[Photoelasticity in dentistry. 2. Influence of the supporting apical base]

Thirty photoelastic dental-periodontal patterns were prepared according to the following criteria: ten with cilindrical shaped roots (series I), ten with conical shaped roots from apical do cervical (series II), ten with conical shaped roots from cervical to apical (series III), all of them having the same superficial area and apical basis. The photoelastic analysis showed that the main factor in determining the force distribution is the apical basis and that the conical roots from cervical to apical (series III) elicit a lateral compression on the periodontal support, which influences the magnitude of the force that is necessary to the formation of the some photoelastic deformations produced in the other series. The spectral distribution of the internal stress in the patterns of the series III showed the formations of incomplete fringes along with the lateral aspects of the roots. It was concluded that to make a comparison of the periodontal distribution of occlusal forces, it is of paramount importance the standardization of all steps concerning the construction of the patterns, the measurement of the photoelastic blocks and mainly the apical area of the root, which is the main determinant factor of the qualitative and quantitative results produced by the photoelastic analysis.