about an effective in-house developed method for obtaining a proper photoelastic model for such implant related studies. Further, the fringe order information extracted is post-processed using digital photoelastic algorithms to understand the biomechanics of the high fringe order zones. An effective methodology for the evaluation of contact zone near the implant bone interface is established and the influence of the geometric attributes of the implant-abutment connection in stress generated is investigated. 6.2 Experimental Methodology 6.2.1 Identification of Proper Photoelastic Model for Implant Dentistry In Two Dimensional (2-D) photoelastic approaches, model analysis is easier as standard photoelastic sheets are available commercially. However a 2-D block does not capture the essential anatomical structure of the mandible/maxilla. This demands a Three Dimensional (3-D) photoelastic model, which has the essential anatomical feature of the real human mandible. Even though photoelastically sensitive models with high accuracy could be made using rapid prototyping methods, due to the layered manufacturing, the fringe definitions are not good and further the model has residual stress [8]. This demanded a proper method in which stress free photoelastic models can be casted, which has the essential features of the human mandible. Different methods are explored and an effective methodology is established for producing stress free photoelastic models for such studies [9]. A human mandibular bone is reproduced with wax pattern and implant system are placed in the wax model as in All-On-Four concept, at predetermined positions. This is then duplicated using liquid silicon such that the mould has uniform minimum thickness around the alveolar ridge. Photoelastic model used in this study is made by a mixture of araldite resin (HUNTSMAN Araldite CY230) and hardener in 10:1 ratio. Uniform heat dissipation is facilitated by providing an ice bath around the mould during pouring and curing stage. Once the stress free photoelastic model with 15 implant inclination is obtained, bar attachments are used to connect the implants along the alveolar bridge. The bars are investment cast using Ni–Cr alloy. In situations involving complex models, even though stress freezing and slicing could be used to view any segment of the 3-D model, if certain key areas of the model could be viewed directly under live load conditions and analyse them directly, it would be then preferable in many situations. The model obtained using the in-house casting procedure are well suited for such analysis [9]. Implants placed in the molar region can be evaluated by adopting an appropriate angled view approach as shown in Fig. 6.1a. Dark field isochromatics observed for a load of 273 N is shown in Fig. 6.1b. Fig. 6.1 (a) Schematic representation of photoelastic model of mandible placed in a circular polariscope for the evaluation of implants in the molar area. (b) Dark field isochromatic observed for a load of 273 N when viewed according to 6.1 (a) 40 M.P. Hariprasad and K. Ramesh
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