Female bony pelvis, pelvic floor, pouches and pelvic viscera

Overview

The bony pelvis is the birth canal and the frame that holds the pelvic organs. The pelvic floor (levator ani with its fascia and the perineal body) closes the outlet and supports the bladder, uterus, vagina and rectum.

PGET questions focus on:

• Diameters of the inlet, mid-pelvis and outlet, and why the head rotates.
• Pelvic shapes and the labour problems each causes.
• Levator ani parts, attachments and nerves; the perineal body.
• Peritoneal pouches and the relations of bladder, urethra, ureter and rectum.

Bones and joints

• Bones: two hip bones (ilium, ischium, pubis), the sacrum and the coccyx.
• Joints: sacroiliac (synovial), symphysis pubis (a fibrocartilage disc between the pubic bodies), sacrococcygeal.
• Ligaments: sacrotuberous and sacrospinous ligaments turn the sciatic notches into the greater and lesser sciatic foramina and stop the sacrum tipping under the trunk's weight.
• The pelvic brim (sacral promontory, ala of sacrum, iliopectineal line, pubic crest, top of symphysis) divides the false (greater) pelvis above from the true (lesser) pelvis below.

In late pregnancy the ligaments of the symphysis and sacroiliac joints soften, mainly under oestrogen, so the pelvis gives a little during labour. Squatting or kneeling can add room.

Female versus male pelvis

Planes and diameters

• The head enters transverse (widest inlet diameter), rotates in the mid-pelvis and leaves anteroposterior (widest outlet diameter).
• The axis of the pelvis (curve of Carus) is the curved path the centre of the head follows.
• The inlet is tilted at about 60 degrees to the horizontal when standing.

Clinical measurements

• Diagonal conjugate: from the lower border of the symphysis to the sacral promontory, felt on vaginal examination; normally about 13 cm. If the promontory is easily reached, the inlet is small.
• The true and obstetric conjugates (from the top and from the back of the symphysis to the promontory) are shorter than the diagonal conjugate and cannot be felt directly; they are estimated from it.
• Intertuberous distance: about 10 cm, assessed with a fist between the tuberosities.

Pelvic shapes

• A contracted pelvis follows short stature, rickets or osteomalacia (the soft promontory is pushed forward and the brim flattens), old pelvic fractures and spinal deformity (for example spondylolisthesis).
• X-ray, CT or MRI pelvimetry is now rarely used because it predicts the outcome of labour poorly.

Pelvic floor

Pelvic diaphragm

Formed by the two levator ani muscles (and the small coccygeus behind), with their fascia. They form a gutter that slopes down and in.

• Insertions: perineal body (and the preanal raphe), wall of the anal canal blending with the deep external sphincter, anococcygeal raphe and coccyx.
• Nerve supply: mainly S3 and S4, with a contribution from the pudendal nerve.
• Functions:
  • supports the pelvic and abdominal organs, especially when coughing or straining;
  • pulls the anterior vaginal wall and upper urethra forward, keeping an angle of about 100 degrees between the back of the urethra and the bladder base; the angle opens when voiding;
  • the puborectalis sling keeps the anorectal angle for continence;
  • in labour, the gutter shape helps the head flex and rotate forward.

Urogenital diaphragm and perineum

• The urogenital diaphragm (triangular ligament) fills the gap between the pubic rami below the levator ani: two fascial layers with the deep transverse perineal muscle and compressor urethrae between them. Its lower layer is the perineal membrane. Urethra and vagina pass through it.
• The perineum is the diamond below the pelvic floor: symphysis in front, coccyx behind, ischial tuberosities at the sides. A line between the tuberosities divides it into the urogenital triangle (front) and anal triangle (back).
• The ischioanal (ischiorectal) fossa is a fat-filled wedge beside the anal canal, below the levator ani. The pudendal canal (Alcock's) runs in its side wall.
• A collection above the levator ani but below the peritoneum is in the pelvirectal (supralevator) space, not the ischioanal fossa.

Perineal body

• A wedge of fibromuscular tissue lying between the lower vagina (its lowest third) and the anal canal; its base runs from the fourchette to the anus.
• Attachments: bulbospongiosus, superficial and deep transverse perineal muscles, external anal sphincter, and fibres of levator ani.
• It is larger in women and supports the posterior vaginal wall.

Peritoneal pouches

• From the front, the peritoneum goes over the bladder, dips into the shallow vesicouterine pouch, covers the front, fundus and back of the uterus, runs down over the upper posterior vaginal wall (posterior fornix), then turns onto the rectum, forming the rectouterine pouch (pouch of Douglas).
• In the male, the peritoneum passes from the bladder directly to the rectum, forming the rectovesical pouch.

• A prolapsed ovary may lie in the pouch, felt through the posterior fornix and causing deep dyspareunia.
• An enterocele is a hernia of the pouch of Douglas through the upper posterior vaginal wall.

Pelvic viscera and relations

Bladder

• Wall of smooth muscle (detrusor) in three layers; lined by transitional epithelium (urothelium); average capacity about 400 mL.
• The trigone is the triangle between the two ureteric openings and the internal urethral opening. Each ureter runs obliquely through the wall for about 1 cm, which acts as a valve against reflux.
• The bladder base lies against the cervix and upper anterior vagina, separated by the pubocervical fascia.

Female urethra

• About 3.5 cm long, lined by transitional epithelium, with paraurethral glands.
• Surrounded by smooth muscle and by striated compressor urethrae in the urogenital diaphragm.
• Upper part mobile, lower part fixed to the pubis (pubourethral ligaments) and fused to the anterior vaginal wall.

Ureter in the pelvis

1. Crosses the pelvic brim in front of the common iliac bifurcation.
2. Runs down the side wall in front of the internal iliac artery, behind the ovary, attached to the back of the broad ligament.
3. Passes beneath the uterine artery and through the ureteric tunnel in the cardinal ligament, about 1 cm lateral to the cervix.
4. Runs near the lateral fornix and enters the bladder at the trigone.

Its blood comes from small branches of the renal, gonadal, iliac, uterine and vesical vessels, so stripping it in radical surgery can cause necrosis and fistula.

Rectum

• Runs from S3 down to the anorectal junction, just in front of the tip of the coccyx; about 11 cm long, following the sacral curve.
• Peritoneum: upper third covered in front and at the sides; middle third in front only; lower third has none and is separated from the vagina by the rectovaginal septum.

Vessels, lymph and nerves (summary)

Quick revision

• Inlet: transverse 13.5 > AP 11 cm. Outlet: AP 13.5 > transverse 11 cm. Mid-pelvis about 12 cm round.
• Diagonal conjugate about 13 cm; intertuberous about 10 cm.
• Station 0 = ischial spines; instruments only at or below.
• Gynaecoid best; android = deep transverse arrest; anthropoid = OP; platypelloid = no engagement.
• Levator ani = pubococcygeus (with puborectalis) + iliococcygeus; nerve S3-S4.
• Perineal body: bulbospongiosus, transverse perinei, external anal sphincter, levator ani.
• Pouch of Douglas: lowest peritoneal point, reached via posterior fornix.
• Urethra 3.5 cm, transitional; bladder about 400 mL; ureter 1 cm lateral to cervix.

Written by The House Job from standard references (Obstetrics by Ten Teachers; Gynaecology by Ten Teachers; Snell's Clinical Anatomy).

FAQ