15.3 Pelvis & Perineum: Bony Pelvis, Pelvic Floor, Pelvic Viscera, Vessels & Nerves, Perineal Pouches & the Anal Canal

Why this area matters

The TOS lists:

• the pelvis: joints, ligaments, walls, floor, nerves, vessels;
• the viscera of the pelvic cavity: urinary organs, male and female internal genital organs, pelvic fascia;
• the perineum: perineal fascia, superficial and deep perineal pouches, pelvic diaphragm, male and female perineum.

This anatomy explains obstetric mechanics, pelvic fractures with urethral injury, pudendal blocks, episiotomy, hysterectomy complications and hemorrhoids.

The bony pelvis

• Bones: two hip bones (ilium, ischium, pubis, fused at the acetabulum), the sacrum and the coccyx.
• Joints and ligaments:
  • sacroiliac joints (with strong posterior sacroiliac ligaments);
  • the pubic symphysis (fibrocartilage);
  • sacrotuberous and sacrospinous ligaments, which convert the sciatic notches into the greater and lesser sciatic foramina.
  • In pregnancy, relaxin loosens the joints.
• The pelvic brim (inlet) separates the false (greater) pelvis from the true (lesser) pelvis.
• Types: gynecoid (most common in women; round inlet, best for delivery), android (heart-shaped, male-type; prone to arrest), anthropoid and platypelloid.
• Obstetric diameters:
  • The diagonal conjugate (sacral promontory to the lower border of the symphysis, about 12.5 cm) is measured clinically.
  • The obstetric conjugate (about 11 cm or more) is the shortest anteroposterior inlet diameter. It is roughly the diagonal conjugate minus 1.5 to 2 cm.
  • The interspinous diameter (about 10 cm) is the narrowest mid-pelvic diameter.
• Walls and floor:
  • Obturator internus forms the lateral wall; piriformis the posterolateral wall.
  • The pelvic diaphragm (levator ani and coccygeus) forms the floor; see below.
  • The obturator canal transmits the obturator nerve and vessels.

Nerves and vessels of the pelvis

• Sacral plexus (L4 to S4): sciatic nerve, superior and inferior gluteal nerves, pudendal nerve, posterior femoral cutaneous nerve, nerve to obturator internus.
• Pudendal nerve (S2 to S4):
  • leaves through the greater sciatic foramen below the piriformis;
  • hooks around the ischial spine and sacrospinous ligament;
  • re-enters the perineum through the lesser sciatic foramen into the pudendal (Alcock) canal;
  • supplies the external anal sphincter, external urethral sphincter, perineal muscles and perineal skin.
  • A transvaginal pudendal block is aimed at the ischial spine.
• Autonomic nerves:
  • Pelvic splanchnic nerves (S2 to S4, parasympathetic): bladder contraction (micturition), erection, motility of the hindgut.
  • Hypogastric (sympathetic) nerves: ejaculation and internal sphincter tone.
  • Injury during pelvic surgery causes urinary retention and erectile dysfunction.

Pelvic Diaphragm, Internal Iliac Branches & Pelvic Ureter Topography

Pelvic Floor & Perineum

• Pelvic Diaphragm: Funnel-shaped muscular sheet separating the pelvic cavity from the perineum. Formed by the levator ani (puborectalis, pubococcygeus, iliococcygeus) and the coccygeus (ischiococcygeus) muscles; innervated by somatic branches of $S_3\text{--}S_4$ and the pudendal nerve. The puborectalis forms a U-shaped muscular sling around the anorectal junction, creating the 80-degree anorectal angle essential for fecal continence.
• Perineal Pouches: Separated by the tough perineal membrane:
  • Superficial Perineal Pouch: Contains the crura of the clitoris/penis, bulb of the vestibule / corpus spongiosum, ischiocavernosus, bulbospongiosus, superficial transverse perineal muscles, and the greater vestibular (Bartholin) glands in females.
  • Deep Perineal Pouch: Contains the external urethral sphincter, deep transverse perineal muscles, membranous urethra, and bulbourethral (Cowper's) glands in males.

Internal Iliac Artery & "Water Under the Bridge"

The internal iliac artery arises at the pelvic brim and bifurcates into:

• Posterior Division: Iliolumbar, lateral sacral, and superior gluteal arteries.
• Anterior Division: Umbilical (gives origin to superior vesical), obturator, inferior vesical (males) / vaginal (females), uterine artery, middle rectal, internal pudendal (supplies perineum and erectile bodies), and inferior gluteal arteries.

