Sciatic Nerve Origin, Variation, & Course
The sciatic nerve is the largest nerve in the body. It arises from the lumbosacral plexus in the pelvis. The ventral rami of L4 to S3 nerve roots unite to form the sciatic nerve. In this way, the sciatic nerve plus S4 is the lumbosacral plexus.
The sciatic nerve has two components: the tibial nerve and the common peroneal nerve. The tibial nerve arises from the ventral divisions of all roots in the lumbosacral plexus from L4-S3, which supplies the posterior muscles of the leg. The common peroneal nerve arises from the dorsal division from L4-S2, and therefore excludes S3. The common peroneal nerve supplies the anterior muscles of the leg.
The sciatic nerve ends just above the popliteal fossa by dividing into the common peroneal nerve and the tibial nerve.
The level of division of the sciatic nerve is variable. Division of the sciatic nerve usually occurs at the middle or the lower third of the thigh, but in about 10% of patients, the tibial nerve and the common peroneal nerve can separate at the greater sciatic foramen, which is higher up, near the buttocks.
There are four or more anatomic variations for the sciatic nerve in relation to the piriformis muscle. The majority of patients (88%) have a normal relationship between the sciatic nerve and the piriformis muscle, with the entirety of the sciatic nerve passing beneath the piriformis muscle. The piriformis can also be divided into two parts by the sciatic nerve, with the sciatic nerve passing between the two parts of the muscle, and this variation occurs in about 11% of patients. One rare (0.86%) variation is when the peroneal division of the sciatic nerve passes over the piriformis muscle and the tibial division of the sciatic nerve passes beneath the undivided piriformis muscle. The last, extremely rare, variation is the entire sciatic nerve passes through the divided piriformis muscle. This only occurs in 0.13% of patients.
The tibial nerve is the medial nerve, and it descends down vertically towards the tibia. Therefore, the common peroneal nerve is lateral (fibular). An easy way to remember this is that the tibial nerve is divided to the tibial aspect and so the common peroneal nerve is divided to the fibular aspect.
The sciatic nerve leaves the pelvis through the greater sciatic foramen, normally below and anterior to the piriformis muscle, to reach the gluteal region under cover of the gluteus maximus muscle. From here, the nerve runs downwards to the back of the thigh. The sciatic nerve lies between the greater trochanter and the ischium. As it descends, it passes anterior to the piriformis muscle and crosses posterior to the tendon of the gemellus muscles, the obturator internus, and the quadratus femoris muscles. Since the sciatic nerve is anterior to the piriformis but posterior to the obturator internus, the obturator internus is therefore between the retractor and the nerve when placing a sciatic nerve retractor in the lesser sciatic notch.
Then, in the thigh, the sciatic nerve descends on the posterior aspect of the adductor magnus muscle in a midline direction. In the upper part of the thigh, the sciatic nerve is covered by the long head of the biceps femoris muscle (the nerve is deep to the bicep femoris). Then, the nerve descends in the thigh close to the midline, crossed obliquely by the long head of the biceps femoris muscle. This crossing looks like an “X”. In the rest of the course in the thigh, the sciatic nerve is covered by the biceps femoris and hamstring muscles.
The sciatic nerve is closely related to the posterior aspect of the acetabulum and therefore can be injured due to hip dislocation or from traction or retractors during surgery.
High in the thigh, before the nerve divides, the sciatic nerve supplies the hamstring muscles, which are the long head of the biceps femoris, the semimembranosus, and the semitendinosus. It also supplies the ischial part of the adductor magnus muscle. All branches to the muscles in the thigh come from the medial side (tibial branch) of the sciatic nerve, except for the short head of the biceps femoris. The short head of the biceps femoris is innervated by the common peroneal nerve. Due to this, the lateral border of the sciatic nerve is safer during dissection or surgery. On the medial side, the sciatic nerve is accompanied by the posterior cutaneous nerve of the thigh and a small artery, called the inferior gluteal artery.
The sciatic nerve is close to the hamstrings tendon, about 1 cm lateral to the proximal hamstring tendons. Therefore during surgery, the nerve should be identified, dissected away, and neurolysis may be needed, especially in cases of chronic repair of the hamstrings, such as cases that require mobilization of the retracted tendons. These chronic cases may cause an increased difficulty in neurolysis of the nearby sciatic nerve.
