In vitro Study Comparing Fracture Resistance of Nanocomposites with and without Fiber Reinforcement with Different Cavity Designs Used for Obliquely Fractured Incisal Edge Restoration.

AIM: The aim of the study is to evaluate fracture resistance of nanocomposites with and without fiber reinforcement with different cavity designs used for obliquely fractured incisal edge restoration. MATERIALS AND METHODS: In the present study, 60 sound extracted maxillary central incisors were mounted on autopolymerizable acrylic resin up to the cementoenamel junction, out of which, 10 intact teeth were kept as control (group 1) and the remaining 50 samples were reduced incisally in an oblique manner up to 3 mm. All incisally reduced samples were divided into five groups (n = 10) based on the restoration techniques as follows: group 2 (conventional bevel), group 3 (single central palatal slot on the incisal edge), group 4 (single palatal slot with central 2 mm fiber), group 5 (two palatal slots on the incisal edge with a distance of 0.5 mm to 1 mm between them), and group 6 (two slots on the incisal edge with two 2 mm fibers). All samples were built incrementally with nanocomposites followed by finishing and polishing. All samples including control were then stored in distilled water before their fracture resistance was measured using a universal testing machine. Failure modes were visually examined and the results were subjected to statistical analysis. RESULTS: The mean fracture resistance among the experimental groups was observed, group 4 with single fiber in the central position had the highest (832.68 N) followed by group 3 (490.84 N), group 5 (446.175), and group 2 (270.1359), and the least in group 6 (223.443). The mean fracture resistance of group 4 is comparable to intact teeth, i.e., group 1 (1096.40). The mean of all samples was compared using the one-way Anova test, and it was found that there is statistically significant difference in fracture resistance among groups (p < 0.001**). CONCLUSION: Fibers certainly have the reinforcing effect and the position of fibers determines their reinforcing effects. A single central slot with fiber (Ribbond) showed maximum fracture resistance almost equivalent to natural teeth. Modifying conventional beveled cavity design with an additional slot in the center also increases the fractural strength of restoration. CLINICAL SIGNIFICANCE: Nanocomposites reinforced with single fiber in the central palatal slot used for restoring fractured incisors provide strength almost equivalent to natural teeth. In case when the fiber is not available for preparing a single palatal slot also, we can increase the fracture resistance.

Determination of the gas-phase acidities of oligopeptides.

Amino acid residues located at different positions in folded proteins often exhibit different degrees of acidities. For example, a cysteine residue located at or near the N-terminus of a helix is often more acidic than that at or near the C-terminus (1-6). Although extensive experimental studies on the acid-base properties of peptides have been carried out in the condensed phase, in particular in aqueous solutions (6-8), the results are often complicated by solvent effects (7). In fact, most of the active sites in proteins are located near the interior region where solvent effects have been minimized (9,10). In order to understand intrinsic acid-base properties of peptides and proteins, it is important to perform the studies in a solvent-free environment. We present a method to measure the acidities of oligopeptides in the gas-phase. We use a cysteine-containing oligopeptide, Ala3CysNH2 (A3CH), as the model compound. The measurements are based on the well-established extended Cooks kinetic method (Figure 1) (11-16). The experiments are carried out using a triple-quadrupole mass spectrometer interfaced with an electrospray ionization (ESI) ion source (Figure 2). For each peptide sample, several reference acids are selected. The reference acids are structurally similar organic compounds with known gas-phase acidities. A solution of the mixture of the peptide and a reference acid is introduced into the mass spectrometer, and a gas-phase proton-bound anionic cluster of peptide-reference acid is formed. The proton-bound cluster is mass isolated and subsequently fragmented via collision-induced dissociation (CID) experiments. The resulting fragment ion abundances are analyzed using a relationship between the acidities and the cluster ion dissociation kinetics. The gas-phase acidity of the peptide is then obtained by linear regression of the thermo-kinetic plots (17,18). The method can be applied to a variety of molecular systems, including organic compounds, amino acids and their derivatives, oligonucleotides, and oligopeptides. By comparing the gas-phase acidities measured experimentally with those values calculated for different conformers, conformational effects on the acidities can be evaluated.

Comparative evaluation of residual monomer content and polymerization shrinkage of a packable composite and an ormocer.

AIM: The aim of this study is to evaluate the residual monomer content and polymerization shrinkage of a packable composite (Surefil) and an ormocer (Admira). MATERIALS AND METHODS: The study was conducted in two parts. In Part I, 10 samples of each material were prepared in a standardized split brass mould, using incremental curing technique. The residual monomer content was measured by observing change in weight before and after Soxhlet Extraction procedure. In part II, the volumetric polymerization shrinkage was calculated by measuring the difference in specific gravities of 10 uncured and 10 cured samples of each material using a modified version of ASTM D-792 method. The data obtained was put to statistical analysis using student's 't' test. RESULTS: Part I - The percentage change in weight for Surefil was 0.525% while that for Admira was 0.374%, which was found to be statistically significant. Part II - The volumetric percentage shrinkage for Surefil ranged between 1.04-3.42% and that for Admira between 1.01-2.31%, which was not found to be significant statistically. CONCLUSION: Admira may be considered more biocompatible than Surefil due to the lower residual monomer content in the former; however, both are comparable with regards to their polymerization shrinkage.