Biomedical subjects

G W Marshall

Publications and source records attributed to G W Marshall.

At least 19 recordsLinked to original sources

PubMed2005

The dentin-enamel junction and the fracture of human teeth.

The dentin-enamel junction (DEJ), which is the interfacial region between the dentin and outer enamel coating in teeth, is known for its unique biomechanical properties that provide a crack-arrest barrier for flaws formed in the brittle enamel1. In this work, we re-examine how cracks propagate in the proximity of the DEJ, and specifically quantify, using interfacial fracture mechanics, the fracture toughness of the DEJ region. Careful observation of crack penetration through the interface and the new estimate of the DEJ toughness ( approximately 5 to 10 times higher than enamel but approximately 75% lower than dentin) shed new light on the mechanism of crack arrest. We conclude that the critical role of this region, in preventing cracks formed in enamel from traversing the interface and causing catastrophic tooth fractures, is not associated with the crack-arrest capabilities of the interface itself; rather, cracks tend to penetrate the (optical) DEJ and arrest when they enter the tougher mantle dentin adjacent to the interface due to the development of crack-tip shielding from uncracked-ligament bridging.

Dental Enamel↗

PubMed2005

The nano-hardness and elastic modulus of sound deciduous canine dentin and young premolar dentin--preliminary study.

The purpose of this study was to compare the nano-hardness and elastic modulus among deciduous and permanent dentin, buccal and lingual sides, incisal, center and cervical areas, and outer, middle and inner layers. Three premolars and three deciduous canines were bucco lingually (BL) sectioned, and three deciduous canines were mesio-distally (MD) sectioned parallel to the long axis at the center of the tooth. Hardness (H), plastic hardness (PH) and Young's modulus (Y) were measured using a nano-indentation tester. The H, PH and Y values from the deciduous canine dentin were significantly lower than those from the premolar dentin at most sites. For deciduous canine dentin, the H and PH values of the MD sectioned dentin were significantly higher than those of the BL sectioned dentin in many layers of many areas. Generally deciduous canine dentin had H, PH and Y values that decreased from outer toward the inner layers and significant differences were obtained among the layers in many areas. For MD sectioned deciduous canine and BD sectioned premolar dentin, the H, PH and Y values of the cervical area were significantly lower than those of the incisal and center areas in many layers. It is possible that optimum bonding may require different treatments for deciduous and permanent dentin and perhaps also for different intratooth locations.

Anisotropy↗

PubMed2005

Amelogenin control over apatite crystal growth is affected by the pH and degree of ionic saturation.

OBJECTIVE: To study the mechanisms which promote the interactions of amelogenin proteins with the forming mineral to establish suitable conditions for the biomimetic synthesis of enamel in vitro. DESIGN: Saturated calcium phosphate solutions were used in conjunction with recombinant amelogenin proteins to induce mineral formation on glass-ceramics substrates containing oriented fluoroapatite crystals (FAP). The height of mineral layers formed on these substrates within 24 h was measured by atomic force microscopy (AFM). EXPERIMENTAL VARIABLES: The effect of protein concentration, pH and degree of saturation (DS) on the growth of apatite mineral was evaluated. Mineralization experiments were performed at 0, 0.4 and 1.6 mg/ml amelogenin concentrations. Mineralization solutions were used at pH values of 6.5, 7.4, 8.0 and 8.8 and DS of calcium and phosphate between 9 and 13. OUTCOME MEASURE: Height and morphology of mineralized layer formed on glass-ceramic substrates as determined from AFM measurements. RESULTS: Homogeneous nucleation and crystal growth of thin layers on the FAP were observed, when calcium and phosphate ions were added. The height of these layers grown on (001) planes of FAP was strongly dependent on the protein concentration and pH. At concentrations of 0 and 0.4 mg/ml crystal grew 5-15 nm on the FAP, while they grew approximately to 200 nm at 1.6 mg/ml. The enhanced crystal growth was observed only at pH 6.5, 7.4 and 8.0, while layers only 20 nm thick were obtained at pH 8.8. An increase in DS resulted in uncontrolled growth of calcium phosphate mineral covering large areas of the substrate. CONCLUSIONS: Protein concentration, pH and the saturation of the mineralizing solution need to be considered carefully to provide suitable conditions for amelogenin-guided growth of apatite crystals.

