Gabriella Bargon, Sue Deacon, Sharon Tsang, Lena Wallin
Obturator internus
The tendon of obturator internus was clearly identifiable, running in the same plane as the gemelli. The entire superior gemelli muscle blended with the obturator internus. In contrast, the proximal, inferior gemelli was separated by fascia from the obturator internus until it merged onto the obturator tendon more distally. The anterior portion of obturator internus was exposed by a semicircular cut along the muscle belly. Once reflected, tendinous bands were visible on its undersurface. The muscle made a right angle through the lesser sciatic notch, which enhances its mechanical efficiency. A bursa was identified deep to the obturator internus, which facilitates the smooth gliding of the muscle. A fine fascial layer was identified running perpendicular to the muscle fibers, from the superior gemelli to the inferior gemelli. The authors propose that this fascia maintains the tension within the obturator internus when the hip extends and external rotates. Furthermore, it is likely that these muscles will behave similarly to the piriformis muscle, given the orientation of their fibers and the location of their insertion onto the femoral head. In a neutral hip position, they will function as external rotators. As with piriformis, their common tendon will translate inferiorly to a position below the axis of rotation, which may explain the reversal of their functions as previously outlined in the biomechanics studies (Delp et al 1999, Dostal et al 1986). They are also likely to exert an abduction force; however, the authors speculate that it would occur at a greater degree of hip flexion than with piriformis. This theory is based on the fact that the fibers of obturator internus and the gemelli are less vertically directed than occurs with the piriformis muscle and therefore, increased hip flexion would occur before these muscles would become parallel to the femur.
