Angular force mechanics of bilateral accessory piriformis muscles with unilateral Type II sciatic nerve involvement

Introduction

The piriformis muscle coordinates external rotation and abduction of the thigh. The superior gluteal and sciatic nerves characteristically enter the gluteal region just superior and inferior to the piriformis, respectively. Variations in piriformis morphology can lesion these nerves but reports of concurrent piriformis aberrations with biomechanical analysis are scarce. The objective of the present study is to investigate a case of bilateral accessory piriformis muscles with concomitant unilateral sciatic nerve involvement via gross examination, biomechanical analyses of angular force mechanics, and intermuscular architectural comparability analyses.

Methods

The present case was discovered during routine human cadaver dissection. The aberrations were cleaned and photographed in situ. Attachment angles of the accessory piriformis muscles and split piriformis belly to the force axis of the typical piriformis were measured and recorded. The mean length of observed sarcomeres in each muscle was measured via light microscopy to calculate normalized maximal isometric forces (F_max) and atypical force vectors on the common piriformis tendon. An intermuscular architectural comparability analysis was also performed.

Results

The left and right accessory piriformis muscles (F_max = 6.52 N and 8.62 N) originated on the gluteal surfaces and inserted onto the tendons of the piriformis muscles (F_max = 46.25 N and 42.44 N) at 30.1° and 39.5°, respectively. Notably, the superior gluteal nerve coursed between the two piriformis muscles. Concurrently, the superior (common fibular) segment of the left sciatic nerve coursed through piriformis (Type II presentation), separating a small muscular belly (F_max = 3.14 N) which joined the piriformis tendon at 30.3° inferiorly, was architecturally similar to both accessory piriformis muscles, and generated 1.46 N of compressive force on the nerve.

Conclusions

Accessory piriformis muscles can influence ipsilateral hip mechanics by imposing atypical angular forces on the piriformis tendon and may instigate contralateral hip instability during the swing phase of the gait cycle (Trendelenburg sign) by lesioning the superior gluteal nerve. Concomitant aberrations, such as a Type II piriformis-sciatic complex, could also compress the sciatic nerve and elicit piriformis syndrome sequalae (e.g., sciatica). This case report may inform medical educators, clinicians, and anatomy researchers about these and other piriformis aberrations when deliberating related cases.