Tooth size is actually the 'seventh key' to normal occlusion. It is clear that the Andrews' non-orthodontic normal models had balanced tooth size. If not, there would have been either spacing in one arch or crowding in the opposing arch.
As stated above, it is common to see a lack of tooth mass in the upper anterior segment relative to the lower anterior segment. Tooth size discrepancy frequently contributes to the situation. The most common anterior tooth size discrepancy consists of small lateral incisors in the upper arch and/or large lateral incisors in the lower arch. In the buccal segments, small upper second premolars frequently contribute to the tooth size discrepancy.
Evaluation of tooth size discrepancy can be carried out by using the Bolton analysis.' Tooth size discrepancy may be corrected either by reducing tooth mass in one arch with inter-proximal enamel reduction (usually the lower incisors) and/or by addition of tooth mass with restorative materials in the opposing arch (usually the upper lateral incisors).
It is more common to find an excess of tooth substance in the lower arch. If the Bolton analysis confirms this, it is often advisable to carry out inter-proximal enamel reduction in the lower anterior region in the initial stages of treatment (Case MS, p. 236). Only minimal amounts of tooth mass should be removed from the upper anterior segment, early in treatment. As the finishing stage of treatment is approached, the relative tooth mass in the upper anterior segment can be evaluated. If there is an excess of upper anterior tooth mass as a result of lower tooth mass reduction, then stripping procedures can be carried out in the upper anterior segment. If enamel reduction is done in the upper arch too early in treatment, spacing may result, which can only be corrected by the addition of bonding material.
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