Pelvic Anatomy (Obstetric Relevance)

1. Quick Answer

Pelvic anatomy encompasses the bony pelvis (ilium, ischium, pubis, sacrum, coccyx), pelvic floor musculature (levator ani, urogenital diaphragm), pelvic viscera (bladder, uterus, ovaries, rectum), and the vascular, lymphatic, and neural structures traversing this region.

Key Concepts:

• The bony pelvis forms the birth canal with defined obstetric diameters (pelvic inlet, midpelvis, pelvic outlet)
• The pelvic floor (levator ani complex) supports pelvic viscera and is stretched during vaginal delivery
• The internal iliac artery provides the primary blood supply to pelvic organs including the uterus
• The uterine artery is the main blood supply to the pregnant uterus and key target in PPH management

ICU Relevance:

• Critical for management of postpartum hemorrhage (uterine artery embolization, B-Lynch sutures, hysterectomy)
• Understanding of neuraxial anatomy for epidural/spinal anesthesia in labor and cesarean section
• Applied anatomy for pelvic trauma assessment and interventional radiology
• Knowledge of pelvic floor essential for understanding obstetric injuries and repair

Exam Focus:

• CICM First Part examiners commonly ask about bony pelvis dimensions, pelvic floor anatomy, uterine blood supply, and sacral plexus anatomy

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2. CICM First Part Exam Focus

What Examiners Expect

Written SAQ:

Common question stems:

• "Describe the anatomy of the bony pelvis with particular reference to its obstetric significance"
• "Draw and label the pelvic floor muscles and explain their function during parturition"
• "Outline the blood supply to the uterus and describe its relevance to postpartum hemorrhage management"
• "Describe the anatomy of the sacral plexus with particular reference to the pudendal nerve"
• "Describe the anatomical basis for neuraxial anesthesia in obstetrics"

Expected depth:

• Detailed anatomical knowledge with named structures, measurements, and relationships
• Clear understanding of obstetric diameters and their clinical significance
• Blood supply including arterial anastomoses and surgical implications
• Clear diagrams with accurate labeling (bony pelvis, pelvic floor, uterine vasculature)
• Explicit ICU application (PPH management, pelvic trauma, regional anesthesia)

Written MCQ:

Common topics tested:

• Obstetric diameters (conjugates, transverse, oblique)
• Pelvic floor muscle attachments and innervation
• Internal iliac artery branches and territories
• Uterine artery course and relationship to ureter
• Sacral plexus formation and branches
• Anatomical landmarks for neuraxial procedures

Difficulty level:

• Applied anatomical scenarios (e.g., "During cesarean hysterectomy, what structure is at risk during uterine artery ligation?")
• Identification of structures from cross-sectional descriptions
• Clinical consequences of nerve injury or vascular occlusion

Oral Viva:

Expected discussion flow:

1. Define/Describe - Overview of pelvic boundaries and contents
2. Detail Structure - Bony pelvis, muscular floor, fascial planes
3. Organ Systems - Female reproductive anatomy, relationships
4. Blood Supply - Internal iliac artery, uterine artery, ovarian artery
5. Apply to ICU - PPH management, pelvic trauma, neuraxial anesthesia
6. Clinical Correlation - Episiotomy, cesarean section, pelvic floor injury

Common viva scenarios:

• "Describe the anatomy relevant to managing massive postpartum hemorrhage"
• "A patient requires emergency cesarean section under spinal anesthesia. Describe the relevant anatomy"
• "Explain the anatomical basis of pudendal nerve block"

Pass vs Fail Performance

Pass Standard:

• Accurate description of bony pelvis with correct obstetric diameters
• Clear understanding of pelvic floor muscle anatomy and function
• Correct description of internal iliac artery divisions and branches
• Ability to describe uterine artery course and ureter relationship
• Draws clear diagrams of pelvic inlet/outlet and pelvic floor

Common Reasons for Failure:

• Confusing pelvic inlet and outlet dimensions
• Not knowing internal iliac artery branches or divisions
• Cannot describe pudendal nerve course or block technique
• Poor understanding of uterine artery-ureter relationship
• Inability to apply anatomy to clinical scenarios (PPH, episiotomy)

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3. Key Points

Must-Know Facts

1. Bony Pelvis Composition: The bony pelvis consists of two hip bones (each formed by fusion of ilium, ischium, and pubis), the sacrum, and the coccyx. The hip bones articulate anteriorly at the pubic symphysis and posteriorly with the sacrum at the sacroiliac joints (PMID: 30969621).

2. Pelvic Inlet (Superior Aperture): Heart-shaped opening bounded by sacral promontory posteriorly, arcuate lines laterally, and pubic symphysis anteriorly. Obstetric conjugate (10 cm) is the shortest AP diameter and clinically most important (PMID: 28892465).

3. Pelvic Outlet (Inferior Aperture): Diamond-shaped opening bounded by coccyx posteriorly, ischial tuberosities laterally, and pubic arch anteriorly. The AP diameter increases during delivery as the coccyx moves posteriorly (PMID: 29630219).

4. Levator Ani Complex: The primary pelvic floor muscle comprising pubococcygeus, puborectalis, and iliococcygeus. Creates a muscular sling supporting pelvic viscera. Innervated by direct branches from S3-S4 and the pudendal nerve. Damage during childbirth leads to pelvic organ prolapse (PMID: 26720449).

5. Internal Iliac Artery: Divides into anterior and posterior divisions. Anterior division supplies pelvic viscera (uterine, vaginal, middle rectal, inferior vesical, internal pudendal). Posterior division supplies pelvic walls (iliolumbar, lateral sacral, superior gluteal) (PMID: 29083568).

6. Uterine Artery: Branches from anterior division of internal iliac. Crosses OVER the ureter ("water under the bridge") approximately 2 cm lateral to the cervix. Provides 90% of uterine blood flow during pregnancy. Key target for surgical ligation or embolization in PPH (PMID: 28698207).

7. Ovarian Artery: Direct branch from abdominal aorta (L2 level). Enters pelvis via suspensory ligament (infundibulopelvic ligament). Anastomoses with uterine artery at ovarian hilum. Important collateral in uterine artery occlusion (PMID: 29939721).

8. Pudendal Nerve (S2-S4): Leaves pelvis through greater sciatic foramen, hooks around ischial spine, enters perineum through lesser sciatic foramen in Alcock's canal. Provides motor and sensory innervation to perineum. Block at ischial spine provides perineal anesthesia for episiotomy/forceps (PMID: 30844173).

9. Pelvic Splanchnic Nerves (S2-S4): Carry parasympathetic fibres to pelvic viscera. Unlike lumbar splanchnic nerves (sympathetic), these are parasympathetic. Damage during radical hysterectomy causes bladder dysfunction (PMID: 27145120).

