Comparison of muscle fiber directions between different levator ani muscle subdivisions: in vivo MRI measurements in women

Abstract

Introduction and hypothesis: This study describes a technique to quantify muscle fascicle directions in the levator ani (LA) and tests the null hypothesis that the in vivo fascicle directions for each LA subdivision subtend the same parasagittal angle relative to a horizontal reference axis.

Methods: Visible muscle fascicle direction in the each of the three LA muscle subdivisions, the pubovisceral (PVM; synonymous with pubococcygeal), puborectal (PRM), and iliococcygeal (ICM) muscles, as well as the external anal sphincter (EAS), were measured on 3-T sagittal MRI images in a convenience sample of 14 healthy women in whom muscle fascicles were visible. Mean ± standard deviation (SD) angle values relative to the horizontal were calculated for each muscle subdivision. Repeated measures ANOVA and post-hoc paired t tests were used to compare muscle groups.

Results: Pubovisceral muscle fiber inclination was 41 ± 8.0°, PRM was -19 ± 10.1°, ICM was 33 ± 8.8°, and EAS was -43 ± 6.4°. These fascicle directions were statistically different (p < 0.001). Pairwise comparisons among levator subdivisions showed angle differences of 60° between PVM and PRM, and 52° between ICM and PRM. An 84° difference existed between PVM and EAS. The smallest angle difference between levator divisions was between PVM and ICM 8°. The difference between PRM and EAS was 24°. All pairwise comparisons were significant (p < 0.001).

Conclusions: The null hypothesis that muscle fascicle inclinations are similar in the three subdivisions of the levator ani and the external anal sphincter was rejected. The largest difference in levator subdivision inclination, 60°, was found between the PVM and PRM.

Fig. 1

a Midsagittal MRI view of the muscles from the left side of the pelvis. The sacrococcygeal inferior pubic point (SCIPP) line is drawn in the midsagittal plane and transposed to all parasagittal slides. b Fibers are demonstrated (lines with round tips) on a parasagittal slide. c Fiber directions were marked and evaluated in respect of the individual SCIPP line and expressed as the angle to the average horizontal line, which is 34° below the SCIPP line with the Pelvic Inclination Correction System (PICS) system. Fiber orientations subtending an angle clockwise to the horizontal line have a negative sign; those with an angle counter-clockwise to the horizontal line have a positive sign. PVM pubovisceral muscle, PCM pubococcygeal muscle, ICM iliococcygeus muscle, PRM puborectal muscle, EAS external anal sphincter muscle

Fig. 2

The thick arrow displays the mean direction to the horizontal line in a two-dimensional graphic. The dashed line is the horizontal line from which the angles are measured. Angles above the horizontal line have a “+” sign and those below the horizontal line a “−” sign. On MRI, the PVM was found medial to the PRM; for graphical reasons their lines of actions are depicted in the same plane

Fig. 3

Horizontal and vertical components of the PVM and PRM in the standing position. The thick arrows show the average direction of the lines of action of the PVM and PRM muscles relative to the horizontal with a theoretical 1 N force. Thin lines indicate the portion of each force related to a closing and lifting function. (Note: vectors are shown larger than the background anatomy to avoid an overlap in the display)