Different protocols to produce artificial dentine carious lesions in vitro and in situ: hardness and mineral content correlation

This study compared dentine demineralization induced by in vitro and in situ models, and correlated dentine surface hardness (SH), cross-sectional hardness (CSH) and mineral content by transverse microradiography (TMR). Bovine dentine specimens (n = 15/group) were demineralized in vitro with the following: MC gel (6% carboxymethylcellulose gel and 0.1 m lactic acid, pH 5.0, 14 days); buffer I (0.05 m acetic acid solution with calcium, phosphate and fluoride, pH 4.5, 7 days); buffer II (0.05 m acetic acid solution with calcium and phosphate, pH 5.0, 7 days), and TEMDP (0.05 m lactic acid with calcium, phosphate and tetraethyl methyl diphosphonate, pH 5.0, 7 days). In an in situ study, 11 volunteers wore palatal appliances containing 2 bovine dentine specimens, protected with a plastic mesh to allow biofilm development. The volunteers dripped a 20% sucrose solution on each specimen 4 times a day for 14 days. In vitro and in situ lesions were analyzed using TMR and statistically compared by ANOVA. TMR and CSH/SH were submitted to regression and correlation analysis (p < 0.05). The in situ model produced a deep lesion with a high R value, but with a thin surface layer. Regarding the in vitro models, MC gel produced only a shallow lesion, while buffers I and II as well as TEMDP induced a pronounced subsurface lesion with deep demineralization. The relationship between CSH and TMR was weak and not linear. The artificial dentine carious lesions induced by the different models differed significantly, which in turn might influence further de- and remineralization processes. Hardness analysis should not be interpreted with respect to dentine mineral loss

Effects of active and passive lacebacks on antero-posterior position of maxillary first molars and central incisors

The purpose of this study was to compare the effects of active and passive lacebacks on antero-posterior position of maxillary first molars and central incisors during leveling phase. Twenty-three subjects with Class I and Class II malocclusion were treated with first premolars extraction using preadjusted appliances (MBT 0.022-inch brackets). The leveling phase was performed with stainless steel archwires only. The sample was divided into 2 groups: 14 subjects received active lacebacks (Group 1) and 9 subjects received passive lacebacks (Group 2). Lacebacks were made from 0.008-inch ligature wire. Lateral cephalometric radiographs were taken pre- and post-leveling phase. Student's t-test was applied to determine the differences between pre- and post-leveling mean values and to determine the mean differences between groups. In Group I, the first molars showed a significant mesial movement, whereas no change was observed in Group 2. In both groups, maxillary central incisor crowns moved to lingual side. In conclusion, active laceback produced anchorage loss of maxillary first molars whereas passive laceback did not affect the position of these teeth. Active and passive lacebacks were effective in preventing central incisor proclination.

Bracket/wire play: what to expect from tipping prescription on pre-adjusted appliances

INTRODUCTION: The opinion on the 'straight-wire' concept has been evolving since its origin, characterized by faithful followers or absolute skepticism. Currently, it seems reasonable to state that most professionals have a more realistic and critical viewpoint, with an attitude that reveals Orthodontics' maturity and greater knowledge on the technique. The most relevant criticisms refer to the impossibility of the both the Straight-Wire and the Standard systems to completely express the characteristics related to the brackets due to mechanical deficiencies, such as bracket/wire play. OBJECTIVES: A critical analysis of this relationship, which is unclear due to lack of studies, was the scope of this paper. METHODS: The compensatory treatment of two patients, using Capelozza's individualized brackets, works as the scenery for cephalometric evaluation of changes in incisor inclination produced by different dimensions of leveling archwires. RESULTS: The evaluation of these cases showed that, while the introduction of a 0.019 x 0.025-in stainless steel archwire in a 0.022 x 0.030-in slot did not produce significant changes in incisor inclination, the 0.021 x 0.025-in archwire was capable of changing it, mainly in mandibular incisors, and in the opposite direction to the compensation. CONCLUSION: Considering compensatory treatments, even when using an individualized prescription according to the malocclusion, the bracket/wire play seems to be a positive factor for malocclusion correction, without undesirable movements. Therefore, it seems reasonable to admit that, until a bracket system can have absolute individualization, the use of rectangular wires that still have a certain play with the bracket slot is advisable.

