Evaluation Of Changes In The Alveolar Bone And Teeth Displacement Of The Upper Incisors Retraction With Mini Implant Anchorage Using Cone-Beam Computed Tomography

Maxillary protrusion is one of the commonest malocclusions especially in Asian orthodontic patients. The position of the anterior teeth has a significant effect on the profile aesthetics. In orthodontic clinical treatment process, orthodontists often choose to retract the anterior teeth extensively to reduce the facial convexity and obtain a pleasing appearance.With the advent and application of mini screw anchorage, the protruding teeth and serious gummy smile can be improved perfectly. Moreover, the incisors can be moved in different directions when the mini implant combines with different anterior arch hook heights.However, there is a dispute about whether the alveolar bone can remold with the teeth movement. Some people think that "bone traces tooth movement" while others argue that excessive retraction of the anterior teeth may result in iatrogenic dehiscence, root resorption, alveolar bone loss and so on. To our knowledge, few studies have studied the changes of alveolar bone morphology in the region of upper incisors after en masse retraction with the mini implant combining with high anterior arch hooks.ObjectiveThe aim of our study is to evaluate the changes of the alveolar bone morphology and tooth displacement during the upper incisors retraction with CBCT.MethodTwenty-two young patients with maxillary protrusion (10 men,12 women, mean age:14.2±2.5 years) were selected. The patients were treated by extracting the 2 upper first premolars. CBCT images were taken before and after incisors retraction. Mini implant and high anterior arch hooks were used. For all upper incisors, the labial and the palatal alveolar bone thickness and the palatal bone area at crest level, midroot level and apical level were measured. In addition, the root length, the labial and palatal vertical bone level and the amounts of horizontal and vertical movement were assessed. Paired t tests were used for statistical analyses to assess the changes during the incisors retraction.Results1. Just very slight root resorption occurred during the retraction of upper incisors;2. There was a slight increase in the labial alveolar bone thickness but the palatal thickness of the alveolar bone showed significant decrease after retraction except the apical level showed significant increase. So is the change of palatal alveolar bone area (P<0.05);3. The total thickness of the alveolar bone at the crest level and midroot level exhibited significant decrease while there is a significant increase at apical level (P<0.05);4. The palatal vertical bone level showed great loss(P<0.05), the labial side increased slightly;5. The amount of horizontal lingual movement of crown edge was greater than that of the apex. The root apex was intruded significantly while the crown edge was extruded slightly.ConclusionFor young patients with maxillary protrusion, the alveolar bone remolding is not very ideal during extensive retraction of the upper incisors. There were significant decreases in palatal bone thickness and area at the crest level and midroot level while there are significant increases at apical level. The upper anterior could be retracted by controlled tipping movement with mini-screw anchorage combining with high anterior arch hooks.