Pelvis — Walls, Pelvic Floor & Perineum

The bony pelvis, its muscular walls and the muscular diaphragm slung across its outlet form the framework for pelvic viscera, support during childbirth and continence. This topic sits at the high-yield interface of Anatomy and Obstetrics & Gynaecology — perineal tears, pudendal block and prolapse support all flow from these relations.

Bony pelvis and orientation

The pelvis is divided by the pelvic brim (linea terminalis) into the greater (false) pelvis above and the lesser (true) pelvis below. The linea terminalis runs: promontory of sacrum → ala of sacrum → arcuate line of ilium → pectineal line (pecten pubis) → pubic crest → upper border of symphysis pubis.

• False pelvis: bounded by iliac fossae and lower lumbar vertebrae; part of the abdominal cavity, supports gravid uterus and abdominal viscera.
• True pelvis: a curved canal between the pelvic inlet and outlet; contains the urinary bladder, rectum and reproductive organs. The birth canal traverses it.

In the anatomical position the pelvis is tilted so that the anterior superior iliac spine and the pubic tubercle lie in the same vertical plane, and the pelvic inlet makes about a 50–60° angle with the horizontal.

Pelvic inlet and outlet — obstetric measurements

The inlet (pelvic brim) is the boundary of the superior aperture; the outlet is the inferior aperture, diamond-shaped and bounded by the pubic arch anteriorly and the coccyx posteriorly. Obstetric diameters are a perennial favourite.

Plane / diameter	Definition	Average value
Anatomical (true) conjugate	Promontory → upper margin of symphysis	11 cm
Obstetric conjugate	Promontory → midpoint/posterior-most point of symphysis	10–10.5 cm (smallest AP, decisive for engagement)
Diagonal conjugate	Promontory → lower margin of symphysis (clinically measurable PV)	12–12.5 cm
Transverse diameter of inlet	Widest side-to-side	13 cm (largest of inlet)
Oblique diameter of inlet	SI joint → opposite iliopubic eminence	12 cm
AP diameter of outlet	Lower symphysis → coccyx tip	11–13 cm (coccyx mobile)
Transverse (bi-ischial/intertuberous) of outlet	Between ischial tuberosities	11 cm

High-yield: The obstetric conjugate (≈10 cm) is the shortest AP diameter of the inlet and the true limiting factor for fetal head engagement. It is not directly measurable clinically — it is estimated as diagonal conjugate − 1.5 to 2 cm.

High-yield: At the inlet the widest diameter is transverse (13 cm) so the fetal head usually engages in the transverse; at the outlet the widest diameter is AP, so the head rotates to occipito-anterior. This "transverse-at-inlet, AP-at-outlet" rule explains internal rotation during labour.

The four classic Caldwell–Moloy pelvis types: Gynaecoid (ideal, round inlet, ~50%), Android (heart/wedge-shaped, male-type, poor prognosis for vaginal delivery), Anthropoid (oval, long AP), Platypelloid (flat, wide transverse, short AP — rarest).

Walls of the pelvis

The true pelvic cavity has an anteroinferior, two lateral, and a posterior wall, plus the floor (pelvic diaphragm).

• Anterior (anteroinferior) wall: the shallowest — pubic bodies, pubic rami and the symphysis pubis.
• Posterior wall: sacrum and coccyx in the midline, with the piriformis muscle and the sacral plexus (anterior to piriformis) on either side.
• Lateral walls: hip bone below the pelvic brim, covered by the obturator internus muscle and its fascia; the obturator membrane closes the obturator foramen. The obturator nerve and vessels run on this wall toward the obturator canal.

High-yield muscle nerves: Piriformis is supplied by branches of S1–S2 (ventral rami); obturator internus by the nerve to obturator internus (L5, S1, S2). Both muscles are lateral rotators of the hip and act as pelvic wall muscles.

