目的：比较正畸治疗中拔除第二磨牙与拔除第一前磨牙后第三磨牙牙胚位置的不同改变。
Objective:To compare the changes in third molar position in patients treated with second molar and first premolar extractions.
上下颌第三磨牙牙胚绝大多数已形成且牙冠已钙化完成，其出现率为上颌84 0 % ，下颌85 2 % ；
Most of the third molar embryo had been formed and their tooth crowns had been calcified, the occurrence ratio was 84 0 % in maxilla and 85 2 % in mandible.
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