Residency · Residency · Urology
Pelvic Floor Anatomy and the Male Continence Mechanism
Overview
This lecture covers the anatomy of the rhabdosphincter, pelvic floor musculature, and neurovascular bundles, all of which are critical to understanding radical prostatectomy and the mechanisms underlying post-prostatectomy continence outcomes.
Pelvic Floor Musculature
Levator Ani Complex
The levator ani muscle complex forms the main muscular component of the pelvic floor, creating a funnel-shaped diaphragm that supports the pelvic viscera. It consists primarily of three parts. The pubococcygeus, also known as the pubovisceralis, includes subdivisions such as the puboprostaticus in males (pubovaginalis in females), puboperinealis, and puboanalis. This muscle originates from the inner surface of the pubis and inserts into the perineal body, anal canal, and anococcygeal raphe. Its primary function is to support the pelvic organs and contribute to continence. The iliococcygeus muscle arises from the tendinous arch of the levator ani (arcus tendineus levator ani, or ATLA) and inserts on the coccyx and anococcygeal raphe. It is thinner and more shelf-like compared to the pubococcygeus. The coccygeus, also called the ischiococcygeus, originates from the ischial spine and inserts on the coccyx and lower sacrum, lying over the sacrospinous ligament.
Innervation of the Levator Ani
The levator ani receives direct innervation from branches of the sacral nerves S3 to S5, known as the nerve to levator ani, which approach from the superior (pelvic) surface. Although the pudendal nerve’s contribution is debated, it likely innervates the external anal sphincter and perineal muscles. Injury to these nerves during surgery or childbirth can lead to pelvic floor dysfunction.
Perineal Membrane (Urogenital Diaphragm)
The perineal membrane is a triangular fibromuscular sheet spanning the anterior pelvic outlet and attaching laterally to the ischiopubic rami. It provides structural support to the membranous urethra and contains the external urethral sphincter, also known as the rhabdosphincter. In females, it also includes the deep transverse perineal muscle and compressor urethrae.
Male Continence Mechanism
Dual Sphincter System
Internal Urethral Sphincter (Bladder Neck)
The internal urethral sphincter is composed of smooth muscle that continues from the detrusor muscle of the bladder. It is under autonomic sympathetic control via the hypogastric nerve (T10-L2) and is rich in alpha-1 adrenergic receptors. This sphincter is the primary mechanism for continence at rest, providing passive continence. It is typically destroyed during transurethral resection of the prostate (TURP) or radical prostatectomy at the bladder neck. When intact, it also prevents retrograde ejaculation.
External Urethral Sphincter (Rhabdosphincter)
Surrounding the membranous urethra, the external urethral sphincter is composed of striated skeletal muscle arranged in an omega-shaped, horseshoe configuration that is open posteriorly, meaning it does not form a complete ring in males. It predominantly contains slow-twitch (type I) muscle fibers that maintain tonic contraction for passive continence, as well as fast-twitch (type II) fibers that enable voluntary contraction during stress, known as the "guarding reflex." Innervation is provided by the pudendal nerve (S2-S4, somatic) and branches of the pelvic nerve (autonomic). This sphincter is the primary continence mechanism after radical prostatectomy. In males, it measures approximately 2 to 2.5 cm in length and generates the maximum urethral closure pressure.
Continence Zones
There are two main continence zones. The proximal continence zone includes the bladder neck and prostatic urethra, which is lost after radical prostatectomy. The distal continence zone consists of the rhabdosphincter at the membranous urethra, which must be preserved during surgery to maintain post-prostatectomy continence.
Supportive Structures
Several supportive structures contribute to continence. The puboprostatic ligaments, also called pubourethral ligaments, anchor the prostate and urethra to the pubic bone; their division during radical prostatectomy can affect continence. The dorsal venous complex (DVC) lies anterior to the urethra and must be carefully controlled during surgery to avoid sphincter injury. Denonvilliers fascia separates the prostate from the rectum posteriorly, while the endopelvic fascia forms the lateral attachments of the prostate to the pelvic sidewall. The arcus tendineus fasciae pelvis (ATFP), or "white line," is a lateral condensation of the endopelvic fascia extending from the pubic symphysis to the ischial spine.
Neurovascular Bundle (NVB)
Anatomy
The neurovascular bundle, first described by Walsh and Donker in 1982, is located posterolateral to the prostate between the layers of the lateral pelvic fascia. It contains cavernous nerves, which are parasympathetic fibers originating from the pelvic plexus (S2-S4) and are responsible for erectile function. The bundle also includes branches of the inferior vesical artery and vein, as well as capsular arteries supplying the prostate.
Pelvic Plexus (Inferior Hypogastric Plexus)
The pelvic plexus lies lateral to the rectum at the level of the seminal vesicles and the base of the prostate. It receives sympathetic input from the hypogastric nerve (T10-L2), which controls bladder neck closure and seminal emission, and parasympathetic input from the pelvic splanchnic nerves (nervi erigentes, S2-S4), which mediate detrusor contraction and erection. The cavernous nerves arise from this plexus and travel within the neurovascular bundle.
Cavernous Nerves
The cavernous nerves course along the posterolateral surface of the prostate, traveling between the prostatic capsule and the prostatic fascia in the interfascial plane. They pass through the urogenital diaphragm lateral to the membranous urethra and enter the corpora cavernosa at the crura. These nerves mediate erection by releasing nitric oxide, which causes smooth muscle relaxation and dilation of the cavernosal arteries.
