Interferometric measurements of cusp deformation of teeth restored with composites

A Michelson interferometry apparatus was used for measurement of the displacement of the buccal cusps of premolars after restoration of MOD preparations with composites. The effects of composite type, cavity size, and hydration conditions were examined. Interferometry permitted real-time measurement of cusp movement as it occurred. Contraction occurred very rapidly, about 1/3 of the 60-minute amount within the two-minute period of exposure to the curing light. Cusp movement was smooth rather than interrupted, indicating lack of microfracturing at deformations of 11-46 microns. Contraction, 0.94% for Heliomolar and 1.2% for P-50, was similar to the linear polymerization shrinkage of the resins. Less cusp movement occurred in small cavities than in large cavities. Hydrated teeth had less cusp movement than dehydrated teeth.

Response of bone and enamel formation to nutritional supplementation and morbidity among malnourished Guatemalan children

The effects of changes in nutritional and health status upon bone and enamel development are examined in a sample of 63 rural Guatemalan children (24 females, 39 males). The number of ossified hand-wrist centers at 3 years and the number of linear enamel hypoplasias (LEH) in approximately 0-3 year zones of developing teeth were used to monitor the response of bone mineralization and enamel matrix formation to illness and nutritional supplementation. Numbers of ossified centers and LEH were compared across sex, supplementation, and morbidity groups. Enamel matrix secretion responded positively to increased supplementation. Children who received less than 34.25 kcal/day in supplement had more LEH than those who received more supplement. No differences in ossification status were found between supplementation groups. These data suggest that enamel formation may be more sensitive to changes in nutritional status than is bone mineralization. Disruptions of bone and enamel formation were both associated with frequent illness. Children who were ill more than 3.6% of the time had more LEH and fewer ossified hand-wrist centers than children who were less frequently ill. Conclusions regarding relative environmental sensitivity must take into account the specific aspects of dental and skeletal development examined.

Mechanical responses of the periodontal ligament in the transverse section of the rat mandibular incisor at various velocities of loading in vitro

Stress-strain curves of the periodontal ligament (PDL) were obtained at various velocities of extrusive loading of 1, 10, 10(2), 10(3) and 10(4) mm/24 h in vitro. Significant increases of the maximum shear stress, tangent modulus and failure strain energy density were found with increases in the velocity of loading. The maximum shear strain increased from a velocity of 1 to 10 mm/24 h but decreased from 10 to 10(4) mm/24 h. It was shown histologically that the free surface of the PDL adhering to the cementum after mechanical testing was rough and irregular at higher velocities and rather smooth at lower velocities. These results showed that the mechanical properties and mode of failure of the rat incisor PDL were greatly dependent on the strain rate. It is possible that the PDL of the continuously erupting rat incisor has mechanical characteristics favourable for resisting weakly to slow and small eruptive forces but strongly to the fast and large occlusal forces as suggested previously [Chiba and Komatsu, The Biological Mechanisms of Tooth Eruption and Root Resorption (1988)].

Cell- and stage-specific expression of vitamin D receptor and calbindin genes in rat incisor: regulation by 1,25-dihydroxyvitamin D3

To investigate the extent of 1,25-dihydroxyvitamin D3 [1,25(OH)2D3] action and its relationships to calbindin gene expression in mineralized tissues, we have analyzed rat incisors with different probes, including a vitamin D receptor (VDR) antibody and specific cDNAs to rat calbindin-D9K and calbindin-D28K. Developmental and hormonal controls of calbindin gene expression were investigated by Northern blot analysis of ameloblast and odontoblast mRNA. Distribution and hormone-induced changes of VDR were also studied by light microscopic immunocytochemistry. A differential tissue- and stage-specific expression of the calbindin genes was observed in microdissected portions of the continuously erupting incisor. The two calbindins were expressed in ameloblasts, whereas only calbindin-D28K was expressed in odontoblasts. Moreover, in ameloblasts, expression of calbindin-D28K preceded that of calbindin-D9K. Immunoreactivity for VDR was present in all progenitor cells and progressively decreased during the differentiation process, whereas, in differentiated tissues, a hormonal upregulation was restricted to hard tissue-forming cells, i.e., ameloblasts and odontoblasts. Furthermore, calbindin gene expression appeared to be regulated by 1,25(OH)2D3. Taken together, these data indicate that ameloblasts and odontoblasts are target cells for 1,25(OH)2D3 and provide the first insights into the hormonal control of tooth genes during development.

