Effect of storage media on subsequent water loss and regain by human and bovine dentine and on mechanical properties of human dentine in vitro

This study was designed to determine whether storage media and time affect any changes in water loss with dehydration of dentine. The effect of storage of prepared bars was also compared with storage of whole teeth. Prepared bovine dentine bars were stored for 3 days or 12 weeks in deionized water, phosphate-buffered saline or neutral-buffered formalin and whole teeth were stored in neutral-buffered formalin for 12 weeks before the preparation of dentine bars. Specimens were removed from storage and weight loss with dehydration, determined gravimetrically, was equated directly to water loss. Tensile and three-point bend tests were made on human dentine bars that had been stored in deionized water or neutral-buffered formalin for 3 days. Results were subjected to analysis of variance and Duncan's new multiple-range test. Weight loss by evaporation from dentine at 22.3 degrees C and 53.2% relative humidity was rapid and non-linear, reaching 5.05 +/- 0.55% for bovine dentine compared with 3.33 +/- 0.63% for human dentine after 7 days. Water loss with dehydration was not affected by 3-day storage of specimens in different media or by 12-week storage of whole teeth, but was affected when prepared specimens were stored for 12 weeks. Different storage media had no significant effect (p > 0.05) on the strength or toughness of human dentine. Water loss in human dentine was associated with shrinkage of specimens, but these dimensional changes were fully recovered with rehydration. Storage of teeth in neutral-buffered formalin followed by minimal storage time for prepared specimens is recommended to minimize changes in dentine after extraction.

Sterilization of teeth by gamma radiation

Clinical simulations and restorative materials research and development conducted in vitro require the use of large numbers of extracted teeth. The simultaneous need for infection control procedures and minimal alterations of structure and properties of the tissue prompted this study of gamma irradiation as a method to eliminate microbes associated with extracted teeth and their storage solutions. Evaluations of potential change in structure of dentin were conducted in terms of permeability, Fourier transform infrared spectroscopy (FTIR), and optical properties. The dose required for sterilization by gamma irradiation was established by means of a tooth model inoculated with Bacillus subtilis (10(8) organisms/mL). Sterilization occurred at a dose above 173 krad with use of a Cesium (Cs137) radiation source. Gamma irradiation did not affect permeability of crown segments of dentin. A comparative evaluation of the effects of four sterilization methods on dentin disks was based on FTIR and ultraviolet-visible-near infrared (UV/VIS/NIR) spectra before and after sterilization by (1) gamma irradiation; (2) ethylene oxide; (3) dry heat; and (4) autoclaving. No detectable changes were found with gamma irradiation, but all other methods introduced some detectable change in the spectra. This suggests that common methods of sterilization alter the structure of the dentin, but gamma irradiation shows promise as a method which both is effective and introduces no detectable changes as measured by FTIR, UV/VIS/NIR, or permeability.

New adhesives in Class V restorations under combined load and simulated dentinal fluid

OBJECTIVES

This study compared the efficacy of three dentinal adhesives using the "all etch" technique (All-Bond 2, Bisco; Scotchbond MP,3M Dental Products Co.; OptiBond, Kerr) with a dentinal adhesive which still uses phosphoric acid to condition enamel and a self-etching primer for dentin (A.R.T.-Bond, Coltene/Whaledent).

METHODS

Eight V-shaped mixed Class V restorations were placed per group in extracted human premolars. The restorations were subjected to 1,200,000 mechanical occlusal cycles (max. force 49 N; frequency 1.7 Hz) and 3,000 simultaneous thermal cycles (5-50-5 degrees C). Dentinal fluid was simulated using 1:3 diluted horse serum and fed into the pulp chamber both during restoration and loading. Percentages of "continuous margin" were assessed on SEM replicas of enamel and dentinal margins at 200x magnification immediately before and after stressing, respectively.

