Discussion of load carrying behavior of a bilayerbi-material plate using CADCAM materials
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Abstract
Objectives: Non-metallic crown restorations featuring a veneer and core layer are coming to the forefront of dentistry due to their combination of esthetics and strength. Use of computer assisted design/computer assisted machining (CAD/CAM) materials can aid the dental professional in the fabrication of these bilayer crowns, reducing the time required to deliver the restoration to the patient. The objective is to investigate the load carrying capacity of a resin-cemented bilayer/bi-material disc composed of In-Ceram Alumina and Paradigm MZl 00 machinable composite. The influence of bilayer specimen orientation (ln-Ceram cemented on top of MZlO0, or vice-versa) and 3 different thickness combinations on the failure load will be determined, as well as compared to their respective unilayer controls. Calculation of the maximum flexural and shear stresses developed upon bilayer disc failure, thorough the transformed section method, will shed light of the failure loads seen. The failure mode determination, a unique method to assess whether a specimen failed due to shear stress or flexural stress, will be thoroughly described specific to the test method utilized. Plotting a curve of the predicted load carrying capacity of bilayer disc specimens composed of In-Ceram and MZ1OO with a full range of thickness combinations will provide an understanding of bilayer behavior.
Methods: 10 disc specimen groups (13-14 disc specimens per group, 2.2 mm total thickness, 14 mm diameter) composed of 6 different In-Ceram/MZlOO combinations (variations on either relative layer thickness and bilayer specimen orientation), 2 unilayer controls and 2 bilayer/uni-material controls were compared statistically to find out if (1) Introducing a resin cement in between two 1.1mm thick disc layers of the same material (bilayer/bi material) will give significantly different values compared to 2.2 mm thick unilayer/uni material disc specimens in terms of load to failure, flexural stress and shear stress. (2) The orientation of a bilayer/bi-material (large modulus material cemented on top of small modulus material versus small modulus material cemented on top of large modulus material) has an influence on the failure load seen and (3) The thickness ratio in the bilayer/bi-material groups has an influence on the failure load seen. Specimens were loaded until failure using a biaxial 3-point supported plate bending test set up and the values recorded. Flexural and shear stress values for unilayer and bilayer/uni-material controls were calculated using the experimental failure load data, the Kirstein and Woolley biaxial flexure stress equation and a free body diagram derived shear stress equation that allowed for statistical analysis (student's t-test at the p[less than]0.05 level). Load to failure values comparing bilayers/bi-materials specimens to each other and to controls were analyzed with one-way ANOVA and the Tukey and Bonferroni pair wise comparison tests (p[less than]0.05). Flexural and shear stress calculations for bilayer disc specimens is more complicated than unilayer samples and involves the transformed section method. Flexural and shear stress diagrams highlighting maximum flexural and shear stresses were plotted for all specimen groups.
Results: Load to failure, flexural and shear stress values were not significantly different when comparing unilayer disc specimens and their cemented bilayer/uni-material counterparts at the p[less than]0.05 level. Load to failure results indicated that in at least one group pairing (Groups 5, 0.8 mm MZlOO veneered over 1.4 mm In-Ceram and Group 6, 1.4mm In-Ceram veneered over 0.8mm MZlOO), the orientation of the bilayer/bi-material specimen composed of In-Ceram and MZlOO had a significant impact (p[less than]0.05 level) on the failure load seen. Load to failure results indicated that the relative thickness heights of In-Ceram and MZlOO had a significant impact (p[less than]0.05 level) of the failure loads seen in bilayer disc specimen with identical overall thickness. Failure loads seen were rationalized by the flexural and shear stresses determined for each specimen group and the influence of the elastic modulus ratio and thickness ratio discussed using the transformed section method. Flexural and shear stress maximums for all specimen groups were compared and the development of the failure mode determination method was realized. An understanding of the different failure modes led to the fabrication of a load carrying capacity curve based upon a full range of thickness combinations of InCeram and MZlOO. Different sections of the load carrying capacity curve are derived from basic flexural and shear stress equations, the failure mode determined, and the calculation of the neutral axis and moment of inertia. The curve reveals a unique mode of failure, specific to bilayer specimens, termed the redundancy mode.
Conclusions: Bilayer systems are unique in that it is the differences in relative thickness and elastic modulus of the materials (ln-Ceram and MZlOO) that have the most impact on the development of flexural and shear stress, and thereby influence the failure loads seen in the 3 point supported plate bending test method. This "influence" on one material on another is now more thoroughly understood after failure mode determination and a predictive load carrying capacity curve for bilayered materials can be plotted. In one bilayer group specimen comparison (Groups 5 and 6), the veneer/core disc orientation had a statistical impact of the failure load. The reason for this difference is unknown.
Future researchers may now have a greater understanding of bilayer systems and proceed to develop material and thickness combinations that will maximize their cost effectiveness, while satisfying their strength criteria and esthetic needs.
Description
Thesis (MSD)--Boston University, Henry M. Goldman School of Dental Medicine, 2004.
Includes bibliographical references (leaves 74-77).
Includes bibliographical references (leaves 74-77).
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This work is being made available in OpenBU by permission of its author, and is available for research purposes only. All rights are reserved to the author.