Bony pelvis and pelvic floor
Bony Pelvis, Pelvic Floor and Pelvic Diaphragm: Anatomy for FCPS
Bony pelvis and pelvic floor for FCPS Part 1: pelvic joints, inlet and outlet, obstetric diameters, pelvic types, levator ani and the perineal body. Covers labour, prolapse, pelvic fractures, avulsions and the landmarks used for nerve blocks.
Bones and joints
- Two hip bones (ilium, ischium, pubis fused at the acetabulum), sacrum and coccyx.
| Joint | Type | Note |
|---|---|---|
| Sacroiliac | Synovial in front, fibrous (interosseous ligament) behind | Transmits body weight; very stable |
| Pubic symphysis | Secondary cartilaginous (fibrocartilage disc between hyaline-covered surfaces) | Widens slightly in pregnancy |
| Sacrococcygeal | Secondary cartilaginous | Coccyx moves back in delivery |
- On the sacrum, the fused spinous processes form the median sacral crest; fused articular processes the intermediate crests; transverse processes the lateral crests.
- In standing, weight passes from the sacrum through the ilium to the acetabula; in sitting, through the ischial tuberosities.
Pregnancy and the pelvic joints
- Relaxin (from the corpus luteum and placenta) softens the pelvic ligaments and symphysis, widening the pelvis for delivery. Progesterone also relaxes smooth muscle but is not the ligament-softening hormone.
- Symphysis pubis dysfunction: pain over the symphysis worse on walking or turning in bed; separation above about 10 mm is diastasis.
Inlet, outlet and false pelvis
- Pelvic brim (inlet): sacral promontory, alae of sacrum, arcuate lines, pectineal lines, pubic crests and the upper border of the symphysis anteriorly.
- Outlet: coccyx behind, sacrotuberous ligaments and ischial tuberosities at the sides, ischiopubic rami and the lower border of the symphysis in front.
- Above the brim is the false (greater) pelvis, part of the abdominal cavity; below it the true (lesser) pelvis. Fluid poured over the pelvic brim runs down into the true pelvis.
Obstetric diameters
| Diameter | Average | Note |
|---|---|---|
| True (anatomical) conjugate | About 11 cm | Promontory to upper border of symphysis |
| Obstetric conjugate | About 10.5–11 cm | Promontory to back of symphysis; shortest AP diameter of the inlet |
| Diagonal conjugate | About 12.5 cm | Promontory to lower border of symphysis; measured on vaginal examination; obstetric conjugate is about 1.5–2 cm less |
| Transverse diameter of inlet | About 13 cm | Widest inlet diameter |
| Interspinous | About 10 cm | Narrowest diameter of the cavity; ischial spines mark station 0 |
- If the examining finger easily reaches the sacral promontory, the diagonal conjugate is short: a contracted inlet.
Male, female and pelvic types
| Feature | Female (gynaecoid) | Male (android) |
|---|---|---|
| Inlet | Round or oval | Heart-shaped |
| Subpubic angle | Wide, about 90° | Narrow, about 60° |
| Greater sciatic notch | Wide | Narrow |
| Ischial spines | Not prominent | Prominent, inverted |
| Obturator foramen | Oval or triangular | Round |
| Sacrum | Short, wide, less curved | Long, narrow, curved |
- Types: gynaecoid (commonest, best for labour), android (heart-shaped inlet, prominent spines, narrow arch; deep transverse arrest), anthropoid (long AP diameter; occipitoposterior positions), platypelloid (flat).
Pelvic diaphragm
- Levator ani: pubococcygeus (including puborectalis), iliococcygeus; plus coccygeus. Together they form the pelvic diaphragm.
- Origin: back of the pubic body, tendinous arch of the obturator internus fascia, ischial spine. Insertion: perineal body, anococcygeal body, coccyx.
- Nerve supply: nerve to levator ani (S4) from above and the pudendal nerve (inferior rectal, perineal branches) from below.
- Puborectalis slings round the anorectal junction to create the anorectal angle; it relaxes during defecation.
- Functions: supports the pelvic organs, resists rises in abdominal pressure, aids continence, and turns the fetal head forwards in labour.
- In childbirth the pubococcygeus (the medial, anterior part) is most often stretched or torn, weakening support of the bladder, vagina and rectum.
- The urethra passing below the pelvic diaphragm (spongy urethra) leaks into the superficial perineal pouch when torn.
Pelvic organ support
| Support | Holds | Defect causes |
|---|---|---|
| Pubocervical fascia | Bladder and anterior vaginal wall | Cystocele: anterior wall bulge, incomplete emptying, cervix not descended |
| Cardinal (transverse cervical) and uterosacral ligaments | Cervix and upper vagina | Uterine prolapse, vault prolapse |
| Rectovaginal fascia | Posterior vaginal wall | Rectocele |
| Perineal body | Lower vagina and anal canal | Perineal deficiency |
Perineal body
- Fibromuscular node between the anal canal and vagina (or bulb of the penis). Muscles meeting here: bulbospongiosus, superficial and deep transverse perinei, external anal sphincter, and fibres of levator ani. Cutting it (median episiotomy, extension of a tear) damages these and the sphincter.
