研究纳入2009年8月至2014年4月于北京大学口腔医院正畸科开始正畸治疗的成人患者共15例, 其中男性5例、女性10例, 年龄范围19~38岁。正畸治疗由两名医师完成, 采用了直丝弓技术矫治, 疗程为18~49个月。

纳入标准:(1)成年患者, (2)牙列完整, (3)咬合稳定, (4)因前突和/或拥挤而拔除了4颗第一前磨牙, (5)治疗前后拍摄了颅面大视野CBCT, 留取了石膏牙颌模型。排除标准:双侧牙颌明显不对称畸形。本研究已通过北京大学生物医学伦理委员会审查批准(审批号:IRB00001052-09010), 并获取了患者知情同意。

患者治疗前后的CBCT由同一技师用同一设备(DCT Pro, Vatech Co., Yongin-Si, Korea)拍摄, 拍摄参数为:拍摄视野20 cm× 19 cm, 管电压90 kV, 管电流7 mA, 拍摄时间15 s, 层距0.5 mm。拍摄时要求患者取自然头位, 下颌处于牙尖交错位。石膏牙颌模型经三维激光扫描仪(3Shape R700, 3ShapeA/S, Copenhagen, Denmark)扫描转化为数字化牙颌模型, 扫描精度为± 0.02 mm。

利用逆向工程软件Rapidform 2006(Inus Technology, Seoul, Korea)首先将治疗前后上颌牙颌模型于腭部稳定区域采用迭代最近点方法进行配准(图1A)^[8]。治疗前后的下颌牙颌模型根据上下颌之间的咬合关系分别与治疗前后的上颌牙颌模型进行咬合定位, 从而获得治疗前后下颌牙颌模型初始配准的结果(图1B~D)。

图1

Figure 1

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图1 基于上颌模型的初始下颌牙颌模型重叠Figure 1 Maxilla-based initial registration of mandibular dental casts A, superimposition of pre- (gray) and post-treatment (red) maxillary dental casts registered on the palatal stable region (black); B, maxillary and mandibular dental casts in maximum intercuspal occlusion; C, pre- and post-treatment mandibular dental casts were transferred to the already superimposed maxillary casts using the occlusal relationship; D, the initially superimposed pre- and post-treatment mandibular dental casts.

利用Dolphin影像软件(Version 11.7, Dolphin Imaging & Management Systems, Chatsworth, Calif)将CBCT图像以正交投照(模拟X线平行投照, 无放大率)方式生成头颅侧位片^{[14, 15]}。对治疗前后头颅侧位片进行描记, 并于前颅底结构(包括蝶鞍前壁、颅中窝和筛板, 因样本为成人, 可同时参照其他颅上颌骨结构)进行重叠, 测量出下颌骨治疗前后相对于前颅底的转动和平移量。

具体操作如下:以平均功能牙合平面(functional occlusal plane, FOP)为X轴, X轴的垂线为Y轴建立坐标系, 在治疗后头颅侧位片上定位髁顶点(Co), 测量正畸治疗后下颌骨绕Co点的旋转量(MCoR)及下颌骨沿X轴和Y轴的平移量(MT_X和MT_Y), 即当治疗后下颌骨绕Co点旋转MCoR后, 其与治疗前下颌骨处于平行状态, 接着沿X轴和Y轴分别平移MT_X和MT_Y后, 其与治疗前下颌骨重合(图2A)。另外, 在治疗后头颅侧位片上记录下第二磨牙最远中点(LM2)至Co点的距离(MCoD)及下中切牙切缘点(LI1)至Co点的距离ICoD(图2B)。

图2

Figure 2

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图2 下颌旋转量和平移量的测量和校正Figure 2 Measurement and adjustment of mandibular rotation and translation A, the post-treatment mandible (red) rotated around the condyle point (Co) until it (green) was parallel to the pre-treatment mandible (black), with the degree of rotation being recorded as MCoR; the green mandible was then translated sagittally and vertically until it coincided with the pre-treatment mandible, with the amounts of translation being recorded as MT_X and MT_Y respectively; B, measurements on the post-treatment cephalogram; C, within the Rapidform software, the Co was located according to the measurements in the cephalorgram, adjustment was done with the initially superimposed post-treatment digital cast rotated around the Co by MCoR and translated by MT_X and MT_Y; D, presentation of the pre- and post-adjustment mandibular digital cast superimposition results. FOP, functional occlusal plane.

