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Preliminary Studies on the Optimum Shape of Dental Bridges

Several pre-existing anterior and posterior dental bridge models using Finite elements and the new ceramic material In-Ceram have been developed. The mechanical behaviour of these models has been compared with optimised profiles obtained from a newly developed evolutionary algorithm known as Evolutionary Structural Optimisation (ESO). The results show that the mechanical behaviour of the bridges was mainly restricted by the properties of the porcelain veneer and the design of the bridges themselves. For the case of the anterior bridge, it was found that there existed a specific thickness of veneer that minimised the maximum principal stress. This was related to peak stresses that occurred at the bridge surface. Peak stresses also occurred in the material interface between the In-Ceram and the veneer. These extreme stresses were attributed to the notch size and shape. For the case of the posterior bridge, it was concluded that the shape of the bottom of the Pontic tooth is crucial in reducing the magnitude of the maximum principal tensile stress. The ESO process produced bridge designs which have uniformly stressed bridge surfaces, and which also have significantly lower maximum principal tensile stresses compared to the pre-existing designs (up to 44%).

Stiffness and inertia multicriteria evolutionary structural optimisation

In continuation of the recent development of Evolutionary Structural Optimisation (ESO) applied to the simultaneous objective to maximise the natural frequency and to minimise the mean compliance, presents the Multicriteria ESO optimisation of two new criteria. This has been done with the use of four different multicriteria methods. Three examples have been used to verify the usefulness and capability of these methods applied to ESO in the context of the aforementioned criteria. Concluded that the ESO weighting method is proficient in presenting the designer with a range of options (of Pareto attribute) taking into account multiple criteria, and the global criterion method has the tendency to produce shapes and topologies that resemble that of the weighted 50 per cent: 50 per cent method. Likewise, the logical OR operator method produced designs that corresponded directly to those of 100 per cent stiffness weighted criteria. No clear resemblance could be concluded with the case of the logical AND operator method.

Investigation of the Role of Dental Ceramic Core Material using Finite Element Analysis

Two different Finite Element Analysis studies were conducted to examine the influence of thickness of a relatively stiff (In-Ceram Alumina) cera mic core on an all-ceramic crown. The first study was that of a biaxial, bi-laminar disc with 5 diff erent ratio thicknesses (where the overall thickness of the disc was maintained). The effect of different s ubstrate materials (ring support, dentine and steel) was also examined. The second study examined t he r sulting stresses inside an axisymmetric model of a first premolar all-ceramic crown. Three different coping thicknesses were modelled (0.3, 0.6 and 0.9 mm). The study found that an increase in the In-Ceram coping thickness ( at the expense of veneer thickness) decreased the resulting stresses, and the increase i n th stiffness of the underlying substrate also, caused a decrease in the resulting stresses. Introduction Because of their excellent aesthetic properties, ceramics are becoming more common in the use of dental restorations. Despite this growth in popularity, little is known about the role of the ceramic core thickness in the behaviour of the restoration. It is known from different studies, that various factors and influences [1-6] affect the construction of the restora tion. These influences create preferred thicknesses of the individual porcelain veneer, coping and underl yi g dentine. These preferences may be in conflict with one another. This study seeks to assist the clinician with the restoration of all-ceramic crowns by clarifying the role of the ceramic core thickness in the behaviour of the restorati on. It shall specifically do so by conducting various analyses using Finite Element Analysis (FEA). He re, the mechanical behaviour is examined. In particular, the maximum principal stress ( σ11) and the radial stress ( σRR) inside different biaxial discs (monolithic and bi-laminar) and axisymmet ric Finite Element (FE) models of a first premolar, all-ceramic crown are examined. Materials and Methods Two different series of FEA were carried out. The first ser ie was based around the axisymmetric analysis of a bi-layer ceramic disc with different ratios of p rcelain veneer to In-Ceram material (5 different combinations with a total combined thickness of 2 mm being m aintained) and different support conditions (ring, dentine (Fig. 1) and steel support) [7]. The di sc radius was 16 mm, and an edge stress that simulated a hemispherical load of 1000 N was appli ed at the centre of the disc, over an area of radius 0.3 mm. The material properties of the bi-lami nar d sc and the support material are displayed in Table 1. No adhesive resin was included in these models. Key Engineering Materials Online: 2003-05-15 ISSN: 1662-9795, Vols. 240-242, pp 871-874 doi:10.4028/www.scientific.net/KEM.240-242.871