Analysis of forces and moments in arch guided molar protraction using Class I and Class II elastics. An in-vitro study.

The use of class I and II elastics in arch guided tooth movement of the lower molars belongs to the proven clinical methods to achieve space closure even though risks are present. The vertical force component of class II elastics tends to interact with the sagittal force and thus the vertical force may change the desired sagittal force and movement direction. The objective of the study presented here was to investigate friction behavior and the movement dynamics of the arch guided protraction of the lower first molar being acted on by differing class I and class II elastic band geometries. The influence of class I and class II elastics at different force levels (1 N and 2 N) were studied. The pattern of the force line varied in the area of angulation from 0 degree to 40 degrees relative to the arch plane. The orthodontic measurement and simulation system (OMSS) was employed to determine force loss due to friction and to analyze side effects. In the arch guided mesialization of the lower first molar, the vertical component of class II elastics induces a minor force loss in comparison with class I elastics. This holds, however, only for the lower 1 N force level. When employing class II bands at a greater force level and with increased angulation, relatively greater force loss and increased side effects, such as extrusion and mesial tipping of the first molar, occur.