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Abstract
Background and aims Intrafascicular injection of a local anesthetic, its toxicity, and direct needle trauma to nerve tissue are generally considered the primary reasons for nerve injury after intraneural injection. We hypothesize that the only way that the local anesthetic can enter the fascicle is if at least 80% of the distal needle orifice is inside the fascicle. the aim of this study was to calculate the theoretical vulnerability of nerve fascicles given their actual sizes and that of different needle openings.
Methods We superimposed microscopic images of two routinely used nerve block needles (a 22-G, 15 ‘StimuplexÒD’ needleand a 22-G, 30 ‘StimuplexÒUltra 360Ò’ needle) over microscopic images of four nerve types taken at the same magnification. We calculated the chances of distal needle orifice overlap of the fascicles and assigned 80%, 90%, and 99% overlap as significant for fascicle vulnerability to intrafascicular injection. We assigned a color for the amount of overlap (vulnerability) to demonstrate this graphically.
Results Figures 1 and 2 demonstrate the overlap vulnerability. table 1 summarizes the sizes of the fascicles and the chances of overlap of the needles and the fascicles. None of the fascicles accommodated 99% of the distal needle orifices.
Typical regional block needle superimposed on a typical nerve at the same magnification. A. Block needle superimposed over a brachial plexus root (C7) to show overlap or occupation calculated percentage of the nerve fascicle by the needle distal orifice. b. Fascicle overlap or occupation by the distal needle opening a color for graphical representation
The percentage of needle distal orifice overlap for different nerves and needles. a) Median nerve and 22-G, 30° ‘Stimuplex® Ultra 360®’ needle; b) Median nerve and 22-G, 15° ‘Stimuplex® D’ needle; c) Brachial plexus root nerve at the interscalene level and 22-G, 30° ‘Stimuplex® Ultra 360®’ needle; d) Brachial plexus root nerve at the interscalene level and 22-G, 15° ‘Stimuplex® D’ needle; e) Popliteal sciatic nerve and 22-G, 30° ‘Stimuplex® Ultra 360®’ needle; f) Popliteal sciatic nerve and 22-G, 15° ‘Stimuplex® D’ needle; g) Subgluteal nerve and 22-G, 30° ‘Stimuplex® Ultra 360®’ needle; h) Subgluteal nerve and 22-G, 15° ‘Stimuplex® D’ needle
Sample and results*
Conclusions The analysis of partial to full occupation of the distal needle orifices inside the nerve fascicles allowed us to document and visually demonstrate how the different microanatomic architecture of each nerve type affects its vulnerability to intrafascicular injection.