Methods: The thickness of enamel and dentin at different positions of 23 upper first and 18 lower first molars were measured by electronic ruler.

方法：用电子测量尺测量23颗上颌第一磨牙和18颗下颌第一磨牙牙合面不同部位牙釉质、牙本质的厚度。

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Permanent teeth begin appearing around age 6, starting with the first molars and lower central incisors. This process continues until approximately age 21.

恒牙萌出于6岁左右，开始于第一恒磨牙以及下颌第一中切牙。这个过程一直持续到将近21岁。

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