The Pelvic Ureter & "Water Under the Bridge": The ureter enters the pelvis crossing anterior to the bifurcation of the common iliac artery at the pelvic brim. In females, it descends along the lateral pelvic wall, passes deep to the broad ligament, and courses directly inferior to the uterine artery ("water [urine in ureter] under the bridge [uterine artery]") approximately $1.5\text{ to }2.0\text{ cm}$ lateral to the supravaginal cervix. During an abdominal or vaginal hysterectomy, clamping and ligation of the uterine artery within the cardinal (transverse cervical / Mackenrodt's) ligament carries a high risk of inadvertent ureteral transection or ligation, manifesting postoperatively with unilateral flank pain, hydronephrosis, or a ureterovaginal fistula. In males, the ureter passes directly inferior to the ductus deferens.

Pelvic viscera

• Urinary bladder:
  • The trigone (smooth, between the ureteric orifices and the internal urethral orifice) is mesodermal.
  • The bladder rises into the abdomen when full, so a suprapubic catheter can be placed above the pubis without entering the peritoneum.
  • Intraperitoneal rupture of a full bladder from a blow to the abdomen leaks urine into the peritoneal cavity. Extraperitoneal rupture with pelvic fracture leaks urine into the retropubic space.
• Male internal genital organs:
  • The ductus deferens joins the seminal vesicle duct to form the ejaculatory duct, which enters the prostatic urethra.
  • The prostate has a peripheral zone (most cancers; palpable on rectal examination) and a transition zone (benign prostatic hyperplasia).
  • The male urethra has prostatic, membranous (the narrowest part, in the deep pouch) and spongy (bulbar and penile) parts.
• Female internal genital organs:
  • The uterus is normally anteverted and anteflexed. It is supported by the cardinal (transverse cervical) ligaments, uterosacral ligaments and the levator ani. The broad and round ligaments add little support. Weakness leads to prolapse.
  • The broad ligament contains the uterine tubes, the uterine vessels, the ovarian ligament, the round ligament and the ureter at its base.
  • Uterine tubes have infundibulum (with fimbriae), ampulla (the usual site of fertilization and ectopic pregnancy), isthmus and intramural parts.
  • The ovary receives the ovarian artery (from the aorta) through the suspensory (infundibulopelvic) ligament. Twisting of this pedicle causes ovarian torsion.
  • The posterior fornix of the vagina lies against the rectouterine pouch (of Douglas), the lowest part of the peritoneal cavity, where blood or pus collects; this is the route of culdocentesis.
• Pelvic fascia: the visceral and parietal layers condense into ligaments (cardinal, uterosacral, puboprostatic, pubovesical) that support the organs.

Perineum

The perineum is the diamond-shaped region below the pelvic diaphragm, bounded by the pubic symphysis, ischial tuberosities and coccyx. A line between the ischial tuberosities divides it into:

• the urogenital triangle in front, containing the perineal pouches;
• the anal triangle behind, containing the anal canal and the ischioanal fossae. Ischioanal abscesses can spread around the anal canal to the opposite side (horseshoe abscess).

Urethral injuries in men:

• Spongy (bulbar) urethra, from a fall astride: urine leaks into the superficial perineal pouch. It spreads into the scrotum, penis and anterior abdominal wall deep to Scarpa's fascia. It cannot enter the thighs, because Scarpa's (Colles') fascia is attached to the fascia lata and ischiopubic rami.
• Membranous urethra (pelvic fracture): urine leaks into the deep pouch or retropubic space. The prostate may be displaced upward ("high-riding"). Blood appears at the meatus, and urethral catheterization should not be attempted before imaging.

Female perineum and episiotomy: the perineal body is the central tendon where the bulbospongiosus, superficial and deep transverse perineal muscles, external anal sphincter and levator ani meet. A mediolateral episiotomy is directed away from the anal sphincter to reduce the risk of third- and fourth-degree tears.

The anal canal and the pectinate line

Sphincters: the internal sphincter is involuntary smooth muscle (sympathetic tone). The external sphincter is voluntary skeletal muscle (pudendal nerve). The puborectalis sling creates the anorectal angle that is essential for continence.