Amelogenesis↗

PubMed2005

Dentin erosion simulation by cantilever beam fatigue and pH change.

Exposed root surfaces frequently exhibit non-carious notches representing material loss by abrasion, erosion, and/or abfraction. Although a contribution from mechanical stress is often mentioned, no definitive proof exists of a cause-effect relationship. To address this, we examined dimensional changes in dentin subjected to cyclic fatigue in two different pH environments. Human dentin cantilever-beams were fatigued under load control in pH = 6 (n = 13) or pH = 7 (n = 13) buffer, with a load ratio (R = minimum load/maximum load) of 0.1 and frequency of 2 Hz, and stresses between 5.5 and 55 MPa. Material loss was measured at high- and low-stress locations before and after cycling. Of the 23 beams, 7 withstood 1,000,000 cycles; others cracked earlier. Mean material loss in high-stress areas was greater than in low-stress areas, and losses were greater at pH = 6 than at pH = 7, suggesting that mechanical stress and lower pH both accelerate erosion of dentin surfaces.

Dental Stress Analysis↗

PubMed2004

Influence of Carisolv on resin adhesion for two different adhesive systems to sound human primary dentin and young permanent dentin.

OBJECTIVE: This study evaluated the influence of Carisolv (Medi Team) for resin adhesion to sound human primary and young permanent dentin. METHODS: The buccal surfaces of 28 primary molars and 64 premolars were used. Two adhesive systems and resin composites were used; SE: Clearfil SE (Kuraray) and Clearfil APX (Kuraray), and SB: Single Bond (3M) and Z250 (3M). Six groups were prepared. Groups 1-2 were primary dentin and Groups 3-6 were permanent dentin. Groups 1 and 3: SE was used. Groups 2 and 4: treated with Carisolv and then primed, SE was used. Group 5: SB was used. Group 6: treated with Carisolv and then etched, SB was used. The microstructural effects of primer or etchant, and Carisolv plus primer or etchant applied to dentin were evaluated by SEM. In addition, the microstructure of the resin-dentin interfaces of each group was studied using SEM. Shear bond strengths (SBS) were tested, and the failed surfaces were observed using SEM. Data was statistically analyzed using ANOVA with subsequent application of Fisher's PLSD at p<0.05. RESULTS: The mean SBS (unit: MPa) of Groups 1-6 were: 27.8, 19.2, 21.3, 21.7, 6.7 and 7.6. The SBS of Group 2 was significantly lower than that of Group 1. There was no significant difference of the SBS among Groups 1 and 4, 2, 3 and 4, and 5 and 6. In SE groups, the hybrid layer for primary dentin was thicker than that for permanent dentin. CONCLUSIONS: Carisolv treatment before priming significantly decreased the SBS to primary dentin in SE groups, but did not influence the SBS to permanent dentin in both SE and SB groups.

Acid Etching, Dental↗

PubMed2004

Micro-Raman spectroscopic investigation of dental calcified tissues.

The purpose of this study was to determine if dental calcified junctions (DEJs/CDJs) in human teeth contain different compositional phases compared to the adjacent dental calcified tissues. Peak positions and intensities were determined from micro-Raman spectra for PO(3-) (4) and the C--H modes and compared among the mineralized tissues and their junctions. Values of width were determined from the intersections of intensity regression lines through the junctions and in the adjacent tissues. The peaks were measured in 1-microm steps along a l00-microm line across the junction. High-resolution analysis revealed that PO(3-) (4) band peaks for dentin, the DEJ, enamel, the CDJ, and cementum were at the same position (959 cm(-1)), while for the C--H stretching mode a significant shift of 4.6 cm(-1) was found between enamel, the DEJ, and dentin. The mean width of the DEJ was 7.6 (+/- 2.8) microm using the PO(3-) (4) band and 8.6 (+/- 3.6) microm using the C--H stretching mode. Across the DEJ, the mineral content monotonically decreased from enamel to dentin while the organic component monotonically increased. The DEJ width was in agreement with prior nanoindentation studies. No width estimate was possible for the CDJ because the compositional differences between cementum and dentin were small.

