Effect of Different Preparation Designs on the Fracture Resistance and Failure Modes of Polymer Infiltrated Ceramic Endocrown Restorations

Abstract

This study was designed to evaluate the effect of different preparation designs including: extension of the finish line, occlusal thickness of the restoration and filling the pulp chamber on the fracture resistance and modes of failure of endodontically treated teeth restored with polymer infiltrated ceramic endocrown restorations. Sixty four recently extracted human mandibular first molars with completely formed apices, without caries or visible fracture lines were selected based on the teeth having similar bucco-lingual (BL) and mesio-distal (MD) dimensions. The teeth were cleaned and stored in distilled water until required for experimentation. Root canal treatment was performed. The selected teeth were divided into 2 main groups according to the extension of the preparation finish line, and then each group of teeth was subdivided into 2 subgroups according to the occlusal thickness of the restoration received. Each subgroup was further subdivided into 2 divisions according to whether the pulp chamber is filled with Fiber reinforced core material or not. A surveyor was used to ensure upright position of teeth in moulds filled with non shrink epoxy resin. Then all teeth were kept in separate closed vials containing distilled water according to grouping. For all teeth, The CNC milling machine was adjusted to reduce the pulp chamber with 8 degree divergence of the walls. The CNC milling machine was adjusted to reduce forty teeth with circular 90º butt margin without any axial reduction (group B) and the Summary & Conclusion 105 other forty with axial reduction and shoulder finish line 1 mm in thickness which was positioned 2 mm below the occlusal surface (group S). After that, the teeth in each of the two groups were prepared into two subgroups. The CNC milling machine was adjusted to reduce sixteen teeth from each group with 2 mm occlusal reduction which received restorations with 2 mm occlusal thickness, and the other sixteen with 3.5 mm occlusal reduction which received restorations with 3.5 mm occlusal thickness. Then, the teeth in each subgroup were further subdivided in two divisions. The pulp chambers of eight teeth in each subgroup were filled with fiber reinforced core material and in the other eight the pulp chambers were not filled. Endocrown restorations were fabricated using The CEREC AC system and milled from VITA ENAMIC blocks. The restorations were polished and the occlusal thickness of each restoration was checked and measured with a caliper after milling and after polishing for verification. Surface treatment of the restorations was performed with hydrofluoric acid and silane before cementation with resin cement. Thermal cycling was performed and all samples were loaded in a universal testing machine until fracture occurred. The fractured specimens and fragments were collected and the mode of fracture was examined visually and under stereomicroscope at magnification 10x to determine different fracture modes according to Burke's classification.