The Histone-Deacetylase-Inhibitor Suberoylanilide Hydroxamic Acid Promotes Dental Pulp Repair Mechanisms Through Modulation of Matrix Metalloproteinase-13 Activity.

Direct application of histone-deacetylase-inhibitors (HDACis) to dental pulp cells (DPCs) induces chromatin changes, promoting gene expression and cellular-reparative events. We have previously demonstrated that HDACis (valproic acid, trichostatin A) increase mineralization in dental papillae-derived cell-lines and primary DPCs by stimulation of dentinogenic gene expression. Here, we investigated novel genes regulated by the HDACi, suberoylanilide hydroxamic acid (SAHA), to identify new pathways contributing to DPC differentiation. SAHA significantly compromised DPC viability only at relatively high concentrations (5 µM); while low concentrations (1 µM) SAHA did not increase apoptosis. HDACi-exposure for 24 h induced mineralization-per-cell dose-dependently after 2 weeks; however, constant 14d SAHA-exposure inhibited mineralization. Microarray analysis (24 h and 14 days) of SAHA exposed cultures highlighted that 764 transcripts showed a significant >2.0-fold change at 24 h, which reduced to 36 genes at 14 days. 59% of genes were down-regulated at 24 h and 36% at 14 days, respectively. Pathway analysis indicated SAHA increased expression of members of the matrix metalloproteinase (MMP) family. Furthermore, SAHA-supplementation increased MMP-13 protein expression (7 d, 14 days) and enzyme activity (48 h, 14 days). Selective MMP-13-inhibition (MMP-13i) dose-dependently accelerated mineralization in both SAHA-treated and non-treated cultures. MMP-13i-supplementation promoted expression of several mineralization-associated markers, however, HDACi-induced cell migration and wound healing were impaired. Data demonstrate that short-term low-dose SAHA-exposure promotes mineralization in DPCs by modulating gene pathways and tissue proteases. MMP-13i further increased mineralization-associated events, but decreased HDACi cell migration indicating a specific role for MMP-13 in pulpal repair processes. Pharmacological inhibition of HDAC and MMP may provide novel insights into pulpal repair processes with significant translational benefit. J. Cell. Physiol. 231: 798-816, 2016. © 2015 Wiley Periodicals, Inc.

Histone deacetylase inhibitors epigenetically promote reparative events in primary dental pulp cells.

Application of histone deacetylase inhibitors (HDACi) to cells epigenetically alters their chromatin structure and induces transcriptional and cellular reparative events. This study investigated the application of two HDACi, valproic acid (VPA) and trichostatin A (TSA) on the induction of repair-associated responses in primary dental pulp cell (DPC) cultures. Flow cytometry demonstrated that TSA (100 nM, 400 nM) significantly increased cell viability. Neither HDACi was cytotoxic, although cell growth analysis revealed significant anti-proliferative effects at higher concentrations for VPA (>0.5 mM) and TSA (>50 nM). While high-content-analysis demonstrated that HDACi did not significantly induce caspase-3 or p21 activity, p53-expression was increased by VPA (3 mM, 5 mM) at 48 h. HDACi-exposure induced mineralization per cell dose-dependently to a plateau level (VPA-0.125 mM and TSA-25 nM) with accompanying increases in mineralization/dentinogenic-associated gene expression at 5 days (DMP-1, BMP-2/-4, Nestin) and 10 days (DSPP, BMP-2/-4). Both HDACis, at a range of concentrations, significantly stimulated osteopontin and BMP-2 protein expression at 10 and 14 days further supporting the ability of HDACi to promote differentiation. HDACi exert different effects on primary compared with transformed DPCs and promote mineralization and differentiation events without cytotoxic effects. These novel data now highlight the potential in restorative dentistry for applying low concentrations of HDACi in vital pulp treatment.

The accuracy and precision of wear testing and wear facet analyses: The two sides of the story.

