The effects of ageing on the biomechanical properties of root dentine and fracture.

OBJECTIVES: Knowledge of the mechanical behaviour of root dentine can facilitate better understanding of spontaneous vertical root fracture (VRF), an age-related disease initiated mainly at the root apex. We tested the hypothesis that the biomechanical properties of root dentine change with ageing. METHODS: Sixteen human premolars were divided into "old" (17-30 years) and "young" (50-80 years) groups. The elastic modulus, nano-hardness, micro-hardness, elemental contents, tubular density/area of root dentine in cervical, middle and apical root regions were evaluated using atomic force microscopy-based nano-indentation, Knoop indentation, scanning electron microscopy and energy dispersive X-ray spectroscopy, respectively. RESULTS: The apical dentine showed a lower nano-hardness, a lower elastic modulus, a lower calcium content, a lower calcium-to-phosphorus ratio and a smaller tubular density/area than the cervical dentine in both age groups, whereas spatial differences in micro-hardness were observed only in old roots. Compared with young dentine, old dentine showed a greater hardness, a higher elastic modulus, a greater mineral content and a smaller tubular size in the cervical portion, whereas the age-induced changes in tubular density were insignificant. Finite element analysis revealed that due to its higher elastic modulus, old apical dentine has a higher stress level than young dentine. CONCLUSIONS: The intrinsic material properties of root dentine have spatial variations, and they are altered by ageing. The higher stress level in old apical dentine may be one reason, if not the most important one, why spontaneous VRFs are more likely to occur in the elderly population.

Effect of calcium hydroxide as a supplementary barrier in the radicular penetration of hydrogen peroxide during intracoronal bleaching in vitro.

AIM: To examine pH changes in the cervical external root surface, when calcium hydroxide was used as a supplementary barrier to the protective base material during intracoronal bleaching. METHODOLOGY: Twenty-eight single-rooted human premolars extracted for orthodontic reasons were instrumented with K-Flex files, obturated with gutta-percha and subjected to thermocatalytic bleaching. The teeth were divided into four groups. In group A, a glass-ionomer cement barrier was placed at the cemento-enamel junction (CEJ) level and in group C, the barrier was placed 1 mm apical to the CEJ. In groups B and D, Ca(OH)2 was placed in contact and apical to the glass-ionomer cement at the CEJ and 1 mm apical to the CEJ, respectively. The teeth were placed in vials containing distilled water and the pH values of the medium surrounding the teeth were recorded after 1, 2, 4, 10 and 15 days, following renewal of the medium. RESULTS: The pH in the medium became acidic in all groups. No statistically significant differences existed between groups for all the experimental days (P = 0.790). CONCLUSION: The placement of Ca(OH)2 as a supplementary barrier during intracoronal bleaching did not have a significant effect in reversing the acidic pH created at the external root surface in vitro. Its potential effect during these procedures in vivo needs to be further investigated.