending occurred. Increasing the length of the introducer from 36mm to 46 mm gave increased shaft support which allowed a greater peak force of 8 Newtons to be applied to the needle tip. Introducer length "36 "38 ~40~42 ~44~46~48 ~50 (mm) Bending Force T59 HI T95 ~6J2 726 TI9 05 ~9~89 of proximal shaft. Discussion: A 36rnm introducer may not offer enough support for penetrating tougher ligaments. Increasing the length of the introducer to 46mm may generate a peak force high enough to overcome the resistance of tough ligaments without proximal shaft bending allowing a higher success rate for spinal anaesthesia.