A critical examination of the literature for dental wear publications highlighted three distinct areas, clinical data, laboratory data and the simulation of the clinical situation data. The imprecision of the clinical data values from direct and indirect in vivo methods renders the clinical data compromised at best. Laboratory data showed a focus on finding a correlation between simplistic laboratory abrasive wear resistance studies and established materials science laboratory techniques, but with no actual correlation identified. Replication of the masticatory cycle in the mouth in the form of an oral wear simulator has focussed more on the wear testing devices rather than the wear quantification methods. As a result, the data acquisition variables in the x- and y-planes need to be examined to consider how they can influence the accuracy and precision of the laboratory wear measurements recorded in the dental literature. The current approach was undertaken using the teaching tool outlined in "The First Three Questions".

Extrusion-mixing compared with hand-mixing of polyether impression materials?

The hypotheses tested were two-fold (a) whether altering the base:catalyst ratio influences working time, elastic recovery and strain in compression properties of a hand-mixed polyether impression material and (b) whether an extrusion-mixed polyether impression material would have a significant advantage over a hand-mixed polyether impression material mixed to the optimum base:catalyst ratio. The polyether was hand-mixed at the optimum (manufacturers recommended) base:catalyst ratios (7:1) and further groups were made by increasing or decreasing the catalyst length by 25%. Additionally specimens were also made from an extrusion-mixed polyether impression material and compared with the optimum hand-mixed base:catalyst ratio. A penetrometer assembly was used to measure the working time (n=5). Five cylindrical specimens for each hand-mixed and extrusion mixed group investigated were employed for elastic recovery and strain in compression testing. Hand-mixing polyether impression materials with 25% more catalyst than that recommended significantly decreased the working time while hand-mixing with 25% less catalyst than that recommended significantly increased the strain in compression. The extrusion-mixed polyether impression material provided similar working time, elastic recovery and strain in compression to the hand-mixed polyether mixed at the optimum base:catalyst ratio.

Are encapsulated anterior glass-ionomer restoratives better than their hand-mixed equivalents?

OBJECTIVES: The performance of encapsulated anterior GI restoratives were compared with their hand-mixed equivalents for the range of powder to liquid mixing ratios routinely encountered clinically. The clinically induced variability of powder to liquid mixing variations of an anhydrous GI restorative formulation was also compared with conventional GI restorative formulations that contained a polyalkenoic acidic liquid. METHODS: Mean compressive fracture strengths, mean elastic moduli and mean total volumetric wear were determined for the encapsulated anterior GI restoratives mechanically mixed in a Capmix or Rotomix machine and the hand-mixed GI restoratives prepared with powder contents reduced from that recommended by the manufacturer (100%) in 10% increments to 50% for a constant weight of liquid. Multiple comparisons of the group means were made using a one-way analysis of variance (ANOVA) and Tukey's multiple range tests employed at P<0.05. RESULTS: For the encapsulated GI restoratives, the mean compressive fracture strength, mean elastic modulus and in-vitro wear resistance were significantly increased compared with their hand-mixed equivalents prepared with powder contents below that recommended by the manufacturers. The conventional GI restoratives resulted in a linear deterioration (R2>0.95) of the mean compressive fracture strength and mean elastic modulus with powder content compared with the bi-modal deterioration for the anhydrous GI restorative. CONCLUSIONS: Encapsulated anterior GI restoratives outperform their hand-mixed equivalents for the range of powder to liquid mixing ratios routinely encountered clinically such that they are advocated for use in clinical practice. Anhydrous GI restorative formulations are more susceptible to clinically induced variability on mixing compared with conventional GI restorative formulations that contained a polyalkenoic acidic liquid.

Strength-limiting damage in lithium silicate glass-ceramics associated with CAD-CAM.

