Anatomical components of continence mechanism in males
Effective urethral closure and continence at rest and during periods of increased intra-abdominal pressure depends on several anatomical factors including: well-vascularized urethral mucosa and submucosa, properly functioning striated and smooth urethral sphincter, pelvic floor muscles and fasciae .
The male urethra is 18-20 cm long and extends from the bladder neck to the external meatus at the end of the penis. It may be considered in two parts (Fig. 1):
Figure (1): Anatomy of male urethra.
The anterior urethra
It is about 16 cm long and surrounded by the corpus spongiosum. It is subdivided into:
The bulbar urethra which is more proximal, surrounded by the Bulbospongiosus muscles and lie entirely within the perineum.
The pendulous urethra which is distal and continues to the tip of the penis .
The posterior urethra
It is about 4 cm long and lies in the pelvis proximal to the corpus spongiosum. The posterior urethra is divided into:
The pre-prostatic part of the urethra is about 1 cm long, extends from the base of the bladder to the prostate and surrounded by the proximal (internal) urethral sphincter.
The prostatic part is the widest and passes through the prostate.
The membranous (sphincteric) part is the shortest and narrowest part. In the deep perineal pouch, it is surrounded by distal (external) urethral sphincter. The membranous urethra is called “sphincteric urethra” as it comprises both striated and smooth muscle components that provide continence at this level .
The urethral mucosa and submucosa
The male urethral mucous membrane is continuous with the transitional epithelium of the bladder. The submucous tissue consists of a vascular layer containing longitudinally arranged collagen fibers and elastin fibers surrounded by a layer of circular smooth muscle .
The male urethral sphincter complex (Fig. 2)
The male urethral sphincter complex is composed of: proximal (smooth) sphincter, distal sphincter (mainly striated) and pelvic ﬂoor (Peri-urethral) muscles .
Figure (2): Male urethral sphincter complex. PUS (lissosphincter) extends from the bladder neck through the prostatic urethra above the verumontanum. DUS (rhabdosphincter) extends from the prostatic urethra below the veru-montanum through the membranous urethra surrounded by Peri-urethral skeletal muscle (Pelvic floor) .
The Proximal (internal) (lisso-) urethral sphincter (PUS)
The proximal urethral sphincter consists of a relatively thick inner longitudinal layer, a thinner outer circular layer of smooth muscles and a lamina propria layer that is less developed than detrusor. PUS extends in the form of a cylinder completely surrounding the urethra from the bladder neck to the perineal membrane with its main part at the bladder neck and becomes thinner in its further course in the urethra. The proximal sphincter is innervated by adrenergic autonomic fibers .
The Distal (external) (rhabdo-) urethral sphincter (DUS)
It is 1.5 to 2.4 cm in length and surrounds the membranous “sphincteric” urethra which extends from the apex of the prostate to the corpus spongiosum of the penis .
The smooth muscle fibers of DUS are continuous with the PUS and lie internal to the striated muscle. The striated muscle fibers lie externally and extend from the base of the bladder and the anterior aspect of the prostate to the full length of the membranous urethra. The striated muscle form a horseshoe or omega configuration around the membranous urethra being deficient posteriorly and bulky anteriorly . The peri-urethral striated muscles of the pelvic floor lie external to the striated urethral muscle fibers .
The innervation of DUS is complex with both autonomic unmyelinated and somatic myelinated nerves (pudendal nerve and pelvic nerve). The autonomic nerves enter at the 5 and 7 O’clock positions, while the somatic nerves enter the striated fibers of the prostatic capsule at the 9 and 3 O’clock positions. Damage to these nerves lead to loss of sphincteric mechanism .
Role of urethral in continence mechanism
The urethral smooth and striated muscles in addition to pelvic floor play the main role in the anatomical support of continence .
The striated muscle fibers of distal urinary sphincter is responsible for resting continence because it contains predominantly type I slow-twitch fibers (see physiology) .
