Aim To analyse the effect of post-core and cement materials and thickness of the cement lute on the stress in post-core systems under ultrasonic vibration at different frequencies and amplitudes using three-dimensional finite element analysis. Methodology Eight three-dimensional finite element models of a maxillary central incisor with postcores were established. Two post-core materials (Au and Ni-Cr alloys), two cements (glass ionomer (GI) and zinc phosphate (ZP)) and two cement layer thicknesses (50 and 150 lm) were considered. Vibration loads were applied near the neck on the buccal side of the core at frequencies of 10-40 kHz and amplitudes of 10-50 lm. The maximum shear stress of the cement layer and maximum principal stress of the roots and their distributions were investigated. Results The stresses on cements and roots increased with an increase in the frequency and amplitude of the vibration load and elastic modulus of the cements, and decreased with increasing thickness of the cement layer and elastic modulus of the post-core. Maximum cement stress was observed on the contralateral upper part of the loading side, whereas the maximum root stress was found on the ferrule where the load was applied. Conclusions In this simulated model, the frequency and amplitude of ultrasound needed to remove a post-core were positively related to the elastic modulus of the post-core and thickness of the cement layer and negatively related to the elastic modulus of the cements.

ABSTRACT:The structural characteristics of poly(vinyl alcohol-b-acrylonitrile) fibers with different AN contents were studied by comparison with that of PVA and PAN fibers. X-ray diffraction analysis showed that both PVA and PAN blocks in the copolymer fibers formed crystals. Two glass transition temperatures corresponding to PVA and PAN components appeared on the dynamic mechanical spectrum of the copolymer fiber, indicative of their incompatibility in the fiber. SEM intuitively exhibited a longitudinal cracked and grooved surface morphology similar to that of PAN fiber and revealed an internal microdomain separation morphology for the block copolymer fibers. TEM showed a morphological structure intermediate between those of PVA and PAN fibers for the block copolymer fibers. It was also found that the copolymer fiber with the lower AN content has a sheath-core structure.