David Patrick

The use of debonding microspheres in electrothermal debonding

The aim of this study was to investigate if debonding microspheres (DM) could enhance electrothermal orthodontic debonding, and specifically to evaluate if the addition of DM, in varying concentrations to the orthodontic adhesive process, will affect bond strength. Bovine teeth (n=8) were mounted in acrylic using a silicone mould. Four test groups of three preparations of primer (Rely-a-Bond) with DM and a control were examined. Five incisor brackets were bonded to each tooth using the assigned primer and cold cure composite (Rely-a-Bond). Bracket tensile bond strength was measured in vitro in an Instron machine and recorded as debonding force (N). Differences between the groups were statistically analysed using analysis of variance, and repeatability was assessed. The mean debonding force of the control group was statistically significantly greater than all the other groups to which DM had been added (P<0.001). Comparison of the mean debonding forces of 1, 2.5, and 5 per cent concentrations of DM revealed no statistically significant difference between the groups. The addition of DM to orthodontic adhesive produced a highly statistically significant reduction in debonding force. There was no statistical difference in debonding force between varying concentrations of DM.

Evaluation of Laminated Structures for Sports Mouthguards

Most of the past studies have concentrated on the properties of mouthguard materials rather than their ability to protect the underlying substructure. Previous work has indicated that the incorporation of a shook absorbing layer into the sports mouthguard reduces the likelihood of injury to the head, neck and oral cavity of the wearer, The purpose of this study is to develop an optimum laminated structure that protects an easily deformable structure during an impact. Dropweight impact tests were conducted on a series of moulded samples which were circularly clamped and force-time and displacement-time plots obtained, Single thickness specimens of ethylene vinyl acetate (EVA), 1-5mm thick were compared with laminated structures of EVA, incorporating 1mm thick layers of polymethylmethacrylate (PMMA) and a silicone or synthetic rubber up to a thickness of 5mm. It was observed that the multi-layered structures exhibited less deformation thereby transmitting less of the harmful effects through the laminate. It was concluded that laminated systems for mouthguards using different materials appear to offer better protection to the wearer.

Fracture resistance of zirconia-composite veneered crowns in comparison with zirconia-porcelain crowns

The objectives were to evaluate the fracture resistance and stress concentration in zirconia/composite veneered crowns in comparison to zirconia/porcelain crowns using occlusal fracture resistance and by stress analysis using finite element analysis method. Zirconia substructures were divided into two groups based on the veneering material. A static load was applied occlusally using a ball indenter and the load to fracture was recorded in Newtons (N). The same crown design was used to create 3D crown models and evaluated using FEA. The zirconia/composite crowns subjected to static occlusal load showed comparable results to the zirconia/porcelain crowns. Zirconia/composite crowns showed higher stress on the zirconia substructure at 63.6 and 50.9 MPa on the zirconia substructure veneered with porcelain. In conclusion, zirconia/composite crowns withstood high occlusal loads similar to zirconia/porcelain crowns with no significant difference. However, the zirconia/composite crowns showed higher stress values than the zirconia/porcelain crowns at the zirconia substructure.

Comparing the optical and mechanical properties of PEKK polymer when CAD/CAM milled and pressed using a ceramic pressing furnace

The aim was to optimise the hot pressing parameters for processing PEKK polymer using a standard ceramic pressing furnace and compare the optical and mechanical properties of pressed samples to PEKK samples produced via CAD/CAM milling. The samples were compared using a spectrophotometer to determine the CIE L*a*b* colour values and ΔE colour difference. A universal testing machine was used for biaxial flexural strength testing and Vickers Hardness machine for measuring the hardness. It was concluded that hot pressed and milled PEKK samples showed no significant difference in the colour values, biaxial flexural strength or hardness.

Analysis of Stress Distribution Caused by Orthodontic Correctional Devices

This investigation has been carried out to support the empirical studies conducted by dentists in orthodontic correction using brackets. Both numerical and experimental methods have been implemented in order to give a clear indication of the intrinsic stress distribution throughout a tooth and its surrounding periodontum when a load is applied to the tooth in its initial translation stage. For optimal accuracy, the tooth and surrounding periodontum in both cases are assigned different values proportional to those present in actuality. From the numerical model developed using Ansys Workbench 12, the possible stress patterns are determined. These are validated by the use of 3-dimensional photoelasticity. Tooth models manufactured from polymethyl-methacrylate are stress-frozen with a known loading configuration and sectioned in order to determine the internal stress pattern. These results are then compared with the finite element model. The choice of material was due to its ability to be manufactured without residual stresses, and its birefringent properties. This investigation will be effective in the improvement of orthodontic correctional procedures, especially with the emergence of more aesthetic bracket designs.