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Effect of molar rotation on arch length
J L McDonald1, F S Shofer, J Ghafari
1University of Pennsylvania, 4001 Spruce Street, Philadelphia, PA.
Clinical Orthodontics and Research
|September 13, 2001
Summary
Maxillary first molar rotation significantly impacts arch length during Class II malocclusion treatment with straight-pull headgear. Molar rotation contributes to space gain, but distal movement involves translation and tipping.
Area of Science:
- Orthodontics
- Craniofacial Biology
- Dental Biomechanics
Background:
- Class II, division 1 malocclusion is a common dental anomaly requiring orthodontic intervention.
- Straight-pull headgear is frequently used to distalize maxillary molars, influencing arch dimensions.
- Understanding molar rotation's effect on arch length is crucial for treatment predictability.
Purpose of the Study:
- To evaluate the impact of maxillary first molar rotation on arch length.
- To quantify the relationship between molar rotation and distal displacement during Class II treatment.
Main Methods:
- Prospective clinical trial involving 19 children with Class II, division 1 malocclusion treated with straight-pull headgear.
- Monthly adjustments of headgear delivered 14-16 oz distalizing force per side.
- Dental casts measured using digital calipers to compute molar rotation and displacement relative to a reference system.
Main Results:
- High correlations observed between molar rotation and tooth displacement (r=0.9, p=0.0001 left; r=0.8, p=0.0001 right).
- Average molar rotation was 15.4 degrees (left) and 10.8 degrees (right).
- Molar rotation's contribution to space gain (approx. 1 mm) was less than total distal movement (approx. 3 mm), indicating combined rotation and translation.
Conclusions:
- Maxillary first molar rotation is a significant factor influencing arch length changes during Class II treatment.
- Distal movement of molars involves both rotation and translation/tipping.
- Molar rotation's contribution to space gain is more pronounced with smaller overall tooth displacement.
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