This study compared the ultimate tensile strength, micropunch shear strength, and microhardness of bleached and unbleached human dentin. Forty-four intact premolars were root canal treated and ...randomly divided into four groups. Bleaching agents were sealed in pulp chambers, as in clinical use. Group 1 (control) was treated with water, group 2 with 30% hydrogen peroxide, group 3 with sodium perborate mixed with water, and group 4 with sodium perborate mixed with 30% hydrogen peroxide. The teeth were stored in saline at 37°C for 7 days. The teeth were then sectioned and biomechanical tests were carried out on dentin specimens that were obtained from all teeth. Intracoronal bleaching with 30% hydrogen peroxide and sodium perborate used either alone or in combination weakened dentin. Hydrogen peroxide alone tended to be more damaging than sodium perborate used alone or sodium perborate mixed with hydrogen peroxide.
Aim To evaluate the effect of repeated cleaning procedures on fracture properties and corrosion of nickel‐titanium (NiTi) files.
Methodology New NiTi instruments were subjected to 2, 5 and 10 ...cleaning cycles with the use of either diluted bleach (1% NaOCl) or Milton's solution (1% NaOCl plus 19% NaCl) as disinfectant. Each cleaning cycle consisted of scrubbing, rinsing and immersing in NaOCl for 10 min followed by 5 min of ultrasonication. Files were then tested for torsional failure and flexural fatigue, and observed for evidence of corrosion using scanning electron microscope (SEM). Four brands of NiTi files were immersed in either Milton's solution or diluted bleach overnight and evaluated for corrosion.
Results Up to 10 cleaning cycles did not significantly reduce the torque at fracture or number of revolutions to flexural fatigue (P > 0.05, two‐way anova), although decreasing values were noted with increasing number of cleaning cycles using Milton's solution. No corrosion was detected on the surface of these files. Files immersed in 1% NaOCl overnight displayed a variety of corrosion patterns. The extent of corrosion was variable amongst different brands of files and amongst files in each brand. Overall, Milton's solution was much more corrosive than diluted bleach. Corrosion of file handles was often extreme.
Conclusions Files can be cleaned up to 10 times without affecting fracture susceptibility or corrosion, but should not be immersed in NaOCl overnight. Milton's solution is much more corrosive than bleach with the same NaOCl concentration.
Objectives. This study was planned to investigate the variations in strains in enamel under different patterns of occlusal loading, using three-dimensional finite element analysis (3D FEA) and strain ...gage measurements in extracted teeth.
Methods. A 3D FEA model of a mandibular second premolar was used to investigate effects of occlusal load on enamel surface strains, particularly in response to oblique directions of cuspal loading. Point loads of 100
N were applied axially and at 45° from the vertical on the buccal or lingual incline of the buccal cusp, either in the bucco-lingual plane or at varying mesio-distal angulations (up to ±20°). Patterns of strain observed in the FEA model were confirmed experimentally using strain gages on extracted premolars mounted in a servohydraulic testing machine.
Results. Strains predicted from the FEA model were in excellent agreement with the strain gage measurements. Strains were concentrated near the cementoenamel junction (CEJ) regardless of load direction. A vertical load on the buccal cusp tip resulted in compressive strains on the buccal surface but small tensile strains in lingual cervical enamel. Strains resulting from oblique loads on buccal cusp inclines were complex and asymmetric, with either tension or compression occurring in any location depending on the site and angle of loading.
Significance. The magnitude, direction and character of strains in cervical enamel are highly dependent on patterns of loading. The asymmetric pattern of strains in buccal cervical enamel in response to oblique occlusal forces is consistent with the common clinical picture of asymmetric non-carious cervical lesions.
An effect of dentinal tubule orientation on mechanical properties of dentin has been difficult to demonstrate. We have tested the hypothesis that ultimate tensile strength (UTS) of dentin is affected ...by tubule (and hence collagen fibril) orientation. The UTS of human root dentin was investigated by direct tensile and diametral testing of specimens of known orientation prepared from extracted teeth. Dumbbell-shaped samples were machined for direct tensile testing (load parallel or perpendicular to tubule direction) and cylinders for diametral testing (load at 0°, 45°, 67.5°, and 90°). Fractured surfaces were examined by SEM. UTS was lowest when the tensile force was parallel to tubule orientation, and greatest at 90° to tubule orientation (fracture parallel to tubule direction). SEM views of fractured surfaces suggested that microstructure contributes to fracture patterns. At least for properties involving disruption of the collagen matrix, root dentin shows a definite anisotropy.
Vertical loads and root surface strains in extracted teeth during lateral condensation using finger and hand spreaders were measured and compared with loads and strains at fracture. Six groups each ...of 10 teeth were tested: maxillary central incisor, premolar and molar; and mandibular incisor, premolar and molar. Root strains were measured using strain gauges mounted on the apical and middle third of the buccal root surface. Statistical analysis was performed at the 95% level of confidence. The maximum loads and strains generated by finger spreaders were significantly lower than those generated using a hand spreader (D11T). These loads and strains were also significantly lower than the values at fracture. Most fracture lines were in a buccolingual direction, but maxillary premolars with two separate roots and the mesiobuccal root of maxillary molars showed more variation in fracture site. The results suggest that lateral condensation alone should not be a direct cause of vertical root fracture. The use of finger spreaders, however, is associated with lower risk.