10. Obstetric Episiotomy Anatomy: Mediolateral episiotomy (from vaginal fourchette at 45-60 degrees) avoids the anal sphincter complex. Structures cut: vaginal mucosa, perineal body, bulbospongiosus muscle, transverse perineal muscles. Median episiotomy has higher risk of third/fourth-degree tears (PMID: 26989196).

Essential Obstetric Diameters

Pelvic Inlet:

True (Obstetric) Conjugate: 10 cm (sacral promontory to closest point on pubic symphysis)
Diagonal Conjugate: 12.5 cm (sacral promontory to inferior border of pubic symphysis - can be measured vaginally)
Transverse Diameter: 13 cm (widest diameter)
Oblique Diameter: 12 cm (sacroiliac joint to opposite iliopubic eminence)

• Clinical significance: Fetal head engages in transverse or oblique diameter

Midpelvis:

Interspinous Diameter: 10 cm (between ischial spines - NARROWEST pelvic diameter)
AP Diameter: 12 cm (S3-S4 junction to pubic symphysis)

• Clinical significance: Most common site of arrest in labor

Pelvic Outlet:

AP Diameter: 9.5-11.5 cm (tip of coccyx to inferior pubic symphysis - increases with coccyx movement)
Transverse Diameter: 11 cm (between ischial tuberosities)

Normal Values Table

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4. Bony Pelvis

4.1 Composition and Articulations

The bony pelvis is composed of four bones that form a complete ring:

Hip Bone (Os Coxae): Each hip bone is formed by the fusion of three bones at the acetabulum (completed by age 16-18):

PMID: 30969621

Sacrum:

• Triangular bone formed by fusion of 5 sacral vertebrae
• Sacral promontory: Upper anterior projection of S1 - key posterior landmark of pelvic inlet
• Sacral ala: Lateral mass articulating with ilium
• Sacral hiatus: Deficiency at S4-S5 from unfused laminae - entry point for caudal epidural
• Anterior sacral foramina: Exit points for ventral rami (S1-S4)
• Sacral curvature: Concave anteriorly; fetal head follows this curve during descent

Coccyx:

• 3-5 fused rudimentary vertebrae
• Articulates with sacrum at sacrococcygeal joint
• Can move posteriorly during delivery, increasing AP outlet diameter by 1-2 cm
• Coccydynia (pain) may follow difficult delivery

Articulations:

PMID: 28892465

4.2 True vs False Pelvis

Linea Terminalis (Pelvic Brim): The linea terminalis is a continuous bony ridge separating the greater (false) pelvis from the lesser (true) pelvis. It is formed by:

• Sacral promontory (posteriorly)
• Sacral ala
• Arcuate line of ilium
• Iliopubic (pectineal) line
• Pubic crest
• Upper margin of pubic symphysis (anteriorly)

Greater (False) Pelvis:

• Above linea terminalis
• Part of abdominal cavity
• Contains abdominal viscera (sigmoid colon, ileum)
• Bounded by iliac fossae laterally
• No obstetric significance for fetal passage

Lesser (True) Pelvis:

• Below linea terminalis
• Contains pelvic viscera (bladder, uterus, rectum, ovaries)
• Bounded by sacrum posteriorly, pubic bones anteriorly, ischial bones/obturator internus laterally
• Forms the birth canal - critical obstetric dimensions

4.3 Pelvic Inlet (Superior Pelvic Aperture)

Shape: Heart-shaped (widest transversely)

Boundaries:

• Posterior: Sacral promontory
• Lateral: Arcuate lines, sacroiliac joints
• Anterior: Pubic crests, pubic symphysis

Diameters:

Fetal Head Engagement: The fetal head typically enters the pelvic inlet in the transverse or oblique diameter (where space is greatest) then rotates during descent.

PMID: 28892465

4.4 Midpelvis (Plane of Least Dimensions)

Boundaries:

• Posterior: S3-S4 junction (or S4 body)
• Lateral: Ischial spines
• Anterior: Inferior margin of pubic symphysis

Diameters:

Clinical Significance:

• Most common site of arrest of labor (cephalopelvic disproportion)
• The ischial spines are palpable vaginally - used to assess fetal station
• Station 0 = fetal head at level of ischial spines (engaged)
• Station -3 to -1 = above spines; Station +1 to +3 = below spines

4.5 Pelvic Outlet (Inferior Pelvic Aperture)

Shape: Diamond-shaped (two triangles with common base at transverse diameter)

Boundaries:

• Posterior: Tip of coccyx
• Lateral: Ischial tuberosities, sacrotuberous ligaments
• Anterior: Pubic arch (inferior pubic rami, pubic symphysis)

Diameters:

Subpubic Angle:

• Angle between inferior pubic rami
• Female: 80-85° (wider)
• Male: 50-60° (narrower)
• Subpubic angle >90°: Gynecoid pelvis (favorable)
• Subpubic angle <70°: Android pelvis (less favorable)

4.6 Pelvic Types (Caldwell-Moloy Classification)

PMID: 29630219

Clinical Application:

• Pure pelvic types are uncommon; most pelves are mixed
• Clinical pelvimetry largely replaced by clinical assessment of labor progress
• CT/MRI pelvimetry rarely used (radiation exposure concerns)
• Trial of labor remains the best test of pelvic adequacy

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5. Pelvic Floor

5.1 Overview

The pelvic floor (pelvic diaphragm) is a muscular partition separating the pelvic cavity from the perineum below. It comprises multiple layers:

1. Pelvic diaphragm (deepest): Levator ani + coccygeus
2. Perineal membrane (urogenital diaphragm): Deep transverse perineal muscle, external urethral sphincter
3. Superficial perineal muscles: Bulbospongiosus, ischiocavernosus, superficial transverse perineal

Functions:

• Supports pelvic viscera (bladder, uterus, rectum)
• Maintains continence (urinary and fecal)
• Allows passage of urethra, vagina, and rectum
• Fetal passage during delivery - must stretch/dilate

5.2 Levator Ani Complex

The levator ani is a broad, funnel-shaped muscle forming the main support of the pelvic floor. It has three components:

Pubococcygeus:

• Origin: Posterior surface of pubic body, arcus tendineus levator ani (thickening of obturator fascia)
• Insertion: Coccyx, anococcygeal raphe
• Subcomponents:
  • "Pubovaginalis (females): Surrounds vagina, forms part of urethral sphincter mechanism"
  • "Puborectalis: U-shaped sling around anorectal junction"
  • "Puboperinealis: Inserts into perineal body"

Puborectalis:

• Origin: Posterior surface of pubic body
• Course: Passes posteriorly around anorectal junction as a sling (no insertion into coccyx)
• Function: Maintains anorectal angle (90-110°) - critical for fecal continence
• Relaxation: Allows straightening of anorectal angle during defecation

