基于数字化技术的正畸诊断性排牙中牙根位置初探

A pilot study of root position in orthodontic diagnosis model set-up

目的 通过数字化技术模拟正畸诊断性排牙的牙根位置,探讨牙根信息在正畸诊断性排牙中的重要性.方法 选择安氏Ⅰ类减数和非减数患者各1例,激光扫描获得治疗前石膏模型牙冠表面三维数据,根据大视野锥形束CT数据,采用Mimics及Geomagic软件重建包含牙根的数字化模型Mo,冠根融合后各牙齿以STL格式独立保存.3名正畸医师和1名高年资正畸技师分别对治疗前石膏模型进行诊断性排牙,激光扫描排牙模型,以牙冠为重叠基准,将冠根融合后各牙齿分别重叠于排牙模型相应牙冠表面三维数据上,得到包含牙根的数字化排牙模型Ma、Mb、Mc、Md.同法构建治疗后数字化模型M.观察Ma- Md牙根平行情况;以磨牙为重叠基准,分别将Ma - Md与Mo重叠,观察牙根外露情况;以磨牙为基准,将Ma- Md分别与M重叠,通过距离偏差色阶图分析差异.结果 4名操作者对2例患者的8个排牙模型中均存在牙根不平行和多处牙根外露现象.排牙模型与治疗后数字化模型的上颌最大差异为8.79 mm,下颌最大差异为9.96 mm.结论 以牙冠为依据的诊断性排牙并不能避免牙根不平行以及牙根外露,将牙根纳入诊断性排牙过程并进行诊断设计非常必要.

Objective To investigate the importance of root information in diagnosis set-up by constructing three-dimensional（3D） digital models with individual anatomic roots.Methods Pretreatment cone-beam CT（CBCT） and laser scanning data were collected from two patients（extraction and non-extraction each） with skeletal Class Ⅰ malocclusion.Threshold segmentation of the CBCT was performed to generate a 3D digital model which has individually isolated tooth.This model and the scan model were superimposed to generate an integrated model（Mo） composed of high-resolution surface scan crowns sutured to the CBCT roots.Pretreatment dentition plaster model were made into set-up model.The diagnosis model set-up was performed successively by three orthodontists and one senior orthodontic technician.Set-up model scan of each patient after tooth alignment was obtained.The isolated composite teeth were individually superimposed onto the set-up model surface scan to creat set-up model（Ma,Mb,Mc,Md） containing root position.These isolated composite teeth were also superimposed onto the posttreatment surface scan depicting the posttreatment model（M）.In order to observe whether diagnosis model set-up would cause exposure of the root,Ma-Md were compared with Mo,which showed the true positions of alveolus.In order to validate the accuracy of the expected root position setup,Ma-Md were compared with the true root position represented by M.Color displacement maps generated to measure the discrepancies of root positions.Results Nonparallel and exposure of the root was found in all setup models.Color displacement maps through molar superimpositions showed maximum differences of 8.79 mm for the maxillary teeth and 9.96 mm for the mandibular teeth.Conclusions Diagnosis model set-up based only on crown can not absolutely ensure the good alignment of roots without root exposure.It is necessary to construct the integrated model including root for diagnosis during tooth arrangement process.