Surgical anatomy of the somatic terminal innervation to the anal and urethral sphincters: role in anal and urethral surgery.

PURPOSE: The gross anatomy of the pudendal nerve branches was studied to identify more precisely the neuroanatomical relationship in the region of the anal canal, bladder neck and proximal urethra. Such knowledge is essential for the development of surgical techniques that avoid nerve injury in sphincteroplasty for anal and urinary stress incontinence, and in pudendal canal decompression.

MATERIALS AND METHODS: The pudendal nerve terminal branches were dissected in 7 female and 5 male formalin fixed cadavers, including 6 fully mature neonates and 6 adults, a mean age of 37.6 years. The nerves were traced from the pudendal nerve to their termination in the anal and urethral sphincters, and pelvic floor muscles.

RESULTS: The inferior rectal nerve occupied the lower half of the ischiorectal fossa. Immediately after emerging from the pudendal canal it extended a motor branch to the levator ani muscle and the cutaneous perianal and scrotal branches. The nerve terminated in the external anal sphincter at the 3 and 9 o'clock positions. Inside the pudendal canal the perineal nerve gave rise to a scrotal branch which joined the scrotal branch of the inferior rectal nerve to form the common scrotal nerve. About 2 to 3 cm. from the pudendal canal the perineal nerve extended a branch to the bulbocavernosus muscle and divided into the terminal scrotal and motor branches, which penetrated the striated urethral sphincter at the 3 and 9 o'clock positions. The deep dorsal nerve of penis or clitoris coursed forward into the ischiorectal fossa, emerged from the deep perineal pouch and penetrated the suspensory ligament to the dorsum of the penis or clitoris.

CONCLUSIONS: The identification of the precise anatomical relation of the somatic nerve termination to the anal and urethral sphincters seems vital to avoid sphincter denervation during surgery for the correction of fecal and stress urinary incontinence.