RESULTS

No significant differences were observed before stress between the materials either in enamel or in dentin. After stress, however, OptiBond and A.R.T.-Bond performed significantly better in dentin than the two other adhesives (Kruskal-Wallis, Mann-Whitney; p < 0.05). Although high initial values were observed, All-Bond 2 and Scotchbond MP were not stress-resistant under simulated physiological conditions.

SIGNIFICANCE

The predicted clinical potential of All-Bond 2 and Scotchbond MP is inferior to that of OptiBond and A.R.T.-Bond.

Induction of dentin formation on canine amputated pulp by recombinant human bone morphogenetic proteins (BMP)-2 and -4

Dental pulp cells have the potential to differentiate into odontoblasts. The molecular mechanisms underlying differentiation are not clear. Demineralized dentin matrix is osteoinductive and contains bone morphogenetic protein (BMP) activity. BMPs have been implicated in embryonic odontogenic differentiation and hence may play a role in the differentiation of adult pulp cells into odontoblasts during pulpal healing. This study examined the hypothesis that BMPs induce dentin formation on amputated canine pulp. Recombinant human BMP-2 and BMP-4 were capped with inactivated dentin matrix on amputated pulp. At two months, the amputated pulp was filled with tubular dentin in the lower part and osteodentin in the upper part. The amount of dentin formed was markedly diminished when dentin matrix alone was implanted. These findings imply that recombinant human BMP-2 and BMP-4 induce differentiation of adult pulp cells into odontoblasts. Thus, BMPs may have a role in dentistry as a bioactive pulp-capping agent to induce dentin formation.

Dependence of mechanical properties on fibre angle in narwhal tusk, a highly oriented biological composite

The successful modelling of the mechanical properties of mineralized tissues depends critically on the knowledge of the off-axis behaviour of individual unidirectional lamellae. Information on this is lacking. In this work we attempt to rectify the situation. Young's modulus, measured in bending and tension, and the tensile strength and ultimate strain to failure of the dentine of the narwhal Monodon monoceros, were determined on specimens that had almost unidirectional fibres, whose direction differed considerably from specimen to specimen. Modulus and strength decreased steadily with the degree of off-angle loading, falling to about 45% of maximum for modulus, and 35% of maximum for strength. Ultimate strain showed a less uniform behaviour, and remained remarkably high at large angles. Differences in mechanical behaviour were not related to the very small differences in mineral content measured between specimens. These findings have strong implications for modelling the anisotropic behaviour of bone, because dentine is very much like bone in most important respects. Predictions using classical composite theory are reasonably satisfactory, as long as the mineral crystals are assumed to be platelets, not rods.

The influence of age and depth of dentin on bonding

OBJECTIVES

The purpose of this study was to investigate what influence the two variables of dentin depth and age may have on the tensile bond strengths of three bonding systems.

METHODS

Dentin discs prepared from human molars were divided into young and old, superficial and deep surfaces. Three bonding systems, Scotchbond Multi-purpose (3M Dental Products), Superbond D-liner (Sun Medical Co.), and Liner Bond II (Kuraray Co.) were the materials tested for tensile bond strength. In addition, the structural variation of the resin-impregnated, or hybrid, layer was compared among the two variables and three bonding systems.

RESULTS

Tensile bond strengths exceeding 10 MPa were obtained for all materials. After ANOVA, an effect on tensile bond strength could be attributed to dentin age or depth for only Superbond D-liner used on deep-young dentin as compared with old-superficial dentin. All other group comparisons failed to show any variation between dentin depth or age. However, specimens bonded to deeper dentin showed slightly lower strengths. SEM observations showed thicker resin-impregnated layers for Scotchbond MP and Superbond D-liner compared with Liner Bond II. Liner Bond II exhibited a thinner and more diffuse resin-impregnated layer, believed to be due to the different dentin conditioning method.

SIGNIFICANCE

Dentin age or depth may not show as great an influence on bond strengths with the newer type of bonding systems. The resin-impregnated layer quality, rather than thickness, is believed to be the most important factor for obtaining high tensile bond strengths.

