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Pelvis - Short Notes

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1. Overview

• The pelvic inlet (brim) opens directly into the abdominal cavity - no clear boundary separates them. Infections, fluids, and organs (e.g., gravid uterus, full bladder) move freely between the two cavities.
• The pelvis is divided into the false (greater) pelvis (above the pelvic brim - part of the abdomen) and the true (lesser) pelvis (below the pelvic brim - contains the pelvic cavity).

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2. Bones of the Pelvis

Ilium

• Most superior component; upper wing = flat fan-shaped structure supporting the false pelvis
• Key landmarks: iliac crest (ASIS, PSIS, tuberculum), iliac fossa (anteromedial), arcuate line (part of linea terminalis), greater/lesser sciatic notches, AIIS (attachment for rectus femoris and iliofemoral ligament)
• Arcuate line separates abdominal (iliac fossa) part from pelvic part

Pubis

• Anterior and inferior part; has a body + superior and inferior pubic rami
• Pubic crest and pubic tubercle on superior surface; body articulates at the pubic symphysis

Ischium

• Posteroinferior part; made of body + ramus
• Ischial tuberosity (weight-bearing when seated) and ischial spine (important landmark separating greater and lesser sciatic notches)

Sacrum & Coccyx

• Sacrum articulates with L5 at the lumbosacral joint and with pelvic bones at sacroiliac joints
• Promontory = anterior projection of S1, the most anterior point of sacrum - key obstetric landmark

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3. Pelvic Inlet (Brim) and Outlet

Obstetric Diameters (True Pelvis)

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4. Pelvic Wall

Ligaments of the Pelvic Wall

• Greater sciatic foramen = superior to sacrospinous ligament and ischial spine
• Lesser sciatic foramen = between sacrospinous and sacrotuberous ligaments

Muscles of the Pelvic Wall

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5. Pelvic Floor (Pelvic Diaphragm)

Levator Ani (main component)

• Innervation: branches of S4 anterior ramus + pudendal nerve (S2-S4)
• Contains a U-shaped urogenital hiatus anteriorly - urethra (both sexes) and vagina (females) pass through it

Coccygeus (ischiococcygeus)

• Triangular; overlies sacrospinous ligament
• Ischial spine → lateral sacrum and coccyx
• Innervation: S3, S4
• Completes the posterior part of the pelvic diaphragm

Anterior Support

• Perineal membrane: thick triangular fascial sheet between arms of pubic arch
• Deep perineal pouch: narrow space superior to perineal membrane

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6. Pelvic Cavity Contents

Contents - Midline Viscera

• Rectouterine pouch (Pouch of Douglas) - deepest in females; site of fluid accumulation
• Vesicouterine pouch - between bladder and uterus
• Rectovesical pouch - in males, between bladder and rectum

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7. Communications of the Pelvic Cavity

• The pudendal nerve exits via the greater sciatic foramen, wraps around the ischial spine/sacrospinous ligament, and re-enters through the lesser sciatic foramen into the perineum

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8. Neurovascular Supply

Arteries

• Internal iliac artery (from bifurcation of common iliac at L5/S1 level) - principal supply to pelvic viscera
  • Anterior division: obturator, inferior vesical, uterine, vaginal, middle rectal, internal pudendal, inferior gluteal
  • Posterior division: iliolumbar, lateral sacral, superior gluteal
• Superior rectal (from inferior mesenteric artery) - supplies upper rectum

Veins

• Internal iliac vein → common iliac vein → inferior vena cava
• Pelvic venous plexuses are extensive and valveless - allow spread of infection/malignancy

Nerve Supply

• Sacral plexus (L4-S4): formed on anterior surface of piriformis; gives sciatic nerve (largest peripheral nerve in body), superior/inferior gluteal nerves, pudendal nerve, nerve to obturator internus, nerve to quadratus femoris
• Pelvic splanchnic nerves (nervi erigentes) - S2, S3, S4: parasympathetic to pelvic viscera; also carry pain from pelvic organs
• Sympathetic supply: hypogastric plexuses from lumbar sympathetics