Iliococcygeus:

• Origin: Arcus tendineus levator ani, ischial spine
• Insertion: Anococcygeal raphe, coccyx
• Function: Forms horizontal shelf supporting pelvic viscera

PMID: 26720449

Innervation of Levator Ani:

• Direct branches from S3-S4 (on superior/pelvic surface)
• Branches from pudendal nerve (on inferior/perineal surface)
• Clinical significance: Pudendal nerve block does NOT fully block levator ani

Levator Hiatus:

• U-shaped gap in levator ani through which urethra, vagina, and rectum pass
• Bounded anteriorly by pubic bones, laterally by puborectalis sling
• Size increases during vaginal delivery - may lead to permanent enlargement

5.3 Coccygeus (Ischiococcygeus)

• Origin: Ischial spine
• Insertion: Lateral margins of S4-S5 and coccyx
• Lies on the same plane as sacrospinous ligament (forms fleshy part of ligament)
• Innervation: S4-S5 ventral rami
• Function: Supports pelvic viscera, pulls coccyx forward after defecation/delivery

5.4 Perineal Membrane (Urogenital Diaphragm)

A triangular fibromuscular sheet spanning the anterior triangle of the pelvic outlet, below the levator ani.

Boundaries:

• Anterior: Pubic symphysis
• Lateral: Ischiopubic rami
• Posterior: Blends with perineal body

Muscles of Deep Perineal Pouch (within perineal membrane):

• Deep transverse perineal muscle: Stabilizes perineal body
• External urethral sphincter (sphincter urethrae): Voluntary control of micturition

Structures Passing Through Perineal Membrane:

• Urethra (+ vagina in females)
• Internal pudendal vessels
• Dorsal nerve of clitoris/penis

5.5 Superficial Perineal Muscles

PMID: 30844173

5.6 Perineal Body (Central Tendon of Perineum)

Location: Midline between vagina/urethra and anus

Structure: Fibromuscular node approximately 3-4 cm in diameter

Muscles Converging on Perineal Body:

• Bulbospongiosus
• Superficial and deep transverse perineal muscles
• External anal sphincter
• Pubovaginalis (part of levator ani)
• Longitudinal muscle of rectum

Clinical Significance:

• Keystone of pelvic floor - damage leads to perineal descent and prolapse
• Cut during episiotomy - must be repaired accurately
• Damaged in 3rd/4th degree tears - associated with fecal incontinence

5.7 Pelvic Floor Changes During Delivery

Physiological Changes:

1. Cervical dilation: Up to 10 cm diameter
2. Levator hiatus expansion: Increases from ~25 cm² to ~35 cm² (40% increase)
3. Puborectalis lengthening: Up to 3-fold increase in length
4. Perineal body distension: Thins from 3-4 cm to <0.5 cm

Risk Factors for Pelvic Floor Injury:

• Prolonged second stage of labor
• Instrumental delivery (forceps > vacuum)
• Macrosomia (>4 kg)
• Occiput posterior position
• Nulliparity
• Episiotomy (mediolateral protective; midline increases risk)

Obstetric Injuries:

PMID: 26989196

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6. Female Reproductive Anatomy

6.1 Uterus

Location: Pelvic cavity between bladder and rectum

Size:

• Non-pregnant: 8 cm long, 5 cm wide, 2.5 cm thick; 30-40 g
• Term pregnancy: 30-35 cm fundal height; 1000 g

Parts:

Uterine Position:

• Anteversion: Cervix angulated forward relative to vagina (normal: 90°)
• Anteflexion: Body bent forward on cervix (normal: 170°)
• Retroverted uterus in 20% of women (usually no clinical significance)

Peritoneal Relations:

• Anterior: Vesicouterine pouch (empty; bladder reflected off lower segment)
• Posterior: Rectouterine pouch (Pouch of Douglas) - deepest part of peritoneal cavity
• Lateral: Broad ligament

PMID: 29939721

6.2 Cervix

Structure:

• Length: 2.5-3 cm
• Supravaginal portion (2/3): Above vaginal attachment
• Intravaginal portion (1/3): Protrudes into vagina (portio vaginalis)

External Os: Opening into vagina

• Nulliparous: Circular (6 mm diameter)
• Parous: Transverse slit

Internal Os: Opening into uterine cavity

Cervical Canal: 8-10 mm; lined by columnar epithelium secreting mucus

Changes in Pregnancy:

• Softens (Goodell's sign)
• Develops mucus plug (operculum)
• Effaces (shortens) and dilates during labor

6.3 Uterine Ligaments

Supports of the Uterus (in order of importance):

1. Pelvic floor (levator ani)
2. Cardinal and uterosacral ligaments
3. Broad ligament (minor support)
4. Round ligament (minimal support)

PMID: 28698207

6.4 Vagina

Structure:

• Fibromuscular tube, 8-10 cm long
• Extends from cervix to vestibule
• Anterior wall shorter (7 cm) than posterior wall (9 cm)

Fornices:

• Recesses around intravaginal cervix
• Posterior fornix: Deepest; adjacent to Pouch of Douglas (culdocentesis site)
• Anterior fornix: Adjacent to bladder
• Lateral fornices: Adjacent to ureters (1.5 cm lateral to cervix)

Relations:

• Anterior: Bladder base, urethra
• Posterior: Rectouterine pouch (upper 1/4), rectum (lower 3/4)
• Lateral: Levator ani, cardinal ligaments, ureter (2 cm lateral to fornices)

Blood Supply:

• Upper 1/3: Uterine artery
• Middle 1/3: Vaginal artery (branch of internal iliac or uterine)
• Lower 1/3: Internal pudendal artery

Innervation:

• Upper 2/3: Uterovaginal plexus (autonomic) - visceral sensation
• Lower 1/3: Pudendal nerve - somatic sensation

6.5 Ovaries

Location: Ovarian fossa on lateral pelvic wall, posterior to broad ligament

Size: 3 × 2 × 1 cm (almond-shaped); 5-8 g

Attachments:

• Suspensory (Infundibulopelvic) ligament: To pelvic wall; contains ovarian vessels
• Ovarian ligament: To lateral angle of uterus; fibromuscular cord
• Mesovarium: To posterior surface of broad ligament; contains ovarian hilum

Blood Supply:

• Ovarian artery: Direct branch of aorta at L2 (below renal artery level)
• Anastomoses with uterine artery at ovarian hilum

Venous Drainage:

• Right ovarian vein → IVC
• Left ovarian vein → Left renal vein

Clinical Significance:

• Ovarian vessels at risk during surgery on suspensory ligament
• Ovarian torsion causes venous congestion before arterial compromise
• Ovarian cysts may rupture causing hemoperitoneum

PMID: 29939721

6.6 Fallopian Tubes (Uterine Tubes)

Length: 10-12 cm

Parts (lateral to medial):