Dentin bond strength after air drying and rewetting

The purpose of this study was to investigate the effect of air drying and remoistening of dentin on resin composite bond strength and to determine the extent to which the outermost, collagenous network and surface area of chemically conditioned dentin contributed to interfacial strength. Three components made up the experimental design: The first component comprised 12 experimental groups each containing 10 caries-free molar teeth. Shear bond strength values with dentin were measured for conditioned and non-conditioned dentin to which All-Bond 2 and BisFil were applied. The following values (MPa) were obtained for the groups conditioned with 10% phosphoric acid for 20 seconds: Group 1 (wet control) 24.20 +/- 2.03; Group 2 (10 seconds dried) 19.83 +/- 5.42; Group 3 (30 seconds dried) 11.56 +/- 4.93; Group 4 (10 seconds dried, 5 seconds. remoistened) 25.38 +/- 3.89; Group 5 (30 seconds dried, 5 seconds, remoistened) 24.66 +/- 4.54; Group 6 (5% NaOCl/wet) 25.97 +/- 3.61. For the non-conditioned groups the values were: Group 7 (wet control) 21.54 +/- 4.07; Group 8 (10 seconds dried) 14.62 +/- 3.55; Group 9 (10 seconds dried, 5 seconds remoistened) 16.95 +/- 4.04; Group 10 (5% NaOCl/wet) 13.67 +/- 4.39; Group 11 (air abrasive action) 18.78 +/- 3.62; Group 12 (air abrasive/5% NaOCl) 19.87 +/- 2.03.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphometric analysis of the deposition and mineralization of enamel and dentine from rat incisor during the recovery phase following a low-calcium regimen

The administration of a low-calcium diet to pups nursed by a mother on the same diet has been shown to induce a slowing of growth. A reduction of the apposition rate of dentine, which was normally mineralized, and a dramatic reduction of the extent of mineralization of enamel, whose organic matrix was otherwise produced in an almost normal amount, was observed in the incisors of these animals. Modifications of the mineral apposition rate of dentine, measured after administration of tetracycline (10 mg/kg), and the thickness and the microhardness of the two tissues, the latter being an expression of the degree of mineralization, were now investigated when hypocalcic pups were restored to a normal-calcium diet for 10 or 60 days. Enamel microhardness was increased by more than 60% after only 10 days of restored diet and had become the same as in the control tissue after 60 days, without any significant increase in thickness. Dentine thickness and mineral apposition rate increased significantly, to become similar to those of the controls after 60 days of restored diet. In dentine there was no significant variation of microhardness between experimental pups and controls, either during the low-calcium diet or the restorative period. These results indicate that the deposition of the organic matrix of enamel is a process independent from that of its mineralization, and that the mineralization of the organic matrix happens by its replacement even a long time after its deposition. In contrast, the deposition and mineralization of dentine are strictly interdependent processes, at least in these experimental conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

The effect of dietary xylitol on dentin formation in ovariectomized rats

Twenty-nine 3-month-old female Wistar rats were labeled by means of a single intraperitoneal tetracycline injection. Nineteen animals were subsequently ovariectomized, whereas a control group of 10 animals underwent sham operations. All the animals received the basal diet, and 10 of the ovariectomized animals were given an additional dietary xylitol supplementation (5%). Three months later the animals were killed by decapitation, and dentinal apposition on the molars was measured. The results indicate that supplementation of the diet with 5% xylitol had an attenuating effect on the enhanced dentin formation caused by ovariectomy, but the mechanism remains unsolved.

A new method to test the cohesive strength of dentin

The cohesive plateau theory states that the strength at a bonded interface should equal the strength, cohesively, of the substrate with which it is bonded. A new method was developed to test cohesive strength of dentin so as to offer an opportunity to compare the data against shear bond strength values of commercially available dentinal bonding agents using the same test assembly. The method applied a shear force to a standard circular column of dentin continuous with the tooth from which it was cut with a tubular diamond drill. The mean cohesive strength of dentin in the shear mode, based on 30 specimens, was 36.18 +/- 6.81 MPa.