Objectives: The aim of this study was to investigate the creep, stress relaxation and strain rate behavior of human root dentin under compressive loading.
Methods: Cylindrical root dentin samples of ...3.5
mm outer diameter, 1.5
mm internal canal diameter and 6–10
mm long were prepared from freshly extracted teeth. The samples were tested in a closed-loop servohydraulic testing machine at constant load or displacement, and varied strain rate. In vivo strain rates were estimated using strain gauges bonded to human teeth.
Results: A family of creep curves, determined at different loads within dentin's elastic region, was found to be consistent with a material having linear viscoelastic behavior. A positive correlation (
r
2=0.79,
P<0.001) was found between creep rate and stress. Young's modulus (
E) was found to be a function of the strain rate with rates of loading in the range 10–500,000
Ns
−1. Loading at constant displacement showed stress to be a decreasing function of time (i.e. stress relaxation).
Significance: Dentin showed linear-viscoelastic behavior under various conditions of compressive loading. Time dependent properties of dentin should be taken into account in restorative dentistry.
Objective: The purpose of this study was to investigate the extent of progressive cuspal displacement during prolonged occlusal loading (either continuous or cyclic) and delayed cuspal recovery ...following removal of the load.
Methods: Extracted maxillary premolars with extensive MOD cavity plus endodontic access were subjected to an occlusal load applied to both cusps, followed by a recovery period. Two types of loading were applied: (a) continuous load of 100, 200 and 300
N for 0.1–1000
s; (b) cyclic loading of 300
N at 0.83
Hz for 1–1000 cycles. Cuspal displacement during loading and subsequent recovery were measured with an extensometer attached to both cusps.
Results: Continuous loading led to progressive cuspal displacement that was both time- and load-dependent. After removal of the load, complete recovery from cuspal deformation was prolonged up to 20
min, also in a time- and load-dependent manner. Cyclic loading resulted in a cumulative increase in cusp displacement but to only a very small extent (approximately 1
μm after 1000 cycles).
Conclusions: Progressive cuspal displacement and delayed recovery occur with prolonged continuous or cyclic occlusal loading. Continuous loading (e.g. as in clenching) is potentially more damaging than cyclic loading (as in chewing).
Objectives: The aim of this study was to compare two techniques of measuring cuspal deformation under occlusal load: Direct Current Differential Transformers (DCDTs) and strain gauges. The study ...investigated the relative dependence of the two techniques on the vertical orientation of the tooth in relation to direction of loading and differences in the patterns of change in response to a cavity preparation sequence.
Methods: Strain gauges and DCDTs were attached to the buccal and lingual surfaces of extracted maxillary premolars and mandibular molars. Premolars were subjected to simulated occlusal loads at varying vertical angulations before and after MOD cavity preparation. Molars were tested at progressive stages of cavity preparation. Cuspal deformation was recorded as linear cusp displacement (in μm, using DCDTs) and as cuspal strains (using strain gauges) and relative stiffness.
Results: Strain gauges were much less sensitive than linear displacement devices to vertical orientation of teeth, and computation of relative stiffness further reduced the effect of angulation. Strain gauges are much easier to use experimentally. DCDTs required precise three-dimensional adjustments for the testing of cuspal deformation. Patterns of change in cuspal flexure following cavity preparation were very different using the two techniques.
Conclusions: Use of both devices simultaneously yielded much more information than when used alone, and showed that cusps do not deform as simple cantilever beams.
It is important to understand subsurface dentin demineralization and caries from the clinical perspective as dentin properties are modified under acidic conditions and mechanical loading. This study ...was conducted to observe the subsurface demineralization of dentin beams at three different pH levels under tension and compression. Bovine dentin beams (10 x 3.75 x 1.45 mm) were fixed at one end and immersed in 0.1 M lactic acid solution at pH levels 4.5, 7, and 10 for 5 days under a load of 6.5 N (663 g), and the subsurface demineralization depth was measured using a polarized light microscope. The mean subsurface demineralization depth was more subjacent to the surface under compression than that under tension at pH 4.5 and decreased as the distance from the fixed end increased. No subsurface demineralization was observed at pH 7 or 10. It was concluded that both stress and low pH are associated with increased subsurface demineralization at the fixed end of the beam.
Noncarious cervical lesions have a multifactorial etiology. Mechanical stress has been identified as one of the factors, but little evidence exists for its cause-effect relationship. This study was ...conducted at three different pH levels to observe the surface loss on dentin beams under tension and compression. Bovine dentin beams (10 x 3.75 x 1.45 mm) were fixed at one end and immersed in 0.1 M lactic acid solution at pH levels 4.5 (n = 20), 7 (n = 20), and 10 (n = 20) for 5 days under a load of 6.5 N (663 g). The mean surface loss was more on the surface under compression than that under tension at pH 4.5 and pH 7 than at pH 10. Also, the surface loss on the beams decreased as the distance from the fixed end increased. It was concluded that stress and lower pH both increase surface loss at the fixed end of the beam, which in a tooth represents the cervical region.
