Morphology of the levator ani muscle

This study was designed to show the morphology and to assess the function of the levator ani and the puborectalis by use of CT defecography in a sitting position. Previous studies have suggested that both muscles lift the anus.

CT defecography datasets from 30 healthy subjects (16 males; aged 22-24 years) were studied. Coronal scans of the pelvis were obtained during the three standard phases of rest, squeeze, and defecation; the multiplanar reconstruction images of the pelvic floor were gained on the workstation, included the coronal, sagittal, and puborectalis planes. Morphometric measurements were noted for the levator ani, puborectalis, genital hiatus, and anus.

The levator ani was funnel-shaped at rest in the sitting position; it ascended, becoming plate-shaped during squeeze, and it descended, becoming basin-shaped during defecation. The mean one-half puborectalis length was 73.8 mm and lower case "u"-shaped at rest, 63.5 mm and "v"-shaped during squeeze, and 94.9 mm and capital "U"-shaped during defecation. The mean area of the genital hiatus was 1,762 mm2 at rest, 1,332.4 mm2 during squeeze, and 3,126.8 mm2 during defecation. The mean inward lift of anus was 14.1 mm during squeeze; the mean downward movement of the anus was 18 mm during defecation.

The anus descends and there is no muscle to lift it during defecation; the results indicate that the puborectalis lifts the anus during squeeze. The levator ani muscle does not lift the anus; its main function is to open the genital hiatus and the anus during defecation. The main function of the puborectalis muscle is to shut the genital hiatus and anus during squeeze.