OBJECTIVE: The fabrication of all-ceramic restorations using Computer Aided Design and Computer Aided Manufacturing (CAD-CAM) most commonly involves subtractive machining which results in strength-limiting, surface and sub-surface damage in the resultant prosthesis. The objective was to explore how clinically relevant machining-process variables, and material variables, affect damage accumulation in lithium silicate glass-ceramics. METHODS: Three commercial lithium silicate glass-ceramics (IPS e.max® CAD, Celtra® Duo and Vita Suprinity®) were selected. For each material, two groups of disk-shaped specimens were fabricated (n=15), using a CAD-CAM process, creating surfaces equivalent to those generated for a dental restoration, or alternatively, using a highly controlled laboratory process generating disk-shaped test specimens with a consistent polished surface. Bi-axial flexure strength (BFS) was determined in a ball-on-ring configuration and fractographic analyses performed. For each material BFS was correlated with machining sequence and with surface roughness. RESULTS: BFS was significantly influenced by material substrate (p<0.01) and by fabrication route (p<0.01). A significant factorial interaction (p<0.01) identified that the magnitude of changes in BFS when comparing the two specimen fabrication routes, was dependent on substrate type. The polished control specimens exhibited a significantly increased BFS when compared with the CAD-CAM counterparts for all materials. IPS e.max® CAD and Celtra® Duo showed a 44 and 46% reduction in mean BFS for the CAD-CAM specimens when compared with the polished counterparts, respectively. In contrast, Vita Suprinity® showed the least disparity in mean BFS (21%) but the greatest variance in BFS data. SIGNIFICANCE: All CAD-CAM specimens showed evidence of machining introduced damage in the form of median and radial cracks at sites either coincident with, or peripheral to the failure origin. Subtractive machining introduced significant strength limiting damage that is not eliminated by heat treatments applied for either microstructure development (IPS e.max® CAD and Vita Suprinity®) or annealing/crack blunting (Celtra® Duo).

Application of analytical stress solutions to bi-axially loaded dental ceramic-dental cement bilayers.

OBJECTIVE: Increased consideration of dental ceramics and dental cements as a singular structural unit for in-vitro mechanical testing has resulted in the reporting of a wide range of analytical methods to calculate the failure stresses. Therefore a comparison of observations between studies is complicated by the use of dissimilar stress solutions despite the employment of a similar testing methodology and specimen geometry. MATERIALS AND METHODS: Three analytical solutions to calculate failure stresses in bi-axially loaded dental ceramic-dental cement bilayers were appraised for a commonly utilized testing geometry (ball-on-ring). Clinically relevant datasets were generated from the bi-axial flexure testing of uncoated and dental cement coated aluminous core ceramic exposed to differing ceramic surface preparations. A Weibull statistical approach was utilized in order to provide insight into the impact of the analytical method on both the scale (sigma0) and distribution (m) of the failure stress data. RESULTS: Calculation of the bi-axial flexure stress utilizing Timoshenko's analysis resulted in an increase in sigma0 for the uncoated (6%) and dental cement coated (11-12%) aluminous core ceramic, when compared with the bilayered solutions reported by Rosenstiel and Hsueh. However, the shape of the failure distributions illustrated by the consistency of m and associated 95% confidence intervals was not influenced by the analytical stress solution employed. SIGNIFICANCE: The choice of the analytical method chosen to calculate failure stresses in bi-axially loaded dental ceramic-dental cement bilayers will impact on the magnitude of the reported strength. Comparison between the failure stresses of uncoated and cement coated dental ceramics is more accurately represented by bilayer solutions, which account for the mismatch between the elastic constants of dissimilar materials. However, within the context of dental cement coated dental ceramics of clinically relevant dimensions, the choice of solution is unlikely to impact on the interpretation of the observations previously reported in the dental literature.

Is encapsulation of posterior glass-ionomer restoratives the solution to clinically induced variability introduced on mixing?

OBJECTIVES: Three GI restorative systems were evaluated to determine if encapsulated GI restoratives performed more favorably than the hand-mixed equivalents prepared with powder contents progressively decreased from that recommended by manufacturers in 10% increments for a constant weight of liquid which are routinely employed in clinical practice. METHODS: Mean compressive fracture strengths, associated Weibull moduli and mean elastic moduli were determined for series of 30 nominally identical cylindrical specimens for three hand-mixed GI restoratives prepared with 100-50% of the recommended powder content for a constant weight of liquid and the equivalent encapsulated GI restoratives mechanically mixed in the Capmix or Rotomix machines. RESULTS: Decreasing the powder content of the three hand-mixed GI restoratives for a constant weight of liquid from that recommended by the manufacturers resulted in a progressive deterioration of the mean compressive fracture strengths and the mean elastic modulus. The elastic modulus was more sensitive than the compressive fracture strength to highlighting the impact of mixing ratio variations in the GI restoratives. There were no significant differences between mean compressive fracture strengths of mechanically mixed encapsulated GI restoratives, however, variations in mean elastic modulus were dependent upon the mixing time employed. CONCLUSIONS: The employment of an intrinsic material property was more sensitive than a non-intrinsic material property to highlight the impact of mixing ratio variations in hand-mixed restoratives and the effect that the method of mechanical mixing has on encapsulated GI restoratives. Encapsulated GI restoratives are a potential solution to the operator induced variability associated with hand-mixed GI restoratives.

