Q1
LONG ANSWER
10M
★ High-yield
Describe the bony pelvis with its divisions and diameters. Add a note on the types of pelvis and the sex differences.
Formation
The bony pelvis is formed by the two hip bones anteriorly and laterally, and the sacrum and coccyx posteriorly, united at the two sacro-iliac joints and the pubic symphysis.
- Each hip bone is formed by the fusion of the ilium, ischium and pubis at the acetabulum, complete by about 16 years
- Its functions are transmission of body weight to the lower limbs, protection of the pelvic viscera, and to form the birth canal
Divisions
| Boundary of the pelvic inlet | From behind forward |
|---|---|
| Sacral promontory and ala of the sacrum | — |
| Arcuate line of the ilium | — |
| Pecten pubis (pectineal line) | — |
| Pubic crest and upper border of the symphysis | — |
| Boundary of the pelvic outlet | Structure |
|---|---|
| Anterior | Pubic arch |
| Lateral | Ischial tuberosities and sacrotuberous ligaments |
| Posterior | Tip of the coccyx |
Diameters
| Diameter | Inlet | Cavity | Outlet |
|---|---|---|---|
| Anteroposterior | 11 cm (true conjugate) | 12 cm | 13 cm (the coccyx moves back) |
| Oblique | 12 cm | 12 cm | — |
| Transverse | 13 cm — the largest at the inlet | 12 cm | 11 cm (interspinous 10.5 cm — the narrowest of all) |
| Conjugate | Definition | Length |
|---|---|---|
| True (anatomical) conjugate | Sacral promontory to the upper border of the symphysis | 11 cm |
| Obstetric conjugate | Sacral promontory to the nearest (innermost) point of the symphysis | 10–10.5 cm — the shortest and most important |
| Diagonal conjugate | Sacral promontory to the lower border of the symphysis | 12–12.5 cm — the only one measurable clinically |
- The head engages in the transverse diameter at the inlet (widest there) and rotates to emerge in the anteroposterior diameter at the outlet (widest there) — the mechanism of internal rotation in labour
Types of Pelvis (caldwell–moloy)
| Type | Frequency | Inlet shape | Obstetric outlook |
|---|---|---|---|
| Gynaecoid | About 50% | Rounded or transversely oval | The ideal female pelvis; favourable |
| Android | About 20% | Heart-shaped (funnel), narrow forepelvis | Unfavourable — deep transverse arrest, difficult labour |
| Anthropoid | About 25% | Long anteroposteriorly, oval | Favours occipito-posterior position; usually delivers |
| Platypelloid | About 3% | Flat, wide transversely, short anteroposteriorly | Least favourable; difficulty at the inlet |
Sex Differences
| Feature | Male pelvis | Female pelvis |
|---|---|---|
| General build | Heavy, thick, marked muscle markings | Light, thin, smooth |
| Cavity | Deep and funnel-shaped | Shallow and broad |
| Inlet | Heart-shaped | Round or transversely oval |
| Subpubic angle | Narrow, 50–60° (V-shaped) | Wide, 80–90° (U-shaped) |
| Greater sciatic notch | Narrow (about 50°) | Wide (about 80°) |
| Obturator foramen | Oval | Triangular |
| Acetabulum | Large, faces laterally | Small, faces more anteriorly |
| Ischial tuberosities | Inturned | Everted |
| Sacrum | Long, narrow, more curved | Short, wide, less curved |
| Preauricular sulcus | Absent or shallow | Deep and well marked |
- The subpubic angle and the greater sciatic notch are the most reliable single features in forensic sex determination
- The pelvis is the most reliable bone for determining sex, more so than the skull
Applied Aspects
- Pelvimetry — clinical assessment of pelvic capacity; the diagonal conjugate and the interspinous diameter are assessed on vaginal examination, and MRI is used where imaging is needed
- Cephalopelvic disproportion — an important indication for caesarean section; suspected when the head fails to engage in a primigravida at term
- Pelvic fracture is a serious injury; the pelvis is a ring, so it usually breaks in two places. Associated injury to the bladder, urethra and iliac vessels causes the mortality
- Symphysis pubis relaxes in pregnancy under the influence of relaxin, increasing the diameters slightly and causing pelvic girdle pain
- The ischial spines are the landmark for the pudendal nerve block and for assessing the station of the fetal head in labour
- Osteomalacia in India from vitamin D deficiency can deform the pelvis, producing a triradiate pelvis and obstructed labour — still seen in some regions
- The sacrum determines sex more reliably than most single bones, and the pelvis as a whole is the first choice in forensic identification
- Weight is transmitted through the sacrum to the acetabula when standing, and to the ischial tuberosities when sitting, along defined trabecular systems
All questions in Pelvis & Perineum
- Q1Describe the bony pelvis with its divisions and diameters. Add a note on the types of pelvis and the sex differences. ★
- Q2Describe the pelvic diaphragm with its parts and functions. Add a note on the perineum, the perineal pouches and the perineal body. ★
- Q3Describe the urinary bladder under relations, interior, blood supply and nerve supply. Add a note on micturition and its disorders. ★
- Q4Describe the uterus under position, supports, relations and blood supply. Add a note on its applied anatomy. ★
- Q5Describe the rectum and anal canal. Explain the importance of the pectinate line and add a note on the ischiorectal fossa. ★
- SN1Describe the prostate with its lobes, zones, relations and applied anatomy. ★
- SN2Describe the course, branches and applied anatomy of the pudendal nerve. ★
- SN3Describe the boundaries and contents of the ischiorectal fossa. Add a note on ischiorectal abscess. ★
- SN4Describe the broad ligament with its parts and contents. Add a note on the ovary and uterine tube. ★
- SN5Describe the male urethra with its parts and constrictions. Add a note on its rupture and the extravasation of urine. ★
- SN6Describe the internal iliac artery with its branches and collateral circulation. ★
- SN7Describe the sacrum with its features. Add a note on caudal anaesthesia and the sacro-iliac joint.