Effects Of Three Different Residual Root Treatments After Post And Core Restoration Of The Mandibular Second Premolar : A 3-Dimensional Finite Element Analysis

Objective: A three-dimensional Finite Elements Analysis(FEA)was used to study the influence of the inner shoulder retention form(ISRF),crown lengthening procedure(CLP)and orthodontic eruption procedure(OEP)on the biomechanics of the mandibular second premolar residual root post-core crown restoration,exploring the feasibility of adding ISRF and the better designs of the ISRF to provide a feasible solution for the post-core crown restoration of residual roots.Methods: Six groups of mandibular second premolar residual root without the ferrule postcore crown restoration models were established using three-dimensional FEA-0.5mm×0.5mm [depth×width] ISRF group(ISRF0.5),1.0mm×1.0mm [depth×width] ISRF group(ISRF1),simulating 2mm crown lengthening procedure(CLP2),simulating 1mm crown lengthening procedure(CLP1),simulating 2mm orthodontic eruption procedure(OEP2)and simulating 1mm orthodontic eruption procedure(OEP1),and 2 groups residual root with 1mm and 2mm ferrule post and core crown restoration models(NPR1 group and NPR2 group)served as control groups.All models applied the 400-N oblique force to the tip of the buccal cusp 30° obliquely in the buccal-to-lingual direction.The values and distribution of von Mises stress(?v M),maximum principal stress(?max),and maximum displacement were calculated in the following structures: root,core,post,cement layer between the post and root and cement layer inside the crown to evaluate the influence of the ISRF on the post-core crown restoration of mandibular second premolar residual roots.Results: The lowest stress peaks of the five restoration structures all appeared in the control group,while the highest stress peaks vary depending on the structure of the restoration.For the root,the highest stress peak occurred in the CLP2 group,followed by CLP1,and the CLP group was similar to the OEP group.For the core,the stress peak of the ISRF group is much higher than the other groups,about twice,and the CLP group is slightly larger than the OEP group.For the post and the two cement layers,the peak stress of the CLP2 group is the largest,and the stress peaks of the CLP1 group and the ISRF group are similar,and both are greater than the OEP group.?(v M)of the restorations appeared on the lingual side of each restoration,and ?(max)appeared on the buccal side of each restoration.Regarding the maximum displacement of each restoration structure,the maximum displacement value appeared in the CLP2 group,followed by the CLP1,OEP group,and the NPR group and the ISRF group had lower displacement values regardless of the restoration structure.Conclusion: Compared with no coronal dentin tissue,the remaining amount of 1-2mm is conducive to the prognosis of residual roots post-core crown restoration.ISRF is an optional residual roots treatment method,compared with ISRF with a width of 1.0mm and a depth of 1.0mm,it is more suitable to prepare ISRF with a width of 0.5mm and a depth of 0.5mm.Residual roots need to undergo crown lengthening procedure or orthodontic eruption procedure,1mm ferrule height is more appropriate,but it has to avoid the formation of a crown-root ratio greater than 1.