The potential of a resin-composite to be cured to a 4mm depth.

OBJECTIVES: The potential of a recently marketed resin-based composite (RBC), namely X-tra fil (shade A3), which claims to be amenable to curing to a depth of 4mm was investigated. METHODS: Disc-shaped specimens (11mm diameter, 2mm thickness) of Filtek Z250, Admira and X-tra fil were tested in bi-axial flexure to determine the strength. Water sorption, water solubility and Vickers hardness measurements were determined following short-term (0.1, 0.5, 1, 4, 24 and 48h) and medium-term (1, 4, 12 and 26 weeks) water immersion on disc-shaped specimens (11mm diameter, 1mm thickness). The top (t) 0-1mm depth and bottom (b) 3-4mm depth of X-tra fil were tested for water sorption, water solubility and Vickers hardness measurements. In addition an analysis of pulpal cell cytotoxicity at 1, 2, 3, 7 and 14 days was also performed on the RBCs. RESULTS: No significant differences in the bi-axial flexure strength or top to bottom hardness ratios were evident between the materials examined. However, the water sorption and water solubility values obtained for Filtek Z250 (12.3+/-1.8 and 2.7+/-1.6microg/mm3, respectively) and Admira (16.0+/-1.5 and 4.3+/-0.2microg/mm3, respectively) were increased compared with X-tra fil (t) (5.4+/-0.7 and 0.8+/-0.2microg/mm3, respectively) and X-tra fil (b) (6.8+/-0.6 and 2.4+/-1.1microg/mm3, respectively) but within the ISO specification standard of < or =40 and < or =7.5microg/mm(3), respectively. No statistically significant differences were identified on cell viability between the RBCs used in the current study. SIGNIFICANCE: The manufacturer claims that the recently marketed material X-tra fil could be cured to a depth of 4mm appear to be vindicated and the performance in terms of flexure strength, water uptake and biocompatibility are comparable with conventional RBCs.

Effects of halogen light irradiation variables (tip diameter, irradiance, irradiation protocol) on flexural strength properties of resin-based composites.

OBJECTIVE: Investigation of halogen light irradiation variables (tip diameter, irradiance, irradiation protocol) on flexural strength and modulus of four methacrylate resin-based composites (RBCs). METHODS: Rectangular bar-shaped specimens (25 mm x 2 mm x 2 mm; n=20 per group) of four RBCs were irradiated at varying irradiances (640 and 790 mW/cm2; standard and boosted mode) with different tip diameters (8, 11, 13 and 25 mm) and for the 8mm Turbo light tip diameter at irradiances of 880 and 1040 mW/cm2 (standard and boosted mode). Following irradiation the specimens were stored in a light-proof container for 24h at 37+/-1 degrees C and tested in three-point flexure. One-way analyses of variance were made at P=0.05, guided as necessary by Tukey's correction in multiple partial analyses, in addition to a Weibull analysis. RESULTS: The mean three-point flexure strengths, Weibull moduli and flexure moduli of the four RBC materials, irradiated with varying irradiances, tip diameters and irradiation protocols highlighted no significant differences although the values were material specific. Similarly the 8mm conventional and Turbo tip diameter resulted in no significant differences for varying irradiances and irradiation protocols. CONCLUSION: Within the limitations of the experiment halogen tip diameter, irradiance and irradiation protocol have no influence on three-point flexural strength and modulus data. The efficacy of the overlapping irradiation regime was upheld for the conditions tested.

Adhesive luting of all-ceramic restorations--the impact of cementation variables and short-term water storage on the strength of a feldspathic dental ceramic.

