Anteriorly, the pelvic bones are joined together by the symphysis pubis. This structure consists of fibrocartilage and the superior and inferior pubic ligaments. The latter ligament is frequently designated the arcuate ligament of the pubis. Posteriorly, the pelvic bones are joined by articulations between the sacrum and the iliac portion of the innominate bones to form the sacroiliac joints.
The pelvic joints in general have a limited degree of mobility. However, during pregnancy, these joints relax remarkably at term. As one result, upward gliding of the sacroiliac joint, which is greatest in the dorsal lithotomy position, may increase the diameter of the outlet by 1.5 to 2.0 cm for delivery (Borell, 1957). Sacroiliac joint mobility also likely aids the McRoberts maneuver to release an obstructed shoulder in cases of shoulder dystocia (Chap. 27, p. 521). These changes may also contribute to the success of the modified squatting position to hasten second-stage labor (Cardosi, 1989). The squatting position may increase the interspinous diameter and the pelvic outlet diameter (Russell, 1969, 1982).
FIGURE 2-15 The innominate bone is composed of the pubis (brown), ischium (red), and ilium (blue). Of the three anteroposterior diameters of the pelvic inlet, only the diagonal conjugate can be measured clinically. The important obstetrical conjugate is derived by subtracting 15 cm from the diagonal conjugate.
• Planes and Diameters of the Pelvis
The pelvis is conceptually divided into false and true components. The false pelvis lies above the linea terminalis, and the true pelvis is below this boundary (Fig. 2-16). he false pelvis is bounded posteriorly by the lumbar vertebra and laterally by the iliac fossa. In front, the boundary is formed by the lower portion of the anterior abdominal wall.
he pelvis is described as having four imaginary planes: 1.
The plane of the pelvic inlet-the superior strait.
2.
The plane of the pelvic outlet-the inferior strait.
3.
he plane of the midpelvis-the least pelvic dimensions.
4.
he plane of greatest pelvic dimension-of no obstetrical significance.
he pelvic inlet, also called the superior strait, is the superior plane of the true pelvis. It is bounded posteriorly by the promontory and alae of the sacrum, laterally by the linea terminalis, and anteriorly by the horizontal pubic rami and the symphysis pubis. During labor, fetal head engagement is deined by the fetal head's biparietal diameter passing through this plane.
Four diameters of the pelvic inlet are usually described: anteroposterior, transverse, and two oblique diameters. Of these, distinct anteroposterior diameters have been described using specific landmarks. Most cephalad, the anteroposterior diameter, termed the true conjugate, extends from the uppermost margin of the symphysis pubis to the sacral promontory (see Fig. 2-15). he clinically important obstetrical conjugate is the shortest distance between the sacral promontory and the symphysis pubis. Normally, this measures 10 cm or more, but unfortunately, it cannot be measured directly with examining fingers. hus, the obstetrical conjugate is estimated indirectly by subtracting 1.5 to 2 cm from the diagonal conjugate. To measure the diagonal conjugate, a hand with the palm oriented laterally extends its index finger to the promontory. The distance from the fingertip to the point at which the lowest margin of the symphysis strikes the same inger's base is the diagonal conjugate.
he transverse diameter is constructed at right angles to the obstetrical conjugate and represents the greatest distance between the linea terminalis on either side (see Fig. 2-16). It usually intersects the obstetrical conjugate at a point approximately 5 cm in front of the promontory and measures approximately 13 cm.