Pelvic Diaphragm

Overview

• Upper level: pelvic diaphragm
• Middle level: deep urogenital muscles (deep perineal pouch)
• Lower level: sphincters and erectile muscles

Composition

1. Levator ani (the major muscle)
2. Coccygeus

Fig. 5.34 - Pelvic diaphragm showing levator ani (iliococcygeus, pubococcygeus, puborectalis) and coccygeus muscle, with the urogenital hiatus anteriorly. (Gray's Anatomy for Students, p. 529)

Levator Ani

Origin (line of attachment to pelvic wall)

• Posterior aspect of the body of the pubic bone
• The tendinous arch - a linear thickening of the fascia covering the obturator internus muscle
• Spine of the ischium

Insertion / Midline attachments

• Posterior to the vagina (in women) and around the anal aperture (both sexes)
• The anococcygeal ligament (anococcygeal body) - where muscles join as a raphe and attach to the coccyx
• Anteriorly, there is a U-shaped gap - the urogenital hiatus - which allows passage of the urethra (in both sexes) and vagina (in women)

Three Parts of Levator Ani

Superior view schematic: 1 = puborectalis, 2 = pubococcygeus, 3 = iliococcygeus. (Thieme Atlas of Anatomy, p. 183)

Innervation

• Direct branches from the anterior ramus of S4
• Inferior rectal branch of the pudendal nerve (S2-S4)

Functions of Levator Ani

• Supports pelvic viscera
• Maintains the anorectal angle (continence mechanism)
• Reinforces the external anal sphincter
• In women, acts as a vaginal sphincter (pubovaginalis)
• Raises the pelvic floor during increased intra-abdominal pressure (coughing, sneezing)

Coccygeus

Relation to Sciatic Foramina

• Greater sciatic foramen - lies above the pelvic floor; connects the pelvic cavity to the gluteal region
• Lesser sciatic foramen - lies below the pelvic floor; connects the gluteal region to the perineum

Urogenital Hiatus

• Urethra (in both males and females)
• Vagina (in females only)

Fasciae of the Pelvic Diaphragm

• Superior fascia of the pelvic diaphragm - forms the floor of the pelvic cavity proper
• Inferior fascia of the pelvic diaphragm - forms the roof of the ischiorectal (ischioanal) fossa laterally, and the roof of the deep perineal pouch anteriorly

Clinical Relevance

• Pelvic organ prolapse - weakening of the levator ani (especially pubococcygeus) allows the bladder, uterus, or rectum to descend through the urogenital hiatus
• Stress urinary incontinence - impaired closure of the urogenital hiatus and loss of urethrovesical angle support
• Perineal flexure - the anorectal angle maintained by puborectalis is critical for fecal continence; loss (as in pudendal nerve injury) leads to fecal incontinence
• Obstetric injury - levator ani tears and avulsion from the pubic bone are common in vaginal delivery, especially with instrumented delivery

Pelvic Diaphragm - Comprehensive Anatomy

Definition and Overview

• Upper level - the pelvic diaphragm (levator ani + coccygeus + their fasciae)
• Middle level - deep urogenital muscles (deep perineal pouch)
• Lower level - sphincters and erectile muscles of the urogenital and intestinal tract

Composition

1. Levator ani (the principal and largest component)
2. Coccygeus (the posterior, smaller component)
3. Superior fascia of the pelvic diaphragm - floor of the pelvic cavity proper
4. Inferior fascia of the pelvic diaphragm - roof of the ischiorectal fossa and deep perineal pouch

Levator Ani

Origin

• Posterior aspect of the body of the pubic bone
• The arcus tendineus levator ani (ATLA) - a thickened condensation of the obturator internus fascia running as a tendinous arch from the back of the pubic body to the ischial spine
• Ischial spine

Three Parts of the Levator Ani

1. Puborectalis

• Origin: Superior pubic ramus on both sides of the pubic symphysis, and the superior fascia of the urogenital diaphragm
• Course: Runs backward alongside the anorectal junction; the two muscles join to form a U-shaped muscular sling around the posterior aspect of the anorectal junction
• Insertion: Loop-shaped around the anorectal junction; interwoven with the deep part of the external anal sphincter
• Key function: Maintains the perineal (anorectal) flexure - normally keeps an angle of approximately 90 degrees between the rectum and anal canal, acting as a "pinch valve" to prevent defecation. When the puborectalis relaxes during defecation, this angle opens to approximately 130-140 degrees. This is the single most important muscle for fecal continence.

2. Pubococcygeus

• Origin: Posterior portion of the pubis and the arcus tendineus (lateral to puborectalis origin)
• Insertion: Anococcygeal ligament, coccyx; visceral organs (forms slings around the vagina and urethra)
• Subdivisions based on visceral relationships:
  • Puboprostaticus (levator prostatae) - in males
  • Pubovaginalis - forms a muscular sling around the vagina in females; functions as a vaginal sphincter
  • Pubourethralis - provides urethral support
  • Puboanalis - inserts into the wall of the anal canal
• During simulated vaginal delivery, the pubococcygeus muscle has been shown to stretch to more than three times its original length.

