◊ BACKGROUND:
Recent clinical reports suggest that intraneural needle placement may not always lead to neurologic injury. To explain the absence of neurologic complications in these reports, we studied the risk and extent of nerve injury after intentional needle-nerve placement in a cryopreserved human sciatic nerve.
◊ METHODS:
The sciatic nerve was dissected from a cryopreserved cadaver through partial exposure. Needles were inserted through the nerve, using blunt-tip (30 degrees beveled) (group A) and sharp-tip (15 degrees beveled) (group D) needles. Five needle insertions were made for each needle type. Subsequently, transverse nerve sections at 10 needle trajectories were processed. Nerve samples were stained with hematoxylin-eosin, Masson trichromic, and immunohistochemical stains. In each section, the following variables were quantified: total number of fascicles and vessels in the immediate vicinity of the needle trajectories and the number of injured fascicles and vessels.
◊ RESULTS:
A total of 520 fascicles were quantified, of which 134 were in contact with the needle trajectories. The numbers of fascicles and vessels per section were 65 +/- 8 and 14 +/- 7, respectively. A mean of 16 +/- 5 fascicles were found in contact with the needle trajectory (group A: 17+/- 3, group D: 15 +/- 6). Of these, 4 fascicles (3.2%) and 1 intraneural vessel were found damaged in group D. No fascicular or vascular injuries were found in group A.
◊ CONCLUSION:
Our findings suggest that intraneural needle insertion may more commonly result in interfascicular rather than intrafascicular needle placement.
◊ Reviewed by W. Harrop-Griffiths, MD
Consultant Anaesthetist
St Mary's Hospital,
London, UK
This paper delves into the relationship between intraneural needle placement and nerve damage. Having thawed out a human cadaver, the authors placed five Braun Stimuplex A (short bevel) needles and five Braun Stimuplex D (long bevel) needles into and through the sciatic nerve at the back of the thigh that had been dissected out in situ. The nerve was then removed and examined histologically, looking for evidence of fascicle damage. The results are fascinating: only 3.2% of the fascicles seen along the needle track produced by D needles were damaged and only one vessel was seen to be damaged. More interesting still was the fact that the A needles caused no identifiable damage to fascicle or vessel. Although this difference was statistically significant, the authors are at pains to point out that the study was not powered “to unambiguously determine whether a blunt- or sharp-beveled needle is more likely to injure fascicles”.
Power aside, to me this constitutes a strong indication that our long-held dogma needs increasingly to be questioned. We have set aside our belief that intraneural needle placement is rare; we now know that it is not. Now we may need to set aside the belief that long-beveled (sharp) needles are more likely to enter nerves but less likely to damage them once in the nerve whereas short-beveled (blunt) needles are less likely to enter nerves but more likely to damage them once in the nerve.
The important question is now: why is the apparent risk of fascicular damage so low? The proportion of a nerve that is actually nerve tissue varies between 30% and 70%, with the sciatic nerve having the most perineural tissue. This in itself could explain why nerve damage seems rarer in the lower limb, but does not fully explain such a low incidence of fascicular trauma. One is drawn to the conclusion that nerve fascicles can move away from an advancing needle tip. Anyone who has tried to skewer a strand of spaghetti with a fork will be familiar with this concept. It is also tempting to believe that nerve fascicles will more readily escape a blunt needle in this way than a sharp needle.
The authors are honest about the limitations of this study. The results may be suggestive but only apply to the dissected sciatic nerve of a thawed and previously frozen elderly and dead human being. Their histological examinations were meticulous but might have missed a small number of incidences of fascicular damage. Needle-nerve angles were all 90 degrees; this would not reflect clinical practice. Notwithstanding these limitations, I think that this study is of considerable interest.
I am left with only one concern, and that a minor one. Anyone who has, like me, tried to skewer a strand of spaghetti (or indeed a pea) with a fork will be aware of one thing: that a swift stab of the needle is much more likely to succeed than a slow one. The fast fork gives the food no chance to move away. I was therefore disappointed that the manuscript does not comment on the speed of needle insertion. I look forward to a similar study in which one group of nerve punctures is performed slowly and one swiftly. If there was a difference, this would support my own innate prejudice (always a joy when reading a paper): that when performing a peripheral nerve block, a blunt needle should be chosen and it should always be moved slowly and carefully.
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