Dynamic imaging of the normal pelvic floor.
PURPOSE: To provide quantitative data by a modern cross-sectional imaging technique (CT) for defining normal physiological values of pelvic floor structures. PATIENTS AND METHODS: Twenty seven subjects, 7 males, 20 females, aged 20-75 yrs (mean 46.3 +/- 5 yrs) without pelvic floor or defection dysfunction underwent Direct Coronal (DC) CT scanning of the pelvis with the patient seated instead of lying. Scans obtained at rest and on straining were compared by bony landmarks. Three anatomical compartments, i.e. anterior, middle and posterior, were identified by two planes drawn tangential to the ischial foramina and the ischial tuberosities, respectively. Measurements of (1) Levator ani muscle length (mm); (2) Levator-anal angle (degrees); (3) Rectal floor-to-ischial line distance (mm) and (4) Supra/Infralevator spaces (square cm) were independently performed twice by two radiologists. The statistical analysis included calculation of intra and interobserver agreement (correlation coefficient). The differences between the means of the resting and straining values from each compartment (Student's t test) and the correlation between parameters (Pearson's coefficient) to evaluate whether resting values allowed a prediction of those on straining were determined. RESULTS: DC scans of diagnostic quality were obtained in all but two patients (92.5%). Both intra- and interobserver agreement indices were always greater than 80% (except for a 0.63 value by one observer obtained in the infralevator space from the anterior compartment at rest). A significant difference between the resting and straining values of all parameters was noted in the three compartments. At rest the levator ani muscle length was significantly shorter and the supralevator space smaller in the posterior compartment (48.3 +/- 7.9 mm vs 48.8 +/- 7 mm vs 42.6 +/- 9.4 mm, P < 0.05 and 70.6 +/- cm2 vs 66.9 +/- 11.5 cm2 vs 27.2 +/- 4.8 cm2, P < 0.01 anterior, middle and posterior respectively). On straining, these two parameters increased by +42% and +17.8%, respectively, in the same compartment, while the most pronounced variation of the infralevator space occurred in the middle compartment (-51.1%). The increase in the supralevator space correlated with a decrease in the rectal floor-to-ischial line distance and widening of the levator-anal angle (r = -0.64, P < 0.01 and 0.48, P < 0.05, respectively). A close correlation between resting and straining values was observed in all parameters, especially in the supralevator space in the three compartments (r = 0.82, 0.93 and 0.88, P < 0.01). CONCLUSIONS: Direct Coronal CT scanning showed that on straining the posterior component of the levator ani muscle, i.e. the coccygeus muscle, undergoes "physiological overstretching" and the supralevator space acts as a "compliant cavity", whose behaviour can be predicted at rest.