Biophysics of the tooth

Synchrotron-radiation-based X-ray micro-computed tomography reveals dental bur debris under dental composite restorations

Dental burs are used extensively in dentistry to mechanically prepare tooth structures for restorations (fillings), yet little has been reported on the bur debris left behind in the teeth, and whether it poses potential health risks to patients. Here it is aimed to image dental bur debris under dental fillings, and allude to the potential health hazards that can be caused by this debris when left in direct contact with the biological surroundings, specifically when the debris is made of a non-biocompatible material. Non-destructive micro-computed tomography using the BioMedical Imaging & Therapy facility 05ID-2 beamline at the Canadian Light Source was pursued at 50 keV and at a pixel size of 4 µm to image dental bur fragments under a composite resin dental filling. The bur's cutting edges that produced the fragment were also chemically analyzed. The technique revealed dental bur fragments of different sizes in different locations on the floor of the prepared surface of the teeth and under the filling, which places them in direct contact with the dentinal tubules and the dentinal fluid circulating within them. Dispersive X-ray spectroscopy elemental analysis of the dental bur edges revealed that the fragments are made of tungsten carbide-cobalt, which is bio-incompatible.

FINITE ELEMENT SIMULATION OF IN VIVO TOOTH MOBILITY IN COMPARISON WITH EXPERIMENTAL RESULTS

The objective of this study was to characterize the material properties of the periodontal ligament (PDL). Since the PDL undergoes the largest deformations when a load is applied to the tooth crown, its material properties mainly govern the resulting tooth deflection. By comparing experiments on tooth mobility with a Finite Element simulation using individual and realistic geometry models of the measured teeth, information about the mechanical properties of the PDL can be obtained. To investigate in vivo tooth mobility, a special experimental setup has been developed. The experimental results showed highly non-linear and time dependent material properties of tooth deflection as they are known for other soft biological tissues.6 Since in vivo tooth deflection is not an uniaxial tensional experiment, it is not possible to determine material parameters of the PDL. For this reason, a geometry model of the measured tooth was generated using computer tomography data and in a Finite Element simulation tooth deflection under external forces was calculated. A comparison of the simulation with the experimental data lead to an optimized characterization of the PDL in view of its mechanical properties.

Novel diamond-coated tools for dental drilling applications

The application of diamond coatings on cemented tungsten carbide (WC-Co) tools has been the subject of much attention in recent years in order to improve cutting performance and tool life in orthodontic applications. WC-Co tools containing 6% Co metal and 94% WC substrate with an average grain size of 1 - 3 microm were used in this study. In order to improve the adhesion between diamond and WC substrates it is necessary to etch cobalt from the surface and prepare it for subsequent diamond growth. Alternatively, a titanium nitride (TiN) interlayer can be used prior to diamond deposition. Hot filament chemical vapour deposition (HFCVD) with a modified vertical filament arrangement has been employed for the deposition of diamond films to TiN and etched WC substrates. Diamond film quality and purity has been characterized using scanning electron microscopy (SEM) and micro Raman spectroscopy. The performances of diamond-coated WC-Co tools, uncoated WC-Co tools, and diamond embedded (sintered) tools have been compared by drilling a series of holes into various materials such as human tooth, borosilicate glass, and acrylic tooth materials. Flank wear has been used to assess the wear rates of the tools when machining biomedical materials such as those described above. It is shown that using an interlayer such as TiN prior to diamond deposition provides the best surface preparation for producing dental tools.

Thermal stimulation causes tooth deformation: a possible alternative to the hydrodynamic theory?

OBJECTIVES

To investigate the relationship between temperature distribution and tooth structure deformation during and after localised application of thermal stimuli used during pulp vitality testing.

METHODS

Strains and temperature changes within tooth structures were recorded when three different thermal stimuli, namely heated gutta percha (120-140 degrees C), carbon dioxide dry ice (-72 degrees C) and refrigerant spray (-50 degrees C), were applied to extracted bovine incisors. Each stimulus was applied for 5s on the labial enamel surface in a random order, with a 30-min interval between tests. Finite element analysis was performed on basic geometrical shapes to investigate structural deformation in relation to temperature change.