PURPOSE: To investigate the impact of resin cement luting variables and short-term water storage on the strength of an adhesively luted all-ceramic restorative material. An understanding of the strengthening mechanisms will result in optimisation of operative techniques and materials selection criteria. MATERIALS AND METHODS: The "fit" surfaces of 480 disk-shaped feldspathic porcelain specimens were alumina air abraded to introduce a clinically relevant surface texture and consistent surface defect population. Thirty specimens randomly allocated to each of 16 groups were coated with silane, unfilled resin or filled resin cement, or a combination. Eight groups were stored either dry or wet for 24 h prior to bi-axial flexure testing (ball-on-ring). Statistical analysis of the flexure strength data involved a three-factor general linear model (p < 0.05) prior to a Weibull analysis. RESULTS: Resin coating the porcelain surface resulted in a significant increase in the characteristic stress (sigma(o)), and strengthening was dependent on coating type (p < 0.001). Silane priming resulted in additional strengthening when preceding filled resin cement coating. Water immersion for 24 h resulted in a strength degradation of both the uncoated control and coated specimens, whereby the magnitude of strength degradation was dependent on coating type (p < 0.001). CONCLUSION: Resin luting of dental ceramics results in significant strengthening likely to impact on clinical performance. The strengthening is dependent on the creation of a resin-ceramic hybrid layer sensitive to cementation variables and clinical placement technique. Short-term water immersion results in a significant degradation of strengthening sensitive to the characteristics of the resin-ceramic hybrid layer.

Methods used in dentine bonding tests: an analysis of 102 investigations on bond strength.

Bond strength testing of resin-based materials to dentine may provide evidence as to the potential effectiveness of a dentine-bonding agent in producing adhesion and minimising microleakage. The purpose of this analysis was to analyse the methods used in 102 recently-published investigations on bond strength tests of resin composite to dentine. One hundred and two papers, published during the years 1998 to 2002 on the subject of bond strength measurement of resin composite to dentine were identified from a literature search. These papers were assessed for their inclusion of the following variables: type of test, thermal cycling regime, tooth notation, storage medium, preparation of dentine surface, rate of load application, type of dentine, film thickness, dimensions of surface area contact and time of testing. Shear testing was the most common type of test, used in 46% (n=47) of the papers analysed, human dentine was used in 77% (n=79) of the investigations, and 24 h post-placement was the predominant time of specimen testing (67%, n=68). Surface area of contact, or specimen diameter, was stated in 65% (n=66) of the papers analysed. A number of variables were not stated in a majority of papers. There was little standardization of test methods and a number of potentially significant variables may not be reported or recorded in bond strength determinations which limit the overall standardization between different test centres.

The impact of modifying alumina air abrasion parameters on the fracture strength of a porcelain laminate restorative material.

OBJECTIVES: The modification of the "fit" surface of porcelain laminate veneer restorations in order to improve adhesion prior to cementation is often indiscriminate. As a consequence, the surface flaw distribution which is implicated in the probability of failure of the restoration is likely to be dramatically modified. The purpose of the current study was to examine the impact of different air abrasion surface treatments on the bi-axial flexure strength and surface roughness of a porcelain restorative material. METHODS: Sets of 30 Vitadur-Alpha dentin porcelain discs (15 mm diameter, 0.9 mm thickness) were alumina abraded with three different grades of alumina particle (25, 50 and 110 microm), utilizing two different air stream pressures (35 and 70 psi) and two distinct angles of incidence of particle delivery (45 degrees and 90 degrees ). Mean bi-axial flexure strengths, standard deviations, the associated Weibull moduli (m) and characteristic stress were determined using bi-axial flexure (ball on ring). RESULTS: A univariate general linear analysis of means revealed a significant difference between the mean bi-axial flexure strength values of the control group and those of groups subjected to alumina particle air abrasion. Further significance (P<0.05) was discovered with the impact of alumina particle size and the interaction between particle size and angle of incidence of particle delivery. The reliability of the fracture strength data generally improved when 50 microm alumina particles were used whereas discontinuities existed at lower strength values when 25 and 110 microm alumina particles were employed. CONCLUSIONS: Alumina particle air abrasion has a significant degradative effect on the bi-axial flexure strength of the porcelain disc-shaped specimens. Variation of alumina size, delivery pressure and angle of particle delivery all impacted on the degree of strength reduction and the shape of the survival probability distributions. It is suggested that alumina particle air abrasion acts to remove/modify the initial flaw distribution replacing it with flaws of differing geometry and stability. The premature failure of porcelain laminate restorations may be markedly influenced by alumina particle air abrasion depending upon the size and distribution of the crystalline phase present in different dentine porcelains materials used in construction of the restoration.