Blood Supply:

• Medial 2/3: Uterine artery
• Lateral 1/3: Ovarian artery

Clinical Significance:

• Ectopic pregnancy (most common site: ampulla)
• Tubal ligation typically performed at isthmus
• Salpingectomy reduces ovarian cancer risk

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7. Blood Supply to the Pelvis

7.1 Internal Iliac Artery

Origin: Bifurcation of common iliac artery at level of sacroiliac joint (L5-S1)

Course: Descends posteriorly into pelvis, divides at upper margin of greater sciatic notch

Divisions and Branches:

Posterior Division (3 branches - supply pelvic walls):

Anterior Division (8 branches - supply pelvic viscera):

PMID: 29083568

7.2 Uterine Artery

Origin: Anterior division of internal iliac (occasionally from internal iliac directly)

Course:

1. Runs anteromedially on levator ani in base of broad ligament
2. Crosses OVER the ureter approximately 2 cm lateral to cervix ("water under the bridge")
3. Reaches lateral aspect of cervix/uterus
4. Ascends along lateral uterine margin (between layers of broad ligament)
5. Anastomoses with ovarian artery at uterine cornu

Branches:

• Cervicovaginal branch: To cervix and upper vagina
• Arcuate arteries: Run circumferentially in myometrium
• Radial arteries: Penetrate myometrium to endometrium
• Spiral arteries: Supply decidua (remodel in pregnancy)
• Tubal branch: To medial fallopian tube

Pregnancy Changes:

• Blood flow increases from 50 mL/min (non-pregnant) to 500-700 mL/min (term)
• Diameter increases to accommodate increased flow
• Spiral arteries remodeled by trophoblast invasion

Clinical Applications:

1. Uterine artery ligation for PPH: Ligate at level of internal os, lateral to uterus
2. Uterine artery embolization: Catheter via femoral artery; for fibroids, PPH
3. B-Lynch suture: Compresses uterus, reducing blood flow
4. Risk of ureteric injury: Always identify ureter before ligation

PMID: 28698207

7.3 Ovarian Artery

Origin: Direct branch of abdominal aorta at L2 level (below renal arteries)

Course:

1. Descends retroperitoneally on psoas major
2. Crosses external iliac vessels (or common iliac) and ureter
3. Enters pelvis via suspensory (infundibulopelvic) ligament
4. Runs in mesovarium to ovary
5. Continues to anastomose with uterine artery

Collateral Significance:

• In uterine artery occlusion (embolization), ovarian artery provides collateral flow
• May require embolization of both vessels for complete uterine devascularization

7.4 Internal Pudendal Artery

Origin: Anterior division of internal iliac artery

Course:

1. Exits pelvis through greater sciatic foramen (below piriformis)
2. Hooks around ischial spine (with pudendal nerve)
3. Enters perineum through lesser sciatic foramen
4. Runs in pudendal (Alcock's) canal on lateral wall of ischioanal fossa
5. Terminates as perineal and dorsal arteries

Branches:

• Inferior rectal artery: External anal sphincter, perianal skin
• Perineal artery: Superficial perineal muscles, posterior labia/scrotum
• Artery of vestibular bulb (females)/bulb of penis (males)
• Deep artery of clitoris/penis: Erectile tissue
• Dorsal artery of clitoris/penis: Glans

7.5 Venous Drainage

Internal Iliac Vein:

• Receives tributaries corresponding to internal iliac artery branches
• Pelvic venous plexuses drain to internal iliac vein

Pelvic Venous Plexuses (interconnected):

• Uterovaginal plexus: Around uterus and vagina → internal iliac
• Vesical plexus: Around bladder → internal iliac
• Rectal plexus: Around rectum → internal iliac (portosystemic anastomosis)

Ovarian Veins:

• Right: Drains to IVC
• Left: Drains to left renal vein (longer course, prone to compression)
• Ovarian vein thrombosis: Postpartum complication, usually right-sided

PMID: 29630220

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8. Nerve Supply to the Pelvis

8.1 Sacral Plexus

Formation: Lumbosacral trunk (L4-L5) + ventral rami of S1-S4

Location: Anterior surface of piriformis muscle, posterior to pelvic viscera

Branches:

PMID: 30844173

8.2 Pudendal Nerve (S2-S4)

Course:

1. Forms in sacral plexus from S2-S4 ventral rami
2. Exits pelvis through greater sciatic foramen (below piriformis)
3. Crosses posterior to ischial spine (lateral to sacrospinous ligament)
4. Enters perineum through lesser sciatic foramen
5. Runs in pudendal (Alcock's) canal on obturator internus fascia
6. Divides into terminal branches

Branches:

Clinical Applications:

Pudendal Nerve Block:

• Site: Medial to ischial spine (palpated transvaginally)
• Technique: Transvaginal or transperineal approach
• Effect: Anesthesia of vulva, perineum, lower vagina
• Uses: Episiotomy, forceps delivery, perineal repair
• Limitation: Does NOT block uterine contraction pain (T10-L1) or upper vaginal pain

Pudendal Nerve Damage:

• Risk during childbirth (prolonged second stage, forceps)
• Causes: Fecal/urinary incontinence, perineal numbness
• Neuropathy may be transient (stretch injury) or permanent

PMID: 27145120

8.3 Pelvic Splanchnic Nerves (Nervi Erigentes)

Origin: S2-S4 ventral rami

Composition: PARASYMPATHETIC fibres (distinct from lumbar splanchnic nerves which are sympathetic)

Course: Join inferior hypogastric plexus → pelvic viscera

Functions:

• Motor to detrusor muscle (bladder contraction)
• Inhibitory to internal urethral sphincter (allows voiding)
• Motor to erectile tissue (via nitric oxide release)
• Motor to distal colon and rectum

Clinical Significance:

• Damaged during radical hysterectomy/rectal surgery → bladder atony, urinary retention
• Spinal cord lesion above S2 → upper motor neuron bladder

8.4 Superior and Inferior Hypogastric Plexuses

Superior Hypogastric Plexus:

• Location: Below aortic bifurcation (L5-S1 level)
• Composition: Mainly sympathetic fibres from lumbar splanchnic nerves (L1-L2)
• Gives rise to hypogastric nerves

Hypogastric Nerves (right and left):

• Connect superior to inferior hypogastric plexuses
• Carry sympathetic fibres

Inferior Hypogastric (Pelvic) Plexus:

• Location: Lateral to rectum and uterus/prostate
• Composition: Sympathetic (from hypogastric nerves) + Parasympathetic (from pelvic splanchnic nerves)
• Gives branches to: Bladder, rectum, uterus, prostate, vagina, erectile tissue

Sympathetic Functions (via inferior hypogastric plexus):

