الفهرس 
AcknowledgmentOnly 14 pages are availabe for public view
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Abstract
Statement of problem: Effect of low temperature degradation (LTD) on yttria stabilized tetragonal zirconia polycrystal (Y-TZP) for monolithic zirconia and core zirconia. However, studies on effect of low temperature degradation on mechanical characteristics of zirconia were insufficient. Aim of study: The aim of the study was to assess the effect of low temperature degradation (LTD) on flexural strength of monolithic zirconia and zirconia core using hydrothermal aging in autoclave condition for 8 hours (relevant to around 30 years of clinical service). Materials and methods: 32 zirconia discs were cut from 2 zirconia blocks(monolithic zirconia and zirconia core) with isomet -1000, then were sintered according to manufacturer instructions to dimensions (12mm diameter and 1.2 mm thickness). Each group of zirconia was subdivided according to hydrothermal aging into aged subgroups and non-aged subgroups. Artificial aging was done in autoclave condition at (134ºᴄ, 0.2 MPa) for 8 hours. Non aged subgroups representing control had no aging. X-ray diffraction (XRD) and scanning electron microscope (SEM) were used to analyze the microstructure of both Y-TZP types before and after aging. All samples were subjected to Biaxial flexural strength test with ball on ring in universal testing machine. Data from biaxial test were analyzed using Graph Pad Instat (Graph Pad, Inc.) software.
Results: Zirconia core group recorded statistically significant higher biaxial flexure strength mean value (1192.76±105.47 MPa) than ZrO2 monolithic group (1033.13±136.68 MPa) before aging. After aging zirconia core group also recorded statistically significant higher biaxial flexure strength mean value (1305.52±159.67 MPa) than zirconia monolithic group (1084.55±91.43 MPa).XRD analysis showed increase of monoclinic content in both types of zirconia surfaces after aging.
Conclusion: Hydrothermal aging induced tetragonal to monoclinic transformation and modified the microstructure of all zirconia types. Moreover, hydrothermal aging didn’t significantly affect the flexural strength of monolithic and core Y-TZP after 8 hours of aging within the limitations of this study.
Clinical Implications: Based on the results of this in vitro study, clinicians may consider leaving zirconia core without veneering exposed in the oral environment in areas adjacent to the soft tissue such as implant abutments and substructures and on the intaglio surface of partial fixed dental prosthesis connectors (which are subjected to tensile stresses) without being concerned about any significant deterioration of the material and subsequent catastrophic failure.
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.