Integration of CT, CAD system and finite element method to investigate interfacial stresses of resin-bonded prosthesis

Abstract

The question of whether prosthesis/abutment tooth interface debonding is associated with inappropriate occlusal force is investigated in this study. A new modeling approach was employed to perform the interfacial stress analyses. Solid models of resin-bonded (RB) prosthesis and abutment teeth were constructed by stacking serious section contours that were obtained from CT images. A 3-D finite element (FE) model of RB prosthesis/abutment teeth was generated in a CAD system after assembling and meshing procedures. An in-house program was developed to combine the FE package (ANSYS) to calculate the interfacial (normal and shear) stresses at the prosthesis/molar interface with the bonding and debonding conditions. After 10 different occlusal force(s) evaluations, three initial opening gaps at the distal margin of the retainer were assumed to examine the possible interface debonding mechanism under the worst loading case. The results indicated that a more accurate FE model of the posterior RB prosthesis/abutments could be generated through combining several computer-aided techniques. The maximum interfacial stresses were obtained when the occlusal force was applied on the buccal slope of the distolingual cusp of the molar. For interface debonding simulations, peak normal and shear stresses were found near the debonding areas and stress values increased gradually with small to large initial opening gaps. From these results, prosthesis/abutment tooth interfacial fatigue damage might arise or accelerate the interface to debond under adequate bonding or initial gap opening conditions after long-term repeated inappropriate occlusal force actions.

Introduction

Compared with conventional fixed partial dentures (FPD), resin-bonded (RB) prostheses that have the ability to preserve dentin substances, reduce the probability of pulp damage and accommodate dentition aesthetics have become an important treatment for replacing missing teeth since Rochette first proposed this approach in 1973 [1], [2], [3]. However, RB fixed partial dentures have a more complicated geometry and unstable mechanical structure in contrast to conventional FPD. After long-term use and under repeated loading conditions, interface debonding between the retainer and abutment tooth usually occurs because of fatigue failures and presents a major problem influencing RB prosthesis failure [4], [5]. In order to increase the bonded strength and reduce the stress level at the prosthesis/abutment tooth interface, many researches [5], [6], [7], [8], [18] have presented several effective techniques to improve the adhesive system or new materials instead of the original rigid metal framework. However, the unfavorable stress levels at the prosthesis/abutment tooth interface caused by parafunction and inappropriate occlusion might also be a significant issue that induces or accelerates the interface to debond [9]. Unfortunately, there is insufficient literature focusing on the relationship between the occlusal forces and prosthesis/abutment tooth interfacial retention. The major reason is because traditional experiments and clinical observations cannot provide enough information to understand the biomechanics of the complicated RB prosthesis system. Consequently, computer simulations based on the finite element (FE) method needs to be employed as the complementary tool for evaluating the detailed mechanical responses, such as the internal stress or basic prediction for interface debonding.

Although the FE method can procee the structural analyses for the irregular geometry, complex material properties and boundary conditions of the biological structures and have become an important tool in dental biomechanics studies [10], [11], [12]. However, the process for generating a three-dimensional (3-D) FE model of biological structures, such as a restored tooth require a large amount of effort and is time consuming due to its complicated geometry and material properties [13]. Few studies have discussed the generation of FE model construction and application to the RB prosthesis system. Although Pospiech et al. [14] employed the FE method to analyze the biomechanics of various anterior all-ceramic RB prosthesis in 1996, the geometry of their 3-D FE model was still not realistic when compared with an intact prosthesis/teeth system. Therefore, computer-assisted techniques should be taken into account to simplify the process for RB prosthesis FE model generation.

The aim of this study was to develop a new approach that combined CT (computer topography), image processing and CAD (computer aided design) system in constructing a more realistic posterior RB prosthesis modeling system. This facilitative process was then combined with linear and nonlinear FE methods to investigate the relationship between prosthesis/abutment tooth interfacial stresses (normal and shear) and various occlusal forces under physiological conditions, i.e. interfacial bonding/debonding.