Abstract
The bony pelvis consists of four bones (two iliac (innominate) hip bones, sacrum and coccyx), and four joints in the articulated pelvis (i.e., two sacroiliac joints, pubic symphysis and sacrococcygeal joint. Both the anterior superior iliac spines and the upper end of the pubic symphysis lie in the same coronal plane, and the tip of coccyx corresponds with the upper margin of the pubic symphysis. The pelvic surface of the body of pubis is directed more upwards than backwards, and the pelvic surface of the sacrococcygeal curve faces more downwards than forwards.
1 Bony Pelvis
The bony pelvis consists of four bones (two iliac (innominate) hip bones, sacrum and coccyx), and four joints in the articulated pelvis (i.e., two sacroiliac joints, pubic symphysis and sacrococcygeal joint). Both the anterior superior iliac spines and the upper end of the pubic symphysis lie in the same coronal plane, and the tip of the coccyx corresponds with the upper margin of the pubic symphysis. The pelvic surface of the body of pubis is directed more upwards than backwards, and the pelvic surface of the sacrococcygeal curve faces more downwards than forwards.
The female pelvis has been classified into four types according to its shape by Caldwell and Moloy. See Figure 3.1 and Table 3.1.
| Gynaecoid | Anthropoid | Android | Platypelloid | |
|---|---|---|---|---|
| Occurrence | 50% | 25% | 20% | Approx. 5% |
| Shape of inlet | Round | Oval anteroposterior | Triangular | Transversely oval |
| Sacrum | Well curved from above, downwards and side to side | Long and narrow | Straight and inclined forwards | Short, straight and inclined posteriorly |
| Sacrosciatic notch | Wide and shallow | More wide and shallow | Shallow | Small and narrow |
| Cavity | Wide and shallow | Wider | – | – |
| Pelvic side wall | Straight or slightly curved | Straight | Convergent | Divergent |
| Ischial spine | Not prominent | Prominent | Prominent | Not prominent |
| Subpubic angle | Wide (85 degrees) | Short and narrow | Narrow | Wide with short pubic arch |
| Bituberous diameter | Normal | Short and narrow | Short | Spacious |
| Outcome | Good for obstetric outcome | High incidence of face to pubis delivery | Pelvic delivery is difficult with higher incidence of perineal tear | Outcome is generally good |
Android is most common in males
Anthropoid and android are more common in white Women
Anthropoid and gynaecoid are common in black Women
Platypelloid is observed in osteomalacia or rickets
1.1 Effects of Hormones on Pelvic Joints in Pregnancy
Increased levels of estrogen, progesterone and relaxin in the latter half of pregnancy leads to increased movement of pelvic joints (due to the softening of the ligaments of the sacroiliac and sacrococcygeal joints and increase in size of the interpubic disc).
Relaxation of the sacroiliac joints and pubic symphysis permits 10–15% increase in transverse diameter).
Relaxation of ligaments leads to less effective interlocking mechanism of sacroiliac joints. This causes greater rotation of the pelvis and the lordotic posture during pregnancy.
1.2 Effect of Age
Obliteration of the cavity in the sacroiliac joint occurs in both sexes after middle age.
2 Function
The primary function of the pelvis is to transfer the body weight from the axial skeleton to the appendicular skeleton for standing and walking. In the female, it is also adapted for childbearing.
Anatomically, the pelvis is divided into greater or false pelvis and lesser or true pelvis. The entrance to the lesser pelvis or the pelvic brim is bounded by the upper part of the pubic symphysis, pubic crest, pubic tubercle, pectineal line, iliopubic eminence, arcuate line of ilium, anterior margin of ala of sacrum, sacral promontory. See Figure 3.2 and Figure 3.3.
Figure 3.2 The pelvis: greater or false pelvis;
lesser or true pelvis.
An inlet/pelvic brim
Pelvic cavity
An outlet
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