Novel Application of Photogrammetry to Quantify Fascicle Orientations of Female Cadaveric Pelvic Floor Muscles

Abstract

Although critical for understanding and simulating pelvic floor muscle function and pathophysiology, the fascicle arrangements of the coccygeus and levator ani remain mostly undetermined. We performed close-range photogrammetry on cadaveric pelvic floor muscles to robustly quantify surface fascicle orientations. The pelvic floor muscles of 5 female cadavers were exposed through anatomic dissections, removed en bloc, and photographed from every required angle. Overlapping images were mapped onto in silico geometries and muscle fascicles were traced manually. Tangent vectors were calculated along each trace; interpolated to define continuous, 3D vector fields; and projected onto axial and sagittal planes to calculate angles with respect to the pubococcygeal line. Contralateral and ipsilateral pelvic floor muscles were compared within each donor (Kuiper's tests) and using mean values from all donors (William-Watsons tests). Contralateral muscles and all but one ipsilateral muscle pair differed significantly within each donor (p < 0.001). When mean values were considered collectively, no contralateral or ipsilateral statistical differences were found but all muscles compared differed by more than 10° on average. Close-range photogrammetry and subsequent analyses robustly quantified surface fascicle orientations of the pelvic floor muscles. The continuous, 3D vector fields provide data necessary for improving simulations of the female pelvic floor muscles.

Keywords: Close-range photogrammetry; Coccygeus; Iliococcygeus; Muscle fascicles; Pubovisceralis; Vector fields.

FIGURE 1.

(a) Sample images of two excised pelvic floor muscle (PFM) cones used to create photogrammetric reconstructions. (b) A virtual, textured surface model of a PFM cone demonstrating the quality of the photogrammetric reconstructions. (c) A virtual PFM cone (light blue) visualized with the exported PFM fascicles (red) that were traced manually from (b). (d) Three images depicting the axial (red) and sagittal (blue) planes and their orthogonality. The dark blue line connects the pubic symphysis and coccyx landmarks, the green line connects the ischial spines, and the light blue line demonstrates the vector that was rotated from the sagittal plane to define the axial plane. (e) Resulting tangent vectors calculated for the right iliococcygeus for the same PFM complex shown in (c) from a similar view. This can be referred to as a discrete vector field.

FIGURE 2.

A photogrammetric reconstruction of a pelvic floor muscle cone shown next to its virtual geometry segmented from MRI. The color map depicts good shape conformity between the photogrammetric reconstruction and pelvic floor muscle complex segmentation as determined by the low co-registration error.

FIGURE 3.

The donor-specific continuous, 3D vector fields defining the entire pelvic floor muscle complex (coccygeus and levator ani) from an anterior-superior view. The corresponding left and right pelvic floor muscles are shown from a medial, sagittal view. Relevant anatomical directions are indicated below each column and each pubococcygeal line (white) is visualized connecting the pubic symphysis landmark (red point) to the tip of the coccyx (white point). The color of each vector is indicative of its 3D orientation: vectors parallel to the pubococcygeal line (anteriorly-posteriorly oriented) are red, left-right vectors are blue, inferiorly-superiorly oriented vectors are green, and those colors are blended to label orientations in between.

FIGURE 4.

Polar histograms depicting the distributions of axial and sagittal angles for each pelvic floor muscle (coccygeus, iliococcygeus, and pubovisceralis from top to bottom, respectively, in each section) and donor. Red indicates right-sided pelvic floor muscles and angles while blue represents those on the left. The black lines denote the mean angle and the standard deviation of the mean. Angle values are shown with respect to the bony pelvis and anatomical directions on the left.

FIGURE 5.

Polar histograms similar to those described in Fig. 4 but generated with angles combined from all 5 donors for more generalizable comparisons of contralateral and ipsilateral pelvic floor muscle fascicle distributions.