The suitability of different FEA models for studying root fractures caused by wedge effect

Abstract

Finite element analysis (FEA) utilizing models with different levels of complexity are found in the literature to study the tendency to vertical root fracture caused by post intrusion ("wedge effect"). The objective of this investigation was to verify if some simplifications used in bi-dimensional FEA models are acceptable regarding the analysis of stresses caused by wedge effect. Three plane strain (PS) and two axisymmetric (Axi) models were studied. One PS model represented the apical third of the root entirely in dentin (PS-nG). The other models included gutta-percha in the apical third, and differed regarding dentin-post relationship: bonded (PS-B and Axi-B) or nonbonded (PS-nB and Axi-nB). Mesh discretization and material properties were similar for all cases. Maximum principal stress (sigma max) was analyzed as a response to a 165 N longitudinal load. Stress magnitude and orientation varied widely (PS-nG: 10.3 MPa; PS-B: 0.8 MPa; PS-nB: 10.4 MPa; Axi-B: 0.2 MPa; Axi-nB: 10.8 MPa). Axi-nB was the only model where all sigma max vectors at the apical third were perpendicular to the model plane. Therefore, it is adequate to demonstrate the tendency to vertical root fractures caused by wedge effect. Axi-B showed only part of the sigma max perpendicular to the model plane while PS models showed sigma max on the model plane. In these models, sigma max)orientation did not represent a situation where vertical root fracture would occur due to wedge effect. Adhesion between post and dentin significantly reduced sigma max.

Dehydration-induced shrinkage of dentin as a potential cause of vertical root fractures

“…Numerous possible reasons for VRF have been reported in the literature including trauma (Pitts et al, 1983), excessive forces during root canal obturation (Yang et al, 1995;Mireku et al, 2010), wedging effects of spreaders during lateral condensation (Mireku et al, 2010;Pitts et al, 1983;Walton et al, 1984), loss of structural integrity of the tooth because of caries or access cavity preparation (Soares et al, 2007;Arola and Rouland, 2003) and decrease in dentine thickness because of intracanal preparation (Meira et al, 2008;Mireku et al, 2010;Cohen et al, 2006;Lertchirakarn et al, 2003a). Furthermore, the application of endodontic posts appears to bear additional risks resulting from excessive pressure during post cementation (Mireku et al, 2010;Yang et al, 1995), wedging effects of endodontic posts (Meira et al, 2008;Pitts et al, 1983;Walton et al, 1984) and volumetric expansion from corrosion of posts (Pitts et al, 1983;Walton et al, 1984;Yang et al, 1995).…”

Stress prediction in a central incisor with intra-radicular restorations

“…Results obtained by Meira et al 10 have indicated that the axisymmetric model presents stress vectors with similar orientation but lower in magnitude than those detected by 2D analysis for elements in the vicinity of restoration post, and stress levels can therefore be overestimated by adopting 2D fi nite element analysis. It is worth mentioning that, despite eventual approximations due to the adoption of 2D rather than 3D analysis, the comparison between the models is considered to be valid, specifi cally in regard to the identifi cation of critical regions where cracks can nucleate and eventually propagate leading to the loss of structural integrity and fi nally ultimate fracture of the restored tooth.…”

“…However, some investigations have demonstrated that root post transmits masticatory loads to the tooth root and supporting structures [5][6][7][8][9] . Accordingly, research works related to the biomechanics of dental materials have been carried out to evaluate the effect of masticatory loads on the stress distribution within endodontically treated teeth restored with intra-radicular posts [10][11][12][13][14][15][16][17][18][19] . This allows one to pinpoint highly stressed regions where fracture is expected to initiate.…”

“…The fi nite element method (FEM) has been used for stress and strain analysis in dental biomechanics for nearly four decades [2][3][4][5][6][9][10][11][12]30,31 . The subdivision of the structure allows one to determine the stress distribution and hence to identify highly stressed regions where cracks are most likely to nucleate and, eventually, lead to partial or total fracture of the restored tooth.…”

“…One should, though, point out that two-dimensional softwares assume that the model's geometry remains unchanged in all sections parallel to the plane of the model. Axisymmetric models are therefore more representative and three-dimensional (3D) models would represent the most faithful simulation of clinical situations 10,16 . Three-dimensional models are surely more accurate in describing the actual state of stress but, at the same time, much more complicated to realize and they do require a much extensive computing time to be resolved 4 .…”