Physical-mechanical effects of Nd:YAG laser on the surface of sound dental enamel

Human dental enamel samples were irradiated using a 5 kHz Q-Switched Nd:YAG laser. An increase in Knoop microhardness and modification of the membrane permselectivity were detached. These results and the changes observed by SEM, can be connected with the fusion of the enamel surface.

Analysis of a central maxillary incisor by using a three-dimensional finite element method

An attempt is made for determining the stress distribution in a maxillary central incisor by using a three-dimensional finite element model. The tooth is assumed isotropic, homogeneous, elastic and unsymmetrical. A load of 450N, 26 degrees to the longitudinal axis is applied on the incisal margin of the tooth. The distribution of compressive, tensile and shear stresses were plotted for the whole tooth structure. It is found that cracks or fractures occur under the given load.

Common epitopes of mammalian amelogenins at the C-terminus and possible functional roles of the corresponding domain in enamel mineralization

The present studies were undertaken to investigate the presence of common epitopes of mammalian amelogenins at the C-terminus and the possible functional importance of the conserved C-terminal domain in enamel mineralization during mammalian amelogenesis. Enamel proteins, including the intact amelogenins and their degraded polypeptides, were isolated from the secretory enamel of pig, cow, rat, and rabbit incisors. Rabbit and rat antipeptide sera, as well as rat anti-25 kD and 20 kD pig amelogenin sera, were used to identify the amelogenins among the isolated matrix proteins of each of the animal species. The antipeptide sera were developed previously (Aoba et al. [19]) using as immunogens the two synthetic peptides, C13 and C25, which correspond to the last 12 (plus Cys for KLH-conjugation) and 25 amino acid residues of pig intact amelogenin, respectively. Reactivity of the enamel proteins with each antiserum was examined by Western blot analysis. The results of immunoblotting showed that a few enamel matrix proteins in each of the mammalian species were recognized by the anti-C13 serum, specifically, pig amelogenin at 25 kD (and trace components at 27, 22, and 18 kD), cow amelogenin at 28 kD (trace components at 26, 22, 19, and 14 kD), rat amelogenins at 28 and 26 kD (and a trace component at 20 kD), and rabbit amelogenins at 24 and 21 kD (and a trace at 13 kD). The anti-C25 serum reacted additionally with pig amelogenin at 23 kD, cow amelogenin at 27 kD (a major matrix constituent), and rabbit protein at 19 kD.(ABSTRACT TRUNCATED AT 250 WORDS)

Enamel structure in astrapotheres and its functional implications

Astrapotheres, large extinct ungulates of South America, share with rhinoceroses vertical prism decussation in the cheek tooth enamel. The similarity extends beyond merely the direction of the planes of decussation. The vertical decussation in astrapotheres is confined to the inner part of the enamel and has uniformly well-defined zones in which the prism direction differs by nearly 90 degrees and the zones are separated by narrow transitional borders of intermediate prism direction. The outer enamel consists of predominantly occlusally and outwardly directed prisms. Within the outer enamel is a region of horizontally decussating prisms; here the angle of decussation is usually smaller than that of the inner vertically decussating prisms. Except for the horizontal decussation in the outer enamel, these conditions match structures that have been described for rhinocerotoids. These features, together with the similarity in premortem crack direction and gross shape of the cheek teeth, imply that astrapotheres and rhinocerotoids shared essentially the same system of cheek tooth mechanics. However, the microstructure of the canine enamel in the astrapotheres is distinct. The lower canine enamel of the Oligocene Parastrapotherium exhibits a form of vertical decussation modified by a wavelike bending of prism zones, whereas the decussation in the rhinocerotoid canine is horizontal. The lower canine in Parastrapotherium was subjected to different loading conditions, judging from multiple sets of premortem crack directions. The modified vertical decussation would in theory resist cracking under different directions of tensile stresses. This is confirmed by the sinuous paths of cracks that run in directions differing by up to 90 degrees. That diverse stresses were generated in the enamel during life is confirmed by the pattern of premortem cracks in Parastrapotherium. The enamel in the upper canine of a late Miocene astrapothere lacks decussation but may have resisted cracking under varied loading conditions by virtue of a 3-dimensional wavelike bending of the prisms.

