Introduction

The sciatic nerve is the largest nerve in the human body. It originates from L4-L5 and S1-S3 spinal segments of the spinal cord and travels posteriorly through the lower limb providing significant portion of innervation to this anatomical region [1, 2]. It provides motor fibers to the posterior compartment of the thigh including biceps femoris, semimembranosus, semitendinosus, adductor magnus. At the level of the popliteal fossa it diverges into the tibial and common peroneal nerve [2]. The tibial nerve provides innervation to the posterior compartment of the leg and foot, while the peroneal nerve innervates the anterior and lateral compartments of the leg and foot [1, 2].

The surgical procedures at the infragluteal region and transgluteal approaches to the sciatic nerve involves different incisions, level of dissection and are based on good knowledge of regional anatomy [3]. Moreover, the low expectations of sciatic nerve repair in the past are replaced by more optimistic approach due to the advances of microsurgery and treatment algorithms [4].

The current case demonstrates an unusual and rare developmental variation of the sciatic nerve.

Case Description

During anatomical dissection of a 75 years-old cadaver, the course of the branches of the sacral plexus, the features of their origin and innervation zones were studied. An atypical variant of the course and branching of the sciatic nerve was encountered. Within the pelvis, the branches of the sacral plexus had a connecting branch with the branches of the lumbar plexus. Thus, the obturator nerve, related to the lumbar plexus, formed by the connection of the spinal nerves L2-L4, had a connecting branch with the sacral plexus (Fig. 1).

Figure 1: The communicating branch connecting the obturator nerve with the sacral plexus: 1 - lateral cutaneous nerve of the thigh, 2 - ilioinguinal nerve, 3 - femoral nerve, 4 - genitofemoral nerve, 5 - external iliac artery, 6 - external iliac vein, 7 - obturator nerve, 8 - communicating branch, 9 - sacral plexus.

In the pelvic cavity, the formation of the nerve corresponded to the classical description. Then the nerve left the pelvis through the greater sciatic opening and enters the posterior part of the thigh behind the internal obturator muscle and the quadratus femoris with two roots, which merged after a small interval into a single 4 cm long trunk. Then the trunk again divided into two roots, which again united into a powerful trunk, finally dividing into the common peroneal and tibial nerves only in the popliteal fossa (Fig. 2). Along its course, the nerve gave branches to the semitendinosus, semimembranosus, biceps femoris and adductor magnus muscles.

Figure 2: The sciatic nerve on the back of the thigh, represented by two roots that connect twice into a single trunk: 1 – Sciatic nerve, 2 – Posterior cutaneous nerve of the thigh.

After the bifurcation of the sciatic nerve, the tibial nerve passed between the heads of the gastrocnemius muscle into the tibial popliteal canal. The nerve followed behind the medial malleolus and exited onto the sole of the foot, dividing into two branches: the medial and lateral plantar nerves. Within the popliteal fossa, the medial cutaneous nerve of the calf branches off from the tibial nerve, which connected with the lateral cutaneous nerve of the calf (a branch of the common peroneal nerve) via the peroneal communicating branch, forming the sural nerve. As a rule, the formation of the sural nerve occurs in the lower third of the leg. In this case, the formation of the sural nerve occurred within the popliteal fossa (Fig. 3).

Figure 3: Nerves of the popliteal fossa and the back of the leg. Formation of the sural nerve: 1 - tibial nerve, 2 - common peroneal nerve, 3 - medial cutaneous nerve of the calf, 4 - peroneal communicating branch, 5 - formation of the sural nerve, 6 - lateral cutaneous nerve of the calf, 7 - sural nerve, 8 - lateral calcaneal branches, 9 - lateral dorsal cutaneous nerve of the foot

The common peroneal nerve ran laterally and downwards. Within the popliteal fossa, the lateral cutaneous nerve of the calf branched off from it. Further, the common peroneal nerve divided into two branches: the superficial peroneal nerve and the deep peroneal nerve.

