Pelvic Walls and Pelvic Floor

Obturator Internus

• Origin: Internal surface of the obturator membrane and surrounding hip bone (pelvic surface).
• Insertion: Medial surface of the greater trochanter of the femur (trochanteric fossa), via a tendon that angles sharply around the lesser sciatic notch (using the ischium as a pulley).
• Innervation: Nerve to obturator internus (L5, S1, S2).
• Action: Lateral rotator of the extended hip; abductor of the flexed hip, a lower limb action, despite the muscle belly's pelvic wall location; it also contributes structurally to the lateral pelvic wall, and its fascia (obturator fascia) is a key attachment point for the pelvic floor and a landmark for the pudendal (Alcock's) canal.
• Relations: Covers most of the lateral pelvic wall (obturator foramen region); the obturator canal (transmitting the obturator nerve/vessels) lies at its superior border.

Piriformis

• Origin: Anterior surface of the sacrum.
• Insertion: Superior border of the greater trochanter of the femur.
• Innervation: Nerve to piriformis (S1, S2).
• Action: Lateral rotator of the extended hip; abductor of the flexed hip, again a lower limb action, though the muscle belly forms part of the posterolateral pelvic wall.
• Relations: Passes through the greater sciatic foramen, dividing it into supra- and infrapiriform parts; its pelvic surface forms part of the posterior pelvic wall.

Both muscles line the lateral/posterior pelvic walls but act primarily on the hip joint, a useful reminder that "pelvic wall muscle" describes anatomical location, not necessarily pelvic function.

The pelvic diaphragm: levator ani and coccygeus together, forms a muscular sling spanning the pelvic outlet, the primary structural support for the pelvic viscera.

Levator Ani

A composite muscle, conventionally described as having three (or four, depending on classification) named parts, sharing a broad, continuous origin and a common general innervation pattern.

Puborectalis:

• Origin: Posterior surface of the pubic bone (and the anterior part of the tendinous arch of levator ani).
• Insertion: Forms a U-shaped sling posterior to the anorectal junction (the two sides unite behind the rectum, without a separate posterior bony/ligamentous insertion).
• Innervation: Direct branches from the sacral plexus (S3, S4) and/or the inferior rectal (pudendal) nerve, innervation of levator ani overall is described inconsistently across sources, with some crediting a direct sacral plexus supply on the pelvic surface and pudendal nerve contribution from below; both are commonly taught.
• Action: Maintains the anorectal angle (the sling pulls the anorectal junction anteriorly, creating an angle that is critical for maintaining faecal continence at rest); relaxes during defecation to straighten this angle.

Pubococcygeus:

• Origin: Posterior pubic bone and the anterior part of the tendinous arch of levator ani (obturator fascia).
• Insertion: Coccyx and the anococcygeal ligament/body (a fibrous raphe between the anus and coccyx); some fibres blend with the walls of the urethra, vagina (in females), and rectum, contributing to sphincteric support of these structures (sometimes further subdivided by name, e.g., pubovaginalis, puboprostaticus, depending on the specific fibres described).
• Action: Supports the pelvic viscera; contributes to sphincteric control around the urethra/vagina/anal canal.

Iliococcygeus:

• Origin: Posterior part of the tendinous arch of levator ani (obturator fascia) and the ischial spine.
• Insertion: Coccyx and the anococcygeal ligament/body, joining with the contralateral side in the midline (often the thinnest, most membranous part of levator ani).
• Action: Supports the pelvic viscera, forming the posterior/lateral part of the muscular floor.

Overall action of levator ani:

• Provides continuous, tonic muscular support to the pelvic viscera against intra-abdominal pressure; contracts further with sudden increases in intra-abdominal pressure (coughing, sneezing, lifting) to prevent prolapse/incontinence; relaxes in a coordinated fashion during micturition and defecation.

Coccygeus (Ischiococcygeus)

• Origin: Ischial spine.
• Insertion: Lateral border of the sacrum and coccyx.
• Innervation: Direct branches from S4, S5 (ventral rami).
• Action: Supports the posterior pelvic floor; assists in flexing the coccyx slightly (e.g., during defecation or childbirth).
• Relations: Lies on the pelvic surface of the sacrospinous ligament (often considered its muscular equivalent/precursor, sharing essentially the same attachment points), together, coccygeus completes the posterior part of the pelvic diaphragm where levator ani's coverage is less extensive.

• Formed by levator ani (puborectalis, pubococcygeus, iliococcygeus) and coccygeus together, spanning the pelvic outlet from the pubic bones anteriorly to the coccyx/sacrum posteriorly, and from one lateral pelvic wall to the other.
• Perforated centrally by the urogenital hiatus (anteriorly) and anal hiatus (posteriorly), through which the urethra (and vagina, in females) and anal canal respectively pass.

• The anterior gap within the pelvic diaphragm (between the two sides of pubococcygeus/puborectalis), transmitting the urethra in males and the urethra plus vagina in females.
• A relative area of weakness in the pelvic floor, given its role as a necessary passage rather than continuous muscle, clinically relevant to pelvic organ prolapse, which characteristically occurs through this hiatus.

• The posterior gap within the pelvic diaphragm, transmitting the anorectal junction/anal canal, bounded by the puborectalis sling.
• The puborectalis sling's maintenance of the anorectal angle here is central to faecal continence.

• Levator ani (and coccygeus) are covered on both surfaces by fascia: the superior fascia of the pelvic diaphragm (continuous with the parietal pelvic fascia above) and the inferior fascia of the pelvic diaphragm (continuous with the perineal fascia below, forming part of the medial wall of the ischioanal fossa).
• The tendinous arch of levator ani: a thickened linear condensation of the obturator internus fascia, running from the pubic bone to the ischial spine, providing the lateral origin line for pubococcygeus and iliococcygeus.

• The pelvic diaphragm provides the primary muscular support for the bladder, rectum, and (in females) uterus/vagina, working in concert with the endopelvic (visceral pelvic) fascia and named ligamentous condensations (transverse cervical, uterosacral, pubocervical ligaments) which provide additional fascial/ligamentous support.
• This dual support system (muscular tone from levator ani + fascial/ligamentous suspension) is essential background for understanding the multifactorial aetiology of pelvic organ prolapse, since either component's failure (or both together) can result in descent of pelvic viscera.

Pelvic floor weakness:

• Results from a combination of factors: ageing, chronic raised intra-abdominal pressure (obesity, chronic cough, heavy lifting), menopausal oestrogen decline (affecting connective tissue quality), and (most significantly) obstetric trauma.
• Produces a spectrum of dysfunction from mild stress urinary incontinence to overt pelvic organ prolapse.

Pelvic organ prolapse:

• Descent of pelvic viscera through the urogenital hiatus, given its structural weakness relative to the surrounding muscular diaphragm.
• Named according to which organ prolapses and the compartment involved (anterior, cystocele/urethrocele; posterior, rectocele; apical, uterine prolapse or vaginal vault prolapse post-hysterectomy).

Obstetric injury:

• Vaginal delivery, particularly with prolonged second stage labour, instrumental delivery (forceps/ventouse), or a large fetus, can directly stretch or tear the levator ani muscle fibres (particularly the puborectalis, given its close relationship to the birth canal) and/or injure the pudendal nerve (stretch neuropraxia, from prolonged compression/traction during descent), both mechanisms contributing to the elevated risk of pelvic floor dysfunction and incontinence following vaginal childbirth compared to those who have not given birth vaginally.
• This obstetric mechanism is considered the single most significant risk factor for pelvic floor dysfunction in women, underlying the clinical and anatomical importance of understanding this muscular/nervous anatomy in detail.