Effect of cavity depth on stresses in a restored tooth

Restorative procedures commonly replace lost tooth structure, but redistribution of functional stresses after treatment is not fully understood. Many restorative methods are dictated by the integrity of the remaining tooth structure, because sparse tooth structure can lead to fracture. It is essential to prevent fractures by having a clear concept of the designs for cavity preparations, and to anticipate the stresses of mastication on the remaining tooth structure. Knowledge of various internal parameters of cavity designs would facilitate selection of the appropriate cavity preparation for a specific clinical situation. Three cavity designs and restorations were examined in this study for stresses using the finite element technique. After placement of restorative materials, the dentin experienced a dramatic change in stress gradient immediately below the pulpal wall, and this response was magnified in deeper cavity preparations. Enamel also exhibited major alterations in the stress gradient in all three designs of cavity preparations. The combination of the changes can cause cracks in the remaining tooth structure, leading to cusp fracture immediately adjacent to the deepest portion of the cavity.

Permselectivity of sound and carious human dental enamel as measured by membrane potential

A microwell technique was used for determination of the permselectivities of sound and carious enamel in the same slice of tooth. The permselectivity determination was accomplished by drilling microwells in the enamel and filling them with a simulated plaque fluid containing lactate, carbonate, and inorganic ions at concentrations similar to those in resting plaque fluid, but with different concentrations of KCl. The electrical potentials developed across the enamel membrane were measured with microreference electrodes placed in the wells or in the solution outside the tooth. The results showed that the membrane potential was a function of the composition of the solutions separated by the enamel membrane and was independent of the composition of the solutions in the adjacent wells. The enamel was found to be cation-permselective, and sound enamel was more permselective than carious enamel. The flux rate of K+ was estimated from the change in the K+ concentration of the well solution as a function of time. The flux rate and the membrane potential data were used in the Nernst-Planck Flux Equation for calculation of the diffusion coefficient of K+ through enamel. The results indicate that the permselectivity of enamel can greatly influence the diffusion of ions through enamel membranes.

Structural and composition studies on the mineral of newly formed dental enamel: a chemical, x-ray diffraction, and 31P and proton nuclear magnetic resonance study

The present report describes a study of the development and maturation of the mineral component of dental enamel. We prepared porcine enamel of different stages of maturation, from the very immature enamel of unerupted teeth, with a mineral content of 45%, to fully mature enamel, with a mineral content of approximately 99%. We fractionated the less mature enamel by density centrifugation and examined the enamel density fractions and unfractionated enamel by a variety of chemical and physical techniques, including conventional and radial distribution function x-ray diffraction analysis, conventional and Fourier transform infrared spectroscopy, 31P and 1H nuclear magnetic resonance spectroscopy, and chemical analysis. The three most immature preparations, from unerupted teeth, had mineral contents of 45, 67, and 91 and Ca/P molar ratios of 1.41, 1.44, and 1.47. Density distribution histograms of the three fractions show that the early maturation of dental enamel mineral is accompanied by an increase in tissue density, reflecting the increase in mineral content. The density distribution in each sample is relatively narrow, indicating that the maturation process occurs at a fairly homogeneous rate, with all enamel in an anatomically defined zone mineralizing to about the same extent. X-ray diffraction studies indicate that even the least mature, least mineralized of these immature samples is considerably more crystalline than the most mature bone mineral studied and that crystalline perfection of the enamel crystals crystals increases further with maturation. Both the a and c axes of the mineral unit cell expand significantly during early stages of maturation. Solid-state 31P nuclear magnetic resonance spectroscopy studies indicate that dental enamel contains a DCPD-like HPO4 component in an apatitic lattice, similar to the component previously observed in bone and some synthetic calcium phosphates. The proportion of this DCPD-like component decreases with maturation but is readily detectable even in fully mature enamel. The infrared spectroscopic studies indicate that the 3570 cm-1 band ascribed to the OH- group of the hydroxyapatite crystals is absent in the least mature enamel but can be detected and becomes progressively stronger as the enamel becomes more mature. The increase in the content of the OH- groups of the apatite crystals is concomitant with the observed increase in unit cell parameters. Similar studies on very young and very old mature bone of four different species failed to detect the presence of OH- groups.

