Counter-nutation movement is deemed crucial during the management of the birth process. It is a combination of lateral ilia expansion and backward displacement of the promontory resulting from the external rotations of the femurs producing an enlargement of the pelvic inlet. However, since its description by Farabeuf, this mechanism has never been challenged and analyzed in a dynamic finite element study.
Based on a female pelvic mesh and sacroiliac ligaments, we simulated external rotations of both femurs with imposed rotation of the two acetabulum centers. We hypothesize that lateral ilia expansion generates a sacrum movement resulting in a backward displacement of the promontory and a pelvic inlet enlargement.
Finite element simulation confirms our hypothesis and reveals that ilio-sacro-transverse and axile ligaments play an essential role in this mechanism. Indeed, the increase in stiffness (ranging from 500 MPa to 750 MPa) of these ligaments accentuates the counter-nutation movement and the opening of the inlet. Instead of the anatomic congruence between the ilium and the sacrum, the sacroiliac ligaments may explain the counter-nutation. After a 6° of femur rotation, the inlet area increases to 11 cm ² (141 cm ² vs. 130 cm ² ). This enlargement could be noteworthy in case of obstructed labor or shoulder dystocia. Moreover, the association between external rotation and flexion of the femurs could be more efficient for opening the pelvic inlet.
Our result did not support the original assumption of Farabeuf. By revealing how postural adjustment increases the bony birth canal, this study provides essential information for the clinical management of the delivery.