An in vitro model was used to determine the force required to pierce bovine dura with a range of new spinal needles and to measure the subsequent leakage rate of cerebrospinal fluid (CSF). A significantly greater force was required to pierce the dura with pencil-point style needles compared to Quincke needles of the same size. Quincke needles caused a greater loss of CSF than their pencil-point equivalents. The results suggest that there is not likely to be a significant reduction in postdural puncture headache (PDPH) using a 27-gauge pencil-point needle compared to a 25-gauge needle that may be easier to use. Different makes of the same design and gauge of needle showed significant differences in the amount of CSF leakage, which may influence the clinician's choice of needle.

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After performing successful continuous lumbar extradural conduction block, we investigated the effects of the extradural insertion technique (midline (M) or paramedian (P)) and patient position during extradural catheter removal (flexed lateral (L) or sitting (S)) on the force required to remove extradural catheters. One hundred parturients were allocated randomly to four groups: ML, MS, PL, PS. The results indicated that neither the midline nor paramedian approach affected withdrawal forces. However, more than 2.5 times as much force was required to remove the catheters when patients were in the flexed sitting compared with the lateral position (P < 0.005). For ease of removal of catheters from the lumbar extradural space we therefore strongly recommend the flexed lateral position.

We have studied magnetic resonance images of the lumbar spine of 39 subjects to examine the anatomy of the lumbar extradural region. The segmental nature of the posterior extradural region at each lumbar level may explain reports of easier cranial passage of extradural catheters introduced by the paramedian approach. This approach may thus provide a more reliable route for rapid introduction of an extradural catheter during the needle-through-needle, combined spinal-extradural technique.