Effects of a modified activator: activator combined with J hook headgear in the treatment of hyperdivergent class II malocclusion during the late mixed teeth period

The aim of this study was to compare the effect of a combination of activator and J hook headgear to that of the headgear activator. The traditional activator was modified to combine with a J hook headgear which had two hooks mesial to the upper canine for extraoral traction instead of the labial arch. In addition, there is an optional screw in the middle of the palatal parts to widen the arch in narrow arch cases. Sixty eight cases (34 boys and 34 girls, with an average age of 10.2 years old) were recruited. They were divided into two groups (Group MA: modified activator and J hook headgear group, Group HA: headgear activator combination group). Lateral standardized cephalograms were taken before and after the treatment. The cephalograms were measured by the same doctor with the Dolphin software. There was no significant difference in the changes of SNA, U1-NA, U1-NA (mm), U1-SN, and U6-PP between the two groups. However, the changes in U1-PP showed that the MA group intrudes the upper frontal teeth while the HA group does not (P<0.001). Regarding the mandible, there were no significant differences in SNB and L1-MP (degrees); however, L1 MP (mm), NP_FH (degrees) and LL-Eline (mm) showed significant differences (P<0.001). The MA group intruded the frontal teeth and rotated the mandible anticlockwise, which was also confirmed by the changes of the OP-SN (degrees), MP-SN (degrees), and np-fh (degrees) (P<0.001). Compared to the headgear activator, the modified activator can level the frontal teeth, decrease the angle of the MP SN and MP FH, and rotate the mandible counterclockwise, which could achieve a better chin and lip outline.