The superficial peroneal nerve ran downwards along the anterolateral surface of the leg, giving off branches to the peroneal muscles. At the level of the lower third of the leg, the superficial peroneal nerve divided into two branches: the medial dorsal cutaneous nerve and the intermediate dorsal cutaneous nerve.

The deep peroneal nerve gave off branches to the long extensor of the fingers, to the anterior tibialis muscle and to the long extensor of the big toe. When passing to the dorsum of the foot, the deep peroneal nerve divided into two branches - lateral and medial. On the dorsum of the foot, the branches of the superficial peroneal nerve connected with the trunk of the deep peroneal nerve, and laterally with the terminal branches of the sural nerve (Fig. 4). Therefore, on the dorsum of the foot, a superficial nerve plexus was formed for the innervation of the skin of the dorsum of the foot and the dorsal venous network. The nerves exchange fibers and formed overlap zones.

Figure 4: Connection of the branch of the superficial peroneal nerve with the trunk of the deep peroneal nerve: 1 - superficial peroneal nerve, 2 - intermediate dorsal cutaneous nerve of the foot, 3 - medial dorsal cutaneous nerve of the foot, 4 - connecting branch of the superficial peroneal nerve with the trunk of the deep peroneal nerve, 5 - deep peroneal nerve

A schematic representation of the anatomy is indicated in figures 5 and 6.

Figure 5: Schematic representation of the anatomy.

Figure 6: Schematic representation of the anatomical variation.

Discussion

In most specimens, the sciatic nerve appears as a thick, single trunk emerging from the infrapiriform opening; its division occurs in the lower 1/3 of the thigh or in the popliteal fossa. Berihu and Debeb, 2015 examined 56 lower limbs from 28 formalin-fixed cadavers (22 males and 6 female) to study the variant anatomy of the sciatic nerve. Of the 56 limbs (100%), 42 (75%) had normal sciatic nerve anatomy. Fourteen lower limbs (25%) had variations in nerve topography. Of the fourteen lower limbs (25%), six (11%) had variations in the nerve in relation to the piriformis muscle, of which in five limbs (9%) the common peroneal and tibial nerves arise separately below the piriformis muscle and join at the level of the quadratus femoris. Following distally, the common trunk bifurcates in the upper angle of the popliteal fossa. In the remaining six cases (2%), the common peroneal nerve emerges above the piriformis muscle, and the tibial nerve below. They follow down the thigh separately from each other. In the remaining 8 lower limbs (14%), variations of the nerve in relation to the popliteal fossa were detected. In three cases (5%), a bifurcation of the sciatic nerve into three branches was detected: the tibial, common peroneal, and accessory nerves. The latter was divided into the lateral cutaneous nerve of the leg and the peroneal communicating branch. Of the 8 lower limbs (14%), 5 limbs (9%) were divided into three branches: the tibial, common peroneal, and sural nerves. In this case, the sural nerve on the three lower limbs originates directly from the common peroneal nerve on the left, and on the other two lower limbs, it originates directly from the tibial nerve on the left and right [5].

Feduro and Matyushchenko, 2021 studied ten fixed cadavers as part of routine dissection. In one case, an atypical variant of the sciatic topography similar in some way to our case was found on the right. In the pelvic cavity, the formation of the nerve corresponded to the classic description. At the level of the piriformis muscle, the nerve divided into two trunks. One trunk emerged from the suprapiriform opening, and the other from the infrapiriform opening. Having rounded the piriformis muscle, these two branches merged into a single trunk, 10 cm long. Having reached the popliteal fossa, the nerve divided into the common peroneal and tibial nerves [6]. The two nerves often pass close to each other, but do not share fibers [7].

The nervous system is characterized by the presence of multiple variations [9]. Beaton and Anson's classification system is still used to report variations of the sciatic nerve, however additional types of variation, or alternative classification systems have also been proposed, providing examples that do not fall within the criteria of the original six categories (Table 1) [10].

Conclusions

The sciatic nerve is the largest nerve in the human body. To our knowledge there is only one report of a similar variation when the sciatic nerve bifurcates and then merges until its division into the tibial and fibular nerves.

References

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