3. Iliococcygeus

• Origin: Tendinous arch of the obturator internus fascia (the posterior and lateral portion of the arcus tendineus levator ani)
• Insertion: Iliococcygeal raphe, anococcygeal ligament, coccyx
• The two iliococcygeus muscles join their partners in the midline to form the levator plate (iliococcygeal raphe extending from the anal aperture to the coccyx). This plate acts as a horizontal shelf upon which the pelvic organs rest.
• Weakening of this plate allows it to sag, opening the hiatus and predisposing to pelvic organ prolapse.

Superior view of female pelvic floor: pubococcygeus m., iliococcygeus m., coccygeus m., ischial spine, obturator internus, and relationship to urethra, vagina, and rectum. (Berek & Novak's Gynecology, p. 154)

Insertion / Midline Attachments of Levator Ani (summary)

• Central tendon of the perineum (perineal body)
• Wall of the anal canal
• Anococcygeal ligament (anococcygeal body/raphe)
• Coccyx
• Vaginal wall (in women)
• Anteriorly: the urogenital hiatus (see below)

The Urogenital Hiatus

• Urethra (both sexes)
• Vagina (females)
• Rectum (passes through the posterior part, the anal hiatus)

Inferior view of female pelvic diaphragm showing pubococcygeus, iliococcygeus, coccygeus, and surrounding perineal muscles including levator ani, obturator internus, superficial transverse perineal, ischiocavernosus, and gluteus maximus. (Berek & Novak's Gynecology, p. 155)

Coccygeus

Innervation

• Traditional teaching: Levator ani muscles are innervated by the pudendal nerve (perineal surface) and direct sacral branches (pelvic surface).
• Current evidence: The levator ani muscles are innervated solely by the levator ani nerve, which travels on the superior (intrapelvic) surface of the muscles without pudendal nerve contribution.
  • Originates from S3, S4, and/or S5
  • Exits sacral foramina and travels 2-3 cm medial to the ischial spine and ATLA
  • Courses across the coccygeus, iliococcygeus, pubococcygeus, and puborectalis
  • Occasionally, a separate branch from S5 innervates the puborectalis independently
• Coccygeus: Direct branches from S4 and S5

Muscle Fiber Composition and Tonic Behavior

• Keeps the urogenital hiatus narrowed
• Draws the distal urethra, vagina, and rectum toward the pubic bones
• Supports pelvic viscera against gravity and intra-abdominal pressure

• Involuntary reflex contractions with sudden increases in abdominal pressure (coughing, sneezing, Valsalva)
• Voluntary contractions (Kegel exercises)

Fasciae of the Pelvic Diaphragm

Superior Fascia (Endopelvic Fascia - Parietal Layer)

• Lines the superior (pelvic) surface of the levator ani
• Continuous with the transversalis fascia of the abdominal wall
• Condensations form important ligaments: arcus tendineus fascia pelvis (ATFP) runs from the lower pubic bone (1 cm lateral to midline) to the ischial spine, providing attachment for the endopelvic fascia supporting the urethra and bladder
• Level II vaginal support is provided by the lateral attachment of the midvagina to the superior fascia of the levator ani via the ATFP (DeLancey's anatomical levels)

Inferior Fascia

• Lines the inferior (perineal) surface of the levator ani
• Forms the medial wall and roof of the ischiorectal (ischioanal) fossa
• The ischiorectal fossa is bounded:
  • Medially: inferior fascia of pelvic diaphragm (levator ani)
  • Laterally: obturator internus fascia and sacrotuberous ligament
  • Content: fat, pudendal nerve and vessels in Alcock's canal

Pelvic Floor Support - DeLancey's Three Levels

Functions of the Pelvic Diaphragm (Summary)

1. Visceral support - bears the weight of pelvic organs against gravity and intra-abdominal pressure
2. Urinary continence - tonic closure of the urogenital hiatus compresses the urethra; reflex fast-twitch contractions resist stress incontinence
3. Fecal continence - puborectalis maintains the anorectal angle; relaxation opens this angle during defecation
4. Vaginal sphincter function - pubovaginalis narrows the vaginal introitus
5. Defecation assistance - relaxation + increased intra-abdominal pressure allows feces passage; levator ani then elevates and returns the anal canal to position
6. Parturition - supports the fetal head during cervical dilation; the pubococcygeus guides fetal rotation
7. Coccyx control - coccygeus pulls the coccyx forward after defecation

Relations to Adjacent Structures

Ischiorectal Fossa

• Lies below and lateral to the pelvic diaphragm
• Bounded medially by the inferior fascia of the pelvic diaphragm and the external anal sphincter
• Bounded laterally by the obturator internus muscle and its fascia, and the sacrotuberous ligament
• The fossa contains fat, the pudendal nerve and internal pudendal vessels (in Alcock's canal on the lateral wall), the inferior rectal nerve and vessels
• Anteriorly it has a recess extending above the urogenital diaphragm
• Posteriorly it communicates through the sciatic foramina into the pelvis, which allows spread of perirectal infections

Lateral Pelvic Wall

• The obturator internus muscle forms the lateral wall of the pelvis and gives origin to the ATLA
• The piriformis lies posterolaterally and exits through the greater sciatic foramen above the pelvic diaphragm

Surgical Relevance

Pelvic diaphragm with levator ani (iliococcygeus, pubococcygeus, puborectalis) and coccygeus muscle; note tendinous arch, anococcygeal ligament, and urogenital hiatus anteriorly. (Gray's Anatomy for Students, p. 529-530)