[Investigation of posterior teeth displacement under normal bite force by an intraoral scanner].

Objective: To quantitatively evaluate the change of the long axis angle and the relative displacement of the crown feature points of the posterior teeth under normal bite force utilizing an intraoral scanner, and to provide clinical reference. Methods: From May to December 2019, fifteen graduate volunteers (5 males and 10 females, aged from 22 to 30, with an average age of 25.7 years) from Peking University School and Hospital of Stomatology were recruited to participate in the study. The surface data (U1, L1) of the maxillary and mandibular posterior teeth were scanned by an intraoral scanner i500, and saved as original data. The volunteers were guided to bite in the intercuspal position with normal bite force. The buccal bite data of the posterior teeth were scanned as the basis for registration. The digital casts were imported into Geomagic studio 2013 software and the boundary lines along the gingival margin and mesial and distal contact area of posterior teeth of data (U1, L1) were determined. Long axis of the crown, crown centroid and mesial functional cusp vertex were establishd. The data (U1, L1) were segmented into single tooth. Single tooth was aligned to buccal bite data separately using best-fit alignment command based on the buccal common area of the crown and new casts data (U2, L2) were obtained as the data under bite force. The long axis angle and centroid distance between adjacent teeth (second premolar and first molar, second molar and first molar) were measured and the deviation between data obtained at mouth-open state and that at biting state was calculated. Negative value meant centroid distance became shorter under bite force. The first molar was set as the common area and registrate the U1, L1 to U2, L2. The angle of long axes, and displacement of centroid and of functional cusp vertex between second premolar of two casts were calculated, as well as between second molar of two casts. Wilcoxon signed rank test was used to analyze the differences of teeth displacement between second premolar and second molar, and between maxillary and mandibular jaws with SPSS 26.0 statistical software. Results: The result of second premolar and second molar in the same jaw had no statistical difference (P>0.05). The centroid distance deviation of mandibular second premolar-first molar [-0.022(0.046) mm] was larger than that of maxilla [-0.006 (0.040) mm] (P<0.05). The long axis angle of second premolar itself [0.913°(0.647°)] and centroid distance of second molar itself [0.102 (0.106) mm] on the mandibular jaw were different from that on the maxillary jaw, which were 0.590°(0.550°) and 0.074(0.060) mm respectively (P<0.05). Conclusions: Under bite force, displacement of the second premolar and second molar was present, including the displacement of centroids and deflection of long axes. The mandibular posterior teeth have larger displacement than the maxillary teeth.目的： 利用口内扫描仪定量评估正常（牙合）力咬合下后牙长轴角度变化量及牙冠特征点相对位移量，以期为临床提供参考。 方法： 招募北京大学口腔医学院·口腔医院研究生志愿者15名（男性5名，女性10名，年龄22~30岁），用口内扫描仪扫描牙列，扫描获得张口状态上颌和下颌后牙表面数据（U1、L1）作为咬合前数据；嘱志愿者以正常（牙合）力咬合，扫描后牙颊侧获得颊侧咬合数据，作为配准依据。在Geomagic Studio 2013软件中沿上下颌后牙表面数据牙冠龈缘及近远中邻接触区绘制边界线，确定上下颌第二前磨牙及第一、第二磨牙的牙冠长轴、质心及近中功能尖顶点，分割上下颌后牙表面数据为单牙数据。通过单牙数据和颊侧咬合数据对应牙冠颊面共同区域，将单牙数据逐一配准至咬合数据，获得新的上下颌后牙数据（U2、L2），作为咬合后数据。以第一磨牙为基准，测量第一磨牙与邻牙间牙冠长轴夹角及质心距离，计算咬合前后变化量，以咬合后质心距离变小为负值，反之为正值。以第一磨牙牙冠表面为共同区域，分别将L1、U1配准至L2、U2，测量咬合前后第二前磨牙或第二磨牙的牙冠长轴偏转角度、质心及近中功能尖偏移量。采用Wilcoxon符号秩检验比较同颌第二前磨牙与第二磨牙相同测量项目间差异以及上下颌同名牙相同测量项目间差异。 结果： 同颌第二前磨牙与第二磨牙相同测量项目的结果差异均无统计学意义（P>0.05）。下颌第一磨牙与第二前磨牙质心距离的咬合前后变化量［-0.022（0.046） mm］显著大于上颌［-0.006（0.040） mm］（P<0.05）。咬合前后下颌第二前磨牙牙冠长轴偏转角度［0.913°（0.647°）］和第二磨牙质心偏移量［0.102（0.106） mm］分别显著大于上颌第二前磨牙和第二磨牙［分别为0.590°（0.550°）和0.074（0.060）mm］（P<0.05）。 结论： 咬合前后上下颌第二前磨牙和第二磨牙均可发生牙冠长轴偏转及质心偏移，下颌变化量显著大于上颌。.