MOULDING OF THE PELVIC OUTLET

THE Hippocratic treatise “The Nature of the Infant” indicated that the pelvic canal of the mother was too small to allow delivery, and that during delivery of the first child a physical separation of the bones of the pelvis occurred (Graham, 1950). By the nineteenth century it had been realized that a reversible moulding of the pelvis occurred and Duncan (1854) noted that all the diameters of the outlet increased during delivery. The mechanism of this expansion was thought to be a simple diastasis of the pelvic joints, in particular at the pubic symphysis (Snelling, 1870). A dissenter was Laborie (1862) who suggested that as the head reached the outlet and the ischial tuberosities were separated, the iliac crests moved towards each other, so that there was a rocking movement about the symphysis pubis, rather than a diastasis. The use of radiology demonstrated that only rarely does significant separation of the pelvic joints occur during labour (Heyman and Lundqvist, 1932; Abramson et al,, 1934). The lower sacrum was found to swing backwards during delivery (Borell and Fernstrom 1957a). Weisl (1955) and Borell and Fernstrom (1957b) showed that this sacral movement, which increases the sagittal diameter of the outlet, occurred not only during delivery but also with postural changes of the mother. Sliding movements of the sacro-iliac joints also occur. Borell and Fernstrom (1960) suggested that measurements of the movement at the sacro-iliac joint gave an indication of pelvic “mouldability”, and that knowledge of the magnitude of this movement might help in the antenal assessment of patients who had small pelves. The sagittal diameter increase was small in cases of protracted labour (Borell and Fernstrom 1957a). ANATOMY The sacro-iliac joint was recognized as a synovial joint by Luschka (1854). The joint space is roughly ) shaped, with the apex pointing forwards, and a shorter upper portion passing backwards and upwards, and the lower portion backwards and downwards. Weisl(l954) describes the upper limb as having an appreciable convexity on the lateral border of the first sacral vertebra which is received by a concavity on the iliac aspect. In the lower half of the joint the anterior surface is more lateral than the posterior, so that the two joints, in their lower half, form a segment of a wedge pointing backwards. It seems likely that a backwards movement of this part of the joint must inevitably lead to a separation of the ischial spines. Similarly, at the top of the joint the coronal diameter is wider than at the lower part, so that a downwards movement of the sacrum into the pelvic girdle will lead to an increase in the coronal pelvic diameters. The rotation of the sacrum with postural changes about horizontal coronal axis has been demonstrated in both sexes, and there is no difference between males and non-pregnant females (Weisl, 1955). Irregularities of the joint surfaces become more marked with increasing age, and in the elderly the movements may cease, and arthrodesis of the joints develops (Weisl, 1954). Weisl found that the axis of rotation was often in front of the sacrum.