Relationship between analysis of laser speckle image and Knoop hardness on softening enamel.

BACKGROUND: In this study is presented the correlation between laser speckle images and enamel hardness loss. METHODS: In order to shift the enamel hardness, a dental demineralization model was applied to 32 samples of vestibular bovine teeth. After they were cleaned, cut and polished, the samples were divided into 4 groups and immersed in 30ml of a cola-based soft drink for 10, 20, 30 and 40min twice a day for 7 consecutive days with half the surface protected by two layers of nail polish. Each sample was analyzed by Knoop hardness and laser speckle imaging. RESULTS: Pearson's correlation analysis demonstrated that the laser speckle image technique presents a strong correlation with the hardness loss of the enamel (r=0.7085, p<0.0001). This finding is corroborated by Blend & Altman analysis, in which the data presented a constant behavior throughout the whole interval. For both analyses, more than 95% of the data is within the confidence interval, as expected. CONCLUSION: This work demonstrates, for the first time to our knowledge, an empirical model for correlating laser speckle images with the loss of tooth enamel hardness.

Correlation between fracture toughness and leucite content in dental porcelains.

OBJECTIVES: To determine the correlation between fracture toughness and leucite content in dental porcelains. The mechanisms by which leucite influences the fracture toughness of dental porcelains were also investigated. METHODS: Six porcelains were tested: A (Ceramco I/Dentsply), B (Ceramco II/Dentsitply), C (Finesse/Dentsply), D (d.Sign/Ivoclar), Cb (Cerabien/Noritake) and V (Vitadur Alpha/Vita). Bar-shaped specimens were produced, and their fracture toughness was determined by means of the single-edge precracked beam (SEPB) method. The test consisted of fracturing the specimen after a precrack was generated by a bridge-anvil device. KIc was calculated based on fracture force and size of the precrack. Microstructural analysis and determination of the leucite volume fraction were performed on polished specimens etched with 2% HF for 15s by means of scanning electron microscopy. Fractographic analysis was performed on fracture surfaces. RESULTS: Porcelains A and B presented the highest leucite contents (22%) and similar KIc values (1.23 and 1.22 MPa m1/2, respectively), significantly higher than the other materials. Porcelains C and D presented similar K(Ic) values (0.81 and 0.93 MPa m1/2, respectively), but different leucite contents (6 and 15%, respectively). Porcelain D presented higher KIc compared to porcelains Cb and V (0.71 and 0.75 MPa m1/2, respectively), which presented similar values and the lowest leucite contents (0%). Fractographic analysis showed that porcelains with higher leucite content presented higher incidence of crack deflection. CONCLUSIONS: For the materials evaluated in this study, the leucite content was directly related to KIc. The main toughening mechanism observed was crack deflection around leucite particles and clusters.