Orthodontic treatment and third molars development: longitudinal study on radiographs

OBJECTIVES The purpose of this study was to evaluate the effects of first premolar extraction on third molar angulation during fixed orthodontic treatment. MATERIALS AND METHODS The sample of this study included pre-treatment and post-treatment orthopantomographs (OPG) of 40 patients, 17 males and 23 women (106 third molars), treated by fixed orthodontic appliances at the Department of Orthodontics, Sapienza University, Rome. All individuals had both first and second mandibular molars present at all stages and initial malocclusion. Records - including dental study models, lateral cephalograms, panoramic radiographs, and facial photographs - were collected before, during and after orthodontic treatment. The sample of patients was divided into two sub-groups: - - a group of 20 patients who have received orthodontic treatment without premolar extractions; - a second group of 20 patients treated with premolar extractions. Pre-treatment mean age of the extraction group was 13.17 years (±1.54), and 12.23 years (±1.45) for the non extraction group. The mean duration of treatment was 5.17 years (±1.12) in the extraction group and 4.53 years (±1.00) in the non extraction group. The inclusion criteria in the study group were the following: - existence of longitudinal series of orthopantomograms (OPG) pre-treatment (T1), during treatment (T2) and post-treatment (T3); - the crowns of the third molar formed at the beginning of the orthodontic treatment; - second premolars and second molars fully erupted into the mouth; - patients treated with fixed orthodontic appliances (maxillary and mandibular). Both pre- and post-treatment radiographs were evaluated by using the following techniques: - tracing the images of the teeth on the acetate paper; - angular measurements of reference lines (the anterior angle formed between the long axis of the third molar and the long axis of the second molar [M3-M2] and the anterior angle between the long axis of the third molar and the occlusal plane [M3-OP]). The post-treatment values were subtracted from pre-treatment values to calculate the degree of changes in the angulation of third molar relative to the occlusal plane and second molars. The data were analysed by Student’s t-test. A p value ≤0.05 was considered as significant. RESULTS AND CONCLUSIONS The data show that the uprighting of the wisdom tooth long axis is more important in patients treated with premolar extractions. The angle between the maxillary third and the second molar has decreased, the third molar axis has uprighted, but only in patients treated with premolar extractions. In the group of patients treated without extractions, the results showed increased values of this angle, pointing an unfavourable change in the wisdom tooth angulation. In both groups of patients, the changes in maxillary third molars angulation resulting from orthodontic treatment showed statistically significant values. CLINICAL SIGNIFICANCE The ability to early predict the future evolution of the developing third molars can be of great help in planning orthodontic, surgical and prosthetic treatment. The extraction of first premolars during orthodontic treatment significantly improved the angulation of third molars toward uprighting. However, an improvement in angulation does not necessarily mean that third molars will erupt in good position. Therefore it may be prudent for orthodontists to inform their patients that premolar extractions may not prevent the need for third molar extractions in future.