An in vitro investigation of the tooth strains associated with four different restorations in Class II cavity

Evaluation of Fracture Strength of Fiber-Reinforced Direct Composite Resin Restorations: An In Vitro Study

This in vitro study aimed to compare the fracture strength of direct non-reinforced class II composite resin restorations and polyethylene fiber-reinforced restorations, and also to investigate the influence of the locations of polyethylene fibers within the cavity on the fracture strength. Sixty freshly extracted human teeth were disinfected and prepared (class II cavity design). The teeth were assigned randomly into four groups (n = 13). Group I (control) was restored with nano-hybrid composite resin. The other three experimental groups were restored with the same composite resin material reinforced by polyethylene fibers (Ribbond) at different locations. Fibers were placed either on the axial wall (Group II), on the gingival floor (Group III), or on the axial wall and pulpal/gingival floor (Group IV) of the proximal cavity. All the teeth were subjected to thermocycling to simulate the oral environment. The fracture strength was measured using a universal testing machine. Group IV had the highest mean fracture strength at maximum load (148.74 MPa), followed by Group II (140.73 MPa), Group III (136.34 MPa), and Group I (130.08 MPa), with a statistically significant difference from the control group (p = 0.008) but not between groups II and III.

Finite element analysis of multi-piece post-crown restoration using different types of adhesives

The multi-piece post-crown technique is more effective in restoring residual root than other restoration techniques. Various types of adhesives have different material properties that affect restoration. Therefore, the choice of adhesive is particularly important for patients. However, the effect of different kinds of adhesives was not too precise by experimental methods when concerning about individual differences of teeth. One tooth root can only be restored with one type of adhesive in experiment. After the mechanical test, this tooth root cannot be restored with other adhesives. With the help of medical imaging technology, reverse engineering and finite element analysis, a molar model can be reconstructed precisely and restored using different types of adhesives. The same occlusal and chewing loads were exerted on the same restored residual root models with different types of adhesives separately. Results of von Mises stress analysis showed that the adhesives with low Young's modulus can protect the root canal effectively. However, a root canal concentration is apparently produced around the root canal orifice when chewing. Adhesives with large Young's modulus can buffer the stress concentration of the root canal orifice. However, the root canal tissue may be destroyed because the adhesive is too hard to buffer the load.

Time-dependent properties of human root dentin

OBJECTIVES

The aim of this study was to investigate the creep, stress relaxation and strain rate behavior of human root dentin under compressive loading.

METHODS

Cylindrical root dentin samples of 3.5mm outer diameter, 1.5mm internal canal diameter and 6-10mm long were prepared from freshly extracted teeth. The samples were tested in a closed-loop servohydraulic testing machine at constant load or displacement, and varied strain rate. In vivo strain rates were estimated using strain gauges bonded to human teeth.

RESULTS

A family of creep curves, determined at different loads within dentin's elastic region, was found to be consistent with a material having linear viscoelastic behavior. A positive correlation (r(2)=0.79, P<0.001) was found between creep rate and stress. Young's modulus (E) was found to be a function of the strain rate with rates of loading in the range 10-500,000Ns(-1). Loading at constant displacement showed stress to be a decreasing function of time (i.e. stress relaxation).

SIGNIFICANCE

Dentin showed linear-viscoelastic behavior under various conditions of compressive loading. Time dependent properties of dentin should be taken into account in restorative dentistry.

Effect of adhesive layer properties on stress distribution in composite restorations--a 3D finite element analysis

OBJECTIVES

Teeth, adhesively restored with resin-based materials, were modeled by 3D-finite elements analysis that showed a premature failure during polymerization shrinkage and occlusal loading.

METHODS

Simulation of Class II MOD composite restorations with a resin bonding system revealed a complex biomechanical behavior arising from the simultaneous effects of polymerization shrinkage, composite stiffness and adhesive interface strain. Due to a polymerization contraction, shrinkage stress increases with the rigidity of the composites utilised in the restoration, while the cusp movements under occlusal loading are inversely proportional to the rigidity of the composites. The adhesive layer's strain also plays a relevant role in the attenuation of the polymerization and occlusal loading stresses.

RESULTS

The choice of an appropriately compliant adhesive layer, able to partially absorb the composite deformation, limits the intensity of the stress transmitted to the remaining natural tooth tissues. For adhesives and composites of different rigidities, FEM analysis allows the determination of the optimal adhesive layer thickness leading to maximum stress release while preserving the interface integrity. Application of a thin layer of a more flexible adhesive (lower elastic modulus) leads to the same stress relief as thick layers of less flexible adhesive (higher elastic modulus).

The influence of resin composite and bonded amalgam restorations on dentine permeability in Class II cavities in vitro

OBJECTIVES

This study was conducted to measure and compare dentine permeability reduction in Class II preparations, after restoration with resin composite or bonded amalgam, using either a multi-step or one-bottle dental adhesive system.

METHODS

An in vitro fluid transport model was used to measure initial dentine permeability in Class II cavities with an intact smear layer in crown segments from extracted human premolars. One week and 3 months after restoration with resin composite or bonded amalgam, using either multi-step or one-bottle dental adhesive system, the measurements were repeated and the reduction in permeability was calculated as a percentage of the initial values. The data were analyzed statistically using a one-way ANOVA and Least Significant Difference tests.

RESULTS

After 1 week the resin composite with one-bottle dental adhesive system provided the highest reduction in dentine permeability, whereas after 3 months the bonded amalgam caused the highest reduction. At both time intervals the resin composite with multi-step adhesive system provided the lowest reduction. (P=0.036 at 1 week, P=0.016 at 3 months). After 3-months storage in water a significant increase in dentine permeability reduction was found in the bonded amalgam group only (P=0.017). The reduction in dentine permeability provided by resin composite with one-bottle dental adhesive system and bonded amalgam in Class II cavities was similar at both time intervals. (P=0.182 at 1 week, P=0.750 at 3 months).

SIGNIFICANCE

The results of this study indicated that on the basis of reducing dentine permeability in vitro, resin composite restorations with the investigated one-bottle adhesive system was superior to its multi-step equivalent and bonded amalgam can also be preferred in Class II restorations.

Stress transfer from polymerization shrinkage of a chemical-cured composite bonded to a pre-cast composite substrate

OBJECTIVES

(1) To develop and test a strain gauge-based method for evaluating the strain transferred through a bonded interface to a deformable substrate; and (2) to develop and test a finite element (FE) model for evaluating the stress development in a chemical-cured composite during polymerization.

METHODS

A generic light-cured resin composite was used to fabricate a rectangular plate with an internal slot filled with a chemical-cured composite. Strain gauges on the surface of the composite in the channel and on the plate adjacent to the channel-plate interface were used to record strain continuously up to 500 s after mixing the composite paste. A quadrant three-dimensional (3D) finite element model used strains measured on the channel to simulate the experimental conditions. The model was used to estimate stresses in the channel and at the bonded interface.

RESULTS

Strain in the plate reached a plateau 200 s after mixing the composite. Strain of the composite paste in the channel continued to rise with time but at a steadily decreasing rate. Maximum principal stress predicted by the FE model on top of the plate, on top of the channel and within the channel was 5.12 MPa, 3.78 MPa, and 5.26 MPa, respectively.

SIGNIFICANCE

Stresses were effectively transferred through the bonded interface in this test configuration, and results were in close agreement with previously reported literature values for polymerization contraction stresses generated in composite configurations with similar bonded to unbonded surface ratios.