The Urethra And Associate Continence Structures

In most descriptions, male continence mechanisms are divided into two continence zones: the internal or proximal complex, consisting of the intraprostatic urethra and bladder neck; the external complex, consisting of the urethra distal to the verumontanum; the puborectalis aspect of the levator ani; and the additional fascial support structures previously mentioned.20-22 The urethral component of the external complex is a sophisticated arrangement of elastic fibers, longitudinal smooth muscle and the external rhabdosphincter.23,24

The rhabdosphincter is a component of the urethra that is composed of slow-twitch and fast-twitch fibers that initially give an 'omega' shape appearance, suggesting muscle deficiency, particularly at the base.19'25'26 Current studies suggest, however, that this is a function of lateral thickening of the muscle with similar dorsal and ventral distributions of muscle fibers.20,21,24 Some of these fibers, however, are not circumferential and do project laterally to flanking fibrous bands, which provide support for the urethra.7 An associated yet separate structure involved with continence is the puboperineales, which by their sling-like arrangement about the urethra and the fast-twitch component of muscle fibers, work as a voluntary fast-stopping mechanism for continence.22

The innervation for this continence region is somewhat complex. Dual innervation of the rhabdosphincter has been suggested by several investigators.25,27 Recent work by Steiner and associates using fresh cadaver dissections has provided greater insight regarding the neuroanatomy of the urethra and suggests some surgical modifications that may aid in optimizing continence in surgical patients.28,29 In general, the autonomic component of the urethra is derived from the pelvic nerve and inferior hypogastric plexus. The somatic component is provided by the pudendal nerve. A perineal component, the perineal nerve, provides somatic innervation for the urinary rhabdosphincter. Additionally, as the pudendal nerve heads toward the ischioanal fossa, a terminal branch of the intrapelvic component of the pudendal nerve can be identified, which innervates the rhabdosphincter at the 5 and 7 o'clock positions (Fig. 28.2). Thus, the somatic innervation of the rhabdosphincter is from two components of the pudendal nerve.

The smooth muscle component of the proximal continence mechanism is innervated by intrapelvic branches of the inferior hypogastric plexus that are distinct from the neurovascular bundle cavernosal nerves and enter the inferiolateral aspect of the rhabdosphincter.28'29 These autonomic fibers also enter at the 5 and 7 o'clock positions along with the intrapelvic branches of the pudendal nerves, and course through the rhabdosphincter to supply the muscle component of the urethra (Fig. 28.3). The intrapelvic component of the pudendal nerve was found to emanate from the pudendal up to 2 cm proximal to the rhabdosphincter or branch of the pudendal at the rhabdosphincter. More recent histochemical studies have also supported the concept of dual (mixed) innervation.30 The practical implication of these findings bears on surgical technique. If a significant proportion of innervation to vital continence mechanisms can be demonstrated at the posterio-lateral portion of the rhabdosphincter, it would be prudent to avoid spreading and dissection of the urethra in that location during the apical portion of the operation. An effort to avoid disruption or traction on the pelvic floor structures during surgery is further rationalized by these data. The 5 and 7 o'clock entry of this dual innervation also suggests that suture placement should be modified to more lateral and medial locations.7

The continence mechanism is often obscured anatomically by the variations in the anatomy of the prostate apex and the effect of benign enlargement in general.31 The impact of variable prostate anatomy on the optimal performance of a radical prostatectomy has been elegantly investigated and described by Myers in several studies and the following descriptions are drawn primarily from his work.19'23'32

What is generally thought of as only the anterior fibromus-cular stroma contains not only the commisure, but also a portion of the preprostatic sphincter and the collection of longitudinal smooth muscle as well as prominent veins referred to as the detrusor apron.9 The apron is an extension of the longitudinal smooth muscle of the bladder, while the preprostatic sphincter is an extension of the circular smooth muscle fibers of the bladder neck. The maturation of the prostate displaces these structures. With the development of benign prostatic hyper-plasia, one also sees the enlargement of the avascular plane beneath the apron.23 This can facilitate vascular control in larger glands, but also suggests that passage of a vascular clamp should be avoided in smaller glands, where the margin for damage to the urethra is greater.32

In addition to structural displacement by general enlargement, there is significant variation in the morphology of the gland at the apex. This is best demonstrated during surgery by downward pressure at the vesicoprostatic junction, which then accentuates the presenting apical anatomy.32 While multiple variations may be encountered, the issue is conceptually simplified by the presence or absence of an anterior apical notch (Fig. 28.4).

Pubourethral ligaments

Dorsal v.

Urethra

Ext. striated urethral sphincter

Median fibrous raphe

Prostate

Intrapelvic branch of pudendal n.

Branch of inf. hypogastric plexus

Cavernosal n. /vascular bundle

Pudendal n. in pudendal canal

Bladder

Pubourethral ligaments

Dorsal v.

Urethra

Branch of inf. hypogastric plexus

Cavernosal n. /vascular bundle

Bladder

Pudendal n. in pudendal canal

Levator ani m

Fig. 28.2. Diagram of pudendal nerve anatomy demonstrating an intrapelvic branch innervating the rhabdosphincter.7

Levator ani m

Fig. 28.2. Diagram of pudendal nerve anatomy demonstrating an intrapelvic branch innervating the rhabdosphincter.7

Ngf Prongf
Fig. 28.3. Diagram of pelvic nerve anatomy demonstrating a distinct branch innervating the rhabdosphincter.

If a significant portion of the periprostatic urethra is enveloped in lateral lobe hypertrophy, division at the apex can sacrifice several millimeters of continence-preserving urethra. In the ideal situation, the apex falls off sharply at the urethral level of the verumontanum, providing optimal length without compromising cancer control. Prostate hypertrophy may be so great, however, as to envelope the periprostatic urethra for several millimeters beyond the verumontanum, and any attempt to recover this length would result in an unacceptable compromise of the prostatic margin.32

The Surgical Anatomy of the Prostate

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