Measurement of the pubic portion of the levator ani muscle in women with unilateral defects in 3‐D models from MR images

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Abstract
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 cross‐sectional 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.