Effect of minor amendments of patient's position on the accuracy of linear measurements yielded from cone beam computed tomography

Statement of Problem: Image distortion in intra and extra-oral radiographs are an unavoidable phenomenon. Patient's positional changes from the routine alignments is an important issue for this unwanted alteration, Therefore the accuracy of the dimensional measurements will be affected

Purpose: Our purpose is to find out the effect of minor changes [possibly happening in the clinic] in the position of a human dry skull on the accuracy of the measurements acquired by Cone Beam Computed Tomography [CBCT]

Materials and Method: In this study, 3 locations on the skull were pointed with radioopaque markers. Imaging process with Kodak 9000 CBCT was performed in standard and 10 analytically miss- oriented positions. Then 2 distances were measured between the centers of markers. Later, these measurements were compared with the standard position values

Results: There was not any imperative difference in the measurements of the 10 altered positions yielded in this study with standard position values

Conclusion: According to our results, apparently, the accuracy of linear measurements in CBCT images is endorsed by unintentional small changes in the patient's position during the projections

[Effect of sodium ascorbate and calcium hydroxide on shear bond strength of composite resin to bleached enamel]

Application of sodium ascorbate as an antioxidant and calcium hydroxide as a buffering agent has been recommended after bleaching .The aim of this study was to investigate the effect of these two materials on shear bond strength of composite resin attached to bleached enamel. This in-vitro investigation used 60 human sound premolars. They were randomly divided into five groups [n=12]. Buccal enamel surfaces were flattened by using a trimmer machine and silicon carbide paper. The samples in negative control [NC] group were not bleached, while all others were bleached with 35% hydrogen peroxide gel for 5 days. Composite cylinders were bonded in positive control [PC] group immediately after bleaching; in delayed bonding [DB] group after one week; in sodium ascorbate [SA] and calcium hydroxide [CH] groups after 40 hours of treatment with these two materials, respectively. The specimens were thermocycled [5-55°C, 500 cycles] 24 hours after bonding and they were subjected to shear bond testing by using universal machine. Statistical analyses were carried out to identify group differences and Kruskal-Wallis and Mann-Whitney tests were used to detect the significance level. Group differences revealed that, the lowst mean of bond strength belonged to the calcium hydroxide group the difference of which was significant compared with the other groups [P<0.006]. The positive control group had lower bond strength in comparison with the negative control, sodium ascorbate and delay bonding groups [p<0/001] .But the difference among other groups was not significant. Both one week delay and using sodium ascorbate treatment could significantly increase the bond strength of composite resin to bleached enamel, while the use of calcium hydroxide had an adverse effect on composite resin bond strength

Efficacy of QTH and LED light sources, in polymerization of dual cure resin cement beneath a simulated ceramic onlay

The dual cured resin cements are conventionally used for cementing indirect restorations, such as composite and ceramic inlays and onlays. The aim of this investigation was to evaluate the efficacy of two light-curing units, LED and QTH in polymerization of dual cured resin cement that were placed beneath the ceramic onlays. This experimental study was conducted using 20 specimens. Ten specimens were cured with QTH and the rest with LED light curing devises. Metal rings with 5mm in diameter and 0.5mm in thickness were placed next to the surface of the curved dentin of the molar teeth that were mounte4 in self cured acrylic resin. A Mylar strip was placed beneath the metal rings and they were filled with Maxcem dual cured resin cement; they were then covered with another Mylar strip. Next, the ceramic onlays with 2mm thickness were placed over the Mylar strips. The specimens were stored in a dry and dark incubator at 37°C for 24 hours, after cemented onlays were cured for 60 seconds. The Vickers hardness was determined for each sample and data were analyzed by using student T-test and SPSS statistical software. The mean Vickers hardness for QTH cured specimens was [24.75 +/- 3.59], and this value was [33.92 +/- 2.28] for the LED light cured specimens. The mean group difference was statistically significant [P < 0.001. Based on these findings, the efficacy of LED light curing unit was significantly more efficient in polymerization of resin cements that were placed underneath the ceramic onlays compared with QTH light-curing unit