Intrapulpal temperatures during pulsed Nd:YAG laser treatment of dentin, in vitro

Lasers are being used for soft tissue removal, caries removal, and treatment of root surface sensitivity. One concern for laser safety is that the heat produced at the irradiated root surface may diffuse to the pulp causing irreversible pulpal damage. To test this heat diffusion, copper-constantan thermocouples were inserted into the radicular pulp canals of extracted teeth. Simulating direct exposure which might occur during gingival excision, superficial caries removal, and modification of the dentin surface for treatment of root surface sensitivity, a 2 mm2 area of the external root surface was uniformly irradiated with a pulsed Nd:YAG laser using a 320 microns diameter fiber optic contact probe. Power was varied from 0.3 to 3.0 W with frequencies of 10 and 20 Hz. Temperature changes during cavity preparations using a high speed handpiece with air coolant were also recorded. Repeated measures ANOVA (P < or = 0.05) indicated that intrapulpal temperatures increased as a function of power, frequency, and time. Intrapulpal temperatures decreased as remaining dentin thickness (0.2 to 2.0 mm) increased for each laser parameter. Irradiation of dentin using a Nd:YAG pulsed laser, within the treatment times, powers, and frequencies with adequate remaining dentin thickness, as outlined in this paper, should not cause devitalizing intrapulpal temperature rises.

A critical reevaluation of stresses generated during vertical and lateral condensation of gutta-percha in the root canal

The finite element method was used to calculate the stresses in a maxillary canine tooth produced during lateral and vertical condensation. Description of the biological domain to be analyzed, and conversion of the manual operations of the endodontist to realistic load representation are intricate problems which must be overcome before stresses are recovered and plotted. The stresses in dentin are in general of comparable magnitudes during lateral or vertical condensation, but these magnitudes generally remain much below those presented in a previous study. From these calculations it appears that root fractures reported by some investigators may be explained in terms of stress concentrations and local irregularities of the samples tested.

Effect of smear layer removal on the diffusion permeability of human roots

Ten human maxillary incisors, extracted because of periodontal disease or nonrestorable caries, were obtained and instrumented to a size #70 K-Flex file at the working length using a standard stepback technique. Tritiated water (3H2O) was placed in the root canals and allowed to diffuse to the external surface of the roots until it reached a constant rate. The smear layer in each of the experimental roots was then removed using 0.5 M EDTA followed by 5.25% sodium hypochlorite (NaOCI). The constant rate diffusion of 3H2O was remeasured. The roots were then stored in deionized H2O for 2 months and the constant rate diffusion of 3H2O was remeasured. A statistically significant difference was noted between all three groups. A decrease in the diffusion permeability of the root to 3H2O was noted immediately after smear layer removal and the highest permeability was recorded after storage in the deionized water for 2 months.

Effects of a supersaturated pulpal fluid on the formation of caries-like lesions on the roots of human teeth

Root surfaces were exposed to demineralizing buffer in a continuous-flow system, with or without simultaneous perfusion of the pulp chamber with a supersaturated surrogate dentine fluid (SDF). Experimental lesions formed with perfusion of the pulp were significantly less deep than control lesions formed without perfusion, but the application of hydrostatic pressure to the SDF (15 or 30 cm water) did not produce significantly greater reductions. The lesion depth reduction was attributed to reduced undersaturation at the advancing lesion front through interdiffusion of the demineralizing buffer and the SDF. Translucent bands, resembling sclerotic dentine, were observed beneath 9 of 24 experimental lesions and 5 of 24 control lesions. Scanning electron microscopy showed that, in most of the bands beneath experimental lesions, dentinal tubules were either partly occluded by granular mineral deposits or were reduced in diameter. Thus, in vivo, dentine fluid may modify the rate of lesion progression and promote sclerosis in the underlying dentine.

