Rationalization of incisor shape: Experimental-numerical analysis

Statement of problem. Moving from the posterior segment in the anterior dir ection within the dental arch, the process of "incisivization" takes place. The occlusal table is gradually replaced by an incisal edge that has the f unction of cutting. Purpose. This study considers these genetically controlled changes by using strain gauge measurements and finite element analyses to rationalize the c linical and biologic advantages of incisal form. A direct clinical link in the common esthetic procedure of anterior veneering is expected. Material and methods. Six maxillary incisors were mounted in a positioning device and equipped with 2 strain gauges bonded to the palatal surface: gau ge 1 (G1) in the concavity and gauge 2 (G2) on the cingulum. A 50 N load wa s applied on the palatal side of the incisal edge, perpendicular to the lon g axis of the tooth. Displacement of the load tip and the palatal strain we re recorded after successively removing one third, two thirds, and the tota l thickness of the facial enamel. The same experiment was reproduced with t he finite element method (FEM). Four additional experimental designs were t ested with the FEM by simulating the progressive thinning and elimination o f palatal enamel and a thickened palatal lobe. Surface tangential stresses and local strain in the area corresponding to gauges 1 and 2 were calculate d from the postprocessing files. Results. The FEM as validated by experimental results considering both disp lacement of the load tip (-120 +/- 30 mu m) and tangential surface strain a t G1/G2. Recorded strains were always higher in the concavity when compared with the cingulum; high tensile strains were recorded at G1 after the tota l removal of the facial enamel. The entire facial surface was submitted to compressive forces. Subsequent compressive stresses were higher (-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 t ensile stresses measured in the palatal concavity; especially in the absenc e of facial enamel (272 MPa). Multiple experimental cracks were generated i n the remaining palatal enamel as a consequence of stress redistribution. H owever, smooth and convex surfaces with local enamel bulk such as the cingu lum, the marginal ridges, and the facial cervical third of the anatomic cro wn 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. Conclusions. Palatal concavity that provides the incisor with its sharp inc isal edge and cutting ability proved to be an area of stress concentration This shortcoming carl be compensated by specific areas that feature thick e namel 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 u sing materials with properties similar to enamel to restore the original bi omechanical behavior of the tooth.