摘要
数字化技术在口腔种植修复中的应用从根本上改变了传统口腔修复的概念，使得口腔种植修复过程得以在术前进行方案规划和术后效果预测，推动着口腔种植理论由传统的定性向定量化方向发展，正在掀起一场口腔种植新技术革命。本文结合口腔种植学应用实际，以提高种植修复的成功率和美学效果为目的，综合运用多种交叉学科的先进理论和方法，在牙齿分割、龈缘提取与重建、种植手术规划、个性化基台设计等几个方面展开深入的研究，论文的主要研究内容和成果如下：
(1)针对CT图像的模糊性和牙体结构的拓扑易变性问题，提出一种基于边缘和局部区域相结合的混合水平集活动轮廓模型的牙齿分割方法。基于层间映射机制，对不同牙层切片采用不同的分割模型：单水平集混合模型分割牙根层切片；双水平集混合模型分割牙冠层切片。同时引入窄带法、先验形状和梯度方向约束以克服周围颌骨的干扰；并针对牙冠层齿间粘连现象，加入区域竞争准则以提取相邻牙齿之间的共同边界。该方法在分割过程中无需额外的监督和干预，就能够很好地处理牙齿拓扑结构分裂合并的问题，有效提取出每颗牙体的组织轮廓。
(2)针对三角网格牙颌模型齿间粘连问题，提出一种单颗牙龈缘轮廓提取方法。根据牙颌模型表面解剖形态特征，采用基于离散曲率分析以及最短路径搜索相结合的方法，快速准确地获取龈缘处的特征线；利用基于能量优化法的B样条拟合以及特征线的微分特性，正确分割出每颗牙齿的龈缘轮廓型值点；为恢复单颗牙龈缘齿间缺失形态，采用牙弓曲线的B样条拓展拟合、节点插入的曲线求交、齿间相切约束的形态建模方法，正确得到齿间无干涉且具有二阶连续的封闭龈缘曲线。实验表明，该方法不仅对排列整齐的牙颌模型有效，而且对排列不整齐或有缺失牙的模型也能获得良好的结果。
(3)为恢复缺牙患者理想的美学龈缘轮廓，提出一种基于统计模型的龈缘重建方法。利用龈缘提取技术，提出一种基于牙齿解剖特征的龈缘形状自动标定方法，对大量患者的牙颌模型建立龈缘数据库，并基于主成分分析技术建立龈缘统计形状模型。在统计模型的引导下，通过统计形变参数的调整、马氏先验、全局相似变换的非线性优化拟合以及基于径向基函数的弹性变形，快速重建出匹配缺牙患者的解剖龈缘轮廓。
(4)在综合考虑口腔修复学、美学和骨质骨量的基础上，提出一种参数化种植手术定位规划方法。利用龈缘轮廓规划种植区域，通过计算种植区域内的骨密度中心和基于剖切平面的径向骨量搜索方法确定种植体定位的最佳位置；为保证种植-骨界面的稳定性，基于图像采样和插值方法对种植体周围骨组织的径向安全距离和骨质骨量进行评估。根据方案规划结果，提出一种参数化的种植导板定位结构网格建模方法，快速准确地将种植规划的信息转移到患者口内。该方法能够为医生提供最优的种植手术方案，有效改善以往靠医生经验进行手术规划和手工设计导板的不精确问题，提高手术的成功率。
(5)为改善种植义齿修复后的龈缘形态，提出一种基于曲面特征识别技术和龈缘重建技术的个性化基台数字化设计方法。通过获取患者带愈合基台的牙颌模型数据，基于几何约束的愈合基台表面特征识别技术快速而准确的获取基台和种植体在牙槽骨中的位置；为纠正种植体植入偏斜现象，通过基于牙体解剖形态特征的牙体长轴建立方法，确定基台的倾斜角度。为使个性化基台具有美学解剖形态，采用龈缘重建技术获取匹配患者的最佳美学龈缘曲线，以指导个性化基台颈部解剖形态设计，从而简化基台设计过程。三维非线性有限元接触分析有效验证了所设计个性化基台结构的合理性。该方法摒弃传统的蜡型制作以及印模转移等繁琐步骤，有效提高了设计的效率和准确性。
The application of digital technology in oral implantology has changed the concept abouttraditional prothsodontics radically and enabled the proe and post surgical simulation to be achieved,which promotes oral implanting theory to develop from traditonal qualitative to quantitative status,and starts a new technological revolution in oral implantology. By combining the practical applicationof oral implantology and various interdisciplinary theories and methods, this dissertation launches theresearches on teeth segmentation, gingival contour extraction and reconstruction, implant surgicalplanning, and custom abutment design in order to improve the successful rate and aesthetic effect. Themain research works and results are as follows:
(1) For the issues of the fuzziness of CT slice images and the variability of tooth topologicalstructure, a novel teeth segmentation method based on a mixed level set active contour modelcombined edge and local area is presented. According to the mapping mechamism between layers,different models are adopted on different slices: the single mixed level set model for dental root layerslices and the double model for dental crown layer slices. Meanwhile, the narrowband method, theprior shape restraint, and the gradient direction restraint are introduced to overcome the disturbance ofthe jawbone. Aimed at the teeth adhesion phenomenon in the dental crown layer slices, regionalcompetitive principle is also added in order to extract the common border between the adjacent teeth.The method can greately deal with splitting and merging of teeth topology structure and therefore caneffectively extract each tooth profile without extra supervision and intervention during thesegmentation process.