Molecular determinants during dental morphogenesis and cytodifferentiation: a review

Craniofacial development provides a number of opportunities to investigate the cellular and molecular biology of morphogenesis, cytodifferentiation, tissue-specific extracellular matrix (ECM) formations, and biomineralization. Regulatory processes associated with mandibular morphogenesis and specifically tooth formation are being investigated by the identification of when and where molecular determinants such as cell adhesion molecules (CAMs), substrate adhesion molecules (SAMs), and tissue-specific structural gene products are expressed during sequential developmental stages. Based upon in vitro organotypic culture studies in serumless, chemically defined medium, instructive and permissive signaling has been found to be required for both mandibular and dental morphogenesis and cytodifferentiation. For example, intrinsic developmental instructions (autocrine and paracrine factors), independent of long-range hormonal or exogenous growth factors, mediate morphogenesis from the initiation of the dental lamina through crown and initial root stages of tooth development. This review summarizes recent results using experimental embryology, organ culture, recombinant DNA technology, and immunocytology to elucidate mechanisms responsive to instructive epithelial-mesenchymal interactions associated with mandibular morphogenesis, tooth positional information, and subsequent tooth crown and initial root development.

Elastic outward bending of loaded buccal and lingual premolar walls in relation to cavity size and form

The aim was to study effects of the size and form of the occlusal part of Class II cavities on elastic outward bending during loading of buccal and lingual walls, separated by the preparation. Twenty newly extracted caries-free premolars were used. A special arrangement for standardized cavity preparation was set up. In order to obtain accurate measurements of the outward bending of loaded cavity walls, an electronic tensile test device was constructed. Methodologic studies showed an insignificant departure from linearity between between outward bending and load. All deformation was elastic. Correction factors were determined for strain of the apparatus as well as for indentation in the tooth substance. Measurement accuracy was calculated to +/- 0.12 N for force and +/- 0.2 micron for movement. In accordance with statistical principles, a few outlying measurements were discarded. The outward bending of buccal and lingual walls of cavities suitable for conservative restoration with dental amalgam was about 0.15 micron per Newton. Outward bending increased dramatically with exaggerated cavity depth and width. In the light of present knowledge of average chewing forces and tooth contacts in the habitual intercuspal position in young dentate adults, there seems to be little risk of leakage between conservative restorations and cavity walls as a result of elastic outward bending, though the risk certainly does increase with the depth and width of the cavity.

Influence of posteruptive age of enamel on its susceptibility to artificial caries

Epidemiologic studies and caries experiments with animals have suggested that caries susceptibility of teeth decreases with age. Observations on the changes in the dental tissues and their environment with age have pointed out that a process commonly referred to as 'posteruptive maturation' of the dental enamel may be responsible. To test this assumption 10 unerupted third molars and 56 erupted human premolar teeth of various posteruptive ages were subjected for 130 days to an acid gel for caries-like lesion formation in vitro. Sections of the teeth were examined with polarized light and 'contour maps' were drawn with the approximate porosity of the lesions. The results were: unerupted = 833 microns (+/- 261); erupted of posteruptive age 0-3 years = 561 microns (+/- 150), 4-10 years = 470 microns (+/- 136), 11-30 years = 459 microns (+/- 192) and over 30 years = 297 microns (+/- 89). These findings show a decrease in the susceptibility of enamel to artificial caries with increasing age, which was especially marked at and shortly after eruption. This may be explained by a completion of mineralization of the outer enamel at about eruption, by a maturation process of the outer enamel posteruptively and by a reduction in the permeability of enamel occurring through to old age. The fluoride content of the mid-coronal buccal surface enamel under study was found to decrease posteruptively with age, therefore not accounting for the decreasing caries susceptibility.

