d) ProblemsA problem arises when considering the space left for molar adjustment. If this value in

the chart is negative, that is, the predicted sizes of 3, 4, and 5 are greater than the space left after the alignment of the incisors,' then crowding will occur in the arch even without any forward molar adjustment. When the first permanent molars are in an end-to-end relationship (i.e., a flush terminal plane of the second primary molars), approximately 3.5 mm of space (one-half a cusp width) is required to convert to a Class I molar relationship. This needed 3.5 mm might be'acquired, without orthodontic intervention, in any of three ways: (1) 3.5 mm more late mesial shift of the mandibular first permanent molar than the maxillary; (2) at least 3.5 mm more forward growth of the mandible than the maxilla; or (3) some combination of dental adjustment and differential skeletal growth. Since,we cannot yet predict accurately the amount of differential skeletal growth that will occur, treatment planning must be based on dental adjustment factors. If differential skeletal growth occurs during this period, alterations in the molar relationship will result and the Mixed Dentition Analysis predictions must be altered accordingly. When there is a Class I molar relationship in the mixed dentition (mesial step of the second primary molars), no part of the arch perimeter need be preempted for molar adjustment and all the space can be made available for incisors, cuspids, and bicuspids.

It has become the fashion in many Mixed Dentition Analysis procedures to assume that every child' will require precisely 1.7 mm of late mesial shift. Such fallacious reasoning is unfortunate, since it leads to errors in treatment planning. One cannot assume average mesial shift or leeway space values any more than one can assume average tooth sizes. As was stated earlier, though some children will require no mesial shift of the first permanent molars (Class I molar relationship), the greatest number of children will require approximately 3.5 mm late mesial shift or differential skeletal growth (end-to-end molar relationship). It is best to quantify the amount of mesial shift necessary to bring the molars into a Class I relationship (see Fig 11-15). The treatment plan is then devised to accomplish the precise amount of correction needed in each arch.

Perhaps the most severe limitation of Mixed Dentition Analyses is their inability to reflect the position of the incisors with respect to the skeletal profile. There are a number of crude rules of thumb for determining how much arch perimeter reduction occurs for each degree or millimeter the incisor edge is changed in the cephalometric visualization of treatment. For example, one degree of tipping or 1 mm of lingual displacement of the mandibular incisal edge is said to be equal to 1 mm of arch shortening on each side. Useful as such estimates are the simple truth remains: We must use some clinical judgment to fit the Mixed Dentition Analysis into the facial skeleton both at the profile and over the apical bases.

A problem is imposed when the occlusal curve is assumed to be a flat plane. Mixed Dentition Analyses assume, by projection to a flat plane, no vertical occlusal curve. Often in the mixed dentition there is indeed a flat occlusal plane, but many times the Curve of Spee is exaggerated or complicated. Enhanced and elaborated occlusal curves are, unfortunately, seen frequently with some of the most severe malocclusions. The arch perimeter is usually longer than it is measured when the Curve of Spee is exaggerated or shows complicated vertical curves. As a

result the clinician may assume there is sufficient space for all the teeth until the arch is made level during treatment. Then it becomes obvious that the Mixed Dentition Analysis is a two-dimensional visualization of a complex three-dimensional problem.