The Effect of Aging Dental Composites in Biologically-Relevant Conditions on Their Mechanical Properties

Dental composites have become the primary choice for dental restorations. Composites consist of a mixture of solid filler particles bound by polymers which are cured in place following placement in the prepared tooth preparation. It is critical to the long-term performance of the restoration that the material properties do not deteriorate during the life of the restoration. However, there is increasing evidence that dental composites deteriorate following placement, leading to failure of the restoration, usually due to secondary dental caries. The conditions in the oral cavity contribute to the break-down of the composites. This degradation is likely caused by four main types of processes: physical material degradation (e.g. fatigue due to cyclic loading during mastication), abiotic chemical degradation due to wetting/drying cycles, aqueous dissolution (particularly under acidic conditions as occurs with tooth enamel), and biodegradation of the organic components of the composite. Recent research has clearly indicated that the latter process, biodegradation of the polymer can be catalyzed by enzymes in the oral cavity, both human derived and bacterial in origin. These studies have typically been performed in vitro with only the polymer component of the dental composite (no filler). While such studies provide a mechanistic understanding of the process, they do not help clarify if these processes are just a decrease in strength or an interaction between the oral environment and the restoration. To address these concerns, a study was undertaken to measure the strength of composites (diametral stress strength and Young’s modulus) following aging under various environmental conditions relevant to the oral cavity; air, artificial saliva (AS), acidified artificial saliva, and artificial saliva with esterase enzyme. A large number of replicates was performed to ensure statistical validity and control for composite curing variability. The aging periods were performed for longer periods of time than is frequently cited in the literature based on an analysis of the maximum amount of time necessary for diffusion penetration of the composite test specimens. The results demonstrate that air incubated samples had the highest strength, followed by a statistically significant reduction in strength in the AS incubated samples. Acidification of AS media (~50 mM free acid) did not result in any statistically significant reduction in strength compared to AS incubations. In contrast, incubation in the presence of esterase enzymes resulted in a marked, statistically significant reduction in strength. Taken together, these results showed that demineralization/dissolution of the composite under acidic conditions did not result in a significant decrease in strength, while exposure to esterase enzyme resulted in a significant deterioration in material properties. These results further show that composites are susceptible to significant deterioration in material strength under conditions relevant to the oral cavity

Combined Damage Index to Detect Plastic Deformation in Metals Using Acoustic Emission and Nonlinear Ultrasonics

Understanding the amount of degradation using nondestructive evaluation (NDE) methods provides an effective way of determining the fitness to service and the residual life of structural components. Due to uncertainties introduced by the single NDE method, a combined damage index using multi-sensor data increases the reliability of damage assessment. In this paper, the outputs of three NDE methods including acoustic emission (AE), linear ultrasonics (LUT), and nonlinear ultrasonics (NLUT) are merged to identify the amount of plastic deformation in aluminum 1100. The sensitivities of individual and combined methods to microstructural changes are evaluated. The coupon samples are loaded up to different strain levels and then unloaded. AE data is recorded in real time and ultrasonic data is recorded from the unloaded samples. The major features combined in the damage index are cumulative AE absolute energy and nonlinear coefficient. The microstructural state is verified with microscopic analysis and hardness testing. The developed damage index can nondestructively assess the amount of plastic deformation with higher reliability