Implant-supported cantilever fixed partial denture in mandibular posterior region: evaluation of vertical misfit, screw loosening, flexural strength and stress distribution by photoelasticity

Abstract

In cases of severe vertical bone loss, especially in posterior region of the mandible, an alternative to bone graft surgery, little used and studied, is the use of implant-supported cantilever fixed partial denture (FPD). Thinking about the infrastructure material that ensures adequate strength to resist the masticatory loads in long term, together with the development of zirconia (Zr) and of CAD/CAM technology, the aim of this study is to evaluate the vertical misfit, torque loss and fracture resistance of cantilever in FPDs screw-retained on Morse taper and external hexagon implants, fabricated with different infrastructure materials (Zr and Co-Cr [CAD/CAM] and Co-Cr [Conventional Casting]) and positioned on the posterior region of the mandible. Additionally, the aim is also to evaluate the stress distribution between planning with mesial and distal cantilever and to verify if the simple substitution of these cantilevers for short implants (5 mm in length) decreases the stress concentration. Seventy-two FPDs with distal cantilever will be fabricated and distributed into 2 major groups (n=36), depending on the implant used: Morse taper and external hexagon. Each group will be divided into 4 subgroups (n=9) according to infrastructure material: 1 - Zr (CAD/CAM); 2 - Co-Cr (CAD/CAM); 3 - Co-Cr (conventional casting - laser welding); 4 - Co-Cr (conventional casting - TIG welding). Analysis of vertical misfit will be performed before ceramic pressing, as well as immediately before and after mechanical cycling by microscope comparator with an accuracy of 1 µm and 40× magnification. Evaluation of screw loosening will be performed before and after mechanical cycling and will be obtained by a digital torque wrench with an accuracy of 0.1 N.cm. To perform the mechanical cycling, FPDs will be positioned in the testing machine, and a load of 50 N will be applied on them, seeking to simulate 2 years of use (600,000 cycles). Maximum resistance to fracture in the cantilever will be determined by the flexural test and, therefore, a perpendicular force on the cantilever will be applied at a crosshead speed of 2.0 mm/min. The 9th FPD of each group will be used to evaluate the stress distribution by photoelasticity, when they are subjected to occlusal distributed (150 N) and punctiform (100 N) loads. In a second stage, the qualitative and quantitative photoelastic analysis will also be used to evaluate the stress distribution of 4 different plannings: A (2 implants [45 and 46 teeth] + distal cantilever [47 tooth]); B (mesial cantilever [45 tooth] + 2 implants [46 and 47 teeth]); C ( 2 implants [45 and 46 teeth] + 1 short implant [47 tooth]); D (1 short implant [45 tooth] + 2 implants [46 and 47 teeth]). (AU)