Apatites↗

PubMed2004

Ultrastructure and nanomechanical properties of cementum dentin junction.

The attachment between cementum and dentin has been given several definitions and nomenclature, including: interzonal layer, intermediate cementum, collagen hiatus, Hopewell-Smith's hyaline layer, and more commonly, cementum-dentin junction (CDJ). Understanding the attachment of two structurally dissimilar hard tissues such as cementum and dentin defined by a junction may provide information necessary to engineer functionally graded materials that can be used for efficient tooth restorations in clinical dentistry and other bioengineering applications. Hence, in this study, as a first step toward understanding the CDJ using a biomechanical approach, it was hypothesized that the CDJ between cementum and dentin is a wide zone with mechanical properties significantly lower than the neighboring tissues. The structure of the CDJ was studied using an atomic force microscope (AFM), and site-specific mechanical response of the three regions; cementum, CDJ, and dentin were determined using an AFM-nanoindenter under dry and wet conditions. The AFM results of the CDJ demonstrated a valley under dry conditions and a peak under wet conditions. The magnitude of the depth of the valley was approximately the same as the height of the peak of the CDJ, ranging from 10 to 40 microm. The nanomechanical properties under dry conditions indicated no significant difference (p > 0.05) in elastic modulus and hardness of the CDJ (Er = 17.5 +/- 2.7 GPa, H = 0.6 +/- 0.1 GPa) and cementum (Er = 18.7 +/- 2.5 GPa, H = 0.6 +/- 0.1 GPa). The mechanical properties of the CDJ were significantly lower (p << 0.05) than dentin (Er = 19.9 +/- 2.9 GPa, H = 0.6 +/- 0.1 GPa) under dry conditions. However, under more relevant hydrated conditions, the mechanical properties of CDJ (Er 3.0 +/- 0.7 GPa, H = 0.1 +/- 0.0 GPa) were significantly lower (p << 0.05) than those of cementum (Er 6.8 +/- 1.9 GPa, H = 0.2 +/- 0.1 GPa) and dentin (Er 9.4 +/- 2.3 GPa, H = 0.3 +/- 0.1 GPa). Based on the results from this study, it can be concluded that the CDJ can be regarded as a wide zone containing large quantities of proteins including collagen that contribute to hydration and significantly reduce mechanical properties, compared with the adjacent hard tissues, cementum, and dentin. The lower mechanical properties of the CDJ may make it possible for it to redistribute occlusal loads to the alveolar bone.

Aged↗

PubMed2004

Resonant ultrasound spectroscopy measurements of the elastic constants of human dentin.

The technique of resonant ultrasound spectroscopy (RUS) was used to measure the second-order elastic constants of hydrated human dentin. Specimens were placed between two transducers, and the resonant frequencies of vibration were measured between 0.5 and 1.4 MHz. The elastic constants determined from the measured resonant frequencies in hydrated dentin exhibited slight hexagonal anisotropy, with the stiffest direction being perpendicular to the axis of the tubules (E11 = 25.1GPA) This hexagonal anisotropy was small (E33/E11 = 0.92), and almost disappeared when the specimens were dried. In addition, there was a pronounced anisotropy in the Poisson's ratio of wet dentin: v21 = 0.45; v31 = 0.29. With drying in air, this anisotropy vanished: v21 = v31 = 0.29. The isotropic Young's modulus of dried dentin was 28.1 GPa. RUS shows promise for determining the elastic constants in mineralized tissues.

Anisotropy↗

PubMed2004

Evaluation of a new modulus mapping technique to investigate microstructural features of human teeth.

