Sciatic Nerve Intercommunications: New Finding R

Original Article

Sciatic Nerve Intercommunications: New Finding R. Shane Tubbs1, Peter G. Collin2, Anthony V. D’Antoni2, Marios Loukas3, Rod J. Oskouian1, Robert J. Spinner4

- OBJECTIVE: Communicating branches between the tibial INTRODUCTION and common fibular divisions of the sciatic nerve have not he sciatic nerve is composed of L4-S3 spinal nerve been previously described. The aim of our study was to contributions and is the thickest nerve in the body.1,2 Its examine such neural connections. width is 2 cm at its origin, ranging across its length from T 1-3 1.5e2.0 cm. It comprises 2 nerves, the common fibular and - MATERIALS AND METHODS: Twenty unembalmed adult tibial nerves, which are encased in a common epineurial sheath.4-6 cadavers underwent dissection of the sciatic nerve. The tibial nerve comprises the ventral branches of the ventral rami Observations were made for interneural communications of L4-S3 while the common fibular nerve comprises the dorsal between the tibial and common fibular divisions of this branches of the ventral rami of L4-S2.1,3 The 2 nerves/divisions are nerve. When present, these were measured and classified. separated within the nerve sheath by the Compton-Cruveilher septum, which is composed of connective and adipose tissues.7 - RESULTS: The majority of sides (75%) had neural com- These 2 components of the sciatic nerve are mixed nerves munications between the parts of the sciatic nerve in the supplying the posterior thigh muscles, the ischial aspect of the gluteal/posterior thigh regions before the normal bifurca- adductor magnus, and most structures inferior to the knee tion of the nerve just above the knee. These connections except for the sensory distribution of the saphenous nerve on were always within 20 cm of the greater sciatic notch. the medial aspect of the leg and foot.1,2 The sciatic nerve also Most connections were represented by Testut inter- sends articular branches to the hip, knee, and ankle joints.1,2 communicating branches types A (14 sides), F (8 sides), and As the sciatic nerve, the 2 encased tibial and common fibular divisions exit through the greater sciatic foramen and inferior to D (2 sides). Most sides were found to have 1 location for 3 sciatic nerve intercommunications. However, 4 sides (13%) the piriformis muscle. Anatomic variations of the sciatic nerve involving the piriformis muscle are well documented, with had multiple locations (up to 3) for these in- particular debt to the early work of Beaton and Anson.4,8-11 More tercommunications. The mean length of the communica- rarely, the common fibular component of the sciatic nerve in- tions was 4.1 cm, and the mean diameter was 2.4 mm. No nervates the gluteus maximus, as demonstrated in a case reported statistically significant difference was found between by Sumalatha et al.12 The sciatic nerve continues to descend deep sides or sexes. to the gluteus maximus muscle and along the posterior aspect of the acetabulum.2,3 It courses over the obturator internus, quad- - CONCLUSIONS: To our knowledge, neural interconn- ratus femoris, and superior and inferior gemelli muscles,2,3 ections between the divisions of the sciatic nerve in the continuing inferiorly between the greater trochanter and the posterior thigh have not been described in the extant ischial tuberosity; halfway between these bony features is the literature. Such data might help explain unusual neurologic surface marking of the nerve.1-3 Through the posterior compart- examinations and alert the surgeon as to the potential for ment of the thigh, the sciatic nerve courses along the posterior encountering such connections at operation. aspect of the adductor magnus.2 It separates into the tibial and common fibular nerves proximal to the knee as it is crossed by the long head of the biceps femoris muscle and enters the apex

Key words Sciences, St. George’s University, True Blue, Grenada; and 4Department of Neurosurgery, - Anatomy Mayo Clinic, Rochester, Minnesota, USA - Interneural connections To whom correspondence should be addressed: Peter G. Collin - Nerve action potential [E-mail: [email protected]] - Neurosurgery Citation: World Neurosurg. (2017) 98:176-181. - Posterior thigh http://dx.doi.org/10.1016/j.wneu.2016.10.118 Journal homepage: www.WORLDNEUROSURGERY.org From the 1Seattle Science Foundation, Seattle, Washington, USA; 2Department of Available online: www.sciencedirect.com Pathobiology, CUNY School of Medicine/The Sophie Davis School of Biomedical Education, The City College of New York, CUNY, New York, New York, USA; 3Department of Anatomical 1878-8750/$ - see front matter ª 2016 Elsevier Inc. All rights reserved.

