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Engagement of the Head does not depend only on the size and shape of the brim, but also on the angle of inclination of the brim. The methods of determining this inclination are considered and the angle between the plane of the brim and the front of the body of the 5th lumbar vertebra is found to furnish the best index of the inclination. Analysis of a series of cases shows that this angle varies considerably. Its postural range is demonstrated.
When the inclination is high the head does not easily engage although the measurements may be normal, and a high inclination is one of the commonest causes of unexpected dystocia.
Because these cases are usually selected for a “trial of labour”, criteria are necessary to select the cases suitable. Success or failure of trial labour in these cases depends on the amount of room in the upper pelvis. A part from the actual size of the true conjugate the amount of room is shown to depend both on the sacral inclination, a method of measuring which is described, and the shape of the upper sacrum, which shows considerable vriation. A common type of pelvis causing dystocia is one in which the inclination of the brim is high and the upper sacrum is relatively vertical and convex. Pelves of this type do not always fit into any of the standard classifications.
The uses and limitations of postural treatment of these cases are discussed.