(2) Aimed at the problem of the adjacent teeth adhesion of dental meshes, a single-tooth gingivalcontour extraction method is proposed. According to the anatomical characteristic of dental model, anapproach combining the discrete curvature analysis and the shortest path search is applied to obtainthe gingiva feature line.The gingival data points of each tooth are then segmented accurately by usingthe B-spline curve fitting based on the energy optimization and by the differential properties of thefeature line. For restoring the missing gingival shape between teeth, the B-spline curve extendedfitting to the dental arch, the curve intersection by knot insertion, and the morphology modeling of thetangent contraints between teeth are adopted to gain the closed gingival curve of each tooth with twoorder continuous characteristic and no interference between them. The experiments show that thepresented method is effective not only for the aligned dental model, but also for the misaligned andthe model with missing teeth.
(3) For restoring the ideal aesthetic gingiva controur for edentulous patients, a gingivalreconstruction method based on statistical model is proposed. Introducing the technology of thegingival contour extraction, an automatic calibration of gingival shape according to the teethanatomical characteristics is presented. Through principal component analysis, the statistical model ofgingival contours is built from a3-D scan dental dataset. Within this statistical model, themorphologically plausible missing contours can be inferred based on a nonlinear optimization fittingfrom the statistical deformation parameters adjustion, the Mahalanobis prior, and the global similaritytransformation, as well as the elastic deformation based on the radial basis function.
(4) On the basis of taking into account the factors about prosthodontics, aesthetics, bone qualityand quantity, a parametrized planning method for preoperative dental implant positioning is presented.Through the gingival contours the implanting region is first restricted. By computing the bone densitycenter in this region of the CT images and searching the radial bone mass on the cutting plane, theappropriate implant positioning is determined. Based on the image sampling and interpolation theradial safety distance of the bone tissue around implant and the bone quality and quantity parametersare evaluated to guarantee the stability of the implant-bone interface. According to the positioningplanning results a parametric mesh modeling method for the positioning structure of surgical guide ispresented, which can fast and accurately transfer the implant positioning information to the oral cavityof patients. The method can offer the optimal surgical planning for dentists, effectively improve thepast inaccurate problem from dentist experience and hand-made design for surgical guide, and raisethe success rate of operation.
(5) For improving the gingival shape after implant restoration, a digital design method for customabutment is proposed based on the suface extraction and gingiva reconstruction technology. Throughobtaining the patient dental model with healing abutment and recognitioning its surface characteristicsbased on the geometric constraint, the position of the abutment and implant in the alveolar bone isobtained quickly and accurately. To correct the inclination of the implant, a method for computingtooth long axis based on the tooth anatomical characteristics is presented to acquire the oblique angleof the abutment. To make the custom abutment has aesthetic anatomical characteristics, the gingivalreconstruction technique is applied to obtain the best matching anatomical gingival curve withpatients, which guides the design for the neck anatomical shape of custom abutment, and thereforesimplifis the design process. The method abandons the troublesome procedures of traditional waxingand impression transfer, and greatly improves the design efficiency and accuracy.
引文
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