The impact of hydrofluoric acid surface treatments on the performance of a porcelain laminate restorative material.

OBJECTIVES: Hydrofluoric (HF) acid etching increases the bond strength between composite resin and porcelain surfaces and has been advocated as a pre-cementation technique for ceramic restorations. The internal surface flaw distribution which is implicated in the premature failure of ceramic restorations is modified by the etching process and little agreement exists amongst researchers as to the appropriate etching regime. The purpose of the current study was to examine the impact of HF acid concentration and etching time on the performance of a low fusing feldspathic porcelain. METHODS: Sets of 30 Vitadur-Alpha dentin porcelain discs (15 mm diameter, 0.9 mm thickness) were etched with HF acid of three different concentrations (5, 10 and 20%) and for three different etching periods (45, 90 and 180s). Mean flexure strengths, standard deviations and the associated Weibull moduli (m) and characteristic stress (sigma(0)) were determined using bi-axial flexure (ball on ring). Contact profilometry was utilised to characterise the roughness of the etched porcelain surfaces. RESULTS: A univariate general linear analysis of means revealed a significant reduction in the mean strength values of the as-fired control compared with groups subjected to HF acid etching. Further significance (P<0.05) was discovered with the impact of acid concentration. Altering etching time also resulted in changes in the reliability of the fracture strength data. Contact profilometry demonstrated an increase in surface roughness following HF acid etching and an increase in roughness associated with increasing HF concentration. CONCLUSIONS: Etching of feldspathic porcelain is a dynamic process and the impact is dependent on substrate constitution, surface topography, acid concentration and etching time. A significant reduction of the flexural strength of a low fusing feldspathic porcelain has been demonstrated to result from etching and clear evidence exists that the nature of surface flaw modification is a function of etching time and HF acid concentration. Favourable combinations of HF acid concentration and etching time have been identified which enhance the reliability of the porcelain utilised although variability in clinical technique will result in the reduced reliability of porcelain laminate restorations in function.

Corrosion of nickel-based dental casting alloys.

OBJECTIVES: To study the microstructure, corrosion behaviour and cell culture response of two nickel-based dental casting alloys before and after a heat treatment to simulate porcelain firing. METHODS: The microstructure was studied using scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX). Corrosion behaviour was evaluated by electrochemical measurements in artificial saliva at different values of pH in the presence of a crevice. 3T3 mouse fibroblasts were exposed indirectly to alloy specimens and the number of viable cells counted after 3 and 6 days compared to a control culture. RESULTS: Small changes in microstructure were observed after heat treatment but had a negligible effect on the corrosion properties in the conditions tested. The alloy with a lower bulk level of Cr (12.6 wt.%) showed lower corrosion resistance, indicated by an increased passive current density and this stability was greatly reduced at pH 2.5, where crevice corrosion was observed. Selective dissolution occurred at regions within the microstructure containing lower levels of Cr and Mo. Furthermore, the proliferation of 3T3 mouse fibroblasts was reduced (p<0.05) when exposed indirectly to this alloy. The alloy containing a higher level of Cr (25 wt.%) showed superior corrosion resistance, which was associated with a more uniform distribution of Cr in the alloy microstructure. SIGNIFICANCE: The presence of crevices combined with an inhomogeneous distribution of Cr in the microstructure can lead to accelerated corrosion of Ni-based alloys with lower Cr contents. This effect can be avoided by increasing the Cr content of the alloy.

The impact of montmorillonite clay addition on the in vitro wear resistance of a glass-ionomer restorative.