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Figure 1. Classiﬁcation system used by Testut.

of the popliteal fossa, but the location of the bifurcation is highly types A (14 sides), F (8 sides), and D (2 sides). Two sides had a variable.1,2,11,13-15 configuration similar to a Testut type H intercommunication. One The aim of the present study was to identify communicating side was similar to Testut’s type B but was more complex by branches between the common fibular and tibial components of having an additional crisscrossing branch traveling in the opposite the sciatic nerve. direction from the 2 parallel nerves. This side was classified as a type B2 intercommunication. Three sides were not represented by MATERIALS AND METHODS Twenty unembalmed adult cadavers (40 sides) underwent dissection of the sciatic nerve from its exit from the greater sciatic foramen to its division into common fibular and tibial components above the knee. The cadavers were aged 46e97 years (mean 77 years) at death. Twelve specimens were male, and eight were female. With the specimen in the prone position, a linear skin incision was made from the top of the iliac crest to the popliteal fossa. The fascia lata was then identified and opened in a cranial to caudal direction. The overlying gluteus muscle was transected, and retractors were placed. The sciatic nerve was identified inferior to the piriformis muscle and followed distally to the popliteal fossa. The outer layer of connective tissue uniting the common fibular and tibial components of the sciatic nerve was dissected with scissors using a spreading motion in a caudal to cranial direction. No specimen showed evidence of prior surgery or trauma to the areas studied. Calipers and a ruler were used for mea- surements. Random nerve communication samples (15) were sent for routine histologic analysis to prove their neural nature. The classification developed by Testut16 (Figure 1) was used to describe the various nerve communications when any were present. Statistica for Windows was used for statistical analysis, with significance set at P < 0.05.

RESULTS Thirty of the 40 sides (75%) had communications between the parts of the sciatic nerve in the gluteal/posterior thigh regions before the normal bifurcation of the nerve just above the knee Figure 2. Left posterior thigh with e sciatic nerve dissected. Note the (Figures 2 7). All neural connections were located in the gluteal simple neural interconnection (black region or proximal thigh. This was always within 20 cm of the arrow) traveling between the tibial greater sciatic notch with the majority of these interconnections nerve medially to the common ﬁbular nerve laterally. This represents a occurring in the proximal superior half of the thigh. Most con- Testut type A interconnection. nections were represented by Testut intercommunicating branches

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Figure 3. Right posterior view of the sciatic nerve in the thigh illustrating 2 branches from the medial tibial nerve fusing to a single branch attaching to the Figure 5. Left-sided sciatic nerve common fibular nerve. This represents a Testut type F seen from behind illustrating 2 interconnection. connections traveling from the tibial to the common fibular nerve and a single branch traveling from the common fibular to the tibial nerve. the Testut classification and were described by us as “complex” or This configuration is not represented type H2 configuration (see Figures 6 and 7). On one right side by Testut’s classification but has from a male cadaver we identified a high bifurcation of the components similar to his type B intercommunication.

sciatic nerve, with clear communication between the tibial and common fibular components in the gluteal region just inferior to the point at which the nerve was split by the piriformis muscle (Figure 8). The majority of sides were found to have 1 location for sciatic nerve intercommunications. However, 4 sides (13%) had multiple locations (up to 3) for these intercommunications (see Figures 5 and 6). The lengths of the communications between the tibial and common fibular nerves ranged from 1.0e9.2 cm (mean 4.1 cm). Their diameters ranged from 1.0e5.5 mm (range 2.4 mm). The number of connections on each side ranged from 1e3, and they traveled in opposite directions in many specimens (i.e., lateral to medial and medial to lateral). Histologically, all the communications examined were found to be neural (Figure 9). No statistically significant difference was found between sides or sexes.