RESULTS

Application of thermal stimuli to the labial enamel surface resulted in rapid development of strain at the pulpal dentine surface before any temperature change was detected at the dentino-enamel junction. The strain pattern was biphasic; heat produced an initial contraction of the pulpal surface, followed by an expansion, and the reverse pattern was found with cold stimulation. Finite element analysis confirmed that the initially pronounced thermal gradient across the enamel and dentine caused rapid flexural deformation before temperature changes reached the dentino-enamel junction. When the temperature changes reached the pulpal dentine and thus reduced the thermal gradient, the direction of the strain was reversed.

CONCLUSION

These results indicate possible alternatives to the hydrodynamic theory for thermal stimuli applied to intact teeth. Mechanically induced dentine deformation may trigger nerve impulses directly, or may exert mechanically induced dentinal fluid flow that triggers nerve activity.

Effect of static loading of dentin beams at various pH levels

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.

Effect of loading and pH on the subsurface demineralization of dentin 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.

Anisotropic fracture in bovine root and coronal dentin

OBJECTIVES

This purpose of this study was to address the fundamental question of how the fracture properties of dentin are related to its composite structure.

METHODS

Strain concentration tests and impression-induced damage tests were designed to compare bovine root dentin with coronal dentin, and to understand the role of individual structural elements in the fracture of dentin.

RESULTS

Absent in peritubular dentin, root dentin is insensitive to cracks and exhibits higher fracture resistance than coronal dentin that has a typical brittle fracture behavior along the peritubular dentin. Fracture analysis and impression damage experiments found that root dentin is highly anisotropic in fracture behavior. Cracking is predominantly controlled by the organization of collagen fibrils, with the incremental lines being the weakest planes. In coronal dentin, highly mineralized peritubular dentin that intersects with the incremental lines creates additional weak orientations that compete with the incremental lines and thus greatly decrease the degree of fracture anisotropy.

SIGNIFICANCE

This study demonstrated that dentin is by no means homogeneous in terms of fracture properties. Location and orientation (especially in terms of incremental lines) should be taken into account when examining tooth failure both in laboratory and in clinical studies.

Phase shifting speckle interferometry for determination of strain and Young's modulus of mineralized biological materials: a study of tooth dentin compression in water

Mineralized biological materials have complex hierarchical graded structures. It is therefore difficult to understand the relations between their structure and mechanical properties. We report the use of electronic speckle pattern-correlation interferometry (ESPI) combined with a mechanical compression apparatus to measure the strain and Young's modulus of root dentin compressed under water. We describe the optomechanical instrumentation, experimental techniques and procedures needed to measure cubes as small as 1 x 1 x 2 mm. Calibration of the method is performed using aluminum, which shows that the measurements are accurate within 3% of the compression modulus reported for standard aluminum 6061. Our results reveal that the compression moduli of root dentin from the buccal and lingual sides of the root are quite different from the moduli of the interproximal sides. Root dentin from interproximal locations is found to have an average modulus of 21.3 GPa, which is about 40% stiffer than root dentin from the buccal and lingual locations, found to have a modulus of 15.0 GPa. Our approach can be used to map deformations on irregular surfaces, and measure strain on wet samples of varying sizes. This can be extended to the study of other biological materials including bone and synthetic biomaterials.

The effect of hydrogen peroxide concentration on the outcome of tooth whitening: an in vitro study

AIM

This in vitro study examined the effect that various concentration of hydrogen peroxide (5-35%) had on tooth whitening.

METHOD

Extracted third molars were sectioned and stained using a standardised tea solution to Vita shade C4. These stained specimens were then bleached with a series of gels containing 5, 10, 15 or 25% w/w hydrogen peroxide. Each specimen was bleached for a number of sessions with one session being defined as 3 x 10 min exposure.

