Effects of First Premolar Extraction in Orthodontic Treatment on the Third Molar Position.

Objective: To study the changes of upper and lower third molar position in patients treated with first premolars extractions.Methods: Fifty-fix adolescents were chosen and divided into first premolar extraction and nonextraction groups,with 30 and 26 patients respectively. The pretreatment and posttreatment panoramic radiographs were taken.The indexes of third molar eruption space,distance from third molar to occlusal plane and the length of mandible were measured and analyzed.Results: All the measurement indexes had statistically significant differences between pretreatment and postreatment in first premolar extraction group(P0.05).The lower eruption space had no statistically significant difference in nonextrction group(P0.05).Compared first premolar extraction group with nonextrction group,the lower eruotion space increased 1.35mm(P0.05),the lower eruption spaces L-ES(R)and L-ES(Xi) increased 2.72mm(P(0.05)) and 3.16mm(P0.01) respectively,the distance from lower third molar to occlusal plane decreased 0.31mm(P0.05),the length of mandible increased 1.68mm(P0.05).The distance from upper third molar to occlusual plane had no statistically difference.Conclusion: First premolar extraction in orthodontics could increase the eruption space of third molar.It could promote the eruption of third molar.