The vertical development of the face, particularly the lower face, has a profound effect on facial aesthetics. When the lower face (Ans to Mn) is deficient in relation to saggital development, the facial contours are adversely affected. This is particularly true when the tegumental tissues are thin, but a thick integument will usually camouflage some of the adverse effects.

When there is little or no increase in Ans to Menton distance, the lower incisors must not be moved posteriorly with relation to the mandible. Lingual movement of the incisors, under these circumstances, will have an adverse effect on facial contours.

When there is excessive increase in Ans to Menton distance, incisors can be moved lingually a great deal without adverse effect on facial contours. Please see Fig. 1.

Fig. 1 These are the profiles of two patients, showing the effects of the vertical dimension of the face. Please see text.

In this illustration is shown the profiles of two patients' before and after records. It may be noted in (A) that 3 mm of lingual movement of the mandibular incisors had almost no effect on lip contours, while in (B) 1.5 mm of lingual movement had a marked adverse effect. Also it may be noted in (A) that the SN-MP angle was 38.5° and increased to 40°; in other words the angle was steep and it became steeper. In (B) angle was 31°, and decreased to 27.5°; it was flat and became flatter. Thus, the size of the original SN-MP angle, plus the amount and direction of rotation, had more effect on facial aesthetics than the actual movement of the incisors on their bases. This is why we must use a different set of diagnostic rules in dealing with individuals with high and low SN-MP angles.

I hope I may be forgiven for a slight digression. The correct inclination and position of the mandibular incisors are associated with: (1) the size of the SN-MP angle, (2) the changes in the inclination of the mandibular plane, (3) the relative length of the mandible, (4) the size of the ANB angle and (5) the size of the effective symphysis. Please see Fig. 2. We can compromise the inclination of the lower incisor with impunity, but we must not compromise its position with relation to line NB.

Figure 2. This shows the effect of the position of the lower incisor on the facial aesthetics. Please see text.

These principles are extremely important, and are easily overlooked. If the orthodontist is unaware of the importance of these principles, he or she may miss a golden opportunity to perform a beautiful, lasting, life long service for a patient; and what is probably more important, fail to make an admiring friend for life.

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