Gabriella Bargon, Sue Deacon, Sharon Tsang, Lena Wallin
Piriformis
An anomaly of the piriformis was identified, in which a superficial and deep belly existed. These bellies joined to form a common tendon prior to its insertion onto the femoral head. The common peroneal nerve exited between these two bellies while the tibial nerve passed underneath the deep muscle belly. The common peroneal and tibial nerves united in a common fascial sheath 4cm distal to the inferior margin of the piriformis muscle. The prevalence of this type of anomaly in the general population is approximately 20% (Chen 1994). The piriformis tendon inserted onto the superoanterior aspect of the greater trochanter. In neutral, the muscle will exert an external rotation force. However, when the hip is flexed to approximately 90 degree, the function will be reversed because the tendon will have moved to a position distal to the coronal axis of rotation and internal rotation will occur. Given the orientation of the piriformis muscle, it appears that it would also play a role in abduction, particularly through positions of hip flexion as the muscle becomes parallel to the line of the femur.