Cyclical changes in pH in bovine developing enamel as sequential bands

Developing enamel was stained with pH indicators and a banded colour pattern exhibiting alternate acidic (5.8-6.0) and neutral (7.0-7.2) staining was clearly visualized. The pH values of the enamel samples scraped from respective bands were confirmed by measuring them in suspensions with a pH meter. Neutral bands corresponded to red stripes of glyoxal bis(2-hydroxyanil) staining and acidic bands to unstained ones, suggested a correlation of the acidic and neutral stripes with the zones of ruffle-ended and smooth-ended ameloblasts.

Amelogenin post-secretory processing during biomineralization in the postnatal mouse molar tooth

The primary structures, molecular genetics and biosynthesis of the amelogenin protein of the developing tooth are established, but knowledge of their subsequent post-secretory processing and its relation to enamel biomineralization is fragmentary. Preparations of tooth matrix proteins were isolated from molars (M1) of mice from birth to 15 days and analysed by SDS-PAGE and immunochemical methods. Amelogenin proteins, isolated and partially purified by HPLC, were characterized by amino acid analysis and SDS-PAGE. At birth a 26 kDa amelogenin was present that during subsequent developmental stages generated a series of 20-25 kDa amelogenins differing in apparent size by approximately 1 kDa. Amino acid analyses showed that all these amelogenins have amino-terminal TRAP sequences; analyses for both glycosylation and phosphorylation were negative. It is suggested that these post-secretory amelogenins are generated by a sequence of specific carboxy-terminal cleavages, and that the observed post-secretory processing of amelogenin is functionally linked to the structure of the enamel matrix and the control of crystallite development.

Studies on fracture toughness of dental ceramics

One important mechanical property of dental ceramics is fracture toughness, KIC, which represents the serviceability in the oral cavity, such as the resistance to marginal fracture. KIC values of several dental ceramics, natural tooth enamel and industrial ceramics were examined by use of the indentation microfracture (IM) method. This technique was based on the series of radial cracks emanating from the corners of the Vickers indentation. It was observed that appropriate load levels should be selected on each specimen to induce radial/median cracks. For feldspatic dental porcelains, larger loads of 10, 20 and 30 kg were needed to determine their KIC values in the range 1.5-2.1 MN.m-3/2. For natural tooth enamel and a new apatite-based castable glass-ceramic, a smaller force of 1 kg was sufficient to decide the respective KIC values of about 0.9 and 1.8 MN.m-3/2. The KIC values of most dental ceramics examined were slightly higher than that of soda lime glass, but less than one-third that of zirconia. It was confirmed that the IM method is simple and cost-effective for evaluation of KIC of dental ceramics.

Clinical implications of the response of enamel and dentin to masticatory loads

The success of restorative procedures is dependent on comprehension of the responses of enamel and dentin, including responses to masticatory forces. The regional variation resulting from masticatory forces is critical because clinically it relates to the thickness of enamel and dentin occlusogingivally. Three-dimensional finite element models of an intact mandibular molar were developed to analyze stresses in enamel and dentin occlusogingivally, buccolingually, and mesiodistally. There were dramatic regional variations in the magnitude and character of different stresses caused by masticatory forces, and despite being organically "bonded," enamel and dentin responded independently. This unique behavior with regional variations of these tissues could have serious clinical implications during restorative procedures.

[Measurement of thermal expansion coefficient of human teeth]

The coefficient of thermal expansion of specimens from 60 freshly extracted sound human teeth was measured by a dilatometer over the range 10-80 degrees C. The coefficient increased rapidly above 50 degrees C, but there was no significant variation with the tooth age between 10 and 70 years. At 10-50 degrees C, the coefficient of thermal expansion was almost constant. The human teeth are usually at this temperature range in oral cavity. The authors suggest that the average values obtained can be used as quotable values.