Teeth contain several calcified tissues with junctions that provide interfaces between dissimilar tissues. These junctions have been difficult to characterize because of their small size. In this work a new technique using a combination of atomic force microscopy (AFM) and a force-displacement transducer was used to simultaneously study the surface topography and map mechanical properties of the junctions and adjacent hard tissues. Prepared specimens from human third molars were scanned by an AFM piezo-tube in contact mode. To measure the dynamic viscoelastic properties of the material a small sinusoidal force was superimposed on the contact force and the resulting displacement amplitude and the phase shift between the force and amplitude were measured. This force modulation technique was used to map the local variation of nanomechanical properties of intertubular dentin, peritubular dentin, enamel, dentin-enamel junction (DEJ) and peritubular-intertubular dentin junction (PIJ). This new technique allowed us to measure the widths of these junctions in addition to local variation in dentin and enamel without causing plastic deformation to the material and with 2 orders of magnitude increase in spatial resolution compared with previous studies that used discrete nanoindentation techniques. Due to the ability to analyze the sample line-by-line, the distribution functions associated with the width of the DEJ and PIJ were conveniently obtained for specific intratooth locations. The data suggested, for three third molar specimens, a DEJ width of 2-3 microm with full-width half-maximum (FWHM) of 0.7 microm and PIJ width of 0.5-1.0 microm with 0.3 microm FWHM. The intertubular dentin storage modulus variation was between 17 and 23 GPa with a mean value of 21 GPa. The range of storage modulus for enamel near the DEJ was between 51 and 74 GPa with a mean value of 63 GPa.

Dental Enamel↗

PubMed2004

Amelogenin-guided crystal growth on fluoroapatite glass-ceramics.

The formation of aligned fibrous apatite crystals in enamel is predominantly attributed to the involvement of amelogenin proteins. We developed a model to study interactions of matrix proteins with apatite mineral in vitro and tested the hypothesis that amelogenin solubility affects the ability to induce protein-guided mineralization. Crystal growth experiments were performed on fluoroapatite (FAP) glass-ceramics in mineralizing solutions containing recombinant full-length amelogenin (rH174) at different concentrations. Using atomic force microscopy, we observed that mineral precipitated randomly on the substrate, but also formed thin layers (height, 10 nm) on FAP within 24 hrs. This growth pattern was unaffected when 0.4 mg/mL of rH174 was added. In contrast, crystals grew on FAP at a rate up to 20 times higher, at 1.6 mg/mL protein. Furthermore, nanospheres and mineral bound specifically to FAP and aligned in strings approximately parallel to the c-axis of FAP, leading us to the conclusion that amelogenin proteins indeed control direction and rate of growth of apatite in enamel.

Amelogenin↗

PubMed2003

Optical spectroscopy and imaging of the dentin-enamel junction in human third molars.

A 351-nm laser excitation source was used to perform autofluorescence microscopy of dentin, enamel, and the dentin-enamel junction (DEJ) to obtain information regarding their morphology and spectral characteristics. The emission spectra of these calcified dental tissues were different from one another, and this enabled the DEJ to be imaged and dimensionalized. The DEJ displayed sharp and clearly delineated borders at both its enamel and dentin margins. The dentinal tubules and the enamel prisms appeared to terminate abruptly at the DEJ. The median DEJ width was 10 microm, ranging from 7 to 15 microm, and it did not appear to depend on intratooth position.

Dental Enamel↗

PubMed2003

The importance of intrafibrillar mineralization of collagen on the mechanical properties of dentin.

