Evaluation of amount of stress distribution on edentulous mandible generated by implant supported hybrid overdentures fabricated with different techniques

The aim of this study is the evaluation of passive fit (misfit) and stress distribution in the implant supported hybrid overdentures following the placement of 4 implants in the interforaminal region as 2 parallel implants in the mesial and 2 long and 30˚ distally inclined implants in the distal, depending on the fabrication technique; (1) traditional lost wax technique, (2) metal framework fabricated via lost wax technique split and soldered on the cast, (3) CAD/CAM milling, (4) CAD/CAM milling from presintered blanks, (5) Laser sintering Materials and Methods:An edentulous mandible model was fabricated from epoxy resin material to be used for the study. Implants were placed on the model in the interforaminal region as 2 parallel implants (4.6mm diameter, 10mm length) in the mesial and 2 long and 30˚ distally inclined implants(4.6mm diameter, 15mm length) in the distal at bone level. Co-Cr hybrid metal frameworks were fabricated on the model by following techniques: 1-traditional lost wax technique (P-1), 2-metal framework fabricated via lost wax technique split and soldered on the cast (P-2), 3-CAD/CAM milling (P-3), 4-CAD/CAM milling from presintered blanks (P-4), 5-Laser sintering (SLS) (P-5). Same acrylic superstructures were fabricated on the metal framework for standardization. The measurement of misfit was done using "Sheffield Test" with a software evaluating the images taken by a digital camera. The stress formed after the application of torque and vertical force (105 N) on different regions of the dentures were calculated with a strain gauge stress analysis technique. The statistical analysis of the results were compared with two-way ANOVA and Tukey's HSD tests. Results:The lowest misfit was measured in P-3 (99,11 µm), followed by P-4 (120,12 µm), P-2 (122,19 µm), P-5 (139,07 µm) and P-1 (151,63 µm). There is a statistically significant relationship between misfit and stress distribution (p<0,05). The lowest misfit group, P-3, had the lowest mean stress values after application of torque on the denture (81.10 MPa) and on the model (374.80 MPa). This was followed by (In terms of stress distribution on the denture) P-2 (133,25 MPa), P-4 (235,13 MPa), P-5 (235,13 MPa) and P-1 (542,75 MPa). In terms of stress distribution, the mean values were;P-2 (496 MPa), P-5 (547,9 MPa), P-4 (636,75 MPa) and P-1 (754,14 MPa). The mean stress distribution values on the dentures during loading in ascending order are; P-4(20.79 MPa), P-5(23,17 MPa), P-2(24,81 MPa), P-3(33,57 MPa) and P-1(40,48 Mpa). As the stress distribution around the implants were evalutated it was observed that there was statistically significant differences between the implants, loading locations and the relationship of these two variables (p<0,05). There was no statistically significant difference between dentures and variables related to dentures (p>0,05) Conclusions:The fabrication method of framework significanly influences the passive fit. Significant relationship was observed between the amount/distribution of misfit and stress. No correlation was seen between stress distribution and misfit during loading. Similarly, no correlation was seen betweenstress distribution and misfit on dentures and model after torque was applied to hybrid overdenture on the model and during loading. Although the amount of prosthodontic misfit which could be tolerated by the bone around the implant is not clear, it is accepted that keeping the misfit as low as possible is beneficial.