A comparative study of translucent apical dentine in vital and non-vital human teeth

Previous studies have shown that dentinal apical translucency increases with age. Although the mechanism by which apical translucency is produced is still uncertain, it has been suggested that it develops at a slower rate in non-vital teeth. In this investigation the amount and distribution of apical translucent dentine in a group of age-matched vital and non-vital teeth were compared. Fifty-five non-vital (root-filled) teeth and 49 vital teeth were used. Freshly extracted teeth were fixed in 2% glutaraldehyde solution, sectioned longitudinally in the buccolingual plane and ground to a thickness of 150 microns. Each section was photographed in polarized light at a standard magnification against a ruled scale background. Black-and-white prints of standard magnification were produced and translucent apical dentine was plotted using an Apple II digitizer. Variables measured were length, length as a percentage of root length, area and area as a percentage of root area. Data were related to age and tooth type and analysed by Mann-Whitney and regression analysis. Significant differences (p < 0.05) were present when tooth types were pooled. There was more apical translucency present in all types of non-vital teeth when compared with vital teeth at any given age.

Dentine permeability and its role in the pathobiology of dentine sensitivity

The classical hydrodynamic theory implicated fluid movement as a transducing mechanism in the production of dental sensitivity. This theory assumes that sensitive dentine must be permeable. Various measurements of dentine permeability are discussed, including: (1) factors that influence diffusive permeation across dentine; (2) factors that influence convective fluid movement across dentine; (3) osmotic activities of solutions; (4) comparison of evaporative and convective fluid movement; (5) the interaction between outward convective fluid flux on the inward diffusive flux of molecules; and (6) the importance of pulpal blood flow in the clearance of noxious substances from dentine and pulp, a balance concept. The variables involved in achieving good penetration of desensitizing agents in the presence of outward movement of dentinal fluid are also discussed, along with the presentation of a new hypothesis which emphasizes the importance of dentine as a dynamic physiological barrier that works in harmony with neurovascular elements in the pulp in an attempt to maintain the health of the pulp-dentine complex.

Interactions between neural and hydrodynamic mechanisms in dentine and pulp

Evidence is presented that the rate of inward diffusion of chemicals through exposed dentine is affected by the rate of outward flow of fluid through the dentinal tubules. Such a flow has been demonstrated in cats. The flow rate appears to depend upon the pulpal tissue-fluid pressure; flow increased during pulp vasodilatation and decreased, even reversing in direction, during vasoconstriction. Pulp vasodilatation can be produced by stimulating intradental afferent nerves, including some of those that seem to be excited by displacement of tubule contents (i.e. by a hydrodynamic mechanism). Thus, when dentine is exposed and these afferents are stimulated they will help to protect the pulp by producing reflex vasodilatation, which will decrease the rate of diffusion of toxins from the mouth into the pulp. The relation between the rate of flow through dentine and the discharge evoked in intradental nerves was investigated in cats. Single fibres were more sensitive to outward than to inward flow. The flow rates required to excite the pulp afferents were greater than those observed during even maximal pulpal vasodilatation.

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.

Stress of tooth and PDL structure created by bite force

Stress is created by the bite force and distributed along the tooth towards the PDL structure. It is of interest to investigate the complex tooth structure, consisting of enamel, dentine, pulp, and thin cementum layer and how it functions in stress distribution. This study was intended to analyze the role of the tooth and PDL structures in stress distribution, by using a three-dimensional finite element method. A mandibular first molar was constructed for the finite element model. The bite forces were measured by Pressensor, and these bite force values were programmed to load down upon the occlusal surface of the model. The results were expressed by stress contours and principal stress graphs. The stress was found to decrease as it distributed from the occlusal surface towards the cervical portion in the dentine and the pulp. In contrast, the stress, especially a compressive stress, increased gradually in the enamel layer in the lower half of the crown, in the same direction. It was apparent in displayed pattern of stress that the stress distributed outward towards the surrounding portion of the lower half of the crown. This resulted in a uniform magnitude of the principal stresses for all aspects of the mesial and distal roots. The stresses of both roots were generally compressive stress. When comparing the stress values of sampling points positioned between the root surfaces and the periphery of the PDL (the alveolar wall), all principal stresses for those of the PDL (periodontal ligament) were less than those of the root surfaces. These findings revealed that the PDL, the dentine, and the pulp functioned in cooperation in stress reduction; and the sequences of enamel, dentine, and pulp influenced the pattern of stress distribution. The different material properties of the tooth structure in sequence was considered a very important factor for stress reduction and for the pattern of stress distribution, especially in the root.

