Closure mechanism of the anal canal in women: assessed by three-dimensional ultrasound imaging.

Purpose: To describe the functional correlates of anal canal anatomy using 3 dimensional ultrasound imaging.

Methods: Ten nulliparous women were studied by using a 10-cm bag of 20-mm diameter. The bag was placed along the anal canal and inflated with 20 to 45 ml water, in 5-ml increments. At each volume, a three-dimensional ultrasound volume of the anal canal was obtained while the subjects were at rest and squeeze. The ultrasound images were analyzed to determine the relationship between the bag cross-sectional area and bag pressure.

Results: At low distension volumes, the bag is shaped like an "hourglass." The flared ends of the funnels correspond with the proximal and distal margins of the puborectalis muscle and external anal sphincter respectively. With increasing bag volumes, the length of completely closed segment of anal canal decreased. The last anal segment to open at rest was the one surrounded by all three structures. Anal contraction resulted in reduction of the anal canal cross-sectional area; the least compliant part of the anal canal was the one surrounded by external anal sphincter.

Conclusion: The internal anal sphincter, external anal sphincter, and puborectalis muscle are all involved in the anal canal closure function. During contraction, the external anal sphincter is the strongest component of anal canal closure mechanism.