An unusually quick onset of dense anesthesia is occasionally observed after a sciatic nerve block in the popliteal fossa. Frequently, anesthesia is profound in the distribution of both divisions of the nerve, although the response to nerve stimulation or paresthesia is obtained only in the distribution of either the tibial nerve (TN) or common peroneal nerve (CPN).
To investigate the possible role of a common continuous epineural sheath in these phenomena, we examined the popliteal fossa and lower leg in cadavers for the existence and continuity of a epineural tissue sheath surrounding the sciatic nerve and its two main divisions.
The sciatic nerve is formed from spinal cord segments L4-S2 (and occasionally S3) and consists of two distinct divisions, the TN and CPN, which share a common epineural sheath from their origin to the popliteal fossa [1,2], where they divide into the TN and the CPN. The TN is the larger of the two branches and runs parallel and slightly lateral to the midline. Inferiorly, it passes between the heads of the gastrocnemius muscle. The CPN follows the tendon of the biceps femoris muscle laterally and travels around the fibular head as it leaves the popliteal fossa. The two nerves innervate the entire leg below the knee except for the anteromedial leg and foot, which are innervated by the saphenous nerve (L2-L4).
The large peripheral nerves are enclosed in three layers of tissue of differing character. From the inside outward, these are the endoneurium, perineurium, and epineurium . Nerve fibers embedded in endoneurium from a funiculus surrounded by perineurium, a thin but strong sheath of connective tissue. The nerve bundles are embedded in a loose areolar connective tissue framework called the epineurium. The epineurium that extends between the fascicles is termed the inner or interfascicular epineurium, while that surrounding the entire nerve trunk is called the epifascicular epineurium . The connective tissue outside the epineurium is referred to as the adventitia of the nerve or epineural tissue . Although the epineurium is continuous with the surrounding connective tissue, its attachment is loose, so that nerve trunks are relatively mobile except where tethered by entering vessels or exiting nerve branches .
The study was performed on 10 adult cadaver legs (5 right and 5 left) free of obvious pathology of the lower extremities. Six- to 18-month-old cadavers, were embalmed for anatomical purposes using a solution of phenol (13%) as the principle fixative and glycerin (28%) for retention of water content. The cadavers were positioned prone on the dissecting Table sothat the long axes of the legs were horizontal to the Table planeand the feet formed a 90 degrees angle to the horizontal plane.
The calf was dissected, and the TN was exposed in the deep posterior compartment under the soleus muscle (Figure 1). The fascial sheath surrounding the nerve was identified, and a 16-gauge catheter was inserted into the sheath 15 cm below the popliteal crease. The catheter was then advanced until the tip of the catheter was positioned at a distance of 10 cm distal to the popliteal crease.
Using the method of sealed envelopes, each leg was randomly assigned to one of two groups. In Group A (n = 5), 15 mL of blue vinyl acetate (BVA) solution (Cabisco[R], Carolina Biological Supply Company, Burlington, NC) was injected into the TN sheath. In Group B (n = 5), 30 mL of BVA was injected into the TN sheath. Upon injection of the dye, the catheters were left in place. One h later, the dissection was extended into the popliteal fossa, and the spread of BVA within the sheath investing the nerves in the fossa was observed. The level of the division of the sciatic nerve into TN and CPN was also determined in each leg.
Representative tissue samples of TN, CPN, and sciatic nerves were excised, embedded in paraffin, crosssectioned, and stained with hematoxylin and eosin or trichrome stains. The samples were examined for anatomical placement of the catheter and distribution of the BVA in relation to the nerves.
The difference in extent of the spread of the BVA within the epineural sheaths and external measurements between the two groups was tested using a one-sided Student's t-test.
The division of the sciatic nerve occurred at a mean of 44 +/- 20 mm (range 0 to 73 mm) above the popliteal crease, with no significant difference between the two groups. In all 10 legs, the sciatic nerve in the popliteal fossa consisted of two separate nerves, the TN and CPN enveloped by a common epineural sheath that accompanied the nerves after their divergence in the fossa. In one leg in each group, the BVA solution did not flow proximally, but leaked at the level of the catheter tip, forming a pool of injectate between the intermuscular fasciae. Careful dissection along the TN revealed that in both legs, the catheter tip perforated the sheath, resulting in extra-epineural injection. These two legs were excluded from further analysis. In the remaining eight legs, the BVA solution traveled 147 +/- 34 mm (Group A) and 172 +/- 50 mm (Group B) proximally within the sheath from the injection point (not significant) (Figure 2). The dye readily filled the sheath with minimal or no apparent leakage of the solution outside the sheath. The injectate reached the apparent division of the sciatic nerve in the popliteal fossa, bathing both the TN and CPN in all eight legs (Figure 2) except one in Group A. In all legs, the epineural tissue sheath continuously surrounded the sciatic nerve and its main divisions, forming a virtual fluid conduit. This conduit appeared to communicate among the sciatic nerve, TN, and CPN in the fossa, as evident from the extent of the spread of BVA within the TN sheath and into the sheath extensions of both the sciatic nerve and CPN. Cross-sectional examination of the specimens containing the catheter documented the placement of the catheter within the epineural sheath and not intraneurally (Figure 3). Histological examination of all 10 nerve specimens at multiple levels from 15 cm to 30 cm above the popliteal fossa showed that the sciatic nerve consists of two distinct nerve bundles, the CPN and TN, which are enveloped by separate epineurium and contained in a common epineural adventitia (Figure 3). The two components of the sciatic nerve could be traced and separated with minimal traction along the entire length of the thigh in all examined extremities.
