Factors influencing the changes of lower third molars' angulation and space/crown width ratio by orthodontic treatment

Abstract

OBJECTIVE: To investigate the factors which influence the changes of lower third molars' angulation and space/crown width ratio by orthodontic treatment. METHODS: Pretreatment and posttreatment panoramic radiographs of 120 orthodontically treated cases were taken. The lower third molars' angulation and space/crown width ratio were examined and the relative factors on changes were analyzed. RESULTS: (1)The changes of the lower third molars' angulation and space/crown width ratio after orthodontic treatment were quite different among the cases. (2)The factors which influenced these changes mainly included extraction, mandibular plane angle, angulation and space/crown width ratio of lower third molar before orthodontic treatment. (3) Extraction was the most significantly variable associated with lower third molars' upright (angulation <30 degree) after orthodontic treatment. CONCLUSION: Orthodontic extraction is helpful to upright the lower third molars and to increase the space for third molars; and the pre-treatment lower third molars' angulation and space/crown width ratio are important variables for the changes after treatment; cases with lower mandibular plane angle are apt to greater possibility for the lower third molars' upright. 目的：研究影响口腔正畸患者治疗前、后下颌第三磨牙状态改变的因素。方法：利用正畸治疗前、后的全口曲面断层片,测量120例正畸患者的120颗下颌第三磨牙[男37例,女83例,年龄在1024岁,平均（14.1±3.3）岁]前倾角度和间隙/宽度比值的改变,统计分析影响正畸治疗前、后改变的因素。结果：（1）正畸治疗前、后下颌第三磨牙前倾角度和间隙/宽度比值的改变个体差别较大;（2）影响正畸治疗前、后下颌第三磨牙前倾角度和间隙/宽度比值改变的因素主要有拔牙、下颌平面角、治疗前的前倾角度和间隙/宽度比值（P〈0.05）,而骨龄和牙龄等因素对下颌第三磨牙状态的影响差异无统计学意义;（3）Logistic回归分析,进入治疗后下颌第三磨牙前倾角度是否〈30°判别方程的因素有拔牙、下颌平面角和治疗前的前倾角度,其中拔牙状况影响程度较大,拔牙使得治疗后的前倾角度〈30°（直立倾向）的可能性更大,是不拔牙的4.210倍。结论：正畸拔牙治疗有助于下颌第三磨牙的直立,并使其萌出间隙增大;第三磨牙治疗前的状况（前倾角度和间隙/宽度比值）也是影响治疗后改变的重要因素;下颌平面角度小的患者预后下颌第三磨牙直立的可能性增加。