- Mediolateral episiotomy is preferred to keep away from the anal sphincter.
Landmarks and nerve blocks
- Ischial spine: landmark for the pudendal nerve block (the nerve crosses the sacrospinous ligament just medial to the spine); also station 0 in labour.
- At the ischial spine level in hysterectomy, the uterine artery crosses above the ureter: the ureter is at risk when the artery is clamped.
- Midway between the ischial tuberosity and the greater trochanter lies the sciatic nerve.
- Ischial bursa lies between gluteus maximus and the tuberosity; prolonged sitting on hard benches inflames it (weaver's or tailor's bottom), which can mimic a hamstring origin injury.
Apophyseal avulsions in adolescents
| Site | Muscle |
|---|---|
| ASIS | Sartorius (and tensor fasciae latae) |
| AIIS | Rectus femoris (straight head) |
| Ischial tuberosity | Hamstrings |
| Lesser trochanter | Iliopsoas |
Pelvic fractures and anomalies
- Open-book (anteroposterior compression) injuries widen the symphysis and tear the presacral venous plexus: major bleeding; apply a pelvic binder over the greater trochanters.
- Associated injuries: membranous urethra in men, bladder, rectum, vagina; fractures through the ilium and sacrum injure the lumbosacral trunk and sacral plexus and superior gluteal vessels.
- Bladder exstrophy: wide separation of the pubic bones with epispadias and an exposed, wet bladder plate in the suprapubic region.
Remember Measure the diagonal conjugate, deduce the obstetric conjugate, and fear the interspinous diameter.
Exam trap The ASIS is pulled off by sartorius; the AIIS by rectus femoris.
Exam trap The obstetric conjugate is the shortest AP inlet diameter but is not measured directly; the diagonal conjugate is.
How FCPS asks about this
- Type of joint at the pubic symphysis: secondary cartilaginous.
- Hormone softening pelvic ligaments in pregnancy: relaxin.
- Diameter measured clinically: diagonal conjugate; narrowest pelvic diameter is interspinous.
- Prominent spines and narrow pubic arch: android pelvis.
- Nerve supply of levator ani: S4 nerve to levator ani plus pudendal nerve.
- Cystocele with normal cervical position: pubocervical fascia defect.
- Landmark for pudendal block: ischial spine.
Test yourself on this topic
5 of the 42 questions in our bank on this topic. Choose an answer to see why every option is right or wrong.
In a primigravida at 38 weeks, the diagonal conjugate measured on vaginal examination is 12.5 cm. The obstetric conjugate, the shortest anteroposterior diameter of the inlet that the fetal head must pass, is best estimated as:
Answer: About 11 cm
The diagonal conjugate runs from the lower border of the pubic symphysis to the sacral promontory and is the only conjugate that can be felt clinically. The obstetric conjugate runs from the promontory to the nearest point on the back of the symphysis and is the narrowest AP diameter of the inlet. It is estimated by subtracting 1.5–2 cm from the diagonal conjugate, giving about 10.5–11 cm here, which is adequate. An obstetric conjugate below about 10 cm suggests a contracted inlet.
Conjugates of the inlet
- True: promontory to top of symphysis
- Obstetric: promontory to back of symphysis, shortest
- Diagonal: felt on VE
- Obstetric = diagonal minus 1.5–2 cm
Remember Feel the diagonal, subtract to get the obstetric.
Exam trap The obstetric, not the true, conjugate is the narrowest AP diameter of the inlet.
Why the other options are wrong
- About 12.5 cm This is the diagonal conjugate itself.
- About 14 cm Too large; this is added, not subtracted.
- About 9 cm Would be a contracted inlet; too much has been subtracted.
- About 13 cm Adds to rather than subtracts from the measurement.
A para 4 woman leaks urine when she coughs or laughs, without urgency. Urodynamics show a stable detrusor. What is the main cause?
Answer: Weak pelvic floor support of the bladder neck
Normally the bladder neck and proximal urethra sit above a firm hammock of levator ani and endopelvic fascia, so a rise in abdominal pressure compresses the urethra as well as the bladder and continence is kept. Childbirth can stretch and damage these supports and the pudendal nerve, so the bladder neck descends and the urethra can no longer be compressed. A cough then pushes urine out: stress incontinence. Pelvic floor muscle training is first-line. Urge incontinence is from detrusor overactivity, and a fistula gives continuous leakage.
Stress incontinence
- Leak with cough, laugh, lift
- Bladder neck descends below the pelvic floor
- Risk: childbirth, menopause, obesity
- First line: pelvic floor exercises
Remember Cough leak with a stable bladder: weak support.
