Whole-field macro- and micro-deformation characteristic of unbound water-loss in dentin hard tissue

Influence of moisture content of frozen and embalmed human cadavers for identification of dentinal microcracks using micro-computed tomography

The aim of this study was to investigate the influence of moisture content in frozen and embalmed human cadavers on the detection of dentinal microcracks using micro-computed tomography (micro-CT). The group of embalmed specimens included three mandibular and two maxillary segments each containing one tooth. The group of frozen cadavers consisted of two frozen mandibular bone-blocks with two teeth and one mandibular segment containing one tooth. The final number of teeth for each preservation method was n = 5. All specimens were scanned with eight different moisture conditions: 48 h wet, 2 h dry, 48 h wet, 24 h dry, 48 h wet, 1 wk dry, 48 h wet, 1 wk dry. Micro-CT images were screened for the presence of dentinal microcracks. Statistical analysis was performed by nonparametric analysis of variance (α = 5%). Only few microcracks were observed in wet and in 2 h dried bone-blocks with no significant differences (p = 0.63 and p = 0.23, respectively). There was a significant and steady increase of microcracks within the groups of dried specimens as follows: 2 h dry < 24 h dry < first wk dry < second wk dry (all p < 0.008). Preservation method had no significant influence on the visibility of microcracks (p = 0.98). Identification of dentinal microcracks on micro-CT images is influenced by moisture content of cadaveric bone-blocks irrespective of the preservation method.

Shrinkage Strains in the Dentin of Endodontically Treated Teeth with Water Loss

INTRODUCTION

Dehydration has been considered as a potential contributor to vertical root fractures (VRFs) after root canal treatment (RCT). A loss of water could cause embrittlement of dentin and detrimental shrinkage strains. Senior patients have the highest risk of VRF. In this study, we characterized the spatial distribution in shrinkage of tooth roots with respect to donor age and prior RCT.

METHODS

Single-rooted human teeth with and without prior RCT were collected from young (age <25 years) and old (age >60 years) adults. Transverse slices were sectioned from the apical, middle, and coronal thirds of the roots, and digital image correlation was used to evaluate shrinkage during free convection. Crack initiation and growth analysis was performed via optical microscopy, and bound water in dentin was characterized by Raman spectroscopy.

RESULTS

The rate of shrinkage was significantly higher (p ≤ .05) in the apical third than in the middle and coronal thirds of all teeth regardless of donor age. The highest shrinkage strain occurred in the apical third of old donor teeth with prior RCT. In addition, the RCT-treated old teeth suffered the highest percentage of water loss with dehydration. Cracks initiated from the root surface and extended toward the canal with loss of water and shrinkage.

CONCLUSIONS

The apical third undergoes significantly larger shrinkage strains with dehydration than the remainder of the root. Prior RCT exacerbates the extent of shrinkage, particularly in the teeth of seniors and after clinical function, which could increase the propensity for VRF.

Contributions of intermolecular bonding and lubrication to the mechanical behavior of a natural armor

Among many dermal armors, fish scales have become a source of inspiration in the pursuit of "next-generation" structural materials. Although fish scales function in a hydrated environment, the role of water and intermolecular hydrogen bonding to their unique structural behavior has not been elucidated. Water molecules reside within and adjacent to the interpeptide locations of the collagen fibrils of the elasmodine and provide lubrication to the protein molecules during deformation. We evaluated the contributions of this lubrication and the intermolecular bonding to the mechanical behavior of elasmodine scales from the Black Carp (Mylopharyngodon piceus). Scales were exposed to polar solvents, followed by axial loading to failure and the deformation mechanisms were characterized via optical mechanics. Displacement of intermolecular water molecules by liquid polar solvents caused significant (p ≤ 0.05) increases in stiffness, strength and toughness of the scales. Removal of this lubrication decreased the capacity for non-linear deformation and toughness, which results from the increased resistance to fibril rotations and sliding caused by molecular friction. The intermolecular lubrication is a key component of the "protecto-flexibility" of scales and these natural armors as a system; it can serve as an important component of biomimetic-driven designs for flexible armor systems. STATEMENT OF SIGNIFICANCE: The natural armor of fish has become a topic of substantial scientific interest. Hydration is important to these materials as water molecules reside within the interpeptide locations of the collagen fibrils of the elasmodine and provide lubrication to the protein molecules during deformation. We explored the opportunity for tuning the mechanical behavior of scales as a model for next-generation engineering materials by adjusting the extent of hydrogen bonding with polar solvents and the corresponding interpeptide molecular lubrication. Removal of this lubrication decreased the capacity for non-linear deformation and toughness due to an increase in resistance to fibril rotations and sliding as imparted by molecular friction. We show that intermolecular lubrication is a key component of the "protecto-flexibility" of natural armors and it is an essential element of biomimetic approaches to develop flexible armor systems.