Absence of more than two first premolars or absence of a first and a second premolar.
缺少超过两个第一前臼齿或者一个第一和一个第二前臼齿。
Conclusion: Mandibular second premolar after pulling, the third molar eruption clearance increased obviously, conduce to the jaw third molar normal eruption.
结论：拔除下颌第二前磨牙后，第三磨牙萌出间隙明显增大，有助于下颌第三磨牙的正常萌出。
Objective:To compare the changes in third molar position in patients treated with second molar and first premolar extractions.
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。
Methods: 76 old people of 218 premolar teeth with wedge-shaped defect were divided into two groups randomly: the teeth were prepared in the first group but not in the second group.
方法：选择相邻的前磨牙楔状缺损患者76例218颗患牙，随机分为2组，第一组修复前预备洞型；第二组修复前不预备洞型。
Methods: 76 old people of 218 premolar teeth with wedge-shaped defect were divided into two groups randomly: the teeth were prepared in the first group but not in the second group.
方法：选择相邻的前磨牙楔状缺损患者76例218颗患牙，随机分为2组，第一组修复前预备洞型；第二组修复前不预备洞型。
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