Ljubomir Petrovic

A model for shrinkage strain in photo polymerization of dental composites.

OBJECTIVES We formulate a new model for the shrinkage strain developed during photo polymerization in dental composites. The model is based on the diffusion type fractional order equation, since it has been proved that polymerization reaction is diffusion controlled (Atai M, Watts DC. A new kinetic model for the photo polymerization shrinkage-strain of dental composites and resin-monomers. Dent Mater 2006;22:785-91). Our model strongly confirms the observation by Atai and Watts (see reference details above) and their experimental results. The shrinkage strain is modeled by a nonlinear differential equation in (see reference details above) and that equation must be solved numerically. In our approach, we use the linear fractional order differential equation to describe the strain rate due to photo polymerization. This equation is solved exactly. RESULTS As shrinkage is a consequence of the polymerization reaction and polymerization reaction is diffusion controlled, we postulate that shrinkage strain rate is described by a diffusion type equation. We find explicit form of solution to this equation and determine the strain in the resin monomers. Also by using equations of linear viscoelasticity, we determine stresses in the polymer due to the shrinkage. The time evolution of stresses implies that the maximal stresses are developed at the very beginning of the polymerization process. SIGNIFICANCE The stress in a dental composite that is light treated has the largest value short time after the treatment starts. The strain settles at the constant value in the time of about 100s (for the cases treated in Atai and Watts). From the model developed here, the shrinkage strain of dental composites and resin monomers is analytically determined. The maximal value of stresses is important, since this value must be smaller than the adhesive bond strength at cavo-restoration interface. The maximum stress determined here depends on the diffusivity coefficient. Since diffusivity coefficient increases as polymerization proceeds, it follows that the periods of light treatments should be shorter at the beginning of the treatment and longer at the end of the treatment, with dark interval between the initial low intensity and following high intensity curing. This is because at the end of polymerization the stress relaxation cannot take place.

A method of improving marginal adaptation by elimination of singular stress point in composite restorations during resin photo-polymerization

OBJECTIVES To reduce the effect of stresses due to volumetric shrinkage the authors propose an incremental technique for placing composite restorations. METHODS The goal of the method is to reduce the volume of the resin that is polymerized and eliminate a stress singular point in the resin that is positioned at the geometric center of the cavity. This is achieved by a two step type incremental technique. In the first step the resin is placed in the cylindrical cavity with a metal pin embedded in the middle of the composite restoration. After polymerization, the metal pin is removed and the cylindrical hole is filled with the second layer of composite. Finally, the second layer in the center of the composite restoration is polymerized. RESULTS This study confirmed that the proposed incremental type placement technique reduces marginal debonding. SIGNIFICANCE The main hypothesis is that the elimination of a stress singular point at the center of the restoration results in the reduction of stresses at tooth-composite interface and therefore improve the marginal adaptation (reduces length of the contraction gap at tooth-composite interface).

Viscoelastic properties of uncured resin composites: Dynamic oscillatory shear test and fractional derivative model

OBJECTIVE In this study we analyze viscoelastic properties of three flowable (Wave, Wave MV, Wave HV) and one universal hybrid resin (Ice) composites, prior to setting. We developed a mathematical model containing fractional derivatives in order to describe their properties. METHODS Isothermal experimental study was conducted on a rheometer with parallel plates. In dynamic oscillatory shear test, storage and loss modulus, as well as the complex viscosity where determined. We assumed four different fractional viscoelastic models, each belonging to one particular class, derivable from distributed-order fractional constitutive equation. The restrictions following from the Second law of thermodynamics are imposed on each model. The optimal parameters corresponding to each model are obtained by minimizing the error function that takes into account storage and loss modulus, thus obtaining the best fit to the experimental data. RESULTS In the frequency range considered, we obtained that for Wave HV and Wave MV there exist a critical frequency for which loss and storage modulus curves intersect, defining a boundary between two different types of behavior: one in which storage modulus is larger than loss modulus and the other in which the situation is opposite. Loss and storage modulus curves for Ice and Wave do not show this type of behavior, having either elastic, or viscous effects dominating in entire frequency range considered. SIGNIFICANCE The developed models may be used to predict behavior of four tested composites in different flow conditions (different deformation speed), thus helping to estimate optimal handling characteristics for specific clinical applications.

The micromorphological characterization of adhesive bond in dentin different locations

The aim of this study was to analyse the interfacial micromorphology of total‐etch adhesives and dentin structures different locations by using SEM. Standardized cylindrical cavities (3mm in diameter, 2mm deep) with all margins in dentin were prepared on occlusal and buccal surfaces of twenty extracted human third molars. A total‐etch dentin adhesive system and a light‐cure flowable composite (Filtek Ultimate Flowable, 3M ESPE, St. Paul, MN, USA) were used in this study. Micro‐morphological SEM analysis of the marginal seal of the original tooth specimens was performed using high magnification of up to 1000×. In this study, we found the difference in interfacial micromorphology in dentin different locations. Also, marginal gap was found in both observed dentin area. Better understanding of complexity and three‐ dimensional variations of the tooth structure is important for prevention of clinical challenges such as postoperative sensitivity, marginal discoloration and secondary caries, which could be prevented by achieving of predictable and long‐lasting adhesive bond.

[One-year clinical evaluation of compomer restorations in cervical lesions of different aetiology].

INTRODUCTION The aim of this one-year prospective clinical study was to evaluate the treatment results of compomer restorations (Dyract eXtral Dentsply/De Trey, Konstanz, Germany) with a single step self-etching dental adhesive (Xeno III Dentsply/De Trey, Konstanz, Germany) used for restoring class V lesions (non-carious and primary carious cervical lesions). MATERIAL AND METHODS A total number of 62 class V restorations (n = 62) were placed by one dentist in 30 patients on incisors, canines and premolars. The fillings were placed due to different indications: non-carious cervical defects (n = 32) and primary carious lesions (n = 30). The restorations were evaluated by a single-blind design, according to the Modified United States Public Health Service system 6 and 12 months following the placement. The following were evaluated: retention, marginal integrity, marginal discoloration, wear; postoperative sensitivity and secondary caries. The statistical analysis compared the ratings of each criteria between materials using the Pearson chi-square or Fishers exact test at a level of significance of 5% (p < 0.05). RESULTS Two restorations of the non-carious lesion group were lost after 6 months, and after 12 months one restoration was lost in the group of primary carious lesions. There were no statistically significant differences between restorations for all evaluated criteria in both groups. CONCLUSION The compomer restorations in combination with a single step self-etching dental adhesive showed acceptable clinical performance in Class V lesions after one year of clinical service.