RESULTS

The number of applications of the various concentrations of bleaching gel varied from 12 applications for the 5% gel to one application for the 35% gel. Plotting the number of applications against hydrogen peroxide concentration showed an exponential response curve.

CONCLUSIONS

The concentration of hydrogen peroxide in a proprietary bleaching gel had a marked effect on the number of applications required to produce an optimal shade outcome.

3D-finite element analyses of cusp movements in a human upper premolar, restored with adhesive resin-based composites

The combination of diverse materials and complex geometry makes stress distribution analysis in teeth very complicated. Simulation in a computerized model might enable a study of the simultaneous interaction of the many variables. A 3D solid model of a human maxillary premolar was prepared and exported into a 3D-finite element model (FEM). Additionally, a generic class II MOD cavity preparation and restoration was simulated in the FEM model by a proper choice of the mesh volumes. A validation procedure of the FEM model was executed based on a comparison of theoretical calculations and experimental data. Different rigidities were assigned to the adhesive system and restorative materials. Two different stress conditions were simulated: (a) stresses arising from the polymerization shrinkage and (b) stresses resulting from shrinkage stress in combination with vertical occlusal loading. Three different cases were analyzed: a sound tooth, a tooth with a class II MOD cavity, adhesively restored with a high (25 GPa) and one with a low (12.5GPa) elastic modulus composite. The cusp movements induced by polymerization stress and (over)-functional occlusal loading were evaluated. While cusp displacement was higher for the more rigid composites due to the pre-stressing from polymerization shrinkage, cusp movements turned out to be lower for the more flexible composites in case the restored tooth which was stressed by the occlusal loading. This preliminary study by 3D FEA on adhesively restored teeth with a class II MOD cavity indicated that Young's modulus values of the restorative materials play an essential role in the success of the restoration. Premature failure due to stresses arising from polymerization shrinkage and occlusal loading can be prevented by proper selection and combination of materials.

Human root dentin: structural anisotropy and Vickers microhardness isotropy

The demanding mechanical functions and the variable structure of dentin make it an invaluable material for studying the structure-mechanical function relations of a mineralized collagen-containing tissue. The mineralized collagen fibril axes in human root dentin are mainly located on the incremental plane. Within this plane there is a preferred orientation in the general root-crown direction. The apatite crystals are aligned in three dimensions within an individual collagen fibril, but this orientation does not necessarily extend to the neighboring fibrils. Crystals are also present as aggregates without any preferred orientation. The structure is therefore clearly anisotropic with respect to the collagen fibril orientation, but less so with respect to overall crystal orientation. Vickers microhardness measurements of the root dentin are essentially the same on the three orthogonal planes with respect to the incremental plane. Knoop microhardness measurements are also the same on all three orthogonal planes when the major diagonal is aligned perpendicular to the collagen fibril axis preferred orientation direction. In-plane variations of up to 20% are observed in the orthogonal direction. The material is thus isotropic in the three main directions with respect to Vickers microhardness, but anisotropic in structure. This paradoxical situation is attributed mainly to the variable modes of crystal organization.

Strain-structure relations in human teeth using Moiré fringes

Teeth are subjected to stress during normal function. The manner in which the resulting strain is distributed within the tooth is related to its structure. The Moiré fringe technique was used to map the in-plane strain distribution in slices from human tooth crowns under compression. The strain inside enamel is much less than in dentin, and there is a roughly 200 microm thick zone in dentin beneath the dentin enamel junction which undergoes larger strain than the central coronal dentin. This zone is softer and less mineralized than the bulk of the dentin. The strain distribution in this zone along the dentin-enamel junction shows localized maxima on both the lingual and the labial sides. This study is consistent with the hypothesis that within the dentin there are structural adaptations for transferring and minimizing stress.

The dentin substrate: structure and properties related to bonding

OBJECTIVES

Dentin is a vital, hydrated composite material with structural components and properties that vary with location. These variations are reviewed along with alterations by physiological and pathological changes that allow classification into various forms of dentin. Structural characteristics and mechanical properties are reviewed and the limitations of our understanding of structure-property relationships for normal and modified forms of dentin are discussed with respect to their impact on dentin bonding. Recent progress in methods available to study dentin and its demineralization are emphasized with their promise to increase our understanding of dentin properties and structure.