Our results demonstrate the existence of a continuous epineural sheath surrounding the sciatic nerve and its major divisions, the TN and CPN. This sheath appears to form a fluid conduit that communicates among the major nerve divisions in the popliteal fossa.
Recently, we described the existence of an adventitial sheath that encompasses the sciatic nerve at the level of its division into the TN and the CPN . Injection of 5 mL of dye into this sheath at the level of the division of the sciatic nerve resulted in spread of the dye 5-10 cm within the sheath without significant leakage outside the sheath. Confirming our previous findings, the present data show that this epineural sheath is virtually continuous; envelopes the sciatic nerve, TN, and CPN; and allows spread of solutions injected into the sheath. The dissection of the sciatic nerve showed that the apparent division of the sciatic nerve into the TN and CPN is a mere separation of two distinct nerves descending from the pelvis in a common adventitial sheath .
These anatomical characteristics of the sciatic nerve and its epineural sheath offer a possible explanation for some clinical phenomena often seen during popliteal nerve block. For example, anesthesia is often obtained in both divisions of the sciatic nerve, although paresthesia or the response to nerve stimulation is obtained in the distribution of either the TN or the CPN. However, under apparently identical clinical circumstances, a block in the distribution of only one division of the nerve can also occur . Since TN and CPN separate at a highly variable distance from the popliteal crease (0-115 mm) , a needle inserted at 5-7 cm above the popliteal fossa crease will frequently be next to only one of the divisions of the sciatic nerve. In a successful block, it is possible that the needle tip is positioned within the epineural sheath above the division of the nerve, resulting in spread of solution of local anesthetic to both divisions of the nerve, or there may be a deposition of the local anesthetic in close proximity to the main trunk proximal to its division. With partial nerve blockade, an extraneural injection (injection outside the epineural adventitia) might result in a pool of local anesthetic in the popliteal fossa fat that is in close proximity to only one division of the nerve.
It is important to emphasize that the placement of an intraepineural catheter is not equivalent to intraneural or intrafascicular placement. This was demonstrated histologically (Figure 3) and confirmed by the absence of significant back pressure during the injection of 15 or 30 mL of the BVA solution into the sheath. In contrast, our attempts to inject BVA into the nerve fasciculi resulted in significant resistance to injection, which promptly disappeared after withdrawal of the needle outside the fasciculus. Thus, as previously demonstrated during intraoperative exposure of the peripheral nerves, an unintentional intraneural injection during neuronal blockade should readily be recognized as an increase in resistance to injection in addition to severe pain or paresthesia . However, if the solution of local anesthetic is injected intraepineurally, it should substantially spread within the sheath, resulting in conduction blockade in the entire sciatic nerve distribution. This mechanism could explain successful block of the entire sciatic nerve trunk when the response to nerve stimulation or paresthesia is obtained in the distribution of only one division of the sciatic nerve. This is analogous to conduction blocks of the lumbar or brachial plexuses [9,10] except that in the sciatic nerve block, the structure that conveys the solution of local anesthetic is an epineural, not a perivascular, sheath. Functionally, however, the sciatic nerve block at the popliteal fossa appears to be a block of both the nerve components, although frequently only one is identified with nerve stimulation.
Our study has several important limitations. First, the conductivity of the sheath and its anatomical characteristics could be somewhat different in patients from that observed in embalmed cadavers. Furthermore, the viscosity of the BVA solution is greater than that of local anesthetic viscosity, and this could have influenced its spread within the sheath. Third, the solution was injected in the tibial nerve sheath 10 cm below the popliteal fossa, while sciatic nerve blockade is done in the popliteal fossa. The TN was chosen because injections into the sheath of the sciatic nerve or its main divisions in the fossa would have required dissection of the fossa, resulting in distortion of the anatomy. This, in turn, could have influenced the spread of the BVA.
In summary, we demonstrated the existence of a common epineural sheath enveloping the major divisions of the sciatic nerve. This structure has features of a fluid conduit, which appears to communicate among the sciatic nerve, TN, and CPN. These characteristics of the epineural sheath may have important clinical implications in sciatic nerve blockade at the popliteal fossa.
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© 1997 International Anesthesia Research Society
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