Assessment of fracture resistance of endodontically treated molars restored with endocrowns fabricated by two different materials : in-vitro study /

Abstract

This in vitro study was conducted to assess the fracture resistance of endodontically treated molars restored with endocrowns fabricated by pressed lithium disilicate and nanohybrid resin composite cemented by conventional etch and rinse resin cement. Forty mandibular second molars were collected according to the inclusion and exclusion criteria. Teeth were divided into four equal groups; Group (1): sound molars as a negative control (sound), Group (2): unrestored endodontically treated molars with occluso-mesial cavities as a positive control (unrestored), Group (3): endodontically treated molars with occluso-mesial cavities and lithium disilicate endocrowns (LDS) and Group (4): endodontically treated molars with occluso-mesial cavities and nanohybrid resin composite endocrowns (NRC). Teeth of unrestored, LDS and NRC were endodontically treated. All groups were embedded in self-cure acrylic resin blocks. Teeth of LDS, and NRC groups received a standardized occluso-mesial cavities. Each tooth of LDS and NRC groups were prepared for endocrown restorations and scanned using digital intraoral scanner. Casts were designed on a software and printed using 3D printer. For LDS group, restorations were designed using CAD/CAM software with same anatomy, 2 mm occlusal heights and 80 μm cement space. Wax patterns were milled, then sprued and invested to produce lithium disilicate endocrowns using heat pressed technique. For NRC group, four coats of 20 μm die spacer were coated on the 3D printed casts. Increments (2 mm) of nanohybrid composite were applied until building the entire shape and thickness using the silicon index. Nanohybrid resin composite endocrowns were removed after initial curing and subjected to additional dry heat curing. Lithium disilicate endocrowns were surface treated using 9.5% hydrofluoric acid followed by application of silane coupling agent. Nanohybrid resin composite endocrowns were sandblasted using 50 μm aluminum oxide particles followed by silanization. While teeth surfaces were etched using 37% phosphpric acid and total-etch adhesive. All endocrowns were cemented using conventional dual cure resin cement. Teeth of all groups were mounted to universal testing machine with a 6 mm diameter ball applying load perpendicularly to the long axis of the tooth to test the fracture resistance. Maximum force required for fracture were recorded in Newton. Fracture modes were also identified under stereomicroscope and divided into repairable fracture which extends above the CEJ and irreparable fractures which extends below the CEJ. Results of this study showed a statistically significant difference between groups (p< 0.001). Sound teeth presented the highest mean fracture resistance followed by NRC then LDS with statistically insignificant difference between the three groups. Unrestored teeth showed the least mean fracture resistance with a statistically significant difference to the other three groups. Mean percent recovery in fracture resistance of both test groups and fracture modes of all groups were also evaluated. Results showed that NRC has higher mean percent recovery than that of LDS, which emphasize the ability of NRC to restore 91.08% of the fracture strength lost after endodontic treatment and occluso-mesial cavity preparation. While LDS restored only 75.66%. On the other hand, Sound teeth showed the highest percentage of samples with repairable fractures (80%) followed by NRC (70%). While LDS showed unfavorable fractures of (80%).