DATA SOURCES

Recent microstructural studies, focusing on scanning electron microscopy, atomic force microscopy and X-ray tomographic microscopy are included. A review of fundamental studies with emphasis on microstructurally sensitive methods, and prior reviews of basic mechanical properties are included with discussion of their correlation to composition and structure.

STUDY SELECTION AND CONCLUSIONS

Emphasis in this work was placed on the major structural components of the tissue, including the collagen based organic matrix and its mineral reinforcement, the distribution of these components and their microstructural organization as related to mechanical properties and response to demineralization. Little information is included on biochemical and developmental studies or on non-collagenous proteins and other organic components for which limited understanding is available with respect to their role in structure-property relations and influence on bonding. In spite of the fact that the complexity of dentin precluded a comprehensive review, it is clear that local structural variations influence properties and impact nearly all preventive and restorative dental treatments. Much more work is needed in order to understand differences between vital and non-vital dentin, and dentin from extracted teeth. Although our knowledge is rudimentary in certain areas, increasingly sophisticated methods of studying dentin should provide the necessary information to model structure-property relations, optimize dentin bonding, and improve many aspects of preventive and restorative dentistry.

The effects of acetone, ethanol, HEMA, and air on the stiffness of human decalcified dentin matrix

During resin-bonding procedures, dentin surfaces are treated with acidic conditioners to remove the smear layer and decalcify the surface to expose the collagen fibrils of the underlying matrix. These decalcified surfaces are then either air-dried or treated with dehydrating solvents, procedures which may modify the physical properties of the dentin matrix. The purpose of this study was to evaluate the effects of dehydration on the stiffness of the decalcified dentin matrix. Small (8 x 1.7 x 0.9 mm) beams of dentin were prepared from mid-coronal dentin of extracted human molars. The ends were covered with varnish for protection, and the specimens were placed in 0.5 M EDTA for 5 days to decalcify. The stiffness was measured by both the cantilever technique and by conventional stress-strain testing. Specimens tested by the cantilever technique were sequentially exposed to water, acetone, alcohol, HEMA, and glutaraldehyde. Specimens tested by conventional stress-strain testing were exposed either to water, acetone, or HEMA, or were allowed to air-dry. The results indicate that the stiffness of decalcified human dentin matrix is very low (ca. 7 MPa), if the specimens are wet with water. As they are dehydrated, either chemically in water-miscible organic solvents or physically in air, the stiffness increases 20- to 38-fold at low strains or three- to six-fold at high strains. These increases in modulus were rapidly reversed by rehydration in water. Exposure to glutaraldehyde also produced an increase in stiffness that was not reversible when the specimens were placed back in water.

Does an incremental filling technique reduce polymerization shrinkage stresses?

It is widely accepted that volumetric contraction and solidification during the polymerization process of restorative composites in combination with bonding to the hard tissue result in stress transfer and inward deformation of the cavity walls of the restored tooth. Deformation of the walls decreases the size of the cavity during the filling process. This fact has a profound influence on the assumption--raised and discussed in this paper--that an incremental filling technique reduces the stress effect of composite shrinkage on the tooth. Developing stress fields for different incremental filling techniques are simulated in a numerical analysis. The analysis shows that, in a restoration with a well-established bond to the tooth--as is generally desired--incremental filling techniques increase the deformation of the restored tooth. The increase is caused by the incremental deformation of the preparation, which effectively decreases the total amount of composite needed to fill the cavity. This leads to a higher-stressed tooth-composite structure. The study also shows that the assessment of intercuspal distance measurements as well as simplifications based on generalization of the shrinkage stress state cannot be sufficient to characterize the effect of polymerization shrinkage in a tooth-restoration complex. Incremental filling methods may need to be retained for reasons such as densification, adaptation, thoroughness of cure, and bond formation. However, it is very difficult to prove that incrementalization needs to be retained because of the abatement of shrinkage effects.