• Inhibits detrusor muscle
• Contracts internal urethral sphincter (continence)
• Mediates ejaculation (males)
• Vasoconstriction to pelvic viscera

Parasympathetic Functions (via pelvic splanchnic nerves):

• Contracts detrusor muscle (voiding)
• Relaxes internal urethral sphincter
• Mediates erection

8.5 Pain Pathways in Labor

Uterine Contraction Pain (First Stage of Labor):

• Afferents travel with sympathetic nerves
• Enter spinal cord via T10-L1 dorsal roots
• Referred to dermatomes T10-L1 (lower abdomen, lower back)

Cervical Dilatation and Vaginal Distension Pain:

• Afferents travel with parasympathetic nerves
• Enter spinal cord via S2-S4 dorsal roots

Perineal Pain (Second Stage of Labor):

• Afferents travel via pudendal nerve (somatic)
• Enter spinal cord via S2-S4 dorsal roots

Implications for Regional Anesthesia:

PMID: 25919621

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9. Clinical Applications

9.1 Postpartum Hemorrhage (PPH) Management

Definition: Blood loss ≥500 mL (vaginal) or ≥1000 mL (cesarean) within 24 hours of delivery

Anatomical Targets for Intervention:

Uterine Artery Ligation:

• Technique: Suture placed 2-3 cm medial to uterine artery at level of internal os
• Anatomy: Artery runs alongside uterus in base of broad ligament
• Effect: Reduces uterine blood flow by 90% (collaterals from ovarian artery maintain viability)
• Risk: Ureteric injury (ureter is 2 cm lateral to cervix)

Internal Iliac (Hypogastric) Artery Ligation:

• Technique: Ligate anterior division distal to posterior division
• Effect: Reduces pulse pressure to uterus by 85%
• Anatomy considerations:
  • Ureter crosses at pelvic brim (identify before ligation)
  • Preserve posterior division (prevents buttock claudication)
  • Internal iliac vein is posterior (risk of venous injury)
• Collateral flow via ovarian, superior rectal, lumbar arteries

Uterine Compression Sutures (B-Lynch):

• Technique: Suture passes around uterus front-to-back, compressing myometrium
• Effect: Mechanical compression of bleeding vessels

Uterine Artery Embolization:

• Technique: Catheter via femoral artery → internal iliac → uterine artery
• Embolic agents: Gelfoam (temporary), PVA particles, coils (permanent)
• Anatomy: Bilateral embolization usually required (cross-filling from ovarian)

PMID: 28698208

9.2 Neuraxial Anesthesia in Obstetrics

Anatomy for Lumbar Epidural:

Layers traversed (posterior to anterior):

1. Skin
2. Subcutaneous tissue
3. Supraspinous ligament
4. Interspinous ligament
5. Ligamentum flavum (midline: 3-5 mm thick; lateral: thinner)
6. Epidural space (target; contains fat, epidural veins)
7. Dura mater → subdural space → arachnoid → subarachnoid space (CSF)

Epidural Space in Pregnancy:

• Distended epidural veins (increased blood volume + IVC compression)
• Reduced epidural space volume → faster onset, lower drug requirement
• Increased risk of bloody tap (epidural vein puncture)

Landmarks:

• Iliac crest: Level of L4 spinous process
• Preferred levels: L3-L4 or L4-L5 (below conus medullaris at L1-L2)

Spinal Anesthesia:

• Needle enters subarachnoid space (contains CSF)
• Local anesthetic deposited directly into CSF
• Faster onset, more reliable block than epidural
• Lower drug dose (hyperbaric bupivacaine 10-15 mg)

Complications Related to Anatomy:

• Post-dural puncture headache: CSF leak through dural hole
• High spinal: Excessive spread of local anesthetic
• Epidural hematoma: Epidural vein puncture in coagulopathic patient
• Direct neural injury: Needle trauma to spinal cord or nerve roots

PMID: 25919621

9.3 Episiotomy Anatomy

Definition: Surgical incision of perineum to enlarge vaginal outlet

Types:

Structures at Risk:

• External anal sphincter (2nd/3rd degree tear extension)
• Internal anal sphincter (4th degree tear)
• Rectum (4th degree tear)
• Pudendal vessels (bleeding)
• Bartholin's gland (lateral episiotomy)

Repair Principles:

• Identify and accurately reappose all layers
• Repair external anal sphincter with overlapping or end-to-end technique
• Close rectal mucosa with absorbable suture if involved
• Careful alignment of perineal body

PMID: 26989196

9.4 Pelvic Trauma

Pelvic Fracture Classification (Young-Burgess):

• Anteroposterior compression (APC): Pubic symphysis diastasis, SI disruption
• Lateral compression (LC): Sacral crush, horizontal pubic rami fractures
• Vertical shear (VS): Hemipelvis displacement, massive hemorrhage

Vascular Injuries in Pelvic Fracture:

Pelvic Packing:

• Preperitoneal approach
• Tamponades venous bleeding
• Combined with pelvic binder for mechanical stability

Angioembolization Targets:

• Superior gluteal artery (most common source)
• Internal pudendal artery
• Obturator artery
• Lateral sacral arteries

Resuscitative Endovascular Balloon Occlusion of Aorta (REBOA):

• Balloon placed in Zone 3 (infrarenal aorta) for pelvic hemorrhage
• Reduces distal blood flow while maintaining proximal perfusion
• Bridge to definitive hemorrhage control

PMID: 28937654

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10. Graphs and Diagrams

Diagram 1: Bony Pelvis - Pelvic Inlet View

Description: View of pelvis from above showing the pelvic inlet (superior pelvic aperture).

Key Labels:

• Sacral promontory (posterior midline)
• Sacral ala (lateral to promontory)
• Sacroiliac joint
• Arcuate line (ilium)
• Iliopubic eminence
• Superior pubic ramus
• Pubic crest
• Pubic symphysis (anterior midline)

Diameters to Mark:

• Obstetric conjugate (10 cm): Promontory to pubic symphysis
• Transverse diameter (13 cm): Widest point between arcuate lines
• Oblique diameter (12 cm): SI joint to opposite iliopubic eminence

Clinical Relevance: The pelvic inlet is the first obstacle the fetal head encounters during labor. The head typically engages in the transverse or oblique diameter.

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Diagram 2: Pelvic Floor (Coronal Section)

Description: Coronal section through pelvis at level of vagina showing pelvic floor muscles.

Key Labels:

• Obturator internus muscle (lateral pelvic wall)
• Arcus tendineus levator ani (white line)
• Pubococcygeus (medial fibres)
• Iliococcygeus (lateral fibres)
• Puborectalis (U-shaped sling)
• Levator hiatus (containing urethra, vagina, rectum)
• External anal sphincter
• Perineal body

Clinical Relevance: The levator ani complex must stretch during vaginal delivery. Injury to puborectalis leads to fecal incontinence; injury to pubococcygeus leads to urinary incontinence and prolapse.

