A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation

Abstract

The anatomy of the levator hiatus and tunnel was studied to elucidate their functional role in defecation, urination and continence. The material comprised 25 cadavers studied by dissection and serial histologic sections. The levator hiatus occupies the anterior portion of the levator plate which consists of 2 crura, that bound the hiatus, and 2 lateral masses. Three crural patterns could be identified: classic, crural overlap and crural scissor. The levator tunnel is a muscular tube which surrounds the intrahiatal organs along their way down from the levator hiatus to the perineum. It is double sheathed, with an inner coat of the suspensory sling and an outer of the puborectalis. Both coats are of striped muscle bundles. The inner coat is a tunnel dilator; the outer is a tunnel constrictor. The puborectalis not only acts as a common tunnel sphincter but provides an individual sphincter for each intrahiatal organ. A detailed study of the hiatal ligament which firmly binds the levator plate to the intrahiatal organs is presented. A tunnel septum could be identified to line the levator tunnel and separate it from the intrahiatal organs. Its surgical significance as a landmark for mobilizing the intrahiatal organs from within the tunnel is stressed. The levator plate consists of 2 functionally separate zones: a lateral visceral support zone and a medial dilator zone. The double sphincteric control provided to each intrahiatal organ by the individual and common sphincters suggest that unless both sphincters are destroyed, continence could be maintained by either. The role of the levator complex which comprises levator crura, tunnel and hiatal ligament in fixation of intrahiatal structures, as well as in mechanisms of defecation, urination and continence, is discussed. The understanding of the anatomic details of the levator hiatus and tunnel could be of value in mobilizing the intrahiatal structures from within the levator tunnel with preservation of their voluntary sphincteric mechanism.