Date of Award

Spring 1996

Document Type

Thesis - Restricted

Degree Name

Master of Science (MS)

Department

Biomedical Engineering

First Advisor

Silver-Thorn, M. B.

Second Advisor

Austin, B. P.

Third Advisor

Dhuru, Virendra

Abstract

Vertical tooth root fractures are diagnostically challenging, frustrating, and an increasingly common cause of failure of tooth restoration. These vertical root fractures have been associated with many causes, including those associated with the endodontic crown replacement process itself. Nonlinear finite element analysis was used lo model the various aspects of crown replacement, including packing the canal with cones of fill er material (gutta percha), positioning the metallic post, and mounting the ceramic crown. The finite element models simulated two common canal preparations: one for posterior teeth (i.e. premolars) and another typically employed when treating anterior teeth (i.e. incisors). Stresses developed during obturation (filling), post positioning, crown placement, and occlusal and masticatory loading were obtained using this analysis method. The initial FE mesh consisted of approximately 1400 nodes and 1200 linear, isopararoetric plane strain quadrilateral elements. The endodontic processes of filling, post positioning, and crown placement resulted in the addition of approximately 1100 nodes and 800 elements. The material properties of all structures were assumed to be elastic and isotropic and were obtained from the literature. The elastic moduli for the dentin, ligament, and bone were, 18,600, 50, 13,800 MPa, respectively. The corresponding magnitudes of Poisson's ratio were 0.26, 0.30, and 0.32, respectively. The material behavior of the gutta percha was modeled using the nonlinear Mooney-Rivlin formulation, with Mooney constants C10 = 0.92 and C01 = 0.23 MPa. For the obturation of the posterior root canal. the two-dimensional FE analyses resulted in lateral stress concentrations in the area of the root apex for the condensation of the first gutta percha cone. These stresses were negligible (less than 2%) relative to those developed as further accessory cones were condensed. The lateral stress induced by the placement of accessory cones increased sharply (i.e. from 31.6 to 376.9 MPa) with 100% spreader penetration. The stress concentrations moved coronally with the condensation of successive cones. For the fully obturated canal, all lateral stress concentrations were near the coronal opening of the root canal. The stress concentrations in the coronal end of the tooth root resulting from post positioning and crown placement were considerably Jess (15 to 20%) than those developed during the obturation process. The stress concentrations generated with masticatory and occlusal loading of the full tooth restorations were also less (50 to 80%) than that which resulted from root canal obturation. Also, the validation of the finite element models resulted in errors less than 16% for obturation. The peak stresses were developed during the obturation process. These stresses were larger than the tensile and compressive strength of human dentin. Therefore, the stresses developed during obturation are believed lo be the primary cause of vertical root fractures. In addition, the spreader used to condense the filler material is believed to be largely responsible for the development of the large stresses during the obturation process. This suggests that extra care should be taken in the obturation of the root canal; the pressure applied during spreader/dentin, contact should be minimized.

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