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Diagram 3: Blood Supply to Uterus and Adnexa

Description: Anterior view of uterus showing arterial supply.

Key Labels:

• Abdominal aorta
• Ovarian artery (from aorta at L2)
• Common iliac artery (bifurcation at L4)
• Internal iliac artery
• Anterior division of internal iliac
• Uterine artery (crossing over ureter)
• Ovarian artery anastomosis with uterine artery
• Ureter ("water under the bridge")
• Cervix (uterine artery at lateral aspect)

Arrows showing:

• Arcuate arteries (in myometrium)
• Radial arteries (penetrating myometrium)
• Spiral arteries (to endometrium)

Clinical Relevance: Uterine artery ligation at the level of internal os is a key surgical intervention for PPH. The ureter is at risk (2 cm lateral to cervix).

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Diagram 4: Sacral Plexus and Pudendal Nerve Course

Description: Lateral view of pelvis showing sacral plexus and pudendal nerve course.

Key Labels:

• Lumbosacral trunk (L4-L5)
• S1-S4 ventral rami
• Sacral plexus (on piriformis muscle)
• Greater sciatic foramen
• Piriformis muscle
• Ischial spine
• Sacrospinous ligament
• Lesser sciatic foramen
• Pudendal (Alcock's) canal
• Pudendal nerve branches (inferior rectal, perineal, dorsal nerve of clitoris)

Clinical Relevance: The pudendal nerve is blocked at the ischial spine for perineal anesthesia. It provides motor supply to external sphincters and sensory supply to the perineum.

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11. Progressive Difficulty Questions

Basic Level (Foundation)

Question 1: Q: What bones form the bony pelvis?

A: The bony pelvis consists of:

• Two hip bones (each formed by fusion of ilium, ischium, and pubis at the acetabulum)
• Sacrum (5 fused sacral vertebrae)
• Coccyx (3-5 fused coccygeal vertebrae)

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Question 2: Q: List the three muscles that comprise the levator ani.

A:

1. Pubococcygeus (includes pubovaginalis, puborectalis, puboperinealis)
2. Puborectalis (forms U-shaped sling around anorectal junction)
3. Iliococcygeus (most lateral and thin portion)

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Question 3: Q: What is the obstetric conjugate and what is its normal value?

A: The obstetric (true) conjugate is the shortest anteroposterior diameter of the pelvic inlet, measured from the sacral promontory to the closest point on the posterior surface of the pubic symphysis. Normal value: 10 cm.

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Question 4: Q: What is the course of the uterine artery in relation to the ureter?

A: The uterine artery crosses OVER (anterior to) the ureter approximately 2 cm lateral to the cervix. This relationship is remembered by the mnemonic "water under the bridge" (ureter = water, passes under the uterine artery bridge).

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Intermediate Level (Application)

Question 1: Q: Explain the anatomical basis for the increased risk of urinary incontinence following vaginal delivery.

A: Vaginal delivery causes stretching and potential injury to:

1. Levator ani muscle: The pubococcygeus component (specifically pubovaginalis) contributes to the urethral sphincter mechanism and bladder neck support
2. Pudendal nerve: Stretching during descent causes neuropraxia; supplies external urethral sphincter
3. Fascia and ligaments: Pubocervical fascia supports bladder base; damage leads to urethral hypermobility
4. Perineal body: Keystone of pelvic floor; damage reduces anterior support

Risk factors include:

• Prolonged second stage
• Instrumental delivery
• Macrosomia
• Median episiotomy

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Question 2: Q: Compare and contrast the anterior and posterior divisions of the internal iliac artery.

A:

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Question 3: Q: A patient develops perineal numbness after a difficult forceps delivery. Explain the anatomical basis.

A: This represents pudendal nerve injury (neuropraxia or axonotmesis).

Anatomical basis:

• Pudendal nerve (S2-S4) passes close to the ischial spine
• During descent, the fetal head compresses the nerve against the bony ischial spine
• The nerve may be stretched during prolonged second stage
• Forceps blades can directly compress the nerve

Expected findings:

• Sensory loss: Perineum, posterior labia (perineal nerve), perianal skin (inferior rectal nerve)
• Motor weakness: External anal sphincter, external urethral sphincter
• Fecal urgency or incontinence possible

Prognosis:

• Neuropraxia usually recovers within 6-12 weeks
• Permanent damage may result from severe stretch injury

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Exam Level (First Part Standard)

SAQ 1: (10 marks)

Time: 12 minutes

Stem: A 32-year-old woman requires emergency cesarean section for fetal distress. Postoperatively, she develops massive postpartum hemorrhage (estimated blood loss 2.5 L) unresponsive to uterotonics.

Question 1.1 (4 marks): Describe the blood supply to the uterus and its anastomotic connections.

Question 1.2 (3 marks): Describe the technique and anatomical considerations for bilateral uterine artery ligation.

Question 1.3 (3 marks): If uterine artery ligation fails, describe the anatomy relevant to internal iliac artery ligation.

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Model Answer:

1.1 (4 marks):

Primary Supply - Uterine Artery (2 marks):

• Branch of anterior division of internal iliac artery
• Crosses over ureter 2 cm lateral to cervix
• Runs along lateral uterine margin between layers of broad ligament
• Gives arcuate arteries (running circumferentially in myometrium) → radial arteries → spiral arteries (to endometrium)
• Blood flow at term: 500-700 mL/min (10× increase from non-pregnant)

Secondary Supply - Ovarian Artery (1 mark):

• Direct branch of aorta at L2 level
• Enters pelvis via suspensory (infundibulopelvic) ligament
• Runs in mesovarium to ovarian hilum
• Anastomoses with ascending branch of uterine artery at cornual region

Anastomotic Connections (1 mark):

• Uterine-ovarian anastomosis (main collateral)
• Vaginal branches from internal pudendal artery
• Collaterals via round ligament vessels
• Cross-midline anastomoses between right and left uterine arteries

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1.2 (3 marks):

Technique (1.5 marks):

• Incise vesicouterine fold; reflect bladder inferiorly
• Identify uterine artery at level of internal os (junction of uterus and cervix)
• Pass suture through avascular window in broad ligament, 2-3 cm lateral to uterus
• Ligate artery and accompanying veins as a single bundle
• Repeat on contralateral side

Anatomical Considerations (1.5 marks):

• Ureter: Runs 2 cm lateral to cervix, crosses under uterine artery; must be identified and retracted
• Bladder: Must be reflected inferiorly to expose lower uterine segment
• Broad ligament: Contains uterine vessels; avascular window exists lateral to uterus
• Effect: Reduces uterine blood flow by 90%; viable due to ovarian artery collateral

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1.3 (3 marks):