The two great foramina of the lateral/posterior wall are formed by the sacrotuberous and sacrospinous ligaments converting the sciatic notches into:

Foramen	Boundaries	Key structures passing
Greater sciatic foramen	Greater sciatic notch + sacrospinous & sacrotuberous ligaments	Piriformis; above piriformis: superior gluteal vessels & nerve; below piriformis: inferior gluteal vessels/nerve, sciatic nerve, posterior femoral cutaneous nerve, pudendal nerve, internal pudendal vessels, nerve to obturator internus & quadratus femoris
Lesser sciatic foramen	Lesser sciatic notch + sacrospinous & sacrotuberous ligaments	Tendon of obturator internus, nerve to obturator internus, and the pudendal nerve + internal pudendal vessels re-entering the perineum

High-yield: The pudendal nerve and internal pudendal vessels leave the pelvis through the greater sciatic foramen (below piriformis), hook around the sacrospinous ligament/ischial spine, and re-enter the perineum through the lesser sciatic foramen to reach the pudendal (Alcock's) canal. This U-turn is the anatomical basis of the pudendal nerve block.

Pelvic floor — the pelvic diaphragm

The pelvic diaphragm is the funnel-/gutter-shaped muscular sheet that forms the floor of the pelvis and the roof of the perineum. It is composed of the levator ani and coccygeus (ischiococcygeus) muscles with their covering fasciae, pierced midline by the urethra, vagina (in females) and anal canal.

Levator ani — components

The levator ani arises from the "white line" (tendinous arch / arcus tendineus levator ani) over the obturator internus fascia, the body of the pubis and the ischial spine. Its named parts (anterior to posterior, medial to lateral):

1. Puborectalis — most medial; forms a U-shaped sling around the anorectal junction. Maintains the anorectal angle (~80–90°) essential for faecal continence; relaxes during defecation.
2. Pubococcygeus — main bulk; passes from pubis to coccyx/anococcygeal raphe. Subdivided in females into fibres related to the urethra/vagina (pubovaginalis) and to the rectum (puborectalis overlaps).
3. Iliococcygeus — most posterolateral; arises from the tendinous arch and ischial spine, inserts into the anococcygeal raphe and coccyx; thin and often partly aponeurotic.

Coccygeus (ischiococcygeus) lies posterior to iliococcygeus, on the pelvic surface of the sacrospinous ligament, running from ischial spine to lateral coccyx/lower sacrum.

High-yield (most-tested): Nerve supply of levator ani — from the pelvic (superior) surface by the "nerve to levator ani" derived from S3–S4 (ventral rami), and from the perineal surface by the inferior rectal/perineal branch of the pudendal nerve (S2–S4). The classic single-line answer is "levator ani — S3, S4" (with pudendal contribution). Coccygeus is supplied by S4–S5.

Functions of the pelvic floor

• Supports the pelvic viscera and resists rises in intra-abdominal pressure (cough, lifting).
• Puborectalis maintains the anorectal angle for faecal continence.
• Sphincteric support to urethra and vagina — central to urinary continence and prevention of prolapse.
• Acts as the birth canal floor; relaxes and stretches in the second stage of labour.

Mnemonic for levator ani parts: "PIP" — Pubococcygeus, Iliococcygeus, Puborectalis (the three named slips), with coccygeus completing the diaphragm.

The perineum

The perineum is the diamond-shaped region below the pelvic diaphragm, bounded by the pubic symphysis (anteriorly), the ischiopubic rami and ischial tuberosities (laterally), the sacrotuberous ligaments and coccyx (posteriorly). A transverse line joining the ischial tuberosities divides it into:

• Urogenital (UG) triangle — anterior; contains the external genitalia, the membranous urethra (and vagina in females), and the perineal membrane with the deep perineal pouch.
• Anal triangle — posterior; contains the anal canal, external anal sphincter and the paired ischio-anal (ischiorectal) fossae.

Urogenital triangle and perineal pouches

Space	Boundaries	Contents (male / female)
Superficial perineal pouch	Between perineal membrane (above) and membranous (Colles') fascia (below)	Crura of penis/clitoris + ischiocavernosus; bulb of penis/bulbs of vestibule + bulbospongiosus; superficial transverse perineal muscle; greater vestibular (Bartholin) glands in female; proximal spongy urethra
Deep perineal pouch	Between perineal membrane and the superior fascia / above	External urethral sphincter, deep transverse perineal muscle, membranous urethra, bulbourethral (Cowper) glands in male, internal pudendal vessels & dorsal nerve of penis/clitoris

High-yield: Cowper's (bulbourethral) glands are in the DEEP pouch in the male; Bartholin's (greater vestibular) glands are in the SUPERFICIAL pouch in the female. Reversing these is a classic distractor.

