Levator ani muscle stretch induced by simulated vaginal birth.
OBJECTIVE
To develop a three-dimensional computer model to predict levator ani muscle stretch during vaginal birth.


METHODS
Serial magnetic resonance images from a healthy nulliparous 34-year-old woman, published anatomic data, and engineering graphics software were used to construct a structural model of the levator ani muscles along with related passive tissues. The model was used to quantify pelvic floor muscle stretch induced during the second stage of labor as a model fetal head progressively engaged and then stretched the iliococcygeus, pubococcygeus, and puborectalis muscles.


RESULTS
The largest tissue strain reached a stretch ratio (tissue length under stretch/original tissue length) of 3.26 in medial pubococcygeus muscle, the shortest, most medial and ventral levator ani muscle. Regions of the ileococcygeus, pubococcygeus, and puborectalis muscles reached maximal stretch ratios of 2.73, 2.50, and 2.28, respectively. Tissue stretch ratios were proportional to fetal head size: For example, increasing fetal head diameter by 9% increased medial pubococcygeus stretch by the same amount.


CONCLUSIONS
The medial pubococcygeus muscles undergo the largest stretch of any levator ani muscles during vaginal birth. They are therefore at the greatest risk for stretch-related injury.
Lien
Kuo-Cheng
KC
Department of Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, USA.
Mooney
Brian
B
DeLancey
John O L
JO
Ashton-Miller
James A
JA
D000328:Adult; D003198:Computer Simulation; D036861:Delivery, Obstetric; D005260:Female; D006801:Humans; D008953:Models, Anatomic; D018482:Muscle, Skeletal / Q000503:physiopathology*; D017773:Pelvic Floor / Q000503:physiopathology*; D011247:Pregnancy