Changes in the jaw relationships during human foetal cranio-facial growth.

Previous investigations into pre-natal cranio-facial growth have either been purely descriptive or have involved the use of cephalometric points, lines and angles. Neither methods have given an entirely clear picture of the changes taking place during the foetal period. The present investigation measured changes in shape, position and orientation of the jaws by using image outlines and their centroids (centres of area) in combination with the analytic method of morphanalysis. The image outlines of the foetuses were related to each other via a rectangular co-ordinate reference grid so that the exact sites where growth changes occurred could be plotted and isolated. It was found that the jaw relationships were constantly changing during the foetal period. Brain growth appeared to predominate which flattened out the cranial base and determined the definitive position of the naso-maxillary segment to which the musculature of the lower face in turn adjusted mandibular growth. The jaw relationships changed in the direction Class III to Class II mainly as a result of forward growth of the naso-maxillary segment, the mandible showing a lag followed by a catch-up regaining its original position. The two jaws exhibited different growth patterns which suggested differences in the underlying mechanisms responsible for their growth. This is almost certainly an important factor in the aetiology of malocclusion resulting from malrelationships of the basal bones and soft tissues.