Gluteal Nerves

SCIATIC/POSTERIOR FEMORAL CUTANEOUS NERVES

Anatomy

The sciatic and the gluteal nerves share common derivations originating from the anterior rami of the fourth lumbar through the third sacral nerve roots, immediately forming the lumbosacral plexus. The sciatic nerve (SN) is a very large single elliptic trunk, 2.0 cm in diameter, that inclines laterally beneath the gluteus maximus muscle while resting on the posterior ischium and the nerve to the quadratus femoris. The posterior femoral cutaneous nerve (PFCN), lying immediately adjacent to the medial edge of the sciatic nerve, provides cutaneous innervation to the posterior thigh. Concomitantly, the sciatic nerve is also accompanied by the inferior gluteal artery (IGA), providing primary blood supply to this nerve. On reaching a point about midway between the ischial tuberosity and the greater trochanter, the SN turns downward over the gemelli, the obturator internus tendon, and the quadratus femoris muscle, separating it from the hip joint to exit the buttock and enter the thigh beneath the lower border of the gluteus maximus, then emerging from the pelvis through the sciatic notch. Here it is found lying just anterior to the piriformis muscle; however, in about 10% to 15% of individuals, part or all of the SN pierces the piriformis muscle.

After passing through the sciatic notch, the sciatic nerve descends into the thigh, where it innervates the semitendinosus (L5, L4-S2), semimembranosus (L5), biceps femoris (S1, 2), and distal part of the adductor magnus (L5) muscles. The sciatic nerve then descends near the midposterior thigh, initially directly posterior to the adductor magnus, the distal portion of which it also innervates. It soon travels obliquely over the long head of the biceps femoris. Just above the apex of the popliteal fossa, it is overlapped by the contiguous margins of the biceps femoris and semimembranosus muscles.

The sciatic nerve trunk has two well-defined divisions, namely the lateral fibular (peroneal), derived from the anterior divisions of the anterior rami of the L4-S2 roots and medial tibial derived from the posterior divisions of the anterior rami of the L4-S3 nerve roots. The tibial division innervates all posterior thigh muscles, with the exception of the short head of biceps femoris, which is innervated by the fibular division. In approximately 90% of individuals, these two divisions share a common sheath from the pelvis to the popliteal fossa. However, in 10% of individuals, the anatomic separation of the sciatic divisions occurs higher in the thigh. Rarely, the common fibular and tibial nerves arise independently from the sacral plexus itself, pursuing similar courses until truly separating at the apex of the popliteal fossa into its two terminal branches, the common fibular (peroneal) and tibial nerves.

Clinical

Acute proximal sciatic neuropathies manifest with distal leg weakness affecting both fibular- and tibial-innervated muscles—in the most severe instances, leading to severe footdrop (secondary to weakness of the tibialis anterior) and weakness of eversion (peroneus longus muscles), plantar flexion (gastrocnemius), and inversion (tibialis posterior muscles). Concomitantly, the more proximal SN-innervated hamstring muscles are weakened. The ankle jerk and hamstring muscle stretch reflexes are usually depressed or absent with primary SN lesions. Sensory loss and painful dysesthesias of the sole and dorsum of the foot and posterolateral lower leg are common concomitant sensory findings. Sciatic nerve lesions in children are as frequent as fibular nerve lesions, in contrast to adults, where the latter are much more prevalent.

In the setting of an acute gluteal compartment syndrome, secondary to an expanding hematoma compressing the SN, there is often increasingly severe pain within the buttocks. The sciatic notch and gluteal musculature must be palpated to search for tenderness or fullness compatible with a hematoma or other infiltrating lesion. Occasionally, because of the SN’s fascicular anatomy, a proximal sciatic neuropathy manifests with a more predominant fibular division deficit manifested by an isolated footdrop requiring differentiation from the more common fibular (peroneal) neuropathy (FN) at the fibular head. The presence of clinical or electromyographic (EMG) evidence of hamstring weakness helps differentiate between primary SN and FN lesions. Involvement of the gluteal muscles will confirm a proximal lumbosacral plexus or L5, S1 nerve root lesion with combined sciatic and gluteal nerve involvement.

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