Preliminary study of treatment mechanism and stability in deep overbite malocclusion with hyperdivergent and hypodivergent skeletal pattern

Comparison of structural characteristic changes between deep overbite malocclusions with hyper- and hypodivergent skeletal pattern during treatment and follow-up stages. To discuss the difference between both in treatment mechanism and characteristic of relapse.59 cases out of 162 finished patients with deep overbite malocclusion followed for more than 2 years, selected from orthodontic clinic in school of stomatology Peking University, were filtered by cephalometrics (MP-SN>40 degrees or <29 degrees)and then two groups were formed. The hyperdivergent group (MP-SN>40 degrees) comprised 10 cases and the hypodivergent group (MP-SN<29 degrees) comprised 9 cases. Measurements and statistics were performed upon the two groups.After orthodontic treatment, the hyperdivergent group exhibited the extrusions of upper and lower molars [U6-PP increased from (10.69+/- 2.10)mm to (12.91+/-2.19)mm; L6-MP increased from (17.34+/-2.73)mm to (20.86+/-3.08)mm]. As to the skeletal change, a decrease of ANB angle from (5.86+/- 1.34) degrees to (4.62+/-1.67) degrees and NA-PA angle from (11.99+/-3.74) degrees to (8.76+/-4.45) degrees reflecting the sagittal change tendency from skeletal Class II to Class I. In vertical direction, the ratio of Ar-Go/ANS-Me increased from (0.58+/-0.03) to (0.62+/-0.05). Compared with the hyperdivergent group, the hypodivergent one only showed the extrusion of lower molars after treatment. The stability of overbite didn't show any statistically significant difference between two groups during the follow-up stage. Also in this stage the change of overbite in the hyperdivergent group was mainly due to the lingual inclination of upper incisors during the treatment stage and in the hypodivergent group it was due to the molar extrusion during treatment.The ramus showed great potential of growth in the hyperdivergent group during the treatment stage, which maintained the inclination of mandibular plane. The relapse of overbite was correlated with the changes of teeth during the treatment and follow-up stages.PubMed04425-83