Assessment of the relationship between the upper and lower arch changes with the opening of the expanding screw after the rapid maxillary expansion.

Objective: The purpose of this study is to assess the relationship of the upper and lower arch dimension changes after the rapid maxillary expansion (RME) with the screw opening of the modified Hyrax® type appliance.

Materials and methods: Initial (T1) and final (T2) models of study of 63 children between 7 years and 6 months and 16 years and 5 months were performed. From these, 21 showed maxillary atresia and/ or posterior crossbite and were submitted to expansion and 42 children formed the control group .The appliance used in this work was the modified Hyrax appliance using the 7mm expanding screws "Dentarum". The measures of the upper intercanine and the upper and lower intermolar distances were performed in plaster models with the help of a digital paquimeter. The opening of the appliance's screw was also assessed.

Results: The RME provoked a statistically significant increase in the upper interdental distances and no statistic significance in the lower interdental distances, i.e. the change in the 16 - 26, 13 - 23 and 36 - 46 distance was of 5,85 mm, 5,31 and 0,34 mm respectively. The average opening of the expander screw, which was of 6,27 mm.

Conclusion: The appliance's opening is related to the interdental distance changes, i.e. for each appliance's opening mm, a 0,94 variation in the upper intermolar distance, a 0,87 in the upper intercanine distance and 0,054 in the lower intermolar distance are provoked.