Magne P, Versluis A, Douglas W H
School of Dentistry, University of Minnesota, Minneapolis, Minn., USA.
J Prosthet Dent. 1999 Mar;81(3):345-55. doi: 10.1016/s0022-3913(99)70279-9.
Moving from the posterior segment in the anterior direction within the dental arch, the process of "incisivization" takes place. The occlusal table is gradually replaced by an incisal edge that has the function of cutting.
This study considers these genetically controlled changes by using strain gauge measurements and finite element analyses to rationalize the clinical and biologic advantages of incisal form. A direct clinical link in the common esthetic procedure of anterior veneering is expected.
Six maxillary incisors were mounted in a positioning device and equipped with 2 strain gauges bonded to the palatal surface: gauge 1 (G1) in the concavity and gauge 2 (G2) on the cingulum. A 50 N load was applied on the palatal side of the incisal edge, perpendicular to the long axis of the tooth. Displacement of the load tip and the palatal strain were recorded after successively removing one third, two thirds, and the total thickness of the facial enamel. The same experiment was reproduced with the finite element method (FEM). Four additional experimental designs were tested with the FEM by simulating the progressive thinning and elimination of palatal enamel and a thickened palatal lobe. Surface tangential stresses and local strain in the area corresponding to gauges 1 and 2 were calculated from the postprocessing files.
The FEM was validated by experimental results considering both displacement of the load tip ( approximately 120 +/- 30 microm) and tangential surface strain at G1/G2. Recorded strains were always higher in the concavity when compared with the cingulum; high tensile strains were recorded at G1 after the total removal of the facial enamel. The entire facial surface was submitted to compressive forces. Subsequent compressive stresses were higher ( approximately 150 MPa) when facial enamel was thin or when the palatal enamel was removed. However, their absolute value never reached the elevated and potentially harmful tensile stresses measured in the palatal concavity, especially in the absence of facial enamel (272 MPa). Multiple experimental cracks were generated in the remaining palatal enamel as a consequence of stress redistribution. However, smooth and convex surfaces with local enamel bulk such as the cingulum, the marginal ridges, and the facial cervical third of the anatomic crown showed the lowest stress level. The optimal configuration with regard to the stress pattern was given by the modified natural tooth that exhibited thick palatal enamel and a mostly convex palatal surface.
Palatal concavity that provides the incisor with its sharp incisal edge and cutting ability proved to be an area of stress concentration. This shortcoming can be compensated by specific areas that feature thick enamel such as the cingulum and the marginal ridges. When enamel is worn or removed from the facial surface, its replacement should be carried out by using materials with properties similar to enamel to restore the original biomechanical behavior of the tooth.
在牙弓内从后段向前段移动时，会发生“切牙化”过程。咬合面逐渐被具有切割功能的切缘所取代。
本研究通过使用应变片测量和有限元分析来考虑这些基因控制的变化，以阐明切缘形态的临床和生物学优势。预期在前牙贴面这一常见美学操作中建立直接的临床联系。
将六颗上颌切牙安装在定位装置中，并在腭面粘贴2个应变片：凹面处的应变片1（G1）和舌隆突上的应变片2（G2）。在切缘的腭侧垂直于牙长轴施加50 N的载荷。在依次去除唇面釉质厚度的三分之一、三分之二和全部后，记录载荷尖端的位移和腭侧应变。使用有限元方法（FEM）重复相同的实验。通过模拟腭侧釉质的逐渐变薄和去除以及增厚的腭叶，用有限元方法测试了另外四种实验设计。从后处理文件中计算出与应变片1和2相对应区域的表面切向应力和局部应变。
通过考虑载荷尖端的位移（约120±30微米）和G1/G2处的切向表面应变的实验结果验证了有限元方法。与舌隆突相比，凹面处记录到的应变始终更高；在唇面釉质全部去除后，G1处记录到高拉伸应变。整个唇面承受压缩力。当唇面釉质薄或腭侧釉质被去除时，随后的压缩应力更高（约150 MPa）。然而，其绝对值从未达到在腭侧凹面测量到的升高的且可能有害的拉伸应力，尤其是在没有唇面釉质的情况下（272 MPa）。由于应力重新分布，在剩余的腭侧釉质中产生了多条实验性裂纹。然而，具有局部釉质增厚的光滑和凸面，如舌隆突、边缘嵴和解剖冠的唇侧颈三分之一，显示出最低的应力水平。具有厚腭侧釉质和大多为凸腭面的改良天然牙给出了关于应力模式的最佳构型。
为切牙提供锐利切缘和切割能力的腭侧凹面被证明是应力集中区域。这一缺点可由具有厚釉质的特定区域（如舌隆突和边缘嵴）来补偿。当釉质从唇面磨损或去除时，应使用具有与釉质相似性能的材料进行替代，以恢复牙齿的原始生物力学行为。