Perineal body (central tendon of the perineum)

A fibromuscular node in the median plane between the anal canal and the vagina/bulb of penis, anterior to the anal canal. It is the convergence point for: bulbospongiosus, superficial and deep transverse perinei, external anal sphincter, levator ani (puboanalis fibres) and the perineal membrane.

High-yield (Obs & Gynae): The perineal body is the keystone of pelvic floor support in the female. It is torn/incised in childbirth; a mediolateral episiotomy is preferred over a midline one in many settings because midline extension can rupture the external anal sphincter and rectum (4th-degree tear).

Obstetric perineal tear (RCOG) classification — stepwise severity:

1st degree (skin/vaginal mucosa only) → 2nd degree (perineal muscles, incl. perineal body, but anal sphincter intact) → 3rd degree (anal sphincter complex involved: 3a <50% EAS, 3b >50% EAS, 3c IAS torn) → 4th degree (sphincter complex + anorectal/anal mucosa).

Anal triangle and ischio-anal fossa

The ischio-anal (ischiorectal) fossa is a wedge-shaped, fat-filled space on each side of the anal canal, between the perineum and the pelvic diaphragm.

• Apex (superomedial): where levator ani meets obturator internus fascia.
• Base: skin of the anal triangle.
• Medial wall: levator ani (sloping) and external anal sphincter, anal canal.
• Lateral wall: ischial tuberosity and obturator internus with its fascia, which splits to form the pudendal (Alcock's) canal.
• Anterior recess extends into the UG triangle above the perineal membrane; posterior recess passes toward the sacrotuberous ligament. The two fossae communicate posteriorly behind the anal canal (deep postanal space) — explaining horseshoe abscess spread.

The fat allows distension of the anal canal during defecation and of the vagina during childbirth. Being poorly vascularised fat, it is a site for ischio-anal abscess and fistula-in-ano.

Pudendal canal (Alcock's canal) and the pudendal nerve

The pudendal canal is a tunnel in the obturator internus fascia on the lateral wall of the ischio-anal fossa. It transmits the pudendal nerve (S2–S4) and internal pudendal vessels and the nerve to obturator internus initially.

Branches of the pudendal nerve (in order, mnemonic – "I Pee Daily" / I-P-D):

1. Inferior rectal (anal) nerve — external anal sphincter and perianal skin.
2. Perineal nerve — superficial & deep perineal muscles, posterior scrotum/labia.
3. Dorsal nerve of penis/clitoris — sensation to glans (deepest branch).

High-yield — Pudendal nerve block: The needle targets the pudendal nerve as it crosses the sacrospinous ligament just medial to the ischial spine. The ischial spine is palpated transvaginally (transvaginal approach) or via the ischial tuberosity (transcutaneous/perineal approach). It anaesthetises the lower vagina, vulva and perineum for instrumental delivery and is given when S2–S4 dermatomes need blocking.

Pelvic organ support and prolapse anatomy

Female pelvic organ support is conventionally divided into DeLancey's three levels:

Level	Structure	Supports / failure produces
Level I (apical)	Uterosacral–cardinal (transverse cervical/Mackenrodt) ligament complex	Uterine/vault support; failure → uterine/vault prolapse, enterocele
Level II (lateral)	Paravaginal attachments to arcus tendineus fasciae pelvis	Mid-vagina; failure → cystocele / rectocele
Level III (distal)	Perineal body, perineal membrane, levator ani	Distal vagina/introitus; failure → deficient perineum, gaping introitus

High-yield: The uterosacral–cardinal ligament complex (Level I) is the principal apical support of the uterus; levator ani provides dynamic muscular support beneath the passive ligamentous support. Chronic levator damage (childbirth, raised intra-abdominal pressure) → pelvic organ prolapse and stress urinary incontinence.

Diagnosis & clinically relevant evaluation

• Pelvic adequacy / cephalopelvic disproportion: clinical pelvimetry estimating the diagonal conjugate (PV) and intertuberous diameter; the obstetric conjugate is derived.
• Pelvic organ prolapse: graded by the POP-Q (Pelvic Organ Prolapse Quantification) system — investigation of choice for staging and standardised reporting.
• Anal sphincter / obstetric injury: endoanal ultrasound is the investigation of choice to demonstrate sphincter defects after obstetric tears.
• Ischio-anal abscess / fistula-in-ano: clinical; MRI pelvis is the investigation of choice to map complex/horseshoe fistula tracks.