Nerve-Sparing Radical Prostatectomy
The goal of nerve-sparing radical prostatectomy is to preserve the neurovascular bundle while ensuring complete cancer excision. Intrafascial dissection occurs closest to the prostatic capsule and offers maximal nerve preservation. Interfascial dissection is performed between the prostatic fascia and lateral pelvic fascia, while extrafascial dissection involves wide excision outside the lateral pelvic fascia and does not preserve nerves. Athermal techniques, such as using clips and sharp dissection, are preferred to minimize thermal nerve injury. It is also recognized that nerves may splay over the anterolateral prostate surface, not strictly posterolateral, necessitating careful anterolateral nerve release.
Functional Anatomy Relevant to Radical Prostatectomy
Continence After RP
Postoperative continence primarily depends on preserving the rhabdosphincter and maintaining membranous urethral length. Preoperative membranous urethral length measured on MRI correlates with continence recovery, with lengths greater than 12 mm generally associated with better outcomes. Factors influencing continence recovery include younger age, good preoperative continence status, preservation of urethral length, nerve-sparing surgical technique, posterior musculofascial plate reconstruction (Rocco stitch), bladder neck preservation or reconstruction, and surgeon experience and volume.
Erectile Function After RP
Erectile function recovery depends largely on preservation of the neurovascular bundles, with bilateral nerve-sparing yielding better outcomes than unilateral or non-nerve-sparing approaches. Age is a significant factor, with younger patients recovering more successfully. Penile rehabilitation strategies include early use of phosphodiesterase type 5 inhibitors (PDE5i), either nightly or on-demand, vacuum erection devices, and intracavernosal injections. Recovery may take 12 to 24 months due to neuropraxia.
Key Surgical Landmarks
Important surgical landmarks include the dorsal venous complex, which is controlled early during radical prostatectomy to minimize bleeding and improve visualization; the seminal vesicles and vas deferens, which are dissected during the posterior approach with close relation to the neurovascular bundles; the lateral pelvic fascia, which is incised during nerve-sparing procedures and determines the plane of dissection; the verumontanum, which serves as a landmark for apical dissection where the membranous urethra begins just distal to it; and the anterior fibromuscular stroma, an avascular plane on the anterior prostate surface.
<image>A detailed anatomical illustration of the male pelvic floor viewed from above (superior view), showing the levator ani muscle complex (pubococcygeus, iliococcygeus, coccygeus) forming the pelvic diaphragm. The urethra, rectum, and prostate are shown passing through the levator hiatus. The arcus tendineus levator ani and arcus tendineus fasciae pelvis are labeled. The puboprostatic ligaments connecting the prostate to the pubis are highlighted. Medical illustration with transparent overlay showing muscle fiber orientation.</image>
<image>A coronal cross-section through the membranous urethra showing the rhabdosphincter (external urethral sphincter) in its omega-shaped configuration surrounding the urethra. The dorsal venous complex is shown anteriorly, the neurovascular bundles are positioned posterolaterally, and the levator ani muscles are shown laterally. The layers of fascia (prostatic fascia, lateral pelvic fascia, endopelvic fascia) are color-coded and labeled. The intrafascial, interfascial, and extrafascial dissection planes for nerve-sparing prostatectomy are indicated with dashed lines.</image>
<image>A posterior view of the prostate, seminal vesicles, and neurovascular bundles showing the course of the cavernous nerves from the pelvic plexus (inferior hypogastric plexus) along the posterolateral surface of the prostate to the corpora cavernosa. The hypogastric nerve (sympathetic) and pelvic splanchnic nerves (parasympathetic) converging on the pelvic plexus are labeled. Denonvilliers fascia is shown between the prostate and rectum. The relationship of the NVB to the seminal vesicles and prostatic capsule is clearly demonstrated.</image>
Clinical Pearls
The rhabdosphincter serves as the primary continence mechanism after radical prostatectomy, making its preservation essential for early continence recovery. Preoperative membranous urethral length measured on MRI, typically greater than 12 mm, is predictive of favorable continence outcomes. The neurovascular bundle is not a discrete structure but rather a diffuse neurovascular hammock draping over the posterolateral prostate, with nerves sometimes splaying anterolaterally. Thermal energy from monopolar or bipolar cautery near the neurovascular bundle can cause delayed nerve injury; therefore, athermal techniques such as clips and sharp dissection are recommended during nerve-sparing surgery. Posterior reconstruction using the Rocco stitch re-approximates the posterior musculofascial plate to the residual rhabdosphincter and may accelerate early continence recovery. Finally, the pudendal nerve (S2-S4) is the primary somatic nerve supplying the external sphincter, summarized by the mnemonic "S2, 3, 4 keeps the pee off the floor."
References
- Walsh PC, Donker PJ. Impotence following radical prostatectomy: insight into etiology and prevention. J Urol. 1982;128(3):492-497
- Myers RP. Practical surgical anatomy for radical prostatectomy. Urol Clin North Am. 2001;28(3):473-490
- Rocco F, et al. Posterior reconstruction of the rhabdosphincter allows a rapid recovery of continence after transperitoneal videolaparoscopic radical prostatectomy. Eur Urol. 2007;51(4):996-1003
- Tewari AK, et al. Anatomical grades of nerve sparing: a risk-stratified approach to neural-hammock sparing during robot-assisted radical prostatectomy. BJU Int. 2011;108(6 Pt 2):984-992
- Campbell-Walsh-Wein Urology, 12th Edition — Chapters 68-69
