The mechanical anisotropy on a longitudinal section of human enamel studied by nanoindentation.

Nanoindentation has been widely used for probing the mechanical properties of tooth, especially for characterizing its complex hierarchical structures. Previous studies have confirmed the anisotropic mechanical behaviors caused by the alternated orientations of enamel rods and the alignment of fibril-like hydroxyapatite crystals, but the longitudinal section of enamel, which was composed of parallel-arranged rods, was regarded as a homogeneous continuum as always. In this study, nanoindentation combined with SEM was carried out with the indenter rotating on the longitudinal section of enamel to evaluate the relativity between the nano-mechanical properties and the orientation of indentation impressions. It has been shown that the enamel presented different elastic modulus and hardness with different angles of indenter on its longitudinal section, and its anisotropy was also confirmed by the remarkable asymmetric morphologies of impressions. We observed that the parallel arrangement of crystal fibrils and enamel rods might trigger the expansion of the micro-cracks in preferred orientation, and result in scalene triangle indentation impressions, altering contact areas as well as inconsistent mechanical behaviors. Consequently, it is considered that the longitudinal sections of enamel should be modeled as anisotropic.

Disturbed enamel biomineralization in col1-caPPR mouse incisor.

During the mineralization process of enamel, gene expression controls the activities of ameloblasts, the secretion and assembly of an extracellular protein matrix, affecting the final structure and functions. In this study, the enamel in the maxillary and mandibular incisors of wild-type and transgenic (col1-caPPR) mice, in which a constitutively active PTH/PTHrP receptor (PPR) was targeted to osteoblastic cells, was observed by scanning electron microscopy (SEM), Fourier transform infrared microscopy (FTIRM), and nanoindentation. The SEM studies showed that several different patterns of aberrations in crystal arrangement, disturbed prism organization without decussation, as well as abnormal enamel distribution were encountered in transgenic enamel. FTIRM analysis revealed poorer crystallinity/maturity after mutation. Nanoindentation measurement disclosed that transgenic enamel had 24.6% lower hardness and 12.3% lower elastic modulus. We attributed the inferior properties to the loosely packing crystals and abnormal prism organization. Furthermore, the col1-caPPR mouse model was substantiated to be useful to study how genes modulate the biomineralization process.

Enamel distribution, structure and mechanical alterations in col1-caPPR mice molar.

Enamel mineralisation is a highly controlled process in which the deposition, growth, and maturation of inorganic crystallites are regulated by secreted matrix proteins at the molecular and cellular level. Maxillary and mandibular first molars from the col1-caPPR mutants as well as normal controls aged for 12 weeks were observed by SEM and nanoindentation, respectively. Several types of aberrations in enamel distribution and crystal organisation were encountered in the transgenic molars. Also, the gene alteration resulted in degradation by as much as 23.42% in hardness and 17.56% in the elastic modulus. These data suggested that gene mutation altered the ameloblastic differentiation and movement, resulting in variations of crystal arrangement patterns, aberrations of enamel distribution, and degradation of mechanical behaviour. Furthermore, the col1-caPPR mouse model was determined as useful for studying how the genes modulate the biomineralisation process.

The enamel softening and loss during early erosion studied by AFM, SEM and nanoindentation.

Enamel dissolution occurs when it contacts with acids produced by plaque bacteria, foods or drinks. There have been numerous and varied studies quantifying and characterizing the rate, extent and chemical aspects of enamel erosion; however, there is still hot debate about the amounts of enamel softening and loss. The objective of this study was to measure the enamel erosion process with high accuracy. Native third molars were partly eroded in citric acid (pH = 3.8) with some domains protected by a Ti coating layer. The surface morphology and structures before and after exposure to citric acid for different time periods were studied by AFM, SEM and nanoindentation, respectively. Based on this, the functions between the amounts of enamel softening, loss and erosion time were established for the first time. It was demonstrated that AFM, SEM and nanoindentation were suitable for measuring the early stages of enamel demineralization qualitatively and quantitatively.