| Objectives: To explore the differences in eruption of the lower third molars of the hyperdivergent adolescent patients receiving different orthodontic treatment with and without premolar extraction.Methods: A total of 54 hyperdivergent adolescents were selected from patients accepted orthodontic treatments in the Department of Orthodontics, Hospital of Stomatology, Hebei Medical University during 2009 to 2013. The chronological age of the patients ranged from 12 to 16 years, vertebral age from CVS2 to CVS4 and the third molar Nolla developmental age from 6 to 8 before the treatment. According to the extraction modality, all the selected patients were divided into non-extraction group(Group A), first premolar extraction group(Group B) and second premolar extraction group(Group C). All the items were performed on panoramic and lateral profile by Wincelph 8.06 Softwear. All the items were measured three times and make the mean values as the ultimate values.First, to explore the correlation of lateral radiographs and panoramic radiographs for distance measurements. Radiographs of 20 patients were selected out of the participates randomly. The following items were measured on both lateral radiographs and panoramic radiographs:R-Id: The distance between the anterior border of the mandibular ramus point and the infradentale point.Co-Me: The distance between the Condylion point and the Menton point.Go-Me: The distance between the Gonion point and the Menton point.Then, the differences of the lower third molar eruption tendency in different treatment modality groups were compared. The following distance and angle measurements were measured on panoramic radiographs taken before and after treatment:RES-R: The distance from the distal point on the distal surface of the lower second molar crown to the anterior border of the mandibular ramus point(R) along the occlusal plane in the right side.RES-Xi: The distance from the distal point on the distal surface of the lower second molar crown to the Ricketts’ Xi point along the occlusal plane in the right side.RDM3: The greatest distance between the mesial and distal surfaces of the lower third molar crown in the right side.RRI: The ratio was calculated by dividing the retromolar space(RES-R) by the mesiodistal crown width of the lower third in the right side.RXi I: The ratio was calculated by dividing the retromolar space(RES-Xi) by the mesiodistal crown width of the lower third in the right side.RM1-MP: The mesial upper angle of the axis of the lower first molar and the MP plane in the right side.RM2-MP: The mesial upper angle of the axis of the lower second molar and the MP plane in the right side.RM3-MP: The mesial upper angle of the axis of the lower third molar and the MP plane in the right side.RM1-PP: The mesial upper angle of the axis of the lower first molar and the palatal plane in the right side.RM2-PP: The mesial upper angle of the axis of the lower second molar and the palatal plane in the right side.RM3-PP: The mesial upper angle of the axis of the lower third molar and the palatal plane in the right side.RM2-M3: The angle of the axis of the lower third molar and the axis of second molar in the right side.The items on the left side were measured in the same way, named LES-R, LES-Xi, LDM3, LRI, LXi I, LM1-MP, LM2-MP, LM3-MP, LM1-PP, LM2-PP, LM3-PP, LM2-M3, respectively.Statistical methods: The data were analyzed with the software SPSS for6Windows(release 13.0). Pearson’ s correlations were calculated for evaluation of the associations between the measured variables on profile and panoramic radiographs to see whether the panoramic radiographs was a reliable tool to measure the retromolar spaces. The pre-treatment values were subtracted from post-treatment values to calculate changes of the angles and distances. The data were analyzed by t-test and ANOVA analysis. A P value<0.05 was considered as significant.Results:1 The measurement of RR-Id on the lateral radiographs and panoramic radiographs were significantly correlated, and the correlation coefficients were 0.625 and 0.606 in the right side and left side respectively. The measurement of Co-Me on the lateral radiographs and panoramic radiographs were significantly correlated, and the correlation coefficients were 0.819 and 0.808 in the right side and left side respectively. The measurement of Go-Me were not statistically correlated between the two radiographs.2 After treatment, the ES-R,M1-MP, M2-MP of the non-extraction group increased by 1.91±0.86 mm, 3.22±1.22°, 2.40±1.14°, respectively(P<0.05).3 After treatment, the measurement of ES-R, ES-Xi, RI, Xi I of the first premolar extraction group increased by 7.61±0.75 mm, 8.32±1.00 mm, 0.22±0.02, 0.20±0.03, respectively. The measurements of M1-MP, M1-PP, M2-MP, M2-PP increased by 4.14±0.58°, 4.32±0.65°, 1.62±0.81°, 1.99±0.84°, respectively(P<0.05).4 After treatment, the ES-R, ES-Xi, RI, Xi I of the second premolar extraction group increased by 11.6±1.06 mm, 12.9±1.15 mm, 0.36±0.03, 0.37±0.03, respectively. The M1-MP, M1-PP increased by 4.55±1.17°, 6.64±1.01°, respectively(P<0.05).5 The measurements of ES-R, ES-Xi, RI, Xi I on panoramic radiographs was statistically different in the right side and left side. The ES-R, ES-Xi was longer 2.55±0.40 mm, 4.43±0.54 mm in the right side than the left side. The RI, Xi I was larger 1.50±0.32, 0.09±0.02 in the right side than the left side.6 The changes of the measurements represented the retromolar spaces was statistically different in the three groups. The increments in the Group B were much more than the other groups and which in Group C were the least. The M1-MP, M1-PP increments were also statistically different grouply. The increments in the Group B was more than A and Group C(P<0.05).Conclusion:1 Panoramic radiograph is a reliable tool for evaluating the eruption of mandibular third molar.2 The extraction treatment can increase the eruption space of the lower third molars in the hyperdivergent adolescents.3 Different extraction sites have different effect on the eruption space of the third molar in hyperdivergent adolescents. 2nd premolar extraction seems to increase the retromolar spaces more effectively than the first premolar extraction, and the lower 3rd molar tends to have more chance of normally erupting. |
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