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BONY PELVIS
The pelvis is the lower part of the abdominal cavity. It looks like a basin (Latin: basin). It is formed by the two hip bones, sacrum, and coccyx joined together. This bony framework is called the pelvis, bony pelvis, or pelvic girdle.
The Pelvis - Two Parts
The pelvis is divided into two parts by the pelvic inlet (also called the pelvic brim):
- Greater pelvis (above the brim)
- Lesser pelvis (below the brim)
The brim is formed by:
- Anterior border of the pubic symphysis (upper margin)
- Ala of sacrum (arcuate line) on each side
- Pectineal line (pecten pubis)
- Pubic crest
- Sacral promontory (posteriorly)
Anatomical Position of the Pelvis
Hold the bony pelvis with a forward tilt so that:
- The anterior superior iliac spines and pubic tubercles are in the same coronal plane.
- The upper border of the pubic symphysis is at the same level as the tip of the coccyx.
- The dorsal wall of the bony pelvis faces mostly upward, while the ventral wall faces downward.
Pelvimetry
Pelvimetry means measuring the pelvis. It is important in:
- Obstetrics (childbirth)
- Forensic science
- Anthropology
Methods of pelvimetry:
- On a skeletonised pelvis (from cadavers)
- Clinical and internal pelvimetry (on living subjects)
- Radiological pelvimetry
Pelvimetry on Skeletonised Pelvis
The relevant diameters are shown in Table 15.2.
TABLE 15.2 - Dimensions of the Female Pelvis
| Region | Diameter | Average (cm) |
|---|---|---|
| I. Pelvic Inlet | 1. Anteroposterior diameter (true conjugate) - from midpoint of sacral promontory to upper border of pubic symphysis | 11 |
| 2. Transverse diameter (maximum) | 13 | |
| 3. Oblique diameter - from one iliopubic eminence to the opposite sacroiliac joint | 12 | |
| II. Pelvic Cavity | 1. Anteroposterior diameter - from midpoint of S3 vertebra to the middle of posterior surface of pubic symphysis | 12 |
| 2. Transverse diameter | 12 | |
| 3. Oblique diameter - from the lowest point of one sacroiliac joint to the midpoint of the opposite obturator membrane | 12 | |
| III. Pelvic Outlet | 1. Anteroposterior diameter - from tip of coccyx to inferior margin of pubic symphysis (maximum) | 13 |
| 2. Transverse diameter (bituberous) - between ischial tuberosities | 11 | |
| 3. Oblique diameter - from midpoint of sacrotuberous ligament on one side to the junction of ischiopubic rami on the other side | 12 |
Clinical Pelvimetry
Pelvic measurements in obstetric cases can be done both externally and internally.
External pelvimetry is mostly no longer used because it has limited value. It is helpful only in diagnosing a grossly contracted pelvis.
Key Conjugate Diameters:
-
External conjugate (Baudelocque's diameter):
- Distance between the upper margin of pubic symphysis and the tip of the spine of S1 vertebra.
- Normal: not less than 19 cm.
-
True conjugate:
- The anteroposterior diameter of the pelvic inlet.
- Extends from the midpoint of the sacral promontory to the upper border of the pubic symphysis.
-
Diagonal conjugate:
- Extends from the midpoint of the promontory to the lower border of the anterior surface of the pubic symphysis.
- Can be measured by digital examination per vaginum (P/V examination).
- Usually 12.5 cm.
- True conjugate is calculated by subtracting 1.5 to 2 cm from the diagonal conjugate.
-
Obstetrical conjugate:
- Distance between the sacral promontory and the inner aspect of the pubic symphysis.
- It is 2 cm less than the diagonal conjugate.
Note - Internal Pelvimetry: This is a digital examination per vaginum (P/V examination) to measure various pelvic diameters. It is used to assess:
- Curvature of sacrum
- Mobility of coccyx
- Length of sacrospinous ligaments It helps determine the diagonal conjugate, interischial spinous diameter, and to calculate the obstetrical conjugate.
Morphological Classification of the Pelvis
(Fig. 15.19, Flowchart 15.5)
There are 4 types:
1. Gynaecoid Type (41.4%) - Normal Female Pelvis
- Average diameters (as already described above).
- Inlet is round or slightly oval.
- Transverse diameter is placed well forward from the sacrum.
- Side walls are more vertical than in the android pelvis.
2. Android Type (32.5%) - Resembles a Male Pelvis
- Inlet is triangular.
- Greatest transverse diameter is placed much nearer to the promontory than in the gynaecoid pelvis.
- Subpubic angle and greater sciatic notches are narrower.
- Cavity is funnel-shaped.
- Outlet is reduced in all diameters.
3. Anthropoid Type (23.5%)
- Resembles the pelvis of anthropoid apes.
4. Platypelloid Type (2.6%)
- Somewhat opposite to the anthropoid pelvis.
Out of all four types, only the gynaecoid pelvis allows normal delivery. The other three are types of contracted pelvis.
Sex Differences in the Pelvis
The differences exist mainly due to the adaptation of the female pelvis for childbearing.
TABLE 15.3 - Male vs Female Pelvis
| Feature | Male Pelvis | Female Pelvis |
|---|---|---|
| Bony feature | Heavier and thicker | Lighter and thinner |
| Muscle attachments | Well-marked | Less marked |
| Subpubic angle | <90° (50°-60°) | 90° or more |
| Pelvic inlet | Heart-shaped | Oval |
| Pelvic cavity | Funnel-shaped (long segment of small cone) | Tubular (small segment of large cone) |
| Acetabulum | Large; diameter equals the distance between acetabulum and symphysis pubis | Small; diameter is smaller than the distance between acetabulum and symphysis pubis |
| Obturator foramen | Large, oval | Small, triangular |
| Greater sciatic notch | ~70° (narrower) | 90° (wider) |
| Preauricular sulcus | Less marked | Well-marked, prominent |
| Ischial tuberosity | Inverted | Everted |
| Base of sacrum | Transverse diameter of anterior surface of S1 body is longer than the ala of one side | Transverse diameter of superior surface of S1 body is shorter than the ala of one side |
| Curvature of sacrum | Smoothly curved anterior surface (C-shaped) from above downward | Abruptly curved lower part of anterior surface (S-shaped) |
| Length of sacrum | Length is more than the maximum width | Length and width are usually equal |
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