Anatomy for Internal Iliac Artery Ligation (2 marks):

• Common iliac artery bifurcates at level of sacroiliac joint
• Internal iliac artery runs posteriorly into pelvis (4-5 cm length)
• Divides at upper border of greater sciatic foramen into:
  • "Anterior division: Supplies pelvic viscera (uterine, vesical, rectal, pudendal)"
  • "Posterior division: Supplies pelvic walls (superior gluteal, iliolumbar, lateral sacral)"
• Internal iliac vein lies posterior to artery (risk of venous injury)
• Ureter crosses at pelvic brim, lateral to artery

Surgical Considerations (1 mark):

• Ligate anterior division distal to posterior division
• Preserves posterior division to avoid buttock claudication
• Effect: Reduces pulse pressure by 85%, converting arterial to venous pressure
• Collateral flow maintained via ovarian, superior rectal, lumbar, median sacral arteries

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SAQ 2: (10 marks)

Time: 12 minutes

Stem: A woman in the second stage of labor requests pudendal nerve block for perineal anesthesia prior to episiotomy and instrumental delivery.

Question 2.1 (4 marks): Describe the anatomy of the pudendal nerve including its formation, course, and branches.

Question 2.2 (3 marks): Describe the technique for pudendal nerve block and the anatomical landmarks used.

Question 2.3 (3 marks): What structures will be anesthetized by a successful pudendal nerve block? What pain will NOT be blocked?

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Model Answer:

2.1 (4 marks):

Formation (1 mark):

• Arises from sacral plexus
• Formed by ventral rami of S2, S3, and S4

Course (2 marks):

1. Passes posterior to sacrospinous ligament
2. Exits pelvis through greater sciatic foramen (below piriformis)
3. Crosses the posterior surface of the ischial spine (lateral to sacrospinous ligament)
4. Re-enters pelvis through lesser sciatic foramen
5. Runs in pudendal (Alcock's) canal on medial surface of obturator internus fascia
6. Terminates as three branches in anterior perineum

Branches (1 mark):

• Inferior rectal nerve: Perianal skin, external anal sphincter
• Perineal nerve: Superficial perineal muscles, posterior labia/scrotum
• Dorsal nerve of clitoris/penis: Skin and glans

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2.2 (3 marks):

Landmarks (1 mark):

• Ischial spine: Palpated transvaginally 4-5 cm posterior and lateral to vaginal introitus
• Sacrospinous ligament: Extends from ischial spine to sacrum
• Pudendal nerve lies 0.5-1 cm medial and posterior to ischial spine

Technique (2 marks):

• Position: Lithotomy or lateral decubitus
• Transvaginal approach:
  1. Guide needle with protective sheath (Iowa trumpet)
  2. Palpate ischial spine with fingertip
  3. Advance needle through vaginal mucosa toward ischial spine
  4. Inject 5-10 mL local anesthetic just medial and posterior to spine
  5. Aspirate first to avoid intravascular injection
  6. Repeat on contralateral side
• Alternative: Transperineal approach (ultrasound-guided)

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2.3 (3 marks):

Structures Anesthetized (2 marks):

• External genitalia: Labia majora, labia minora, clitoris
• Perineal skin and subcutaneous tissue
• Vagina (lower 1/3): Below level of levator ani
• External anal sphincter and perianal skin
• Superficial perineal muscles (bulbospongiosus, ischiocavernosus, transverse perineal)

Pain NOT Blocked (1 mark):

• Uterine contraction pain: Afferents travel via T10-L1 (sympathetic pathway)
• Upper vaginal pain: Autonomic innervation (uterovaginal plexus)
• Cervical dilatation: S2-S4 parasympathetic afferents (some overlap but incomplete)
• Levator ani pain: Partially innervated by direct S3-S4 branches (not pudendal)

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MCQ Practice (5 questions):

Question 1: The narrowest fixed bony diameter of the pelvis is:

A. Obstetric conjugate
B. Transverse diameter of inlet
C. Interspinous diameter
D. Transverse diameter of outlet
E. AP diameter of outlet

Answer: C

Explanation: The interspinous diameter (10-10.5 cm) is the narrowest fixed bony diameter and the most common site of arrest in labor. The obstetric conjugate (10 cm) is comparable but at the inlet level. The AP diameter of the outlet can increase during delivery as the coccyx moves posteriorly.

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Question 2: The uterine artery is a branch of which vessel?

A. External iliac artery
B. Posterior division of internal iliac artery
C. Anterior division of internal iliac artery
D. Ovarian artery
E. Common iliac artery

Answer: C

Explanation: The uterine artery arises from the anterior division of the internal iliac artery. The posterior division supplies pelvic walls (superior gluteal, iliolumbar, lateral sacral). The ovarian artery is a direct branch of the aorta at L2.

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Question 3: During cesarean hysterectomy, which structure is at risk during ligation of the uterine artery?

A. Obturator nerve
B. Sciatic nerve
C. Ureter
D. Rectum
E. Bladder

Answer: C

Explanation: The ureter crosses under the uterine artery ("water under the bridge") approximately 2 cm lateral to the cervix. It is the structure most at risk during uterine artery ligation. The ureter must be identified and protected before any clamping or suturing in this region.

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Question 4: A pudendal nerve block is performed transvaginally. Which landmark is used?

A. Sacral promontory
B. Ischial tuberosity
C. Ischial spine
D. Pubic symphysis
E. Coccyx

Answer: C

Explanation: The pudendal nerve crosses posterior to the ischial spine as it exits the pelvis through the greater sciatic foramen and re-enters through the lesser sciatic foramen. The ischial spine is palpated transvaginally and local anesthetic injected 0.5-1 cm medial and posterior to it.

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Question 5: First stage labor pain is transmitted via which spinal cord levels?

A. T1-T5
B. T5-T9
C. T10-L1
D. L2-L4
E. S2-S4

Answer: C

Explanation: First stage labor pain (uterine contractions) is transmitted via sympathetic afferents that enter the spinal cord at T10-L1. This is why epidural analgesia at T10 level provides relief from contraction pain. Second stage perineal pain is transmitted via S2-S4 (pudendal nerve).

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12. Viva Practice Questions

Viva Scenario 1: Pelvic Anatomy for Obstetric Anesthesia

Stem: "You are asked to explain the anatomy relevant to neuraxial anesthesia in a pregnant patient."

Expected Discussion (10 minutes):

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Examiner: "Describe the anatomy of the lumbar vertebral column relevant to epidural insertion."

