Oct. 9 (Tue.), 2001
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9. Differential diagnosis of Class III malocclusions -- 1
Class III malocclusions have traditionally been divided into easy-to-treat and difficult-to-treat categories.
Pseudo mandibular prognathism True mandibular prognathism
Easy to treat Difficult to treat
Classification Angle Class I Angle Class III
Facial profile Relatively good Protrusive mandible and chin
Concave mid-face
Occlusion Deep anterior overbite Shallow anterior overbite
(open bite)
Upright upper incisors Labially inclined upper incisors
Pseudo mandibular prognathism
 True mandibular prognathism
Easy to treat
 Difficult to treat
Classification
 Angle Class I
 Angle Class III
Facial profile
 Relatively good
Concave mid-face
 Protrusive mandible and chin
Occlusion
 Deep anterior overbite
 Shallow anterior overbite
(open bite)
Upright upper incisors
 Labially inclined upper incisors
Labially inclined lower incisors
 Lingually inclined lower incisors
Normal molar relationship Mesially positioned lower dentition
Cephalogram
 ANB: Normal or slightly smaller
 Negative
FMA: Normal or slightly smaller
 Generally large
Gonial: Normal
 Generally large
Construction
 bite Able to retrude the mandible
 Unable to retrude the mandible
Others
 Dento-aleveolar or functional
 Structural

In day-to-day clinical practice, some crossbite cases do not respond to early crossbite correction starting in the primary dentition. Others respond to early treatment but relapse back to a crossbite in the late mixed dentition, requiring orthognathic surgery later due to mandibular overgrowth. I therefore conducted a longitudinal study in which patients treated in similar ways for similar periods of time were followed over a period of time, rather than a cross sectional study based only on initial data, in order to obtain indices for differential diagnosis.



1) Purpose of the study

The purpose of this study was to investigate the possibility of differential diagnosis based on the assessment of crossbites treated in the early mixed dentition with rapid expansion and protraction of the maxilla to stimulate maxillary growth and forward movement of 'A' point. The following orthodontic appliances were used in the study.
The rapid maxillary expander used in the study was a fixed resin-block appliance with a bite plane effect that fully covers the buccal teeth in the early mixed dentition. After a given amount of expansion, the expansion screw was fixed to initiate maxillary protraction with a commercially available appliance called Face Crib.
2) Study subjects

The study included 40 boys and girls aged 6 to 9 who came to my clinic in and after 1989 and who had all the necessary orthodontic records taken such as initial cephalogram (Ceph 1), dental casts and appliance placement records. After initial records were taken, the rapid maxillary expander was placed to achieve a target of 5mm lateral expansion in 16 to 20 days. Soon after the end of lateral expansion, each patient wore the protractor for about 6 months. Both appliances were then removed to take a cephalogram (Ceph 2).
In the meantime, each patient was instructed to chew hard on Chompers for 1 minute per day in order to achieve occlusal stability. (More recently, I have also been using FR-3 of Frankel.) Ceph 3 was taken 1 to 2 years later when the occlusion appeared to be stable.


The table shows a breakdown of the 40 subjects included in the study.