Prostatectomy, either radical or transurethral, results in a destruction of the proximal smooth muscle sphincteric mechanism. Continence in post-prostatectomy patients continues to be maintained through the action of the distal urethral sphincter .
The urethral mucosa and submucosa also play a role in continence. The ﬂow of blood into the large submucosal venules can be controlled assisting in forming a water-tight closure of the mucosal surfaces. So, the urethral mucosa and submucosa function as a filler substance to effectively close the urethral lumen after the urinary sphincters narrow the urethral space .
Role of the prostate in continence
In terms of urinary continence the prostate gland plays an important role. Its enlargement due to benign cause or prostatic cancer causes voiding difficulties and surgical excision of the prostate can be complicated by urinary incontinence .
The pelvic ﬂoor lies at the bottom of the abdomino-pelvic cavity and forms a support for the abdominal and pelvic viscera and plays important role in continence. The pelvic ﬂoor has three layers of support: the endopelvic fascia, the levator ani muscles and the perineal membrane .
1- The endopelvic fascia
The endopelvic fascia (outer stratum of the pelvic fascia) is a viscero-fascial layer that lies immediately beneath the peritoneum and connects the viscera to the pelvic sidewalls. It presents an extension of the transversalis fascia which drapes on the pelvic floor. It can be considered the ﬁrst layer of the pelvic ﬂoor .
The endopelvic fascia becomes condensed to form the urethropelvic and puboprostatic ligaments. The Urethropelvic ligaments are an anterior medial condensation of the endopelvic fascia, which combines with fibers from the pubococcygeus muscle to span the area from the anterior aspect of the tendinous arc to the bladder neck and proximal urethra. The puboprostatic ligament attaches the inferior surface of the pubic symphysis to the junction of the prostate and the external sphincter.
2- The levator ani muscles (Fig 3)
The levator ani muscle is the second layer of pelvic floor and considered the true muscular floor of the pelvis that provides the main support for the pelvic organs. The layer formed by the muscle and its fascial layers (superior and inferior) is referred to as the “Pelvic diaphragm” .
The levator ani can be divided into three parts: the pubococcygeus, iliococcygeus and ischiococcygeus muscles but the boundaries between each part cannot be easily distinguished and they perform many similar physiological functions .
Figure (3): Pelvic diaphragm muscles and pelvic walls .
a- The Pubococcygeus muscle
Pubococcygeus forms a U-shaped thick band being deficient at the ventral aspect of the urethra. It arises from the back of the body of the pubis on either side of the midline and passes back almost horizontally surronding the urethra and rectum.
Pubococcygeus is often subdivided into two parts:
The pubourethralis (Puboprostaticus) is the most medial fibers of pubococcygeus that runs directly lateral to the urethra and its sphincter and inferio-lateral to the prostate.
The puborectalis is a thick muscular sling that wraps around the anorectal junction and behind the rectum .
b- The iliococcygeal muscle
It is attached to the inner surface of the ischial spine. The most posterior fibers are attached to the tip of the sacrum and coccyx but most join with fibers from the opposite side to form a raphe which provides a strong attachment for the pelvic floor posteriorly.
c- The ischiococcygeal muscle (also called coccygeus)
It is the most posterosuperior portion of levator ani and arises as a triangular musculo-tendinous sheet with its apex attached to the tip of the ischial spine and base attached to the lateral margins of the coccyx .
3- The perineal membrane (urogenital diaphragm)
This is the third layer of pelvic floor and it provides some weak support for urethra .
Role of the pelvic floor in continence mechanism
The levator ani muscle plays an important role in urinary continence and support of the striated DUS but the exact anatomical relationship between this muscle and external urethral sphincter remains incompletely understood .
The levator ani muscle fibers are responsible for the voluntary active continence during stress conditions such as cough, abdominal straining or interruption of the urinary stream by contracting forcefully and rapidly but for a short time because it contains predominately fast-twitch type IIa fibres .
The pelvic fasciae also play a role in continence and pelvic viscera support. The puboprostatic ligaments, in conjunction with the pubourethralis muscle prevent the rotational decent of the proximal urethra .