Complications / clinical correlations

• Cephalopelvic disproportion / obstructed labour when the contracted pelvis (esp. small obstetric conjugate, android/platypelloid types) cannot accommodate the head.
• Obstetric anal sphincter injuries (OASIS) — 3rd/4th-degree perineal tears damaging EAS/IAS → faecal incontinence; warrant primary surgical repair.
• Pelvic organ prolapse, stress urinary incontinence and faecal incontinence from pelvic floor / pudendal nerve injury during labour.
• Ischio-anal abscess progressing to fistula-in-ano (Goodsall's rule for tract direction).
• Pudendal neuralgia — entrapment in Alcock's canal → perineal pain worse on sitting.

Key differentials / commonly confused points

• Pubococcygeus vs puborectalis — puborectalis is the sling maintaining the anorectal angle and continence; pubococcygeus is the supportive bulk.
• Pelvic diaphragm vs urogenital diaphragm/perineal membrane — pelvic diaphragm = levator ani + coccygeus (true floor); the perineal membrane is the fibrous sheet of the UG triangle below it.
• Superficial vs deep perineal pouch contents (Bartholin vs Cowper glands).
• Greater vs lesser sciatic foramen contents and the pudendal nerve's path through both.

Recently asked / exam angle

• Obstetric conjugate value and how it is derived (diagonal conjugate − 1.5–2 cm) — recurring NEET PG/Obs MCQ.
• Nerve supply of levator ani (S3–S4 ± pudendal nerve, S2–S4) — repeatedly tested single-best-answer.
• Branches of the pudendal nerve in order and the site of pudendal nerve block (medial to ischial spine, at the sacrospinous ligament).
• Gland in superficial vs deep pouch (Bartholin superficial / Cowper deep).
• Structure torn in a "complete perineal tear" (4th degree) — anal sphincter + anal mucosa; perineal body significance.
• Contents of the pudendal (Alcock's) canal and the lateral wall of the ischio-anal fossa (obturator internus).
• DeLancey levels of vaginal support and ligament responsible for apical (Level I) support — uterosacral–cardinal complex.
• Widest diameter of pelvic inlet (transverse) vs outlet (AP).

Rapid revision

• Obstetric conjugate ≈ 10–10.5 cm, the smallest AP of inlet; cannot be measured directly — derived from diagonal conjugate (12–12.5 cm) minus 1.5–2 cm.
• Inlet: transverse diameter widest (13 cm); Outlet: AP diameter widest — basis of internal rotation.
• Pelvic diaphragm = levator ani (pubococcygeus, iliococcygeus, puborectalis) + coccygeus.
• Levator ani nerve supply: S3–S4 (nerve to levator ani) + pudendal branch (S2–S4).
• Puborectalis maintains the anorectal angle → faecal continence; relaxes on defecation.
• Pudendal nerve (S2–S4) leaves via greater sciatic foramen, loops the sacrospinous ligament at the ischial spine, re-enters via lesser sciatic foramen into Alcock's canal.
• Pudendal nerve branches: Inferior rectal → Perineal → Dorsal nerve of penis/clitoris.
• Pudendal block site: just medial to the ischial spine over the sacrospinous ligament.
• Cowper (bulbourethral) glands — deep pouch (male); Bartholin (greater vestibular) glands — superficial pouch (female).
• Lateral wall of ischio-anal fossa = obturator internus + fascia (Alcock's canal); fat-filled → site of abscess/fistula.
• Perineal body is the central tendon of the perineum; 4th-degree tear = sphincter complex + anal mucosa; mediolateral episiotomy spares the sphincter.
• DeLancey Level I = uterosacral–cardinal (Mackenrodt) ligament = apical/uterine support; Level II = paravaginal; Level III = perineal body/levator ani.
• Caldwell–Moloy types: Gynaecoid (best), Android (poor), Anthropoid, Platypelloid (rarest).
• POP-Q for prolapse staging; endoanal USG for sphincter defects; MRI for complex fistula.