A stress compatible finite element for implant/cement interface analyses

Evaluation of a reinforced slot design for CEREC system to restore extensively compromised premolars

OBJECTIVES

The structural stability and adhesive strength of a large-cavity premolar with a CEREC restoration is a frequent problem in long-term use. This study sought to determine whether an additional reinforced slot could increase tooth/ceramic retention using finite element (FE) analysis and fracture testing.

METHODS

The cavity was designed in a typical MODL restoration failure shape when the lingual cusp has been lost. Two FE restored-tooth models with different cavity designs were created using image processing, contour stacking, and mesh generation. Interfacial (normal and shear) stresses were then calculated with and without the slot design for restored teeth under lateral and axial forces and different interfacial conditions (bonded and de-bonding). For validation, a fracture experiment was performed with and without reinforced slot designs for large ceramic CEREC restorations.

RESULTS

The maximum stresses at the buccal wall increased when a lateral occlusal force acted on the restored tooth with a slot design. Conversely, the interfacial stresses decreased when the restored tooth received a uniform axial occlusal force. After de-bonding on the buccal tooth/ceramic interface, the stresses increased by an average factor of three over those obtained with a bonded interface. The fracture forces were consistent with the tendencies predicted in FE analyses.

CONCLUSIONS

An additional reinforced slot for the CEREC restoration of a large cavity could increase retention when a restored tooth receives an axial occlusal load. However, the benefits of a slot seem to be doubtful for a premolar often subjected to a lateral load.

Finite element models in tissue mechanics and orthopaedic implant design

This article attempts to review the literature on finite element modelling in three areas of biomechanics: (i) analysis of the skeleton, (ii) analysis and design of orthopaedic devices and (iii) analysis of tissue growth, remodelling and degeneration. It is shown that the method applied to bone and soft tissue has allowed researchers to predict the deformations of musculoskeletal structures and to explore biophysical stimuli within tissues at the cellular level. Next, the contribution of finite element modelling to the scientific understanding of joint replacement is reviewed. Finally, it is shown that, by incorporating finite element models into iterative computer procedures, adaptive biological processes can be simulated opening an exciting field of research by allowing scientists to test proposed 'rules' or 'algorithms' for tissue growth, adaptation and degeneration. These algorithms have been used to explore the mechanical basis of processes such as bone remodelling, fracture healing and osteoporosis. RELEVANCE: With faster computers and more reliable software, computer simulation is becoming an important tool of orthopaedic research. Future research programmes will use computer simulation to reduce the reliance on animal experimentation, and to complement clinical trials.

Stress analysis of bone modeling response to rat molar orthodontics

The purpose of this project was to determine if alveolar bone modeling could be associated with altered mechanical environment. Finite element stress analysis of an orthodontically tipped rat molar periodontium was performed. The distributions of mechanical components within the periodontal ligament and cortical bone were compared to the well-documented bone formation and resorption patterns in the alveolus of the tooth. It was concluded that in orthodontically induced bone modeling activity, locations of osteogenesis uniquely coincided with increased tension within the periodontal ligament, while bone resorption areas could be associated with increases in other components (minimum principal and maximum shear stresses, strain energy density, and von Mises) within the bone itself.

A large-deformation, finite-element study of chondrodiatasis in the canine distal femoral epiphyseal plate

A large-deformation, finite-element analysis was conducted to model stress fields around the developing growth plate as a first approach to comprehend the clinical application of traction for limb lengthening procedures. The model chosen was a cross section through the distal femoral growth plate of a 14-week-old dog. The chosen section passed through two of the conformational bends (mammillary processes) formed by the natural convolutions of the physis. Three different loading conditions were applied to the distal femoral epiphyseal model. The model also examined the effects of different values of Young's modulus of the growth-plate cartilage. The cortical bone in all cases, experienced the highest stresses. In the growth plate, the highest principal stresses occurred where the physis intersects cortical bone. There were localized stresses that were higher than those that caused fracture in a rabbit model [Guse et al., J. Orthop. Res. 7, 667-673 (1989)]. Results also indicated the following: a small tilt of 0.1 degree in loading application increases the maximum principal stresses and the von Mises stresses in certain regions of the growth plate by about 8%; the shearing stresses in the growth plate are sensitive to Young's modulus of the growth plate; and traction pins that do not grip the cancellous bone in the epiphysis will increase the regions of high principal stress in the growth plate.

Interface fixation analysis of artificial joints

Implant/cement interface stresses, which affect the quality of implant fixation to the surrounding materials and therefore the long-term performance of the total joint replacement, are addressed. It is noted that previous modeling approaches result in stresses that are discontinuous at the interface between adjacent elements. Three distinct formulations that yield accurate continuous stresses as well as displacements are presented. These are a displacement-based interface element with an extra node on the interface, a penalty-modified compatible formulation, and a mixed stress-displacement formulation. Results for a tibial fixation model subjected to a nonaxisymmetric compression load are presented. The results are predicted using the penalty-based continuous stress formulation. The model has been analyzed for two types of loading conditions. One condition consists of two loaded areas, simulating a situation when the femur is exerting pressure evenly on the two tibial condyles. The second condition consists of a single loaded area simulating the loading of only one condyle. Results for the latter case are reported. The proposed formulations have performed satisfactorily in the test examples and are, therefore, considered as reliable tools to predict the interface mechanics in the total joint replacement.

Dynamic stress response of the implant/cement interface: an axisymmetric analysis of a knee tibial component

To determine the adverse effects on the implant/cement interface stresses caused by a dynamic load on the implant, an axisymmetric dynamic finite element analysis was performed for an idealized knee tibial component assuming perfect bonding at the interface. The component, consisting of a metal plate with a central stem, was subjected to a compressive load that varied with time as a terminated ramp function. At first, the reliability of the interface stress predictions was assessed by computing the effects of a number of method-related parameters, viz., the finite element mesh density, the assumed bone properties. Analysis was then performed considering the stem length, cement mantle thickness, and the type of implant metal as design variables. The analysis predicts high-frequency (600 Hz) stress oscillations of significant amplitude at those locations of the interface that are also subjected to high static stresses: near the stem-plate junction and the stem tip for normal stress, and at the stem tip for shear stress. However, the predicted stress amplitude has been found to be particularly sensitive to the assumed rise time of the input load function. With a rise time of 2.0 ms, an input condition considered to be severe enough to represent the most vigorous dynamic activity, the maximum stress augmentation, because of stress oscillations, is predicted to be less than 25%. In general, the design variables have been found to affect the static stresses much more than the dynamic ones. It has been concluded that for the cases studied, dynamic effects are relatively small and a static analysis is sufficient to characterize the interface stress condition.