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Introduction

Composites are currently the material of choice for tooth restorations . They have many advantages including rapid polymerization, good adhesion to tooth tissue, easy handling, easy to repair, good mechanical properties, the possibility of tissue-saving cavity preparation and the fact that they are relatively cheap compared to indirect restorations. In practice, patients often prefer composites because of their pleasing esthetics. Despite their many advantages, there are also some disadvantages such as biological effects and biocompatibility.

In the clinical situation, composites will never completely polymerize, which results in the release of residual monomers in the oral cavity. There is a possibility that these eluted ingredients are systemically absorbed by the host via the mucosa or the pulp tissue, or through ingestion . Moreover, released monomers have the potential to elicit allergic reactions, most often due to intraoral T-cell mediated delayed hypersensitivity (type IV) and more rarely due to IgE-mediated immediate hypersensitivity (type I) .

In spite of their introduction in the mid-1980s, chairside indirect restorations produced by computer-aided design and computer-aided manufacturing (CAD/CAM) have only recently become a viable alternative to restore teeth with extensive tooth loss. Compared to large composite restorations, they have the advantage of more easy anatomical reconstruction and better mechanical properties . Compared to conventional crowns, the success of chairside restorations lies in the shorter workflow, and thus reduced patient-time and costs. Obtaining optimal adhesive bonding to both tooth and the CAD/CAM material is, however, the prerequisite for success. Typically, a composite cement is used for luting, but also restorative composites can be used as they exhibit better mechanical properties .

Both ceramic and indirect composite are used for CAD/CAM restorations . Ceramics are harder than composite materials and subsequently more wear-resistant. However, this hardness might also lead to increased wear of the opposing dentition and restorations. On the other hand, like glass, ceramics are brittle and more prone to fracture compared to composites . Recently, hybrid ceramics have been brought onto the market as fundamentally new combinations of ceramic and polymer, in an effort to combine the esthetics and wear resistance of ceramics with the advantages of composite resins. The ceramic ingredients provide for the mechanical stability, the polymer part for the elasticity.

Although conventional direct composite restorations may exhibit good esthetic and mechanical properties, indirect resin-composite blocks may represent a good alternative to direct composite restorations, especially in large posterior restorations. Not only do these industrially pre-polymerized indirect composite restorations have a higher degree of conversion (DC), they also redress the negative effect of polymerization shrinkage, consequently improving the overall mechanical and physical properties . Moreover, indirect composites exhibit better interproximal contacts, higher wear resistance, and simplified creation of natural and anatomical shapes in large defects .

The aim of this study was to investigate whether indirect adhesive restorations produced by chairside CAD–CAM technology can provide a more biocompatible way to restore teeth. For this purpose, the release of monomers from identically shaped crown restorations either made from direct (conventional) composite or CAD-CAM blocks (indirect composite blocks, feldspar ceramic blocks and polymer-infiltrated-feldspathic (hybrid) ceramic blocks) was quantified using ultra-performance liquid chromatography-tandem mass spectrometry (UPLC-MS/MS). The null hypothesis of this study was that the indirect restorations result in lower quantities of released monomers.

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Materials and methods