将1.3小节中头颅侧位片前颅底重叠获得的下颌变化量转移到1.2小节完成的数字化下颌模型初始重叠上, 从而实现下颌数字化模型的最终重叠。

具体操作如下:(1)建立平面及三维坐标系。在Rapidform软件中, 对于初始配准后的下颌牙颌模型, 建立平均FOP和正中矢状平面。平均FOP为治疗前后上下颌第一磨牙、第二前磨牙及第一前磨牙的颊尖点和舌尖点拟合而成; 正中矢状平面为过腭中缝且垂直于FOP的平面。在正中矢状面上根据头颅侧位片中的定义建立X轴和Y轴, 即以FOP与正中矢状面的交线为X轴, X轴的垂线为Y轴, 两者的垂线为Z轴, 从而建立三维坐标系。(2)下颌旋转量和平移量的校正。将治疗后下颌模型双侧牙列投影至正中矢状面上, 获得下牙列轮廓线。参照头颅侧位片中的定义定位LM2点和LI1点, 并根据MCoD和ICoD的距离定位Co点, 然后根据头颅侧位片重叠测得的MCoR、MT_X和MT_Y值对治疗后下颌牙颌模型进行沿Co点的旋转和沿X轴、Y轴的平移(图2C), 从而完成校正并获得下颌模型重叠的最终结果(图2D), 此时治疗前后下颌牙齿移动的评估是基于下颌骨重叠的状态下。

将治疗前后的CBCT图像导入Mimics软件(version 10.01, Materialise, Leuven, Belgium)中, 对下颌骨和下牙列进行分割和三维重建并以STL格式输出, 在Rapidform软件中将治疗前后的下颌骨模型以下颌骨升支和位于牙根下方5 mm以下的体部进行区域重叠(图3A), 再将治疗前后的数字化牙颌模型经牙冠区域配准替代CBCT来源的下牙列(图3B~D), 从而获得基于CBCT的下颌数字化牙颌模型重叠结果(图3E)。

图3

Figure 3

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图3 基于CBCT的下颌牙颌模型重叠Figure 3 CBCT-based registration of mandibular dental casts A, registration of mandibular models (gray, pre-treatment; red, post-treatment), the average distance deviation being less than 0.3 mm; B-C, the dental arch from the digital dental cast (green) was superimposed onto the mandibular model by dental crown surface superimposition, the average distance deviation being less than 0.4 mm; D, the location of the dental arch was replaced by the whole dental cast (yellow); E, the superimposition result of mandibular models was transformed to superimposition of dental casts (gray, pre-treatment; yellow, post-treatment) after the pre- and post-treatment dental casts were superimposed onto the pre- and post-treatment mandibular models respectively; F, integration of the superimposition results from the lateral cephalogram-based mandibular digital cast superimposition (gray and red) and the CBCT-based mandibular digital cast superimposition (gray and yellow).

研究以基于CBCT的下颌牙颌模型重叠为参照, 验证基于头颅侧位片的下颌牙颌模型重叠方法的准确性。在Rapidform软件中的共同坐标系下, 于治疗前牙颌模型上选定两侧下第一磨牙的近中颊尖(LM和RM)、下尖牙的牙尖(LC和RC)和下中切牙切缘中点(LI和RI), 并通过牙冠配准方法将相同标志点转移至治疗后牙颌模型上, 避免重复定点误差。测量治疗前后各标志点三维坐标的变化以分别代表下第一磨牙、尖牙和中切牙的三维位移, 对两种重叠方法下三维牙齿移动量进行比较。

利用PASS软件(Version 11.0; NCSS, Kaysville, UT, USA)进行样本量计算, 对于采用配对t检验比较基于头颅侧位片和基于CBCT的两种下颌牙颌模型重叠方法下牙齿三维位移的差异, 根据临床意义水平设定两组差值为0.5 mm, 根据研究初期测量结果设定方差为0.5 mm, 在P< 0.05(双侧)、把握度80%的情况下, 研究所需最低样本量为11例。统计分析在SPSS软件(Version 19.0; IBM, Armonk, NY, USA)中进行。下颌模型的重叠和测量由同一名研究者重复操作两次, 利用组内相关系数(intraclass correlation coefficient, ICC)评价操作者自身的可靠性, 结果ICC值均大于0.97。经Kolmogorov-Smirnov检验, 所有数据均符合正态分布, 采用配对t检验比较两种配准方法下颌牙齿移动量的差异, P< 0.05为差异有统计学意义。