It is widely held that the hardness and modulus of dentin increase in proportion to the mineral concentration. To test this belief, we measured hardness and modulus of normal dentin and an altered form of dentin without gap-zone mineralization in wet and dry conditions by AFM nanoindentation to determine if the modulus and hardness scale linearly with mineral concentration. Mineral concentrations in the mid-coronal location of the normal and altered dentins were 44.4 vol% and 30.9 vol%, respectively. Surrounding the pulp of the altered dentin was a region of higher mineralization, 40.5 vol%. The indentation modulus of normal dentin was 23.9 (SD = 1.1) GPa dry and 20.0 (SD = 1.0) GPa wet. In mid-coronal regions of the altered dentin, the indentation modulus was 13.8 (SD = 2.0) GPa dry and 5.7 (SD = 1.4) GPa wet. In the more mineralized regions of the altered dentin, the modulus was 20.4 (SD = 1.8) GPa dry and 5.3 (SD = 0.8) GPa wet; the properties of the altered wet dentin did not correlate with mineral concentration. The results of this study raise doubt as to whether mineral concentration alone is a sufficient endpoint for assessing the success or failure of remineralization approaches in restorative dentistry.

Adolescent↗

PubMed2003

The mechanical properties of human dentin: a critical review and re-evaluation of the dental literature.

The past 50 years of research on the mechanical properties of human dentin are reviewed. Since the body of work in this field is highly inconsistent, it was often necessary to re-analyze prior studies, when possible, and to re-assess them within the framework of composite mechanics and dentin structure. A critical re-evaluation of the literature indicates that the magnitudes of the elastic constants of dentin must be revised considerably upward. The Young's and shear moduli lie between 20-25 GPa and 7-10 GPa, respectively. Viscoelastic behavior (time-dependent stress relaxation) measurably reduces these values at strain rates of physiological relevance; the reduced modulus (infinite relaxation time) is about 12 GPa. Furthermore, it appears as if the elastic properties are anisotropic (not the same in all directions); sonic methods detect hexagonal anisotropy, although its magnitude appears to be small. Strength data are re-interpreted within the framework of the Weibull distribution function. The large coefficients of variation cited in all strength studies can then be understood in terms of a distribution of flaws within the dentin specimens. The apparent size-effect in the tensile and shear strength data has its origins in this flaw distribution, and can be quantified by the Weibull analysis. Finally, the relatively few fracture mechanics and fatigue studies are discussed. Dentin has a fatigue limit. For stresses smaller than the normal stresses of mastication, approximately 30 MPa, a flaw-free dentin specimen apparently will not fail. However, a more conservative approach based on fatigue crack growth rates indicates that if there is a pre-existing flaw of sufficient size (approximately 0.3-1.0 mm), it can grow to catastrophic proportion with cyclic loading at stresses below 30 MPa.

Biomechanical Phenomena↗

PubMed2002

Dentin shear strength: effect of distance from the pulp.

OBJECTIVE: Dentin structure varies with orientation and location. Ultimate shear strength (USS) has also been found in previous studies to vary with location. The present study further explores this relationship between USS and various locations in coronal dentin as well as distance from the pulp. METHODS: Stick specimens were prepared from coronal dentin located in the center or under cusps of human molar teeth. These were tested in the shear mode at various distances from the pulp. RESULTS: Median values ranged from 52.7 (range 29.0-73.1) MPa near the pulp to 76.7 (range 53.9-104.0) MPa near the dentino-enamel junction. No differences were found among the buccal, central or lingual locations, however, the USS near the pulp was found to be significantly lower. SIGNIFICANCE: The properties of coronal dentin vary with distance from the pulp, which may affect adhesion and other aspects of restorative dentistry.

Anisotropy↗

PubMed2002

In vitro behavior of silicate glass coatings on Ti6A14V.

The in vitro response in simulated body fluid (SBF) of silicate glass coatings on Ti6A14V was evaluated. Glasses belonging to the SiO2-CaO-MgO-Na2O-K2O-P2O5 system were used to prepare 50-70 m thick coatings on Ti6Al4V, employing a simple enameling technique. Glasses with silica content higher than 55 wt% can be used to prepare coatings that do not crack or delaminate and exhibit good adhesion to the alloy. It has been found that coatings with silica content lower than 60 wt% are more susceptible to corrosion and precipitate carbonated hydroxyapatite on their surface during in vitro tests. However, these coatings have a higher thermal expansion than the metal and are under tension. After 2 months in SBF cracks grow in the coating that reach the glass/metal interface and initiate delamination. Glasses with silica content higher than 60 wt% are more resistant to corrosion and have lower thermal expansion. These coatings do not crack but they do not precipitate apatite even after 2 months in SBF.

