Patterns of change in mandibular and facial shape associated with the use of forces to retract the maxilla

Abstract

A method is presented for generating simplified graphic-numeric representations of patterns of change in craniofacial size and shape through time. The procedure uses as input any desired set of linear head film measurements made along different directions of biologic interest at different time points. By a simple arithmetic operation, the rate of change of each measure between two time points is expressed as a proportion of the measure's original value. The proportions for the several measures are then re-expressed in terms of a common reference standard yielding a numerical pattern of change through time. This pattern can be used to distinguish regular enlargement of the structure (change in size) from disproportionate enlargement (change in shape). Using this method, we have examined as a unified entity the data for the nine linear measures of mandibular and facial change which were considered separately in the preceding article. Between-treatment differences in patterns of change among groups of measures have been demonstrated somewhat more clearly than was possible with the conventional univariate analyses which we had performed previously. An important general observation is that the rate of increase in the reference measure, sella nasion distance, is smaller than that of any of the other measures with the exception of pogonion-menton. This strongly corroborates the notion that there is far more “downward” than “forward” displacement of facial structures during the developmental stage being studied. A second general observation is that in each of these four groups of Class II subjects, lower face height values are greater as compared to upper face height than is usually considered to be the case in more nearly “ideal” subjects. But, to a large extent at least, this finding is an artifact of the way in which we measure lower face height. This is because when lower face height is defined as the distance from ANS to menton, the more menton is retruded with respect to the facial profile, the longer the measured value will become and the larger its undesired horizontal component will be. The most striking finding of pattern of change in the cervical group is that all measures which have a major vertical component appear to increase relative to the reference standard at rates greater than those observed in the untreated control sample. The largest rate of increase among the mandibular measures is that for ramus height. Such a change would tend to keep the mandibular plane orientation relatively unchanged in the presence of the observed increase in anterior face height. The most striking finding in the high-pull face-bow group as compared to the untreated control group is the reduction in the rate of increase in lower face height while the rate of change of total face height remains essentially unaltered. The most parsimonious explanation for these apparently contradictory findings is that the reduction in the lower face height value in the high-pull group results from restraint upon the forward movement of ANS rather than upon the downward movement of menton. Perhaps the most interesting finding in the intraoral group is the observation that condylepogonion distance increases relative to sella nasion distance at a much more rapid rate than in the control group. Concurrently, the analogous values for mandibular body length are lower in the high-pull group while those for ramus height are higher. Taken together, these findings argue strongly that the intraoral treatment does, in fact, produce alterations in biologic activity in the region of the condyle. The observed patterns of change in the mandible appear to be generally similar in the control group and the high-pull face-bow group on the one hand and in the intraoral group and the cervical group on the other hand. The similarity between the cervical group and the intraoral group is unexplained and is particularly puzzling. The main limitations of the transformational method that we have used here arc that it is inappropriate for use in individual case analysis, that it contains no measure of within-treatment variability, and that anomalies in the reference measure may cause systematic errors in the values of all other measures when growth rate coefficients are employed. Used with due regard for these limitations, we believe that the method is a useful addition to the repertoire of quantifying tools in roentgenographic cephalometry.