Advances in digital impression technology and manufacturing processes have led to a dramatic paradigm shift in dentistry and to the widespread use of computer-aided design/computer-aided manufacturing (CAD/CAM) in the fabrication of indirect dental restorations. Research and development in materials suitable for CAD/CAM applications are currently the most active field in dental materials. Two classes of materials are used in the production of CAD/CAM restorations: glass-ceramics/ceramics and resin composites. While glass-ceramics/ceramics have overall superior mechanical and esthetic properties, resin-composite materials may offer significant advantages related to their machinability and intra-oral reparability. This review summarizes recent developments in resin-composite materials for CAD/CAM applications, focusing on both commercial and experimental materials.

Resin-composite Blocks for Dental CAD/CAM Applications

OBJECTIVES
Resin-based dental materials are not inert in the oral environment, and may release components, initially due to incomplete polymerization, and later due to degradation. Since there are concerns regarding potential toxicity, more precise knowledge of the actual quantity of released eluates is necessary. However, due to a great variety in analytical methodology employed in different studies and in the presentation of the results, it is still unclear to which quantities of components a patient may be exposed. The objective of this meta-analytical study was to review the literature on the short- and long-term release of components from resin-based dental materials, and to determine how much (order of magnitude) of those components may leach out in the oral cavity.


METHODS
Out of an initial set of 71 studies, 22 were included. In spite of the large statistical incertitude due to the great variety in methodology and lack of complete information (detection limits were seldom mentioned), a meta-analytical mean for the evaluated eluates was calculated. To relate the amount of potentially released material components with the size of restorations, the mean size of standard composite restorations was estimated using a 3D graphical program.


RESULTS
While the release of monomers was analyzed in many studies, that of additives, such as initiators, inhibitors and stabilizers, was seldom investigated. Significantly more components were found to be released in organic than in water-based media. Resin-based dental materials might account for the total burden of orally ingested bisphenol A, but they may release even higher amounts of monomers, such as HEMA, TEGDMA, BisGMA and UDMA. Compared to these monomers, similar or even higher amounts of additives may elute, even though composites generally only contain very small amounts of additives. A positive correlation was found between the total quantity of released eluates and the volume of extraction solution.


SIGNIFICANCE
There is a clear need for more accurate and standardized analytical research to determine the long-term release from resin-based materials. Several guidelines for standardization are proposed.

How much do resin-based dental materials release? A meta-analytical approach (vol 27 pg 723, 2011)

3D Printed Versus Conventionally Cured Provisional Crown and Bridge Dental Materials

Characterization of a polymer-infiltrated ceramic-network material.

This study compared commercial composite resin blocks with one ceramic block for use in computer-aided design/computer aided manufacturing (CAD/CAM). Four composite resins, one composite ceramic, and one feldspar-ceramic block were investigated. Flexural strength (FS), flexural modulus (FM), and Vickers hardness (VH) were determined under three conditions: dry storage; immersion in water at 37°C for 7 days; and immersion in water at 37°C for 7 days followed by 10,000 thermocycles. After dry storage, FS ranged from 127 to 242 MPa, FM from 9.6 to 51.5 GPa, and VH from 64 to 455. Two-way ANOVA was performed for FS, FM and VH followed by Tukey's multiple comparison (α<0.05). Results demonstrated that the materials degraded after water immersion and thermocycling, but their properties were within the acceptable range for fabrication of single restorations according to the ISO standard for ceramics (ISO 6872:2008).

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Mechanical properties of composite resin blocks for CAD/CAM

Indirect composites have been undergoing an impressive evolution over the last few years. Specifically, recent developments in computer-aided design-computer-aided manufacturing (CAD-CAM) blocks have been associated with new polymerization modes, innovative microstructures, and different compositions. All these recent breakthroughs have introduced important gaps among the properties of the different materials. This critical state-of-the-art review analyzes the strengths and weaknesses of the different varieties of CAD-CAM composite materials, especially as compared with direct and artisanal indirect composites. Indeed, new polymerization modes used for CAD-CAM blocks-especially high temperature (HT) and, most of all, high temperature-high pressure (HT-HP)-are shown to significantly increase the degree of conversion in comparison with light-cured composites. Industrial processes also allow for the augmentation of the filler content and for the realization of more homogeneous structures with fewer flaws. In addition, due to their increased degree of conversion and their different monomer composition, some CAD-CAM blocks are more advantageous in terms of toxicity and monomer release. Finally, materials with a polymer-infiltrated ceramic network (PICN) microstructure exhibit higher flexural strength and a more favorable elasticity modulus than materials with a dispersed filler microstructure. Consequently, some high-performance composite CAD-CAM blocks-particularly experimental PICNs-can now rival glass-ceramics, such as lithium-disilicate glass-ceramics, for use as bonded partial restorations and crowns on natural teeth and implants. Being able to be manufactured in very low thicknesses, they offer the possibility of developing innovative minimally invasive treatment strategies, such as "no prep" treatment of worn dentition. Current issues are related to the study of bonding and wear properties of the different varieties of CAD-CAM composites. There is also a crucial need to conduct clinical studies. Last, manufacturers should provide more complete information regarding their product polymerization process, microstructure, and composition, which significantly influence CAD-CAM material properties.

From Artisanal to CAD-CAM Blocks: State of the Art of Indirect Composites.

Resin composite blocks via high-pressure high-temperature polymerization

The aim of this study was to produce composite blocks (CB) for CAD/CAM applications by high-temperature-pressure (HT/HP) polymerization of resin-infiltrated glass-ceramic networks. The effect of network sintering and the absence/presence of initiator was investigated. Mechanical properties were determined and compared with those of Paradigm MZ100 (3M ESPE) blocks and HT/HP polymerized experimental "classic" CB, in which the filler had been incorporated by conventional mixing. The networks were made from glass-ceramic powder (VITA Zahnfabrik) formed by slip casting and were either sintered or not. They were silanized, infiltrated by urethane dimethacrylate, with or without initiator, and polymerized under HT/HP (300 MPa, 180°C) to obtain resin-infiltrated glass-ceramic network (RIGCN) CB. HT/HP polymerized CB were also made from an experimental "classic" composite. Flexural strength (σf), fracture toughness (KIC), and Vickers hardness were determined and analyzed by one- or two-way analysis of variance (ANOVA), Scheffe multiple-means comparisons (α = 0.05), and Weibull statistics (for σf). Fractured surfaces were characterized with scanning electron microscopy. The mechanical properties of RIGCN CB were significantly higher. Sintering induced significant increases in σf and hardness, while the initiator significantly decreased hardness. The results suggested that RIGCN and HT/HP polymerization could be used to obtain CB with superior mechanical properties, suitable for CAD/CAM applications.

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Michaël Sadoun

Papers

Resin-composite Blocks for Dental CAD/CAM Applications

From Artisanal to CAD-CAM Blocks: State of the Art of Indirect Composites.

Resin composite blocks via high-pressure high-temperature polymerization

High-temperature-pressure Polymerized Resin-infiltrated Ceramic Networks

Residual stress measurement in veneering ceramic by hole-drilling