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The efficacy of ISO 4049 for flexural strength testing of light activated resin-based composites |
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The overlapping curing procedure associated with the ISO 4049 standard for flexural strength testing of dental RBCs has been shown previously to demonstrate a significant decrease in the reliability of three-point flexural strength data compared with a one-hit irradiation performed in an oven light curing unit (LCU). This phenomenon may be attributed to differences in polymerization efficiency associated with the quantity of useful light energy emitted from the handheld compared with the oven LCU. Consequently, the efficacy of ISO 4049 for flexural strength testing of light activated RBCs should be investigated further to improve standardisation between test centers and operators.
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The in-vitro clinical failure of all-ceramic crowns and the connector area of fixed partial dentures |
| Summary |
Manufacturers have adopted novel engineering processing routes to produce dental all-ceramic crown core materials with strengths in excess of those achieved using metallic substructures for metal-ceramic crowns. The average fracture strengths have been variously reported as 80-180MPa for aluminous porcelain, 130-300MPa for Dicor and 350-680MPa for In-Ceram. Whilst the application of a veneering porcelain over a high strength core ceramic results in the creation of an aesthetically more acceptable all-ceramic crown, the annual clinical failure rate for bilayered laminated crown structures is remarkably consistent at 3%. These results emphasise the poor correlation between the average fracture strength of the core material determined by conventional material science techniques and the resultant clinical performance of all-ceramic crowns. Testing bilayered laminate disc-shaped specimens with the core layer in tension corresponded with the failure of all-ceramic crowns in clinical practice as demonstrated by in-vitro studies on planar samples, finite element analysis (FEA) studies and quantitative fractography techniques. Alternatively, when the reinforcing core layer of bilayered laminate disc-shaped specimens was tested in compression the conditions corresponded with the in-vitro failure of FPDs which was consistent with quantitative fractography techniques and FEA studies of FPDs where the highest tensile stresses were located at the connector area. In the current study employing various core materials including IPC, pressable, and CAD/CAM manufactured ceramics the hypothesis examined were that the core thickness and core to dentine thickness ratio have a critical influence on design and therefore premature clinical failures in all-ceramic crown and bridge structures. |
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The strengthening mechanism of resin-based luting cements to dental ceramics |
| Summary |
Previous studies have advocated the use of resin-based cements rather than acid-base cements for luting all-ceramic crowns in clinical practise. The corrosive effect of the increased acidity of acid-base cements during the early stages of setting could be responsible for the poor survival rates of crowns cemented with acid-base cements and in particular zinc phosphate. The mechanism behind the strengthening of porcelain by resin cements is unknown and several theories have been postulated. Marquis in 1992 suggested the resin cement modified the surface flaw population by crack healing, while Nathanson (1993), Rosenstiel et al (1993) and Pagniano et al. (2005) proposed the polymerisation shrinkage of resin cements stress the molecules together and prevents cracks from opening freely thereby strengthening the porcelain. More recently Fleming et al. proposed that the substantial pre-cementation treatment the resin cement requires, namely acid-etching and resin priming, produces an intimate bond between the porcelain surface and cement lute. The authors proposed the intimate bond acted to move the fracture origin from the porcelain/cement interface to the cement surface so that the enhanced energy requirement to fracture the resin cemented specimens was the energy required for a crack to travel through the resin cement to the porcelain/cement interface. However, the apparent strengthening mechanism behind the proposed theories has not been proven beyond doubt. The aim of the current study was to substantiate the apparent strengthening mechanism of resin luting cements, by manipulating the acid concentration and etching time with different controlled defect populations.
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The development of improved glass-ionomer restoratives |
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Glass Ionomer Cements (GICs) are restorative dental materials consisting of two separate components - a glass powder and a polymeric liquid element. GICs are routinely employed in dental applications as cavity linings and bases, luting cements and as anterior filling materials especially in paediatric dentistry due to their minimally invasive nature. More recently, there is an increased interest in the use of glass-ionomer restoratives as posterior filling materials due to aesthetic demands of patients and the potential fear of mercury toxicity from dental amalgams. Unfortunately, conventional GIC cement formulations are not advocated for large messio-occlusal-distal (MOD) cavities due to their reduced mechanical properties (compressive fracture strength, fracture tougness and wear resistance) when compared with dental amalgam. The purpose of the study will be to validate the hypothesis that the mechanical properties of conventional GI restoratives can be improved to produce increased performance compared with conventional GI restoratives when placed in the posterior region of the mouth. The experimental GI restoratives will also be trailed for clinical performance, namely compressive fracture strength, fracture toughness and wear resistance. Consequently by targeting and eliminating the key failure mechanisms of posterior GI restorative and filling materials it is proposed that an experimental, commercially viable reinforced experimental GI restorative may improve conventional GI restorative performance in the mouth. |
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| Fleming GJP, Maguire FR, Bhamra G, Burke FM, Marquis PM., The influence of dental cements on controlled porcelain surface preparations. , Journal of Dental Research, 85, (3), 2006, p272 - 276 |
| Palin WM, Fleming GJP, Marquis PM. , The reliability of standardised flexure strength testing procedures for a light-activated resin-based composite. , Dental Materials, 21, (10), 2005, p911 - 919 |
| Palin WM, Fleming GJP, Burke FTJ, Marquis PM, Randall RC. , The influence of short and medium-term water immersion on the hydrolytic stability of novel low-shrink dental composites following medium-term immersion., Dental Materials, 21, (9), 2005, p852 - 863 |
| Fleming GJP, Nolan L, Harris JJ., The in-vitro clinical failure of all-ceramic crowns and the connector area of fixed partial dentures: the influence of interfacial surface roughness. , Journal of Dentistry, 33, (5), 2005, p405 - 412 |
Barralet JE, Fleming GJP, Campion C, Harris JJ, Wright AJ., Formation of translucent hydroxyapatite ceramics by sintering in carbon dioxide atmospheres. , Journal of Materials Science , 38, (19), 2003, p3979 - 3993
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| Fleming GJP, Shelton RM, Landini G, Marquis PM., The influence of mixing ratio on the toughening mechanisms of a hand-mixed zinc phosphate dental cement., Dental Materials , 17, (1), 2001, p14 - 20 |
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