A new concept of the anatomy of the anal sphincter mechanism and the physiology of defecation. VIII. Levator hiatus and tunnel: anatomy and function.

The anatomy of the levator hiatus and tunnel has been studied, aiming at the elucidation of their functional role in mechanisms of 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 two "crura," that bound the hiatus, and two "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 hiatius 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," whereas 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 two functionally separate zones: a lateral "visceral support" zone and medial "dilator" one. The double sphincteric control provided to each intrahiatal organ by the "individual" and "common" sphincters would 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.