Effects of cryogenic and stress relief treatments on temper carbide precipitation in AISI D2 tool steel

The effects of cryogenic and stress relief treatments on temper carbide precipitation in the cold work tool steel AISI D2 were studied. For the cryogenic treatment the temperature was −196°C and the holding time was 2, 24 or 30 h. The stress relief heat treatment was carried at 130°C/90 min, when applied. All specimens were compared to a standard thermal cycle. Specimens were studied using metallographic characterisation, X-ray diffraction and thermoelectric power measurements. The metallographic characterisation used SEM (scanning electron microscopy) and SEM-FEG (SEM with field emission gun), besides OM (optical microscopy). No variation in the secondary carbides (micrometre sized) precipitation was found. The temper secondary carbides (nanosized) were found to be more finely dispersed in the matrix of the specimens with cryogenic treatment and without stress relief. The refinement of the temper secondary carbides was attributed to a possible in situ carbide precipitation during tempering.

One and two degrees-of-freedom Vortex-Induced Vibration experiments with yawed cylinders

Vortex-Induced Vibration (VIV) experiments were carried out with yawed cylinders. The purpose was to investigate the validity of the Independence Principle (IP) for properly describing the flow characteristics and the dynamics of structures subjected to oblique flow. Five yaw angles in relation to the direction perpendicular to the free stream velocity were tested, namely View the MathML sourceθ=0°,10°,20°,30° and 45°. Both the upstream and downstream orientations were tested. The models were mounted on a leaf spring apparatus that allows experiments with one or two degrees of freedom. The Reynolds numbers based on the component normal to the cylinder axis fell in the interval 3×103and the cylinder aspect ratio was L/D≈13L/D≈13 for all the experiments. Time histories of displacement and hydrodynamic forces were acquired. Considering only the component of the free stream which is normal to the cylinder axis, the results of amplitude and force coefficients agreed reasonably well with the non-yawed ones for yaw angles up to 20° for both one and two degrees-of-freedom experiments. This indicates the validity of the IP for this yaw angle range. For yaw angles larger than 20°, a decrease in the maximum amplitude was observed. The decrease in the oscillation amplitudes was related to a larger modulation in the phase shift between force and displacement. Differences in the results for upstream and downstream were observed and were more evident for the larger yaw angle. These differences can be associated to the asymmetric cylinder end conditions.

First principles calculations for vacancies and antisites in PbSe and PbTe: bulk and nanowire.

The energetic stability and the electronic properties of vacancies (VX) and antisites (XY) in PbSe and PbTe are investigated. PbSe and PbTe are narrow band gap semiconductors and have the potential to be used in infrared detectors, laser, and diodes. They are also of special interest for thermoelectric devices (TE). The calculations are based in the Density Functional Theory (DFT) and the General Gradient Approximation (GGA) for the exchange-correlation term, as implemented in the VASP code. The core and valence electrons are described by the Projected Augmented Wave (PAW) and the Plane Wave (PW) methods, respectively. The defects are studied in the bulk and nanowire (NW) system. Our results show that intrinsec defects (vacancies and antisites) in PbTe have lower formation energies in the NW as compared to the bulk and present a trend in migrate to the surface of the NW. For the PbSe we obtain similar results when compare the formation energy for the bulk and NW. However, the Pb vacancy and the antisites are more stable in the core of the NW. The intrinsec defects are shallow defects for the bulk system. For both PbSe and PbTe VPb is a shallow acceptor defect and VSe and VT e are shallow donor defects for the PbSe and PbTe, respectively. Similar electronic properties are observed for the antisites. For the Pb in the anion site we obtain an n-type semiconductor for both PbSe and PbTe, SeP b is a p-type for the PbSe, and T eP b is a n-type for PbTe. Due the quantum con¯nement effects present in the NW (the band gap open), these defects have different electronic properties for the NW as compared to the bulk. Now these defects give rise to electronic levels in the band gap of the PbTe NW and the VT e present a metallic character. For the PbSe NW a p-type and a n-type semiconductor is obtained for the VP b and P bSe, respectively. On the other hand, deep electronic levels are present in the band gap for the VSe and SePb. These results show that due an enhanced in the electronic density of states (DOS) near the Fermi energy, the defective PbSe and PbTe are candidates for efficient TE devices.