An in-vitro investigation into the etiology of porcelain chipping in zirconia based all-ceramic restorations

PLEASE NOTE: This work is protected by copyright. Downloading is restricted to the BU community: please click Download and log in with a valid BU account to access. If you are the author of this work and would like to make it publicly available, please contact open-help@bu.edu.Thesis (MSD) --Boston University, Henry M. Goldman School of Dental Medicine, 2010 (Department of Restorative Sciences and Biomaterials).Includes bibliographic references: leaves 79-86.Statement of the problem: Zirconia is frequently used as a core material for all-ceramic crowns and fixed partial dentures, but a reported relatively high rate of chipping of porcelain veneered to the zirconia is a continuing problem. Objective: To investigate the etiology of porcelain chipping in zirconia based all-ceramic restorations by studying the effect of coping material on veneer porcelain sintering. Materials and methods: An experimental tooth die was used to make zirconia and base metal copings identical in shape. Zirconia copings were milled from VITA YZ zirconia blocks (VITA Zahnfabrik, Bad Sackingen, Germany) to a 0.5mm thickness and LAVA zirconia (3M ESPE, St. Paul, Minnesota, USA) to 0.5mm and 0.8mm thicknesses. Metal copings were milled from acrylic blocks, invested, and cast using ARGELOY NP SUPREME base metal alloy (ARGEN, San Diego, Califomia, USA) to 0.5mm thickness. Veneering porcelains, VM9 (VITA Zahnfabrik, Bad Sackingen, Germany) and LAVA Ceram (3M ESPE, St. Paul, Minnesota, USA) were applied to ViTA YZ zirconia and LAVA zirconia copings and VM13 (VITA Zahnfabrik, Bad Sackingen, Germany) was applied to metal copings by hand condensation in layers following manufacturer instructions. All crowns were embedded in epoxy resin and sectioned vertically to reveal a cross section of the coping and veneering porcelain. Microhardness of the veneering porcelain was tested in regions extending from the coping to the external layer using the Knoop microhardness test. Standardized squares of veneering porcelain were sintered and embedded in epoxy resin and the microhardness of these specimens was tested. A standardized horizontal groove was cut in the veneering porcelain of the PFM and all-ceramic specimens and resistance to shear force was tested. Standardized bars of veneering porcelain were fabricated using hand condensation and the flexural strengths of the different veneering porcelains were tested using the 3-point flexural strength test. SEM images were taken of specimens used for microhardness testing and specimens used for resistance to shear force test. Results: Porcelain microhardness data was tabulated relative to distance in microns from the coping. For the Vita VM9 veneered to Vita YZ zirconia and VM13 veneered to base metal groups, there was a slight increase in microhardness with increasing distance from the coping. For the LAVA Ceram veneered to Vita YZ zirconia group, old and new LAVA Ceram veneered to LAVA 0.5mm zirconia coping groups the microhardness data showed a slight decrease in microhardness with increasing distance from the coping. The microhardness data for the LAVA Ceram veneered to LAVA 0.8mm zirconia coping group showed almost no difference between the microhardness of the inner and outer layers of porcelain. Multiple regression analysis of the microhardness data for all groups showed a statistically significant difference between microhardness of the different porcelains, with the VM13 group showing statistically significantly higher microhardness values (P[less than]0.01 than the VM9 group, and the VM9 group showing higher microhardness values than the LAVA Ceram groups (p[less than]0.01). Microhardness data of the standardized porcelain squares fabricated without coping material showed no difference in microhardness data between the different veneering porcelains. Multiple regression analysis of microhardness vaiues for Vita VM13 porcelain fused to base metal copings and VM13 porcelain sintered without coping material showed that VM13 veneered to base metal copings had statistically significantly higher microhardness values than VM13 sintered without coping material (p[less than]0.01). Resistance to shearing force of the veneering porcelain for the PFM group was higher than the VM9 group, which was higher than the LAVA groups. Pearson product-moment, correlation coefficient was used to measure the correlation between the microhardness values for the LAVA Ceram, VM9 and VM13 veneering porcelains and the resistance to shear force. The coefficient value was calculated as 0.504 with p[less than]0.05 indicating a correlation between the microhardness and resistance to shearing force of the veneering porcelains. Conclusions: Veneering porcelains tested showed no difference in microhardness values when not veneered to a coping material. Porcelains veneered to zirconia and base metal coping materials had significantiy different microhardness values according to coping material (p[less than]0.01). The tested all-ceramic veneering porcelains were differentIy affected by the zirconia coping materials, with the Vita VM9 veneering porcelain veneered to Vita YZ zirconia copings showing higher values for microhardness than LAVA Ceram veneering porcelain veneered to Vita and LAVA zirconia copings (p[less than 0.01). Microhardness of the tested veneering porcelain on the zirconia and base metal copings was correlated with resistance to shearing force of the veneering porcelain on the zirconia and base metal copings (Pearson product-moment correlation coefficient of 0.504 with p[less than]0.05)