随着人类的进化,颌骨及牙齿有退化的倾向,牙齿数目趋向于减少,最常见的是第三磨牙的缺失,其次是第二前磨牙和侧切牙。Graber 报道6%的人有先天性缺牙;成田令博报道日本人中智齿缺失占37.5%。少数牙的缺失几乎没有任何损害,无重要意义.无牙畸形,是指口内的乳恒牙列牙胚完全缺失或近似于完全缺失,影响到面部外形、咀嚼、发音等功能。依牙列牙胚的完全缺失与否,无牙畸形可分为完全性和部分性。无牙畸形,临床所见者多伴有其他外胚叶发育不全的症状,如皮肤、皮肤附属器及指甲等的异常,而作为单独异常发现是少见 As humankind evolves, jaws and teeth tend to degenerate, the number of teeth tends to decrease, the third most common being the absence of third molars, followed by the second premolar and lateral incisors. Graber reported that 6% of people have congenital missing teeth; Narita Naribo reported Japanese missing wisdom teeth accounted for 37.5%. A small number of missing teeth almost no damage, no significance .Dental malformations, refers to the mouth of the permanent dentition tooth germs completely missing or approximately complete loss, affecting the facial features, chewing, pronunciation and other functions. According to complete absence of tooth germs, tooth-free deformities can be divided into complete and partial. No tooth-shaped deformity, the clinical findings are often accompanied by other symptoms of ectodermal hypoplasia, such as skin, skin appendages and nails and other abnormalities, and as a separate abnormal findings are rare