OBJECTIVES: The in vitro wear resistance of a glass-ionomer (GI) restorative ChemFil Superior (Dentsply DeTrey, Kanstanz, Germany) reinforced with either a pristine calcium montmorillonite (Ca-MMT) or an organically modified 12-amino-dodecanoicacid treated montmorillonite (ADA-MMT) clay was evaluated to investigate the potential of MMT reinforced GI restoratives as posterior filling materials. METHODS: The wear resistance of a GI restorative reinforced with 0.5-2.5 wt.% Ca-MMT and ADA-MMT was assessed for groups of four disc-shaped specimens using the OHSU oral wear simulator operating at 20 N abrasion and 90 N attrition forces for 50,000 cycles at a frequency of 1 Hz in the presence of a poppy seed and PMMA micro-sphere slurry. The wear facet of each specimen was quantified by measuring the mean wear depth and mean volumetric wear in the abrasion (40-60% of wear facet) and attrition (80-90%) regions in conjunction with the mean total wear depth and mean total volumetric wear. RESULTS: The addition of Ca-MMT clay resulted in a significant increase in the mean total volumetric wear and mean total wear depth of the GI restorative as the concentration of clay was increased utilising the one-way ANOVA and Tukey's test comparisons at a 95% significance level. When ADA-MMT clay was added to the GI no significant differences in mean total volumetric wear and mean total wear depth were evident (P<0.05). CONCLUSIONS: The addition of Ca-MMT clay to the GI restorative resulted in a significant decrease in the in vitro wear resistance. ADA-MMT clay additions to the powder element of the GI restorative did not significantly enhance the in vitro wear resistance.

Resin strengthening of dental ceramics- the impact of surface texture and silane.

OBJECTIVES: To examine the effect of porcelain surface texture achieved utilising three clinically relevant surface preparation methods prior to silane and unfilled resin application on the flexure strength of a low fusing feldspathic porcelain. MATERIALS AND METHODS: Five hundred and forty dentine disc-shaped specimens (15.0mm diameter and 0.9mm thickness) were condensed, fired and allocated to 18 groups. Six groups were stored as-fired, six were alumina abraded and six were acid-etched. Samples were coated with silane, unfilled resin or both prior to bi-axial flexure testing. Group means were compared utilising a three factor design general linear model and post hoc all paired Tukey tests at P<0.05. Weibull analysis was employed to examine the reliability of the strength data. Profilometry was used to characterise the surface texture of the specimen surfaces. RESULTS: Alumina abraded and acid-etched specimens had significantly lower flexure strengths than the as-fired specimens. A significant difference between the degree of strengthening observed following unfilled resin application on the three surface texture investigated was observed (P<0.001). Silane application prior to resin coating resulted in no further significant increases in the mean bi-axial flexure strengths of the three surface textures. The as-fired surfaces had a low frequency of irregular amplitude defects, alumina abraded surfaces had an increased frequency of regular amplitude defects whilst the acid-etched surface consisted of an increased frequency of irregular amplitude defects. CONCLUSION: The strengthening mechanism whilst dependent on surface texture was independent of defect severity. No significant strengthening occurred following silane priming suggesting that, for the unfilled resin utilised, the strengthening mechanism was not enhanced by improved resin-ceramic adhesion.

The impact of resin-coating on sub-critical crack extension in a porcelain laminate veneer material.

OBJECTIVES: Characterisation of the interaction between crack extension, crack stabilisation and stress/strain relaxation in the polymeric matrix, the interplay between stress corrosion cracking and the mechanical response of a resin-based luting adhesive within a surface defect population could extend PLV restoration longevity by optimising cementation protocols. The aim was to investigate the influence of stress corrosion cracking and the viscoelastic behaviour of a resin-based luting adhesive independently by controlling the environmental conditions operative during test specimen fabrication. METHODS: The effects of stress corrosion at ceramic crack tips and potential viscoelastic responses to loading of the resin-coated impregnating cracks were isolated. Resin-coated feldspathic ceramic test specimens were fabricated in ambient humidity or following moisture exclusion. Bi-axial flexure strengths of groups (n = 20) were determined at constant loading rates of 2.5, 10, 40, 160 or 640 N/min and data was compared with uncoated controls. Fractographic analyses were performed on all fractured test specimens. RESULTS: Resin-cement coating resulted in significant ceramic strengthening in all conditions tested (p < 0.01). A two-way ANOVA demonstrated that the exclusion of moisture during resin- coating significantly increased mean BFS (p<0.01) but post-hoc Tukey tests identified that moisture exclusion resulted in significant increases in BFS values only at intermediate loading rates with no significant differences observed at either the fastest or slowest loading rates (640 and 2.5 N/min, respectively). SIGNIFICANCE: Mechanical reinforcement of PLV materials by resin-cement systems is yet to be optimized. The viscoelastic behavior of the resin-cement itself can influence the magnitude of reinforcement observed and sub-critical crack growth.

