Background and Objectives The cylindrical shape of the dura in vivo, as well as the needle tip deviation known to occur with beveled needle insertion, might predispose to geometric effects of needle orientation on hole size and shape during dural puncture. The object of this study was to investigate such possible effects.

Methods Standard xerographic paper was used to simulate a dura mater membrane with random fiber orientation. Rigidly mounted paper cylinders of 2-cm diameter were transfixed at 90° angles to the cylinder axis with 22-gauge Quincke point spinal needles. A nonrotating drill press effected linear insertion, creating entry and exit perforations at median and paramedian positions. The bevel direction was rotated at 90° angles during punctures in order to determine the effects of lateral versus transverse bevel orientation (relative to the cylinder axis) on the resultant hole morphology.

Results With median perforation, all holes (entry and exit) were of uniform size and shape regardless of bevel orientation. Paramedian perforations of the cylinder at near tangential positions, with the bevel directed lateral to the cylinder axis, resulted in formation of a flap overlapping the margins of either the entry or exit hole but not both. Flaps formed only when the bevel faced the cylinder membrane's surface during paramedian, near tangential puncture (n = 10, P = .00001).

Conclusions The geometric interactions of membranes with Quincke needles lend support to the practice of needle insertion with the bevel facing laterally in order to produce smaller holes. Geometry may help to explain the reduced rate of postdural puncture headache found with Quincke bevels oriented to face laterally during midline approach and during paramedian technique, particularly when a single puncture results in aspiration of cerebrospinal fluid.