Alloys↗

PubMed2002

Bonding to Er-YAG-laser-treated dentin.

Er-YAG laser irradiation has been claimed to improve the adhesive properties of dentin. We tested the hypothesis that dentin adhesion is affected by Er-YAG laser conditioning. Superficial or deep dentin from human molars was: (a) acid-etched with 35% H3PO4; (b) irradiated with an Er-YAG laser (KaVo) at 2 Hz and 180 mJ, with water-cooling; and (c) laser- and acid-etched. Single Bond (3M ESPE) and Z100 composite (3M ESPE) were bonded to the prepared surfaces. After storage, specimens were tested in shear to failure. Bonded interfaces were demineralized in EDTA and processed for transmission electron microscopy. Two-way ANOVA revealed that conditioning treatment and interaction between treatment and dentin depth significantly influenced shear bond strength results. Acid-etching alone yielded shear bond strength values that were significantly higher than those achieved with laser ablation alone, or in combination with acid-etching. The Er-YAG laser created a laser-modified layer that adversely affects adhesion to dentin, so it does not constitute an alternative bonding strategy to conventional acid etching.

Acid Etching, Dental↗

PubMed2001

Collagen orientation and crystallite size in human dentin: a small angle X-ray scattering study.

The mechanical properties of dentin are largely determined by the intertubular dentin matrix, which is a complex composite of type I collagen fibers and a carbonate-rich apatite mineral phase. We performed a small angle X-ray scattering (SAXS) study on fully mineralized human dentin to quantify this fiber/mineral composite architecture from the nanoscopic through continuum length scales. The SAXS results were consistent with nucleation and growth of the apatite phase within periodic gaps in the collagen fibers. These mineralized fibers were perpendicular to the dentinal tubules and parallel with the mineralization growth front. Within the plane of the mineralization front, the mineralized collagen fibers were isotropic near the pulp, but became mildly anisotropic in the mid-dentin. Analysis of the data also indicated that near the pulp the mineral crystallites were approximately needle-like, and progressed to a more plate-like shape near the dentino-enamel junction. The thickness of these crystallites, approximately 5 nm, did not vary significantly with position in the tooth. These results were considered within the context of dentinogenesis and maturation.

Collagen Type I↗

PubMed2001

Mechanical properties of the dentinoenamel junction: AFM studies of nanohardness, elastic modulus, and fracture.

The dentinoenamel junction (DEJ) is a complex and poorly defined structure that unites the brittle overlying enamel with the dentin that forms the bulk of the tooth. In addition, this structure appears to confer excellent toughness and crack deflecting properties to the tooth, and has drawn considerable interest as a biomimetic model of a structure uniting dissimilar materials. This work sought to characterize the nanomechanical properties in the region of the DEJ using modified AFM based nanoindentation to determine nanohardness and elastic modulus. Lines of indentations traversing the DEJ were made at 1-2 microm intervals from the dentin to enamel along three directions on polished sagittal sections from three third molars. Nanohardness and elastic modulus rose steadily across the DEJ from bulk dentin to enamel. DEJ width was estimated by local polynomial regression fits for each sample and location of the mechanical property curves for the data gradient from enamel to dentin, and gave a mean value of 11.8 microm, which did not vary significantly with intratooth location or among teeth. Nanoindentation was also used to initiate cracks in the DEJ region. In agreement with prior work, it was difficult to initiate cracks that traversed the DEJ, or to produce cracks in the dentin. The fracture toughness values for enamel of 0.6-0.9 MPa . m(1/2) were in good agreement with recent microindentation fracture results. Our results suggest that the DEJ displays a gradient in structure and that nanoindenation methods show promise for further understanding its structure and function.

Algorithms↗