Tensile properties of resin-infiltrated demineralized human dentin

The ability of adhesive resins to restore the physical properties of demineralized dentin has not been well-documented. The unfilled resins that are used for adhesion have relatively low moduli of elasticity and limited ability to increase dentin stiffness, although they may increase the ultimate tensile strength of dentin. This study tested the hypothesis that resin infiltration of demineralized dentin can restore its tensile properties to those of mineralized dentin. Small (ca. 0.5 mm thick x 0.5 mm wide) specimens of demineralized human dentin were infiltrated with one of five different dentin bonding resins over many hours, to determine how these resins altered the tensile properties of dentin. Tensile stress and strain were measured in these and control (mineralized and demineralized) specimens until their ultimate failure. The results indicate that some adhesive resins, after infiltrating demineralized dentin, can restore and even exceed the ultimate tensile strength of mineralized dentin. These resins increased the modulus of elasticity of resin-infiltrated dentin to values equal to or greater than those of the resins but far below those of mineralized dentin. Although the conditions in this experiment were far removed from the manufacturer's recommendations or clinical practice, the results support the potential of resin infiltration for reinforcing dentin.

Tooth stiffness with composite veneers: a strain gauge and finite element evaluation

OBJECTIVES

This study was conducted to determine the impact of composite veneer procedures on the functional properties of incisors.

METHODS

Ten extracted human maxillary central incisors were mounted in pairs in a nylon ring. One strain gauge was bonded along the long axis of each tooth on the center of the lingual surface. Each pair formed half of a Wheatstone bridge circuit and was wired to eliminate all but the voltage resulting from experimentally applied procedures. The teeth were ramp-loaded to 50 N near the incisal edge on the lingual surface. Loading was performed on the unaltered teeth, teeth with preparations and restored teeth. Two-dimensional finite element (FE) models were generated to evaluate each test condition. Relative stiffness, compared with the unaltered tooth, was calculated from measurements with the strain gauge steps and from the FE models.

RESULTS

A relative stiffness value of unity represents recovery of stiffness to the level of the unaltered tooth. Both methods of evaluation demonstrated a decrease in mean relative stiffness with each subsequent reduction in tooth structure. The composite restoration increased its mean relative stiffness compared to its corresponding preparation but never to the level of the unaltered tooth. Across all procedures, the two-dimensional FE model correlated well in both direction and magnitude with the experimental strain gauge method (R = 0.83).

SIGNIFICANCE

A resin composite veneer does not restore the stiffness to the level of an unaltered tooth.

The elastic modulus of dentine determined by static and dynamic methods

The purpose of this investigation was to determine a static elastic modulus of dentine using a three point beam test and a dynamic modulus in the frequency range of 0.1 Hz and 10 Hz across a temperature range of 27-37 degrees C. At body temperature, the mean static modulus was 8.6 GPa, (standard deviation 0.86 GPa). The dynamic test produced a range of modulus values. At 0.1 Hz the modulus ranged from 14.3 to 15.2 GPa; for 1.0 Hz the range was 14.6-15.5 GPa and for 10 Hz the range was 14.9-15.8 GPa. The results are of value in the design and analysis of restorative materials.

Temperature-dependence of compressive properties of human dentin

The effect of temperature on compressive stress/strain behavior of human dentin obtained from recently extracted permanent lower molar teeth has been determined over the range 0-80 degrees C. Dentin specimens were loaded uni-axially in a direction perpendicular to the tubule orientation. A statistically significant, linear regression relationship was found between modulus (E) and temperature (T): E (GPa) = 15.55-0.0734 . (T degrees C). The observed temperature coefficient of the modulus is in close agreement with that observed for cortical bone. Proportional limit, compressive strength, and resilience were also found to undergo a linear decrease with increasing temperature. Mechanical failure of specimens generally occurred along lines determined by maximum shear stresses, approximately 45 degrees to the axial load direction.