Morphological Influence Of The Impacted Mandibular Third Molar Extraction On The Angulusmandibulae Using Cone-Beam Computed Tomography

Extraction treatment is very common in orthodontics for dealing with the contradiction of teeth and alveolar bone to align the arch and improve the profile.The probability of extraction for adult orthodontic is higher than adolescent for the lower proficiency of their teeth and alveolar bone reconstruction. It is the most important thing to renew stabilized occlusion for adult patients,while the probable change of facial type along with extraction treatment is also received much attention.The ratio of impaction for third molar is high in modern population and the rate for mandibular is larger than maxillary. Some studies favour the extraction of the mandibular third molar for the reason of erecting the posterior distally during treatment and stabilizing in retention period.As the increase of adult orthodontic patients in clinic,the occurrence of mandibular third molar extraction is more and more universal.The close relationship between mandibular third molar(MTM) and gonial angle which constitute the lower face illustrated the situation of MTM may influence the morphology of face.Orthodontics should realize this relationship so as to predict the influence of MTM extraction on facial type of adult patients.The development of cone beam computed tomography(CBCT) in oral field has settled some defects of traditional X-ray,such as magnification distortion and overlap problem.It provide one appropriate method for oral research in small scale compared with CT owing to its lower radiation dose and expense.Objective:To observe whether there are changes in the morphology of gonial angle after the extraction of mandibular third molars,investigate the effect of the MTMs extraction on the morphology of the gonial angle and provide theoretical basis for clinical treatment.Methods and results:Part one:Research for the bony morphology of regional mandibular in lower third molar.35adult orthodontic patients (19males,16females)with an average age of22.7years old were selected whose panoramic radiographs(PA) showed the existence of unilateral or bilateral MTMs and divided into sagittal bony type Ⅰ, Ⅱ and Ⅲ according to the ANB angle measured with Onyx on lateral cephalograms(LAC).All the patients received CBCT scanning and the data were analyzed by Mimics software.The result showed that the width of the bone where MTMs located was higher in bony type Ⅲ than other types;male with type Ⅲ had larger hight than other types;the length of the bone where MTMs located in female with type Ⅲ was bigger than types;and male of type Ⅰ or Ⅲ had greater width than female with the same bony type.Part two:Influence of the mandibular third molar extraction on the morphology of gonial angle.30adult orthodontic patients (17males,13females) with an average age of23.6years old were selected whose panoramic radiographs(PA) showed the existence of bilateral MTMs which should be extracted for the reason of orthodontic treatment.Before the extraction of MTMs all the patients received CBCT scanning and then again6months later.The width,hight and length of the bone where MTMs located were measured with Mimics software and analyzed by statistical software.The result suggested that after extraction,the hight and width of the bone where MTMs located had reduced,while there was no significant changes in the length6months later.Conclusion:1、The morphological difference in the bone of impacted mandibular third molar region exists in gender and sagittal bony type.2、The extraction of the impacted mandibular has some influence on the morphology of gonial angle bone.