Objective: Develop a method to quantify the cross- sectional area of the pubic portion of the levator ani muscle, validate the method in women with unilateral muscle defects, and report preliminary findings in those women. Method: Multi- planar proton density magnetic resonance images of 12 women with a unilateral defect in the pubic portion of their levator ani were selected from a larger study of levator ani muscle anatomy in women with and without genital prolapse. Three- dimensional bilateral models of the levator ani were reconstructed (using 3- D Slicer, version 2.1b1) and divided into iliococcygeal and pubic portions. Muscle crosssectional areas were calculated at four equally spaced locations perpendicular to a line drawn from the pubic origin to the visceral insertion using the I- DEAS computer modeling software. Results: The cross- sectional area of the muscle on the side with the defect was smaller than the normal side at all the four locations. The average bilateral difference was up to 81% at location 1 (nearest pubic origin). Almost all of the volume difference (13.7% , P = 0.0004) was attributable to a reduction in the pubic portion (24.6% , P < 0.0001), not the iliococcygeal portion (P = 0.64), of the muscle. Conclusions: A method was developed to quantify cross- sectional area of the pubic portion of the levator ani perpendicular to the intact muscle direction. Significant bilateral cross- sectional area differences were found between intact and defective muscles in women with a unilateral defect.
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