Harradine N W, Pearson M H, Toth B
Division of Child Dental Health, University of Bristol Dental School, U.K.
Br J Orthod. 1998 May;25(2):117-22. doi: 10.1093/ortho/25.2.117.
The problem of late mandibular incisor crowding is a well established phenomenon, the cause of which has been the substance of considerable debate over the years. A central issue is the possible role of the third molars though no definitive conclusions have been consistently drawn. This prospective study was designed to investigate the effects of randomly assigned early extraction of third molars on late crowding of the mandibular incisors. One-hundred-and-sixty-four patients entered the study from 1984 following completion of retention after orthodontic treatment. Seventy-seven patients (47%) returned for records up to a mean of 66 months later, and their start and finish study casts were digitized on a reflex microscope to determine Little's index of irregularity, intercanine width and arch length. Forty-four of the patients had been randomized to have third molars removed. There was no evidence of responder bias. Where third molars were extracted the mean increase in lower labial segment irregularity was reduced by 1.1 mm from a mean of 2.1 mm for the group where third molars were retained (P = 0.15, not statistically significant). This difference was also not considered to be clinically significant. The principal conclusion drawn from this randomized prospective study is that the removal of third molars to reduce or prevent late incisor crowding cannot be justified.
下颌切牙迟发性拥挤问题是一个已被充分证实的现象，多年来其成因一直是激烈争论的焦点。一个核心问题是第三磨牙可能发挥的作用，尽管尚未始终得出明确结论。这项前瞻性研究旨在调查随机分配的早期拔除第三磨牙对下颌切牙迟发性拥挤的影响。1984年，164名患者在正畸治疗结束并完成保持后进入该研究。77名患者（47%）在平均66个月后返回进行记录，他们开始和结束时的研究模型在反射显微镜下数字化，以确定利特尔不规则指数、尖牙间宽度和牙弓长度。其中44名患者被随机分配拔除第三磨牙。没有证据表明存在应答偏倚。拔除第三磨牙的患者下唇段不规则度的平均增加量从保留第三磨牙组的平均2.1毫米减少了1.1毫米（P = 0.15，无统计学意义）。这种差异也不被认为具有临床意义。从这项随机前瞻性研究得出的主要结论是，拔除第三磨牙以减少或预防切牙迟发性拥挤是不合理的。