DISCUSSION In our study, 75% of cadaveric sides had neural intercommunications between the tibial and common ﬁbular components of the sciatic nerve in the gluteal or proximal thigh Figure 4. Left sciatic nerve viewed from behind regions. However, the extant medical literature included no ac- illustrating a Testut type D interconnection. counts of such communicating branches within the common

178 www.SCIENCEDIRECT.com WORLD NEUROSURGERY, http://dx.doi.org/10.1016/j.wneu.2016.10.118 ORIGINAL ARTICLE R. SHANE TUBBS ET AL. SCIATIC NERVE INTERCOMMUNICATIONS: NEW FINDING

Figure 7. Dissection of the sciatic nerve from behind at the left greater sciatic foramen. The common fibular nerve is lateral and has multiple connections traveling in a superolateral to inferomedial direction. Although not exact, this configuration is closest to a Testut type H configuration.

sciatic nerve, and there are similar statements in many anatomy textbooks and published literature.1-3,23,27 Although the tibial and common fibular components of the sciatic nerve communicate at, for example, the posterior leg via the communicating sural nerve, no other source to our knowledge has reported communications between these parts within the sciatic nerve in the gluteal region and posterior thigh. This division of the sciatic nerve can occur as the nerve leaves Figure 6. Right sciatic nerve from the posterior thigh and seen from behind. the greater sciatic foramen, where it can be segregated into its Multiple interconnections are seen. tibial and common fibular components by the piriformis muscle. The upper and middle connections This is found in 4.0%e20.9% of specimens.13,14,28-30 There have represent Testut type A and F interconnections, and the lowest been no reports of neural communicating branches between the connections are not represented by tibial and common fibular nerves in the gluteal or thigh regions this classification system. after such early divisions.5,13,14 However, on 1 right side, we identified a high bifurcation of the sciatic nerve with clear communication between the tibial and common fibular parts in the gluteal region just inferior to the point at which the nerve was epineurial sheath of the sciatic nerve.1,3,14,17-26 On the contrary, split by the piriformis muscle. The connections found in our study such scant comments as could be found stated that no neural could have been missed in previous anatomic descriptions of the connections were present. For example, the 2nd edition of sciatic nerve because of the dissection necessary to access the Hollinshead’s Anatomy for Surgeons: Volume 3 (1969) states: “In spite common fibular and tibial nerves within the common epineurial of their [the tibial and common fibular nerves] common con- sheath.3,4,7,10,31-37 Additionally, though the intercommunications nective sheath, these 2 nerves do not interchange fibers.” The were prevalent, their disruption may go clinically unnoticed second edition of Nerve and Nerve Injuries by Sunderland (1978) potentially due to the nerve conveying only minor innervation. states: “There is no exchange of bundles between the 2 in the Alternatively, neural loss resulting from damage to the inter- sciatic trunk and so no difficulty is experienced in identifying and communications may have previously been attributed to compli- separating them macroscopically.” Sunderland’s statement has cations of either the lesion of the sciatic nerve that necessitated served as a foundation for anatomic and clinical studies of the the surgery or the surgery itself.

WORLD NEUROSURGERY 98: 176-181, FEBRUARY 2017 www.WORLDNEUROSURGERY.org 179 ORIGINAL ARTICLE R. SHANE TUBBS ET AL. SCIATIC NERVE INTERCOMMUNICATIONS: NEW FINDING

Figure 9. Neural histology of an intercommunication found between the tibial and common ﬁbular nerves as parts of the sciatic nerve in the posterior thigh.

Figure 8. Right-sided posterior dissection illustrating a sciatic nerve prematurely divided into medial tibial dissecting between the 2 nerves and stated that they should be and lateral common ﬁbular parts due handled independently. In the 2nd edition of Management of Pe- to the common ﬁbular nerve piercing the piriformis muscle (white arrow). A ripheral Nerve Problems (1998), the authors highlight a similar neural intercommunication (Testut point, stating: “At the junction of the upper and middle thirds of type A connection) is seen just distal the thigh, the common peroneal and posterior (sic) tibial nerves black arrow to the split ( ). have formed separate structures and can be easily separated into their components though they are loosely held together until approximately the mid-thigh.”

For lesions of the sciatic nerve, Kline et al21,22,38 describe sciatic nerve dissection so that the extent of a lesion can be CONCLUSIONS assessed by nerve action potential recordings. In order to Contrary to descriptions in common anatomy and surgical text- perform nerve action potential recordings for the tibial and books, we have demonstrated that neural communications be- common fibular components separately, the 2 divisions are split tween tibial and common fibular divisions of the sciatic nerve exist proximally and distally relative to the site of the sciatic nerve in most specimens. Knowledge of such connections could help to lesion.21,22 Both divisions are then independently stimulated, explain unusual clinical presentations and help surgeons avoid and recordings are monitored distal to the site of the lesion.21,22 injuring such fibers during surgical procedures involving the Millesi (1987) and Aydin et al. (2009) also indicated the ease of sciatic nerve.

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