Factors Influencing Superimposition Error of 3D Cephalometric Landmarks by Plane Orientation Method Using 4 Reference Points: 4 Point Superimposition Error Regression Model

Superimposition has been used as a method to evaluate the changes of orthodontic or orthopedic treatment in the dental field. With the introduction of cone beam CT (CBCT), evaluating 3 dimensional changes after treatment became possible by superimposition. 4 point plane orientation is one of the simplest ways to achieve superimposition of 3 dimensional images. To find factors influencing superimposition error of cephalometric landmarks by 4 point plane orientation method and to evaluate the reproducibility of cephalometric landmarks for analyzing superimposition error, 20 patients were analyzed who had normal skeletal and occlusal relationship and took CBCT for diagnosis of temporomandibular disorder. The nasion, sella turcica, basion and midpoint between the left and the right most posterior point of the lesser wing of sphenoidal bone were used to define a three-dimensional (3D) anatomical reference co-ordinate system. Another 15 reference cephalometric points were also determined three times in the same image. Reorientation error of each landmark could be explained substantially (23%) by linear regression model, which consists of 3 factors describing position of each landmark towards reference axes and locating error. 4 point plane orientation system may produce an amount of reorientation error that may vary according to the perpendicular distance between the landmark and the x-axis; the reorientation error also increases as the locating error and shift of reference axes viewed from each landmark increases. Therefore, in order to reduce the reorientation error, accuracy of all landmarks including the reference points is important. Construction of the regression model using reference points of greater precision is required for the clinical application of this model.

在口腔医学领域，影像叠合（superimposition）常被用作评估正畸或颌面矫形治疗后组织变化的方法。随着锥形束计算机断层扫描（cone beam CT, CBCT）的问世，通过影像叠合技术评估治疗后的三维结构变化成为可能。四点平面定向法（4 point plane orientation）是实现三维影像叠合的最简方法之一。为探究四点平面定向法下影响头影测量标志点（cephalometric landmarks）影像叠合误差的相关因素，并评估用于分析叠合误差的头影测量标志点的可重复性，本研究选取20例具有正常颌面骨骼与咬合关系、因颞下颌关节紊乱病（temporomandibular disorder）接受CBCT扫描的患者作为研究对象。本研究以鼻根点（nasion）、蝶鞍点（sella turcica）、颅底点（basion）以及蝶骨小翼左右侧最靠后点的中点作为参照，构建三维解剖参考坐标系。另外15个头影测量参照标志点也在同一影像中重复测定3次。通过包含3个影响因子（描述各标志点相对于参考轴的位置以及标志点定位误差）的线性回归模型，可对各标志点的重定向误差作出较大程度的解释（解释率达23%）。四点平面定向系统所产生的重定向误差量，会随标志点与X轴之间的垂直距离变化而改变；同时，重定向误差也会随标志点定位误差以及从各标志点视角下参考轴的偏移量增大而升高。因此，若要降低重定向误差，包括参照标志点在内的所有头影测量标志点的定位精度均至关重要。若要将该回归模型应用于临床，需使用定位精度更高的参照标志点来构建模型。