Favorable residual stress induction by resin-cementation on dental porcelain.

OBJECTIVES: Despite developments in polycrystalline ceramics, glassy dental-ceramic materials provide the optimum cosmetic option in most clinical situations to mimic the natural dentition. The clinical success of glassy dental-ceramic materials is often attributed to resin-adhesive bonding techniques. In this study we explore whether shrinkage stresses generated on photo-polymerisation of the resin-cement are sufficient to induce ceramic surface defect stabilization, and we quantify the transient nature of the induced stresses. METHODS: Stress-induced changes in a feldspathic ceramic over a range of thicknesses (0.5-2.0mm: n=20 per thickness) were measured using a profilometric technique at baseline for each disc-shaped specimen (mean of the maximum deflection (δbaseline)) and again following polymerisation of a controlled resin-cement thickness on the contra-lateral surface. Measurements were repeated at 30, 60, 90 and 1440min following photo-polymerization (δ30, δ60, δ90 and δ1440, respectively) before bi-axial flexure strength (BFS) determination at 24h. RESULTS: A repeated measures ANOVA and post-hoc Bonferroni tests determined that δ1440 was significantly different from δbaseline (p=0.02), δ30 (p<0.01) and δ60 (p<0.01) but not δ90 (p=0.61). Data exploration revealed that there were differences in directionality of the independent variable (mean of the maximum deflection (δ)) with a proportion of specimens increasing in deflection and others reducing. The directionality of the effect strongly correlated with the BFS values. Where δ1440-δbaseline was low and/or negative, BFS values were also low. High BFS values were observed when δ1440-δbaseline was large and positive (indirectly inferring high shrinkage-stress-induced-deformation). SIGNIFICANCE: A link between shrinkage stresses associated with the photo-polymerization of dental resin-based cements and the reinforcement of dental porcelain has clearly been established.

Implications of resin-based composite (RBC) restoration on cuspal deflection and microleakage score in molar teeth: Placement protocol and restorative material.

OBJECTIVE: To assess the cuspal deflection of standardised large mesio-occluso-distal (MOD) cavities in third molar teeth restored using conventional resin-based composite (RBC) or their bulk fill restorative counterparts compared with the unbound condition using a twin channel deflection measuring gauge. Following thermocycling, the cervical microleakage of the restored teeth was assessed to determine marginal integrity. METHODS: Standardised MOD cavities were prepared in forty-eight sound third molar teeth and randomly allocated to six groups. Restorations were placed in conjunction with (and without) a universal bonding system and resin restorative materials were irradiated with a light-emitting-diode light-curing-unit. The dependent variable was the restoration protocol, eight oblique increments for conventional RBCs or two horizontal increments for the bulk fill resin restoratives. The cumulative buccal and palatal cuspal deflections from a twin channel deflection measuring gauge were summed, the restored teeth thermally fatigued, immersed in 0.2% basic fuchsin dye for 24h, sectioned and examined for cervical microleakage score. RESULTS: The one-way analysis of variance (ANOVA) identified third molar teeth restored using conventional RBC materials had significantly higher mean total cuspal deflection values compared with bulk fill resin restorative restoration (all p<0.0001). For the conventional RBCs, Admira Fusion (bonded) third molar teeth had significantly the lowest microleakage scores (all p<0.001) while the Admira Fusion x-tra (bonded) bulk fill resin restored teeth had significantly the lowest microleakage scores compared with Tetric EvoCeram Bulk Fill (bonded and non-bonded) teeth (all p<0.001). SIGNIFICANCE: Not all conventional RBCs or bulk fill resin restoratives behave in a similar manner when used to restore standardised MOD cavities in third molar teeth. It would appear that light irradiation of individual conventional RBCs or bulk fill resin restoratives may be problematic such that material selection is vital in the absence of clinical data.