Changes in canine position during maxillary molar distalization with mini-implant and CD appliance: an in vitro study

To investigate changes in the three-dimensional position of the maxillary canine during the distal movement of the maxillary first molar by a mini-implant combined with a CD appliance.

Ten typodont models of class Ⅱ malocclusion were selected, and one side was randomly chosen as the experimental group. The CD appliance was bonded to the maxillary canine and first molar of the experimental group, and 1.2 mm stainless steel wire was bent as the anchorage, which was fixed on the model to simulate mini-implants implanted in the zygomatic alveolar ridge. Then, 180 g orthodontic force was applied to the canine of the experimental group; the other side was recognized as the control group and was not used for strengthening. Six mini-implants were implanted in different parts of the model and used as a reference before and after the experiment. The models were placed in an incubator and heated at 56 ℃ in a water bath for 2 min. The models were scanned before and after thermostatic water bath treatment with a 3-shape scanner. Then, the digital models were overlapped through the reference points, and the positions of the canines and first molars were measured before and after the experiment.

The sagittal movement distance of the first molar in the experimental group was ( 0.25 ± 0.33) mm, and the vertical movement distance was (0.25 ± 0.28)mm, which was significantly different from the control group (P < 0.05), while the transverse change was not significantly different (P > 0.05). There was no significant difference in the root position of the first molar in the experimental group (P > 0.05). The lateral and vertical displacement distances of the maxillary canine crown in the experimental group were (4.03 ± 2.11) mm and (1.86 ± 1.01) mm, respectively, which were significantly different from those in the control group (P < 0.001), while the sagittal changes showed no significant differences (P > 0.05). In the experimental group, there was no significant difference in the position of the apex of the maxillary cusp (P > 0.05).

Our in vitro study showed that the maxillary canines inclined buccally accompanied by a small amount of intrusion during molar distalization by a mini-implant combined with a CD appliance.