Answer: The lumbar spine consists of 5 vertebrae (L1-L5) with the following relevant anatomy:

Vertebral Body and Disc:

• Large vertebral bodies increasing in size inferiorly
• Intervertebral discs between adjacent vertebrae

Posterior Elements:

• Spinous processes: Large, quadrangular, horizontal
• Laminae: Join to form spinous process posteriorly
• Pedicles: Connect vertebral body to posterior elements
• Transverse processes: Lateral projections

Ligaments Traversed (superficial to deep):

1. Supraspinous ligament: Connects tips of spinous processes
2. Interspinous ligament: Between spinous processes
3. Ligamentum flavum: Yellow elastic tissue connecting laminae (3-5 mm thick)

The epidural space lies between the ligamentum flavum and the dura mater.

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Examiner: "What are the contents of the epidural space?"

Answer: The epidural space is a potential space containing:

1. Fat: Provides cushioning, varies in amount
2. Epidural venous plexus (Batson's plexus): Valveless, connects to systemic veins
3. Spinal arteries: Segmental branches
4. Lymphatics
5. Spinal nerve roots: Traversing to exit via intervertebral foramina
6. Connective tissue: Loose areolar tissue

In pregnancy:

• Epidural veins are distended (increased blood volume, IVC compression by gravid uterus)
• Epidural space volume is reduced
• Results in faster onset and greater spread of epidural local anesthetic
• Increased risk of intravascular injection

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Examiner: "At what level does the spinal cord end?"

Answer: The spinal cord (conus medullaris) ends at the level of L1-L2 in adults. Below this level, the spinal canal contains only:

• Cauda equina (nerve roots L2-S5, coccygeal nerves)
• Filum terminale (anchors cord to coccyx)
• CSF

Clinical significance:

• Epidural/spinal at L3-L4 or L4-L5 avoids risk of cord injury
• Landmark: Iliac crests correspond to L4 spinous process (Tuffier's line)
• In pregnancy, the lordosis changes landmarks slightly

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Examiner: "How does spinal anesthesia provide analgesia for cesarean section?"

Answer: Spinal anesthesia involves injection of local anesthetic into the subarachnoid space (intrathecal).

Level required for cesarean section: T4-T6 (nipple line)

Reason:

• Peritoneum is innervated to T4 level
• Uterus innervated T10-L1
• Need complete sensory block to T4 for comfortable surgery

Mechanism:

• Local anesthetic blocks sodium channels in nerve roots
• Blocks sensory (afferent) and motor (efferent) fibres
• Sympathetic blockade occurs first (causes hypotension)
• Motor block allows muscle relaxation

Drug: Hyperbaric bupivacaine (0.5%) 10-15 mg with opioid (fentanyl 15-25 mcg)

Duration: 90-120 minutes

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Examiner: "What are the anatomical reasons for hypotension in spinal anesthesia?"

Answer: Hypotension occurs due to sympathetic blockade:

Mechanism:

1. Preganglionic sympathetic fibres (T1-L2) are blocked
2. This causes:
   • Arteriolar vasodilation → reduced SVR
   • Venodilation → reduced venous return → reduced preload
   • Bradycardia (if block reaches T1-T4 cardioaccelerator fibres)

Pregnancy factors exacerbating hypotension:

• Aortocaval compression by gravid uterus (reduces venous return)
• Already reduced SVR due to pregnancy hormones
• Higher sympathetic tone at baseline (more affected by block)

Prevention/Treatment:

• Left uterine displacement (avoid aortocaval compression)
• IV fluid preload/coload (controversial, crystalloid or colloid)
• Vasopressors: Phenylephrine (first-line), ephedrine

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Viva Scenario 2: Postpartum Hemorrhage Anatomy

Stem: "A 35-year-old woman has massive postpartum hemorrhage following vaginal delivery."

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Examiner: "What is the blood supply to the uterus?"

Answer: The uterus has a dual blood supply:

Primary: Uterine Artery

• Origin: Anterior division of internal iliac artery
• Course: Runs medially in base of broad ligament, crosses OVER ureter 2 cm lateral to cervix
• Ascending branch: Runs along lateral uterine margin to cornu
• Branches: Arcuate arteries → radial arteries → spiral arteries (endometrium)
• Blood flow: 50 mL/min (non-pregnant) → 500-700 mL/min (term)

Secondary: Ovarian Artery

• Origin: Aorta at L2 (below renal arteries)
• Course: Descends retroperitoneally, enters pelvis via suspensory ligament
• Anastomoses with ascending branch of uterine artery at ovarian hilum/cornu

Collaterals:

• Vaginal artery branches
• Round ligament vessels
• Cross-midline anastomoses

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Examiner: "If uterine artery ligation is performed, why doesn't the uterus infarct?"

Answer: The uterus maintains viability due to extensive collateral circulation:

1. Ovarian artery anastomosis: Provides blood via cornual anastomosis
2. Vaginal artery: Supplies lower uterine segment/cervix
3. Contralateral uterine artery: Cross-midline anastomoses in myometrium
4. Round ligament vessels: Minor contribution

Effect of ligation:

• Reduces uterine blood flow by ~90%
• Converts arterial pressure to venous pressure
• Sufficient to stop hemorrhage while maintaining viability
• Uterine artery can recanalize over time

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Examiner: "Describe the anatomical relationship of the ureter to the uterine artery."

Answer: The ureter passes under the uterine artery - "water under the bridge":

Course of ureter in pelvis:

1. Enters pelvis by crossing external iliac vessels at bifurcation
2. Descends retroperitoneally on pelvic sidewall
3. Passes through base of broad ligament
4. Crosses 2 cm lateral to cervix (under uterine artery)
5. Enters bladder base (intramural 1-2 cm)

Clinical significance:

• At risk during hysterectomy, cesarean hysterectomy, uterine artery ligation
• Must identify ureter before clamping uterine vessels
• Injury rate: 0.5-1% in hysterectomy

Injury prevention:

• Reflect bladder inferiorly to expose lower uterine segment
• Identify ureter in broad ligament
• Ligate uterine artery 2-3 cm lateral to uterus (above ureter crossing)

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Examiner: "If uterine artery ligation fails, what is the next surgical option?"

Answer: Internal iliac (hypogastric) artery ligation:

Anatomy:

• Common iliac bifurcates at L5/sacroiliac joint level
• Internal iliac runs 4-5 cm before dividing
• Anterior division: Supplies pelvic viscera
• Posterior division: Supplies pelvic walls (must preserve)

Technique:

1. Incise peritoneum lateral to iliac vessels
2. Identify ureter (crossing pelvic brim) and retract
3. Identify bifurcation of internal iliac
4. Ligate anterior division distal to posterior division
5. Double-ligation with absorbable suture
6. Repeat contralateral

Effect:

• Reduces pulse pressure by 85%
• Does NOT stop all bleeding (collaterals)
• Converts to venous-type bleeding (easier to manage)

Collaterals maintaining pelvic perfusion:

• Ovarian artery (from aorta)
• Superior rectal (from IMA)
• Median sacral (from aorta)
• Lumbar arteries
• Deep circumflex iliac

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