The morphological variability of the piriformis muscle

Background: The aim of the study is to create several classifications of the piriformis muscle (PM): proximal and distal attachments, potential fusions, and the relationship with the sciatic nerve. It is the first comprehensive anatomical examination of this subject. Materials and methods: One hundred and twenty-four lower limbs from 62 cadavers, fixed in 10% formalin, were examined. Results: The piriformis muscle was present in 120 limbs (96.8% of cases). Four types of proximal attachment were described (I–IV). The most common type was Type I, in which the proximal attachment was at the anterior surface of the sacrum, between S2 and S4 (52 lower limbs; 43.3%). The rarest type was Type IV, in which the proximal attachment was at the gluteal surface of the ilium near the margin of the greater sciatic notch and from the gluteus medius (12 cases; 10%). Three types of distal attachment were distinguished. The most common was Type 1, a single tendon. This type comprised two subtypes: A and B (105 lower limbs; 87.5%). The other two types accounted for 12.5% of the total. Fusions were noted between the piriformis muscle and adjacent muscles in 31.7%. Four patterns were observed in which the sciatic nerve ran against the piriformis muscle. The most common variation in the relationship was the common fibular nerve exiting superior to the piriformis muscle and the tibial nerve passing inferior to it (10 cases; 8.3%). Conclusions: The piriformis muscle is highly morphologically variable in both its proximal and distal attachments and its relationship with the sciatic nerve. There are four types of proximal attachment and three types of distal attachment. The piriformis muscle shows numerous fusions with its adjacent muscles: gluteus medius or minimus or superior gemellus. A new (fourth) type of relationship was demonstrated between the piriformis muscle and sciatic nerve. The piriformis muscle was absent in four cases.