Cephalo-Pelvic Disproportion

Abstract

At birth the foetal spine and sacrum are almost in a straight line and there is little forward projection of the promontory. The pelvic cavity is narrow transversely, owing to the poor development of the lateral masses of the sacrum. The whole pelvis is long, narrow and funnel-shaped, while the pelvic inclination is marked. In the growing child trunk pressure is applied to the first sacral vertebra and any tendency for the lower end of the sacrum to move backwards is checked by the sacro-sciatic ligaments; this produces curvature of the sacrum at the level of its third vertebra. As the sacrum is pushed down into the pelvis by body weight the ilio-sacral ligaments are stretched and the posterior iliac spines tend to approximate to the mid-line. These changes would produce separation of the innominate bones at the symphysis pubis, if it were not for the strong ligaments there which resist the outward and upward pressure on the acetabula exerted by the femora. The pelvis is widened by the gradual growth of the lateral masses of the sacrum, and muscular action assists this by everting the edges of the iliac crests and descending pubic rami. Harmonious co-operation between these factors is essential for the production of the normal pelvis; distorted forms are due to abnormalities in growth and application of body pressure, and to diseases of bones and joints, abnormal endocrine influences and reduced resistances.