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AN48.1-8 | Pelvic wall and viscera — Part 1
CLINICAL SCENARIO
A 32-year-old primigravida from a rural Tamil Nadu district is brought to the district hospital after 24 hours of obstructed labour. On examination, she has a contracted pelvis and the fetal head is not engaged. An emergency caesarean section is performed.
Why does the shape of the female pelvis matter for childbirth? What muscles support the pelvic viscera during pregnancy? What nerve supply must the surgeon identify before incising the lower uterine segment?
The pelvis is the bony and muscular basin that supports the abdominal viscera, transmits body weight to the lower limbs, and in the female provides the birth canal. Mastery of pelvic anatomy is essential for every doctor practising in India.
WHY THIS MATTERS
The pelvic structures you learn today underpin some of the most common clinical encounters in Indian practice:
- Obstetric emergencies: obstructed labour, caesarean section, obstetric fistula — India has the highest global burden of maternal mortality; pelvic anatomy is lifesaving knowledge
- Benign prostatic hypertrophy (BPH): affects ~50% of Indian men over 50; suprapubic cystostomy is a key emergency procedure
- Cervical cancer: most common gynaecological cancer in India (second overall); radical hysterectomy and pelvic node dissection require precise anatomical knowledge
- Colorectal surgery: abdominoperineal resection, anterior resection — knowing the autonomic nerves of the pelvis prevents disastrous complications
- Ectopic pregnancy: tubal pregnancy is the leading cause of maternal death in the first trimester — understanding tubal and ovarian anatomy is essential
- Uterine prolapse: extremely common in multiparous Indian women (high parity, poor pelvic floor support); pelvic floor anatomy is central to surgical repair
Cross-reference OG: obstetrics, gynaecology. Cross-reference SU: colorectal surgery, urology.
RECALL
Before we begin, recall:
- The pelvis is bounded by the hip bones, sacrum, and coccyx
- The pelvic inlet (superior pelvic aperture) is bounded by the promontory of sacrum, arcuate line, pectineal line, and pubic crest
- The pelvic outlet (inferior pelvic aperture) is bounded by the coccyx, sacrotuberous ligaments, ischial tuberosities, and pubic arch
- The true pelvis (lesser pelvis) lies below the pelvic inlet — it contains the pelvic organs
- The false pelvis (greater pelvis) lies above the pelvic inlet — it is part of the abdominal cavity
- Female pelvis is wider (gynaecoid) with a rounded inlet and wider subpubic angle (>90°); male pelvis is narrower (android) with a heart-shaped inlet and angle <90°
Pelvic Diaphragm — Muscles and Function (AN48.2)
The pelvic diaphragm is the muscular floor of the pelvis, shaped like a funnel, with the opening for the urethra, vagina (in females), and anal canal.
Muscles:
| Muscle | Origin | Insertion | Action |
|---|---|---|---|
| Levator ani (three parts) | |||
| — Pubococcygeus | Posterior pubis | Anococcygeal body + coccyx | Supports pelvic viscera; raises pelvic floor during Valsalva |
| — Puborectalis | Posterior pubis | Forms a sling behind anorectal junction | Maintains anorectal angle (90°) — continence |
| — Iliococcygeus | Ischial spine + tendinous arch | Coccyx | Elevates pelvic floor |
| Coccygeus (ischiococcygeus) | Ischial spine | Sacrum + coccyx | Supports pelvic floor; flexes coccyx |
Nerve supply: Levator ani — nerve to levator ani (S3, S4); also perineal branch of pudendal nerve. Coccygeus — S4, S5.
Clinical relevance:
- Uterine prolapse: weakening of levator ani + puborectalis + perineal body after multiple vaginal deliveries → descent of uterus through vaginal introitus
- Faecal incontinence: injury to puborectalis during childbirth or surgery → loss of anorectal angle maintenance → incontinence
- Levator ani syndrome: chronic pelvic pain, often in South Indian women, from hypertonic levator ani
Internal Iliac Artery — Branches (AN48.3)
Origin: Common iliac artery divides at the lumbosacral junction (L5/S1) into external and internal iliac arteries.
Internal iliac artery — descends into the pelvis and divides into anterior and posterior divisions.
Posterior division (3 branches — go upward/backward):
- Iliolumbar artery → iliopsoas, quadratus lumborum
- Lateral sacral arteries → sacral canal, overlying skin
- Superior gluteal artery → exits via greater sciatic foramen above piriformis → supplies gluteus medius, minimus
Anterior division (8 branches — mnemonic: OISVUIPI):
| Branch | Supplies |
|---|---|
| Obturator artery | Medial thigh muscles; hip joint (may replace inferior epigastric in 30% → "corona mortis") |
| Inferior gluteal artery | Exits below piriformis → gluteus maximus |
| Superior vesical artery | Fundus of urinary bladder; upper ureter |
| Vaginal/Inferior vesical artery | Base of bladder; vagina (F) / prostate + seminal vesicles (M) |
| Uterine artery | Uterus; crosses ureter 2 cm lateral to cervix ("water under the bridge") |
| Internal pudendal artery | Exits via greater sciatic foramen below piriformis → re-enters via lesser sciatic → perineum |
| Middle rectal artery | Middle rectum |
| Inferior epigastric artery | Anterior abdominal wall (variable — sometimes from external iliac) |
Key surgical points:
- Uterine artery ligature: during hysterectomy, ligate medial to ureter (avoid ureteric injury — "water under the bridge")
- Corona mortis: anastomosis between obturator and inferior epigastric arteries over the pectineal line — at risk during ilioinguinal approach to hip/acetabular surgery
- Internal iliac ligation: for massive PPH (postpartum haemorrhage) — reduces pulse pressure in pelvic vessels by 85%
Sacral Plexus (AN48.4)
Formation: Lumbosacral trunk (L4, L5) + anterior rami of S1, S2, S3, S4.
Location: On piriformis muscle, in front of the sacrum.
Branches:
| Nerve | Roots | Exit | Supplies |
|---|---|---|---|
| Superior gluteal | L4, L5, S1 | Above piriformis | Gluteus medius, minimus, tensor fasciae latae |
| Inferior gluteal | L5, S1, S2 | Below piriformis | Gluteus maximus |
| Sciatic nerve | L4–S3 | Below piriformis | Entire lower limb below knee |
| Posterior cutaneous nerve of thigh | S1–S3 | Below piriformis | Posterior thigh and perineum |
| Nerve to piriformis | S1, S2 | Direct | Piriformis |
| Nerve to obturator internus | L5, S1, S2 | Below piriformis | Obturator internus + superior gemellus |
| Nerve to quadratus femoris | L4, L5, S1 | Below piriformis | Quadratus femoris + inferior gemellus |
| Perforating cutaneous nerve | S2, S3 | Through sacrotuberous ligament | Medial buttock skin |
| Pudendal nerve | S2, S3, S4 | Below piriformis → re-enters via lesser sciatic | Entire perineum |
| Pelvic splanchnics | S2, S3, S4 | Direct (no exit through foramina) | Parasympathetic to pelvic viscera (detrusor, rectum, erectile tissue) |
| Nerve to levator ani | S3, S4 | Direct | Levator ani |
Piriformis as landmark: Everything that exits via the greater sciatic foramen either runs above piriformis (superior gluteal vessels + nerve) or below piriformis (all others).