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Introduction

In modern dentistry one of the most interesting advances was CAD/CAM development and application in the design, analysis and manufacture of fixed prostheses such as inlays, onlays, crowns, implant abutments etc. [ ]. Its use in removable prostheses re-awakened the interest of the Italian National Health Service due to their longer life-spans, reduced costs and potential for widespread application [ , ].

In creating removable prostheses CAD/CAM technology includes two patient appointments. In the first, all required clinical data for denture construction are registered and in the second the dentures are inserted. With CAD/CAM anatomical information is digitized, and the denture base and occlusions are designed virtually [ ]. Two processes are then available. Rapid prototyping, also known as the additive process, layers resinous materials on a support structure which is cured by visible light, UV light, heat or laser. Alternatively, the subtractive technique mills the prosthetic bases and teeth from a pre-polymerized commercially-manufactured polymethylmethacrylate (PMMA) disk. The teeth are then either manually fixed into the sockets using methacrylate-based bonding agents [ ] or base and teeth are made in one blank without bonding. Among the advantages of CAD/CAM technology compared with standard denture construction are simpler laboratory process, fewer dental appointments, better clinical outcomes in terms of, for example, patient satisfaction, soft tissue adaptation etc. and more precise tooth alignment [ , ]. Disadvantages include doubts about denture adaptation to soft tissue, costs and the steep learning curve for clinicians and for dental technicians [ ].

Whether produced by standard methods or CAD/CAM technology all removable prosthetics are usually obtained from acrylic resins (PMMA) due to their physical and chemical properties, low costs, ease of processing, aesthetic properties, repairability, low solubility and low water adsorption [ ]. Monomeric PMMA residues may, however, reduce denture mechanical properties and have major repercussions on prosthesis biocompatibility with ensuing allergic reactions, symptoms such as burning stomatitis, edema, oral mucosal ulcers and greater susceptibility to bacterial infections and superinfections [ ]. Disappointing results were achieved in attempts to reduce monomeric PMMA residues by raising pressure, extending processing times and using water bath post-polymerization [ ]. A more promising approach is use of CAD/CAM pre-polymerized PMMA billets. Polymerized under high temperature and press values, residual monomers are reduced in number as assessed by infrared spectroscopy, high performance liquid chromatography and gas chromatography [ ]. The present study compared the mechanical and physical properties of new CAD/CAM and standard resins and assessed their biocompatibility with oral cell populations.

Many studies have focused on the mechanical properties of dental materials [ ]. Static tests [ , ] determined maximum strength, but provided limited information on structure and related physical properties and were not suitable for measuring elastic properties or viscoelastic behavior under load [ ]. Furthermore, sample destruction precluded repeated testing [ ]. Since dental composites are exposed to dynamic, rather than static, loads dynamic tests appeared more useful [ ]. The Dynamic Mechanical Analysis (DMA), for example, determines the elastic and viscous responses of a sample in one experiment, and is particularly well-suited for visco-elastic materials [ ]. Furthermore, since dynamic tests simulate cyclic masticatory loading of dental composites better than static tests [ ], they might be better predictors of clinical performance.

Since substrate mechanical properties are however, known to influence cellular behavior, microscale is the appropriate scale length for cellular interactions [ ]. Brillouin micro-spectroscopy, a valuable tool in tissue engineering and the mechanical characterization of biomaterials [ ] is an innovative technique that tests cellular mechanics [ , ].

Working at high frequency (GHz), it measures the real and imaginary parts of the longitudinal elastic modulus on the cellular length scale [ ], assessing viscoelastic properties and determining tensile micro-stresses [ ]. Moreover, since it is a non-contact method it was used to evaluate elastic heterogeneity in cells and tissues and to study dentin micromechanics [ ]. The present study characterized the mechanical properties of an innovative dental resin polymer by means of Dynamic Mechanical Analysis and Brillouin microscopy, flanked by biological viability assays. After polymer exposure, scanning electronic microscopy (SEM) assessed cell morphology and substrate adhesion while the MTT assay, which was reported to be more sensitive than the sulforhodamine B (SRB) assay [ ] and the lactate dehydrogenase assay (LDH) [ ], investigated cell viability and cytotoxicity in accordance with ISO-10993-5 recommendations [ ].

Since the MTT assay is not informative on the mechanism of cell damage or death, apoptosis and expression of two key apoptosis-related genes (p53 and BCL2) were analyzed [ ]. Cell cycle analysis and expression of cell cycle regulatory proteins (p16 and p21) provided data on cell proliferation patterns [ ].

The first null hypothesis was that the two resins did not present any mechanical differences in terms of the elastic modulus, rigidity and morphology. The second null hypothesis was that no differences could be detected in in vitro biological behavior of oral mucosa cells grown on resin.

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Materials and methods