Exam trap Anticholinergic drugs treat urge incontinence, not stress incontinence.
Why the other options are wrong
- Uninhibited detrusor contractions Excluded by urodynamics; causes urge incontinence.
- A vesicovaginal fistula Gives continuous leakage day and night.
- Overflow from bladder outlet obstruction Gives dribbling with a large residual, mainly in men.
- Loss of pelvic splanchnic supply to the detrusor Causes a large atonic bladder with overflow.
After a crush injury, a man has an open-book pelvic fracture and is hypotensive. Where does most of the bleeding come from, and what is the first mechanical step?
Answer: Presacral veins; pelvic binder
Pelvic ring fractures can bleed massively into the retroperitoneum. About 85–90% of bleeding is venous (presacral and paravesical venous plexuses) or from cancellous fracture surfaces; arterial bleeding (branches of the internal iliac such as the superior gluteal and internal pudendal) accounts for the remainder. In an open-book (anteroposterior compression) injury, the pelvic volume increases. A pelvic binder centred over the greater trochanters (not the iliac crests) closes the ring, reducing volume and tamponading bleeding. Further steps: balanced transfusion, angioembolisation for arterial bleeding, preperitoneal packing and external fixation. Do not spring the pelvis repeatedly.
Pelvic fracture haemorrhage
- Mostly venous (presacral plexus) + bone surfaces
- Arterial: internal iliac branches (superior gluteal)
- Pelvic binder at greater trochanters
- Then: transfusion, angioembolisation, packing, ex-fix
- Don't spring the pelvis repeatedly
Remember Close the book to stop the bleed.
Why the other options are wrong
- External iliac; laparotomy Rarely the source; laparotomy increases bleeding.
- Aorta; thoracotomy Not correct.
- Femoral artery; tourniquet Not correct.
- Bladder; urinary catheter Catheter only after excluding urethral injury.
A 60-year-old para 5 had poorly repaired perineal tears in her deliveries. She has a bulge in the lower posterior vaginal wall and must press on it to empty her bowel. The perineum is short with a gaping introitus. Which of the following structures is most likely deficient?
Answer: Perineal body
The perineal body is a fibromuscular node between the lower vagina and the anal canal. The bulbospongiosus, superficial and deep transverse perineal muscles, external anal sphincter and parts of levator ani attach to it, and it supports the lower posterior vaginal wall. If it is torn and not properly repaired, the lower rectum bulges into the vagina (a low rectocele), the perineum shortens and the introitus gapes. The cardinal and uterosacral ligaments support the cervix and upper vagina, and pubocervical fascia supports the bladder.
Perineal body
- Node between vagina and anus
- Many perineal muscles attach to it
- Torn: low rectocele, gaping introitus
- Repair it carefully after tears
Remember Central point of the perineum holds the back wall up.
Exam trap A high rectocele or enterocele reflects failure of upper vaginal supports, not just the perineal body.
Why the other options are wrong
- Cardinal ligament Supports the cervix; weakness causes uterine prolapse.
- Pubocervical fascia Supports the bladder; weakness causes a cystocele.
- Round ligament Holds the uterus forward; not a true support.
- Uterosacral ligament Supports the cervix from behind; weakness gives enterocele or prolapse.
At 32 weeks of pregnancy, a woman has pain over the pubic symphysis that is worse when she walks or turns in bed. Ultrasound shows the symphysis gap widened to 12 mm. Which of the following hormones mainly causes this softening of the pelvic ligaments?
Answer: Relaxin
Relaxin, made by the corpus luteum and placenta, together with progesterone and oestrogen, remodels collagen in the ligaments of the pubic symphysis and sacroiliac joints. This allows the pelvis to widen slightly for delivery but can cause pelvic girdle pain and, if excessive, symphysis pubis dysfunction. Oxytocin causes uterine contraction and milk ejection, prolactin milk synthesis, hCG maintains the corpus luteum, and placental lactogen causes insulin resistance.
Relaxin
- From corpus luteum and placenta
- Softens symphysis and sacroiliac ligaments
- Allows pelvic widening
- Can cause pelvic girdle pain
Remember Loose pelvis in pregnancy: relaxin.
Exam trap Progesterone also relaxes tissues, but relaxin is the hormone named for ligament softening.
Why the other options are wrong
- Oxytocin Contracts the uterus and myoepithelial cells.
- Prolactin Stimulates milk production.
- Human chorionic gonadotrophin Maintains the corpus luteum in early pregnancy.
- Human placental lactogen Raises maternal glucose by insulin resistance.
Where this sits in the CPSP syllabus
- Paper 1: Pelvis and perineum, Bony pelvis, pelvic floor and pelvic diaphragm
- Surgery & Allied, Paper 2: Pelvis and perineum, Bony pelvis, pelvic floor and pelvic diaphragm