Morphological variations of sciatic nerve and piriformis muscle in gluteal region during fetal period

Intramuscular drug injection in the gluteal region is often the most frequent cause of sciatic nerve injury in preterm newborns. Local anatomic variation is one of the predisposing causes of iatrogenic sciatic nerve injury. The aim of this morphological study was to assess the relationship of sciatic nerve with the piriformis muscle and to elucidate variations of fusion of piriformis with neighboring muscles in the gluteal region of Indian human preterm fetuses. Four types of relationship of the sciatic nerve with the piriformis muscle were observed in one hundred gluteal regions of fifty spontaneously aborted, formalin-fixed fetuses, aged 20 to 36 week (24 males and 26 females). In 85% of the gluteal regions, the classic pattern was found, in which the two components of the sciatic nerve fuse with each other proximal to the piriformis, and the fused sciatic nerve emerges at the lower border of the piriformis. In the remaining 15% of the gluteal regions, variations in relationship were found. The most common variation, characterized by the passage of the common peroneal component through the piriformis and the emergence of the tibial part at the lower border of the piriformis, was seen in 9% of the gluteal regions. Common peroneal and tibial components passed above and below the muscle respectively in 3%, and the unsplit sciatic nerve passed through the piriformis in 3% of the gluteal regions. Four types of fusion of the piriformis with the neighboring muscles were seen: namely, no fusion; fusion with superior gamellus; fusion with gluteus medius, or fusion with gluteus medius and obturator internus complex in 28%, 43%, 26% and 3% of the gluteal regions respectively. Anatomical variations of the sciatic nerve in relation to the piriformis muscle should be kept in mind while performing medical or surgical interventions in this region.