Einfluss von Präparationsform und Schichtdicke auf Bruchfestigkeit und Randverhalten von Keramikveneers

Objective and purpose Perfection in aesthetics and function of teeth is becoming increasingly important in society. Experience has shown that it is not easy to replace lost toothstructure with all its characteristics. Therefore adhesively luted labial ceramic laminate veneers are a clinically well-suited kind of restoration. For a long time veneers have only been bonded to enamel. It has only been possible since some years to also adhere veneers partially or completely to dentin. The adhesion of composite to enamel is much more effective than adhesion to dentin so far. Thus, it is quite a question for the clinician to determine whether it is advantageous to obtain as much enamel as possible at the cost of material thickness, aesthetic appearance and final tooth shape. It is questionable whether to trust in dentine adhesion instead and to better fulfill the listed criteria. Furthermore, dealing with pre-existing composite fillings is relevant and thus the question how far they can influence results. The purpose of the present study was to investigate the influence of five different preparation designs and two different ceramic thicknesses on margin quality and fracture resistance of ceramic laminate veneers after thermomechanical loading in vitro. Materials and methods 80 human central incisors were randomly assigned to 10 groups (n=8) with five different preparation designs: non-prep (NP), minimally invasive (MI) = enamel-bonded only, semi-invasive (SI) = 50% in dentin, invasive (I) = 100% in dentin, and semi-invasive with two additional Class III composite resin restorations (SI-C). IPS InLine veneers were fabricated in two thicknesses (L1 = 0.2-0.5 mm; L2 = 0.5-1.2 mm). After adhesive luting (OptiBond FL, Variolink Veneer) with light curing and polishing, specimens were stored in aqua dest at 37°C for 21 days, subjected to thermocycling (2,000 cycles between +5° and +55°C), and mechanically loaded at the incisal edge at an angle of 45° for 2,000,000 cycles at 50 N und further 1,000,000 cycles at 100 N. Impressions were taken initially, after thermocycling, and after every 250,000 mechanical cycles in order to evaluate cracks and margin quality under a SEM. Fractography was performed according to cracks, chippings, partial, and catastrophic fractures. � Results Margin quality after 3 Mio. cycles revealed medians for the criterium „continuous margin“ of 82-95% without significant differences among groups, neither at the ceramic/composite, nor at the tooth/composite interface (p>0.05). Visual inspection of veneers exhibited 22 cracks, 11 chippings, 4 partial and 4 catastrophic fractures in 38 of 80 veneers. The statistical ranking regarding fracture risk (p<0.05) was: IL1=SIL1=MIL1=IL2=CL1=CL2 ,MIL2=NPL1=NPL2=SIL2 ,IL2=CL1=CL2=MIL2=NPL1=NPL2 =SIL2. Conclusions Even after 3 Mio. cycles with up to 100 N, all groups showed high survival rates. However, the risk of fracture increases with thin veneers and preparations with medium to high dentin portions when compared to thicker veneers with preparations in enamel or partially in dentin (p<0.05). Preexisting resin composite restorations did not show any significant influence on margin quality and risk of facture (p>0.05).