The purpose of this study was to investigate the stresses in different proximal margins and to measure, quantitatively, the effect of different modifications in the design of preparations on the stresses using two-dimensional photoelasticity.
Photoelastic stress analysis is bases on the phenomenon, exhibited by most transparent solids, of becoming birefringent, or doubly refracting, when strained. Two birefringent materials were used in this study, PSM-1 and PSM-5 in standard sheet (10″×10″×¼″thickness), PSM-1(polyester) was used for constructing the substructure, and PSM-5(epoxy resin) was used in making the restorations to be investigated. Two birefringent materials were used in the construction of composite photoelastic model. Seven variable models were constructed. The peripheral dimensions of all model were constant and the models represent an occlusomesial section of a lower posterior molar. Model 1 represents the knife edge margin(shoulderless), Model 2 represents the chamfer, Model 3 represents a rounded shoulder(no sharp angle between the axial wall and gingival floor), Model 4 represents a flat shoulder(axial wall is a 90°angle to the gingival wall), Model 5 represents +15°angulation, Model 6 has a-15°angulation, and Model 7 is the same as Model 4 except that it has a 45°bevel. Improved artificial stone was used to represent dental cement in luting the composite photoelastic model. Static loading procedures (100 pounds) were used at preplanned sites. The results were as follows; 1. The stresses in the proximal portion of all tested models were compressive in nature when the proximal shoulders were loaded vertically on the same proximal marginal ridge. 2. The round and chamfered preparations were the optimum designs in proximo-occlusal restorations. They showed the lowest stress concentration factor, I.e. 2.16 and 2.23, respectively. The knife edged shoulder had the highest value K=5.39. Round type shoulder geometry experiments reduced the stress concentration factor(S.C.F.) 3. The gingival portion of proximal shoulder geometry was a critical location for stress concentration.
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