In vivo evaluation of a polyester and fiberglass composite intramedullary nail for femoral osteosynthesis in calves.

The objective of this study was to test a composite of polyester resin and fiberglass in the form of an intramedullary nail for osteosynthesis of femoral fractures in calves. The methodology was established based on a previous study that used a bovine femur finite element model to simulate fractures, which were then stabilized by the same nails as proposed in this study. General anesthesia was induced in six calves followed by fracture creation via an oblique incision in the middle third of the femoral diaphysis, and osteosynthesis was immediately performed by retrograde insertion of the composite nail. Locking was achieved by drilling the bone and nail without using a jig and introducing two stainless steel screws proximal and two distal to the fracture line. Five of the six calves achieved complete fracture healing after 60 days. No signs of incompatibility or toxicity of the composite were observed. However, limitations were observed during the surgery, such as difficulty in drilling the nail and trimming the remainder portion of the nail that extended beyond the length of the bone. Small fragments produced by these maneuvers were considered irritating to soft tissues during the postoperative period. It was also found that small cracks in the nail tended to propagate in the form of longitudinal fractures. In conclusion, an intramedullary nail made of polyester resin and fiberglass (a low-cost and easy-to-acquire material) was considered biocompatible and capable of allowing bone healing of femoral fractures in young cattle. However, the development of solutions for the reported limitations is crucial prior to recommending the proposed composite for clinical use.

A finite element model to simulate femoral fractures in calves: testing different polymers for intramedullary interlocking nails.

OBJECTIVE: To verify if the finite element method can correctly estimate the performance of polyacetal and polyamide 6 intramedullary nails in stabilizing a femoral fracture in calves and to estimate the performance of a polypropylene nail in same conditions. STUDY DESIGN: Computational and experimental study. SAMPLE POPULATION: Finite element models (FEMs). METHODS: Based on a 3-dimensional finite element method (FEM) of the femoral diaphysis, 3 models were constructed to simulate an oblique simple fracture stabilized by an intramedullary nail composed of 1 of 3 distinct polymers. Models were tested under 6 loading conditions that simulated a static calf or a calf in different walking phases. Maximum bone and implant stresses were compared to yield and rupture stresses of specific materials. RESULTS: Under static conditions, all polymers were resistant to critical deformation and rupture because maximum von Mises stresses were lower than the respective yield and rupture stresses. However, during walking, maximum stresses exceeded the yield and rupture limits of the polymers, in agreement with a previous in vivo study, which used polyacetal and polyamide nails. CONCLUSIONS: FEM correctly estimated that polyacetal and polyamide 6 nails would fail to immobilize an oblique femoral diaphyseal fracture in calves that were allowed to walk freely during the early postoperative period. FEM can be useful in the development of new bovine orthopedic devices.

Stress distribution in a premolar 3D model with anisotropic and isotropic enamel.

The aim of this study was to compare the areas of stress concentration in a three-dimensional (3D) premolar tooth model with anisotropic or isotropic enamel using the finite element method. A computed tomography was imported to an image processing program to create the tooth model which was exported to a 3D modeling program. The mechanical properties and loading conditions were prescribed in Abaqus. In order to evaluate stresses, axial and oblique loads were applied simulating realistic conditions. Compression stress was observed on the side of load application, and tensile stress was observed on the opposite side. Tensile stress was concentrated mainly in the cervical region and in the alveolar insertion bone. Although stress concentration analyses of the isotropic 3D models produced similar stress distribution results when compared to the anisotropic models, tensile stress values shown by anisotropic models were smaller than the isotropic models. Oblique loads resulted in higher values of tensile stresses, which concentrate mainly in the cervical area of the tooth and in the alveolar bone insertion. Anisotropic properties must be utilized in enamel stress evaluation in non-carious cervical lesions.

Abfraction and anisotropy--effects of prism orientation on stress distribution.

This work discusses the effect of enamel anisotropy in the stress concentration at the cement-enamel junction (CEJ), a probable cause of fracture in enamel leading to abfraction. Usual simplifications when developing computer models in dentistry are to consider enamel isotropic, or that the direction of the prisms is orthogonal to either the dentine-enamel interface or the tooth outer surface. In this paper, a more refined model for the material behavior is described, based on laboratory observation and on the work of Fernandes and Chevitarese. The material description is used in a two-dimensional (2D) finite element model of the first upper premolar, and the analysis is performed for two different situations: vertical loads, typical of normal mastication and horizontal loads, dominant in bruxism. The analyses were performed using a unit load, which under the hypothesis of linear response of the tooth, allows the combinations described in the text to simulate different functional and parafunctional loads. The results indicate that a realistic enamel description in terms of mechanical properties and spatial distribution of its prisms alters significantly the resulting stress distribution. For all cases included in this study, the detailed description of prism orientation and resulting anisotropy led to improved response in terms of stress distribution, even when loading was horizontal.

Analysis of stresses during the polymerization shrinkage of self-curing resin cement in indirect restorations: a finite-element study.

BACKGROUND: Adhesive cementation is essential for the longevity of indirect esthetic restorations. However, polymerization shrinkage of resin cement generates stress, which may cause failures in the tooth-restoration interface. So, understanding of the biomechanics of resin cement is important for predicting the clinical behavior of an esthetic indirect restoration. AIMS: To analyze the stresses generated during polymerization shrinkage of self-curing resin cement in ceramic and in indirect resin (IR) restorations, using the finite-element method (FEM). SETTINGS AND DESIGN: Numerical study using the finite-element analysis. MATERIALS AND METHODS: A three-dimensional (3D) model of a second molar restored with ceramic or IR onlay restoration was designed. The polymerization shrinkage of self-curing resin cement was simulated in FEM software using an analogy between the thermal stress and the resulting contraction of the resin cement. The localization and values of tensile stresses in the dental structure, cement, and adhesive layer were identified. RESULTS: The location and value of the tensile stresses were similar for the two restorative materials. High tensile stresses were identified in the axiopulpal wall and angles of the tooth preparation, with the major stresses found in the cement located in the axiopulpal wall. CONCLUSIONS: The high stresses values and their concentration in the angles of the prepared tooth emphasize the importance of round angles and the use of cements with lower rates of shrinkage.

3D finite element analysis on esthetic indirect dental restorations under thermal and mechanical loading.

Thermo-mechanical finite element analyses in 3-D models are described for determination of the stress levels due to thermal and mechanical loads in a healthy and restored tooth. Transient thermo-mechanical analysis simulating the ingestion of cold and hot drinks was performed to determine the temperature distribution in the models of the teeth, followed by linear elastic stress analyses. The thermal loads were applied on the occlusal and lingual surfaces. Subsequently, coupled variation of the temperature and mastication loading was considered. The vertical loading was distributed at occlusal points, adding up to 180 N. Maximum stresses were verified in resin restoration under thermal loads. When studying coupled effect of mechanical loading with that arising from thermal effects, higher tensile stress values occurred in porcelain restorations, especially at the restoration-dentin interface. Regions of high tensile stress were detected and their possible clinical significance with respect to restoration damage and microleakage were discussed.