The pelvic girdle supports and protects the contained viscera and affords surfaces for the attachments of the muscles of the trunk and lower limb. Its most important mechanical function, however, is to transmit the weight of the trunk and upper limbs to the lower extremities.

It may be divided into two arches by a vertical plane passing through the acetabular cavities; the posterior of these arches is the one chiefly concerned in the function of transmitting the weight of the trunk. Its essential parts are the upper three sacral vertebra and two strong pillars of bone running from the sacro-iliac joints to the acetabular fossa. For the reception and diffusion of the weight each acetabular fossa is strengthened by two additional bars running towards the pubis and the ischium. In order to lessen concussion in rapid changes of distribution of the weight, joints (sacro-iliac articulations) are interposed between the sacrum and the iliac bones: an accessory joint (symphysis pubis) exists in the middle of the anterior arch.

Figure 548
Anterior sacral segment coronal section - Figure 548
The sacrum forms the summit of the posterior arch; the weight transmitted falls on it at the lumbosacral joint and, theoretically, has a component in each of two directions. One component of the force is expended in driving the sacrum downwards and backwards between the iliac bones, while the other thrusts the upper end of the sacrum downwards and forwards towards the pelvic cavity.

The movements of the sacrum are regulated by its form. Viewed as a whole, it presents the shape of a wedge with its base upwards and forwards. The first component of the force is therefore acting against the resistance of the wedge, and its tendency to separate the iliac bones is resisted by the sacroiliac and iliolumbar ligaments and by the ligaments of the symphysis pubis.

Figure 549
Middle sacral segment coronal section - Figure 549
If a series of coronal sections be made through the sacro-iliac joints, it will be found possible to divide the articular portion of the sacrum into three segments: anterior, middle, and posterior. In the anterior segment (fig. 548), which involves the first sacral vertebra, the articular surfaces show slight sinuosities and are almost parallel to one another. In the middle segment (fig. 549) the width between the dorsal margins of the sacral articular surfaces is greater than that between the ventral margins, and in the center of each surface there is a concavity intro which a corresponding convexity of the iliac articular surface fits, forming an interlocking mechanism. In the posterior segment (fig. 560) the ventral width of the sacrum is greater than the dorsal; and the articular surfaces are only slightly concave.

Dislocation downwards and forwards of the sacrum by the second component of the force applied to it is prevented therefore by the middle segment, which interposes the resistance of its wedge-shape and that of the interlocking mechanism on its surfaces; a rotatory movement, however; is produced by which the anterior segment is tilted downwards and the posterior upwards the axis of this rotation passes, through the dorsal part of the middle segment. The movement of the anterior segment is slightly limited by its wedge-form, but chiefly by the posterior and interosseous sacro-iliac ligaments that of the posterior segment is checked to a slight extent by its wedge-form, but the chief limiting factors are the sacrotuberous and sacrospinous ligaments. In all these movements the effect of the sacroiliac and iliolumbar ligaments and the ligaments of the symphysis pubis in resisting the separation of the iliac bones must be recognized.

Figure 550
Posterior sacral segment coronal section - Figure 550
During pregnancy the pelvic joints and ligaments are relaxed, and capable therefore of more extensive movements. When the fetus is being expelled the force is applied to the front of the sacrum. Upward dislocation is prevented by the interlocking mechanism of the middle segment. As the fetal head passes the anterior segment the latter is carried upwards, enlarging the anteroposterior diameter of the pelvic inlet; when the head reaches the posterior segment this also is pressed upwards against the resistance of its wedge, the movement being rendered possible only by the laxity of the joints and the stretching of the sacrotuberous and sacrospinous ligaments.


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