To evaluate marginal adaptation, fracture load and failure types of CAD/CAM polymeric inlays.
Standardized prepared human molars (48) were divided into four groups ( n = 12): (A) PCG (positive control group); adhesively luted glass–ceramic inlays, (B) TRX; CAD/CAM polymeric inlays luted using a self-adhesive resin cement, (C) TAC; CAD/CAM polymeric inlays luted using a conventional resin cement, and (D) NCG (negative control group); direct-filled resin-based composite restorations. All specimens were subjected to a chewing simulator. Before and after chewing fatigue, marginal adaptation was assessed at two interfaces: (1) between dental hard tissues and luting cement and (2) between luting cement and restoration. Thereafter, the specimens were loaded and the fracture loads, as well as the failure types, were determined. The data were analysed using three- and one-way ANOVA with post hoc Scheffé test, two sample Student’s t -test ( p < 0.05).
Before and after chewing fatigue, marginal adaptation for interface 1 showed significantly better results for TRX and PCG than for TAC ( p = 0.001–0.02) and NCG ( p = 0.001–0.047). For interface 2, marginal adaptation for TAC was significantly inferior to TRX ( p < 0.001) and PCG ( p < 0.001). Chewing fatigue had a negative impact on the marginal adaptation of TAC and NCG. No significant differences in fracture load were found between all tested groups.
Self-adhesive luted polymeric CAD/CAM inlays showed similar marginal adaptation and fracture load values compared to adhesively luted glass–ceramic inlays.
The choice of whether polymer or ceramic material is the best for tooth colored CAD/CAM inlay restorations has already been the topic of deliberations during development and introduction of the first clinically applicable CAD/CAM system in 1985 . Using material blocks which are prefabricated under controlled conditions by the manufacturer, CAD/CAM offers the chance to use materials at their highest obtainable quality, assuming that the material is not weakened by the automatic machining process. The concept of bonded esthetic ceramic CAD/CAM inlays has been successfully established through clinical long-term studies . Today, as an alternative to esthetic silicate ceramics, composite and PMMA blocks have been introduced for CAD/CAM dental reconstructions . The CAD/CAM resin blocks are cured under high pressure and temperature and therefore yield higher mechanical properties compared to conventionally polymerized resins . One study of CAD/CAM three-unit polymeric fixed partial dentures (FPDs) observed significantly higher fracture load results compared to glass–ceramics, as well as conventionally polymerized resins, after different aging regimens . In several studies, CAD/CAM fabricated composite overlays and crowns for premolars and molars showed high fatigue resistance and were recommended for long-term restorations .
Fasbinder et al. investigated the clinical performance of CAD/CAM fabricated resin-based composite inlays and observed that their color match with natural teeth was significantly better than that of glass–ceramic CAD/CAM inlays after 3 years. One in vitro study observed no differences in color stability between CAD/CAM polymeric and glass–ceramic FPDs . Lehmann et al. evaluated clinical failures, the presence of occlusal contacts, plaque accumulation and the patients’ ratings of the esthetics and functional efficiency of 114 composite single crowns for 5 years. They concluded that their complication rate and the increased plaque accumulation restrict the indication of composite crowns to temporary, or at most semi-permanent, use.
Interestingly, with regard to thin dental restorations, CAD/CAM-milled high-density polymers show higher fracture resistance than restorations out of glass–ceramics . Concerning enamel wear characteristics, PMMA-based materials cause less enamel wear in antagonists than glass–ceramics . Moreover PMMA-based materials showed lower discoloration rate and higher fracture load compared to composite restorations . Given all this material-dependent factors and the possibility to produce dental restorations with lower costs and time exposure , the question arises whether these resins can be seated for a longer period into the patient’s mouth. In this moment, PMMA-based polymers are being increasingly used in complex clinical cases for an extended pretreatment period .
To the best of our knowledge, at present, there is no information available on the fracture load of molars restored with adhesively luted PMMA-based CAD/CAM inlays, whereas fracture load and marginal adaptation of CAD/CAM ceramic inlays has been investigated under various conditions in recent studies . The first steps in evaluating whether unfilled PMMA-based CAD/CAM block material may be suitable to be used for permanent occlusion bearing molar inlays should assess marginal integrity after mechanical loading and thermal stressing by a chewing fatigue test . This is because the elastic and thermal expansion properties of polymers differ significantly from those of the natural dental hard tissues, as well as from those of esthetic silicate ceramics, which represent the standard CAD/CAM inlay material . The physical and chemical properties of PMMA-based FPDs also raise the question of whether adequate adhesion can be established between the polymer and current cementation systems and how resistant it may be to thermal and mechanical stress. Conventional resin cement systems and self-etch resin cements represent the current standard of adhesively seating inlays and are expected to durably restore the stability of the restoration-tooth system . Testing marginal adaptation of class II-restorations seated in natural extracted molars with a chewing fatigue test is considered to provide useful information on the integrity of the type of restoration .
The aim of this study was to evaluate marginal adaptation and fracture load of PMMA-based CAD/CAM inlays. The null-hypotheses tested were whether marginal adaptation and fracture load of PMMA-based CAD/CAM inlays would be similar to that of glass–ceramic inlays and direct resin-based composite fillings.
Materials and methods
Preparation of specimens
For this in vitro study, 48 extracted caries-free molars were collected, cleaned from periodontal tissue residues and stored in 0.5% chloramine T at room temperature for 1 week . Subsequently, the teeth were embedded with their roots parallel to the tooth axis in autopolymerising resin (Palapress, Heraeus Kulzer, Hanau, Germany) using a special holding device. Non-beveled mesio-occlusal-distal (MOD) class II-cavities were prepared with similar dimensions under constant water-cooling ( Fig. 1 ). Initially, an 80 μm diamond bur (No. 8422, Intensiv SA, Grancia, Switzerland) was used for preparation, and a 25 μm diamond bur (No. 3526, Intensiv SA) of the same size and form was used for finishing at 12× magnification (Stemi 1000, Carl Zeiss AG, Oberkochen, Germany). The proximal boxes ended mesially 1 mm above and distally 1 mm below the cemento-enamel junction. After preparation, the teeth were randomly assigned to four groups ( n = 12 per group):
PCG, positive control group: teeth restored with glass–ceramic CAD/CAM inlays (Empress CAD, Ivoclar Vivadent, Schaan, Liechtenstein) luted using a conventional resin cement (Variolink II, Ivoclar Vivadent).
TRX: teeth restored with unfilled PMMA-based CAD/CAM inlays (artBloc Temp, Merz Dental, Lütjenburg, Germany) luted using a self-adhesive resin cement (RelyX Unicem, 3M ESPE, Seefeld, Germany).
TAC: teeth restored with unfilled PMMA-based CAD/CAM inlays (artBloc Temp) luted using a conventional resin cement (artCem GI, Merz Dental).
NCG: negative control group, teeth directly filled with resin-based composite (Filtek Supreme XT, 3M ESPE, Seefeld, Germany).