Anatomy of Anal Sphincters and Related Structures in Continent Women Studied with Magnetic Resonance Imaging

Five anally continent nulliparas of reproductive age were studied with magnetic resonance imaging. The internal and external anal sphincters could be easily delineated, as could the intervening longitudinal musculature, puborectalis muscle, anococcygeal raphe, anorectal lumen, vagina, uterus, bladder, urethra, coccyx, and pubis. The shape of the sphincters was nearly cylindrical, with an anterior component averaging 18.3 mm thick and 28.0 mm long. Fifty-four percent of this anterior thickness was attributable to the internal sphincter. The anorectal angle varied considerably, with a mean of 86.8 +/- 19.1 degrees (range 60-112). The angle between the portion of the rectal lumen supported by the anococcygeal raphe, or levator plate, and the plane of the puborectalis muscle was consistent at 149.0 +/- 6.3 degrees (138-154). The finding of anterior anal sphincters with substantial thickness and length contrasts markedly with a view often pictured in the literature of a female anal sphincter that narrows anteriorly to half its posterior length and forms a small bundle of muscle rather than a broad band. Knowledge of these relationships is important in primary repair of obstetric sphincter lacerations as well as in surgical correction of anal incontinence.