Anal Sphincter Complex

PURPOSE: Recent clinical studies on the anal sphincter complex have criticized the lack of reliable morphologic concepts. The purpose of this study was to determine the anatomy and histology of the anal sphincter complex with the help of undisturbed anatomic preparations. METHODS: The anal sphincter complex was studied in axial, sagittal, and coronal sections of human fetal, newborn, and adult pelves. RESULTS: The anal canal was surrounded by the internal sphincter, the longitudinal muscle layer, and an external sphincter that turned in to become continuous with the internal sphincter and with it to enclose the longitudinal muscle bundles. The classical tripartite subdivision of the external sphincter was not confirmed. The external sphincter seems not to be a complete circle in certain planes, neither in the male nor in the female. Sexual differences of the ventral part of the external sphincter were already present in fetuses. Large lamellated corpuscles were embedded within the interlacing smooth and striated muscles. Branches of the pudendal nerve innervated them. CONCLUSION: Our anatomic and histologic findings highly correlate with the results of magnetic resonance imaging and endosonography as well as with the physiologic findings. Furthermore, they are of great clinical importance for the understanding of sphincter defects during vaginal delivery and for anorectal operations in the adult as well as in the child.