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9. Male vs. Female Pelvis

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10. Clinical Points

• Bone marrow biopsy: iliac crest is the preferred site - superficial, easily palpated
• Obturator hernia: can occur through the obturator canal - more common in elderly, thin females; may compress obturator nerve
• Ischial tuberosity fracture in adolescents: avulsion at hamstring origin
• Pudendal nerve block: infiltrated at ischial spine (accessible transvaginally or transperineally)
• Pouch of Douglas: first site of fluid/pus accumulation in the female peritoneal cavity; drained via posterior fornix
• Piriformis syndrome: compression of sciatic nerve by piriformis muscle - mimics sciatica
• Levator ani tears: occur during difficult vaginal delivery - predispose to pelvic organ prolapse and incontinence
• Defecation: puborectalis relaxation increases anorectal angle from ~90° to ~130-140°, combined with increased intraabdominal pressure

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CN III - Oculomotor Nerve & CN VI - Abducens Nerve

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CRANIAL NERVE III - OCULOMOTOR NERVE

Fiber Types

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Nucleus

• Located in the rostral midbrain (at the level of the superior colliculus), in the periaqueductal grey
• Composed of multiple subnuclei, one for each muscle it supplies
• Edinger-Westphal nucleus = parasympathetic nucleus; provides preganglionic fibers for pupil constriction and accommodation
• Key nuclear feature: levator palpebrae superioris is controlled by a single midline subnucleus - so a nuclear lesion causes bilateral ptosis
• Superior rectus subnucleus is crossed - it supplies the contralateral superior rectus

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Course

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Branches in the Orbit

• Levator palpebrae superioris
• Superior rectus
• Medial rectus
• Inferior rectus
• Inferior oblique

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Parasympathetic Pathway

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Complete CN III Palsy - Clinical Features

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Partial vs. Complete Palsy - Pupil Rule (High-Yield)

A painful CN III palsy with pupil involvement = compressive cause until proven otherwise - urgent neuroimaging + CT/MR angiogram

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Midbrain Syndromes (Fascicular CN III Palsies)

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Causes of CN III Palsy by Location

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Aberrant Regeneration (Post-CN III Palsy)

• Eyelid elevates on downgaze or adduction (lid-gaze dyskinesis)
• Pupil constricts on attempted adduction, elevation, or depression
• Suggests previous CN III compression - primary aberrant regeneration without prior palsy suggests cavernous sinus meningioma

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CRANIAL NERVE VI - ABDUCENS NERVE

Fiber Type

• General Somatic Efferent (GSE) only - purely motor

Nucleus

• Located in the caudal pons, beneath the floor of the fourth ventricle (forms the facial colliculus with the looping fibers of CN VII)
• Contains two types of neurons:
  • Motor neurons → lateral rectus muscle (ipsilateral)
  • Interneurons → cross to the contralateral medial longitudinal fasciculus (MLF) → reach the contralateral CN III nucleus (medial rectus) - this mediates conjugate horizontal gaze

A lesion of the abducens nucleus causes an ipsilateral conjugate gaze palsy (not just lateral rectus palsy), because both the motor neurons and the interneurons are destroyed.

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Course

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Action

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CN VI Palsy - Clinical Features

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Diplopia Fields

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Causes of CN VI Palsy

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CN VI as a "False Localizing Sign"

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Internuclear Ophthalmoplegia (INO) - CN VI Nucleus Connection

• Impaired adduction of the ipsilateral eye on lateral gaze (medial rectus does not fire properly)
• Nystagmus of the contralateral (abducting) eye
• Most common cause: MS (bilateral INO); also brainstem infarct
• One-and-a-half syndrome: ipsilateral gaze palsy + ipsilateral INO = lesion in PPRF + MLF on same side

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Summary Comparison Table

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