Abstract
Background:The human mandible, like all long bones in the body, deforms when loaded. The force exerted by the muscles of mastication during various movements on the body of the mandible has been suggested to play a significant role in mandibular flexure. The effect of the lateral pterygoid muscle on mandibular deformation is controversial with some authors believing that the location of insertion and action of the inferior head of the lateral pterygoid muscle contributes most to mandibular deformation during opening. Restoration of mandibular edentulous areas with implant prostheses is increasing in popularity. Osseointegrated implants behave differently to natural teeth due to the absence of a periodontal ligament, and therefore strain introduced to an implant is transmitted directly to the body of the mandible. Different stages of implant treatment, including implant placement, impression technique and the material used to fabricate the implant prosthesis can be affected by mandibular flexure and alter the outcome of implant treatment. The significance of mandibular flexure in implant dentistry, however, is unclear at this time and it is hoped that this study will contribute to knowledge in this area.
Aim:To measure the extent of mandibular deformation and strain distribution within a mandible model restored with a screw-retained implant prosthesis.
Methodology:Two Certain® Biomet 3i implants were placed in a partially dentate resin mandible model and four cobalt-chromium alloy implant fixed frameworks were constructed. Eight strain gauges were attached to the body of the mandible and eight adjacent to the implants. The mandible was suspended from a customized jig simulating the temporomandibular joint with elastic cords that enabled simulation of the glenoid fossa and the muscles of mastication. Using a universal testing machine, mastication was simulated to generate a unilateral 50 N force on the occlusal surface of the mandibular first molar. The strains were continuously recorded using a computer, first without the implant framework, then during and after framework tightening to 20 Ncm.
Results:Under unilateral loading without the implant framework in place, the area of strain distribution was similar to that of a finite element analysis of a human mandible developed at the School of Dentistry, University of Otago. The results showed that the epoxy resin mandible model used in this study behaved similarly to a human mandible under unilateral loading. Unilateral loading of the epoxy resin mandible model with the implant framework in place showed a strain distribution on the body of the mandible model that was similar to that without implant frameworks. Each of the four frameworks showed a different pattern of peri-implant strain distribution after they were tightened to the implants. The difference in strain could be contributed by variation in casting conditions and solidification shrinkage. The strain distribution patterns at the peri-implant level more closely resembled the strain distribution after implant framework placement. Peri-implant strain was framework-specific depending on the extent of initial misfit of the individual framework.
Conclusions:This study showed that mandibular flexure occurred during unilateral loading. The amount of peri-implant strain during unilateral loading before implant framework placement was within the limits that the mandible is able to repair micro-damage occurring within the body of the mandible and around the implants. Placement of misfitting implant frameworks affected the strain distribution at peri-implant level. Unilateral loading, combined with the strain generated by mandibular flexure, as well as the existing strain resulting from a misfitting implant framework, can increase the strain at peri-implant level. This amount of strain could be harmful to the peri-impant bone during cyclic loading. The clinical implications of the results of this study are unclear at this stage and further research is required.