The effects of enamel anisotropy on the distribution of stress in a tooth

Enamel is thought to have highly anisotropic stiffness characteristics, because of its prismatic structure. It is probable that the enamel is stiffer in the prism direction compared with a direction perpendicular to it. The prisms are thought to run approximately perpendicular to the enamel-dentin junction. The curvilinear anisotropy that will result can readily be modeled by TOMECH, a finite element program developed at the University of Sheffield, since curvilinearity of mechanical properties is available as an automated feature of this program. The patterns of stress due to an external load were investigated in two-dimensional abstract models, and in a model of a mandibular second premolar, for both anisotropic and isotropic enamel. Results were compared with the commercial code ANSYS and good agreement obtained. Enamel with anisotropic properties was found to have a profoundly different stress distribution under load when compared with models with isotropic enamel. For isotropic enamel, the load path is directed through the stiff enamel shell, while for anisotropic enamel, the load path is directed into the dentin, as the load path follows the stiff direction of the enamel prisms. Thus, if enamel is indeed anisotropic, its function differs greatly from that suggested in previous hypotheses. Enamel with anisotropic material characteristics would provide a hard-wearing protective surface-coating while simultaneously diverting the load away from this brittle, low-tensile-strength phase, thus reducing the potential for tooth fracture.

[Adhesion of composite to dentin. Mechanical and clinical results]

Although an efficient bond of resin composites to enamel can be realized since quite a long time, reliable dentine bonding is nowadays still a clinical problem. After the failure of the dentine-etch technique, followed by the misfortunes of the chemical dentine adhesion technique, modern dentine adhesive systems are believed to function by a micromechanical attachment mechanism. Based on a morphological study of the resin-dentine interface, a broad selection of dentine adhesive systems was classified following their adhesion-strategy. In a second part, eight dentine adhesive systems were clinically tested in terms of retention.

[Amalgam. X. Glass-ionomer cement: a biocompatible substitute for amalgam?]

The biocompatibility of glass-ionomer depends upon the components released. The consequences of this releasing for the health of the pulp seem to be moderate, provided that there is a rather thick layer of dentin (> 1 mm) to protect the pulp. Glass-ionomer shrinks upon setting, but it adheres to the dental hard tissues. Yet, bacteria are found along and underneath the material. Systemic effects due to toxicity and allergy are largely unknown, possibly because glass-ionomer is a recent, as yet insufficiently investigated material.

Ultrastructure of wound healing following direct pulp capping with calcium-beta-glycerophosphate (Ca-BGP)

Tissue changes in rat incisors after direct pulp capping with calcium-beta-glycerophosphate (Ca-BGP) have been studied using light and electron microscopy. Immediately after pulp capping, Ca-BGP was converted to hydroxyapatite (HAP) at the cavity floor. At Day 1, the exposure site was covered with Ca-BGP-mediated mineralized tissue. Osteodentine had developed below this mineralized tissue at Day 3. Matrix vesicles (MV) were observed in the extracellular matrix between large cells and osteodentine. At Day 5, tubular dentine was observed below the osteodentine. Three days after the application of Ca(OH)2 in a control experiment, osteodentine had formed below the necrotic zone; however, tubular dentine was not observed at Day 5. These findings suggest that the applied Ca-BGP might be the source of Ca and inorganic phosphate (Pi) through hydrolysis by alkaline phosphatase (ALP), and that Ca-BGP-mediated mineralized tissue induces the early formation of tubular dentine.