3,974 research outputs found
Using Margin Elevation with Bonded Ceramics: A Case Report.
PosterThirty years ago, glass ionomer was first used as a means of bonding resin matrix composite to dentin. Today this method is used to elevate the margin of a preparation to a level which gives the clinician more access to the operating field. This technique has been described in the dental literature with resin composites bonded with resin adhesives. There are still inherent problems with this approach, however, since resin adhesives are subject to hydrolysis, marginal leakage, and recurrent caries. Studies have demonstrated the ability of glass ionomer to chemically bond to dentin; glass ionomer can also be dissolved/etched by phosphoric acid and predictably bonded to resin composites, eliminating the problem of hybrid layer hydrolysis which occurs with resin bonding agents. Margin elevation takes advantage of the favorable properties of glass ionomer cements (adhesion through chemical bond to dentin, fluoride release, biocompatibility, coefficient of thermal expansion similar to tooth structure, and decreased interfacial bacteria penetration/caries activity) while allowing overlaying of a suitable direct or indirect restorative material. This technique should be utilized when a preparation stands an increased risk of contamination or has a gingival margin on dentin/cementum. This case describes restoration of a tooth with a deep subgingival margin located on cervical dentin. The tooth was prepared for a ceramic onlay. Resin-modified glass ionomer was then inserted into the mesial proximal box and re-prepared with the occlusal wall of the glass ionomer becoming the new gingival margin, allowing significantly increased access and isolation. The tooth was then restored with an e.max onlay and cemented with RelyX Unicem. The restoration has been examined at a 6-month recall. With proper case selection and attention to detail, glass ionomer margin elevation is an excellent technique for bonding ceramics to teeth which cannot be isolated adequately for impression and/or resin bonding
Resonant Absorption in the AGN spectra emerging from photoionized gas: differences between steep and flat ionizing continua
We present photoionization models accounting for both photoelectric and
resonant absorption. Resonance absorption lines from C, O, Ne, Mg, Si S and Fe
between 0.1 and 10 keV are treated. In particular we consider the complex of
almost 60 strong Fe L absorption lines around 1 keV. We calculate profiles,
intensities and equivalent widths of each line, considering both Doppler and
natural broadening mechanisms. Doppler broadening includes a term accounting
for turbulence of the gas along the line of sight. We computed spectra
transmitted by gas illuminated by drastically different ionizing continua and
compared them to spectra observed in flat X-ray spectrum, broad optical
emission line type 1 AGN, and steep X-ray spectrum, narrow optical emission
line type 1 AGN. We show that the keV absorption feature observed in
moderate resolution X-ray spectra of several Narrow Line Seyfert 1 galaxies can
be explained by photoionization models, taking into account for resonance
absorption, without requiring relativistic outflowing velocities of the gas, if
the physical properties of these absorbers are close to those found in flat
X-ray spectrum Seyfert 1 galaxies.Comment: 22 pages, 10 figures. Accepted for publication on Ap
Time-reversible Born-Oppenheimer molecular dynamics
We present a time-reversible Born-Oppenheimer molecular dynamics scheme,
based on self-consistent Hartree-Fock or density functional theory, where both
the nuclear and the electronic degrees of freedom are propagated in time. We
show how a time-reversible adiabatic propagation of the electronic degrees of
freedom is possible despite the non-linearity and incompleteness of the
self-consistent field procedure. Time-reversal symmetry excludes a systematic
long-term energy drift for a microcanonical ensemble and the number of
self-consistency cycles can be kept low (often only 2-4 cycles per nuclear time
step) thanks to a good initial guess given by the adiabatic propagation of the
electronic degrees of freedom. The time-reversible Born-Oppenheimer molecular
dynamics scheme therefore combines a low computational cost with a physically
correct time-reversible representation of the dynamics, which preserves a
detailed balance between propagation forwards and backwards in time.Comment: 4 pages, 4 figure
- …