Effects of fluoride on matrix proteins and their properties in rat secretory enamel

This publication concerns the selective adsorption of rat enamel proteins onto hydroxyapatite, their solubility in aqueous solutions, and the effect that systemic fluoride has on these properties. The enamel proteins used as adsorbates were extracted in 0.5 mol/L acetic acid from the secretory enamel of the upper and lower incisors of SD rats (females, 200-220 g body weight). Equilibration of the proteins with hydroxyapatite was performed in two solutions: (i) 50 mmol/L acetate buffer at pH 6.0 and 0 degrees C, and (ii) 50 mmol/L Tris buffer containing 4 mol/L guanidine at pH 7.4 and room temperature. Enamel was dissected from animals, which were given either de-ionized water (control group) or water containing 25, 50, 75, or 100 ppm fluoride as NaF for four weeks. From these enamel samples, the proteins were extracted in sequence with 160 mmol/L NaCl and 3 mmol/L phosphate (pH 7.3), 50 mmol/L carbonate buffer (pH 10.8), and finally, with 0.5 mol/L acetic acid for dissolution of the enamel mineral. The F, Ca, and P contents of the various enamel samples were determined.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes in the mode of calcium and phosphate transport during rat incisal enamel formation

The distribution of 45Ca, 32PO4, 22Na, and calcein in the freeze-dried sections of rat lower incisor was examined. Also, the ratio of 45Ca to 32PO4 transported into the enamel at various developmental stages was studied after the simultaneous injection of 45Ca and 32PO4. The distribution of calcein fluorescence indicated the presence of an extracellular route from capillary to enamel in the areas of both the secretory and smooth-ended ameloblasts. Autoradiograms showed that the 45Ca incorporation into the enamel in the smooth-ended ameloblast region was higher than that into the secretory enamel, and a remarkably high incorporation was observed in the enamel of the apical two-thirds of the ruffle-ended ameloblast region. Although the 32P incorporation into the enamel of the smooth- and ruffle-ended ameloblast region was higher than in the secretory enamel, the differences between these two regions were not so evident as that observed in the case of 45Ca. The high labeling of 45Ca and 22Na was observed in the apical two-thirds of the ruffle-ended ameloblasts. The 45Ca/32PO4 ratio in the secretory enamel was significantly lower than that in the blood, but in the enamel of the smooth-ended ameloblast region the ratio was not significantly lower. Contrarily, the ratio in the enamel of the ruffle-ended ameloblast region was much higher than that in blood. These results indicate that the mode of transport of these ions into enamel is altered in relation to the morphological changes of the ameloblasts.

[Homeostasis in the dental enamel-saliva system]

The formulae of micelles of salivary calcium phosphate are offered to explain the molecular mechanisms of functioning and breakdown of homeostasis in the saliva-enamel system, and the pathogenesis of dental caries and petrifaction formation. This ensured the possibility of controlling the oral mechanisms using goal-directed therapeutic and preventive means.

Caries protective aspects of saliva and enamel

Saliva may be considered as being similar to enamel but in a liquid phase. Saliva, the mechanics of remineralization and demineralization, the acquired pellicle and the enamel surface all act to maintain the status quo, resisting mineral loss from the enamel.

Measurement of thermal expansion coefficient of human teeth

The coefficient of thermal expansion of specimens from 60 freshly extracted human teeth were measured over the range 10-80 degrees C using a dilatometer. The coefficient increased rapidly above 50 degrees C, but there was no significant variation with tooth age between 10 and 70 years. The average values obtained were 16.96 (SD 3.83) x 10(-6)/degrees C for enamel, 10.59 (SD 2.38) x 10(-6)/degrees C for dentine, 11.90 (SD 4.42) x 10(-6)/degrees C for crowns and 9.44 (SD 0.61) x 10(-6)/degrees C for roots.