Metameric effect between natural teeth and resin teeth of A2 shade / 华西口腔医学杂志

<p><b>OBJECTIVE</b>The aim of this study was to determine the metameric color differences between natural teeth and three brands of commercially available resin teeth.</p><p><b>METHODS</b>The spectral reflectance and color coordinates of natural teeth and three brands of commercially available resin teeth of A2 shade were measured with a spectrophotometer (PR-650) according to the CIE L*, a*, b* and CIE XYZ color scale relative to illuminant D65, A, cool white fluorescent (CWF) and ultraviolet (UV), and the metameric indices were calculated to determine the metameric color differences between natural teeth and resin teeth.</p><p><b>RESULTS</b>CIE L*, a*, b* values were influenced by the type of illuminants in both natural teeth and resin teeth. The pattern of spectral reflectance curves for natural teeth and resin teeth of A2 shade were different, while there were more than three crossing points among each curves, which meant the color of natural teeth and resin teeth of A2 shade might be the same under certain illuminant. The metameric indices between natural teeth and resin teeth of A2 shade were 3.48, 2.52 and 3.36 under illuminant A; 1.21, 1.90, and 2.79 under illuminant CWF; 1.59, 2.07, and 4.07 under illuminant UV. The metameric indices between resin teeth of different brand were 1.08, 0.10, and 1.01 under illuminant A; 1.46, 2.23, and 0.94 under illuminant CWF; and 2.55, 2.69, and 4.64 under UV.</p><p><b>CONCLUSION</b>Changes in optical properties of resin teeth of A2 shade relative to the different illuminants were different from those of natural teeth, the metameric effect between natural teeth and resin teeth of A2 shade were significant. Therefore, shade matching between natural teeth and resin teeth should be performed under more than one illuminant.</p>

Comparison of casting precision of two kinds of commercially available gypsum-bonded investment with different setting and thermal expansion characteristics / 华西口腔医学杂志

<p><b>OBJECTIVE</b>To investigate expansion characteristics of two kinds of commercial investment (Cristo balite Micro II and Cristo Quick), and their effects on cast accuracy of a dental Ag-Pd casting alloy.</p><p><b>METHODS</b>Two kinds of investment were mixed at the ratio of 1:3. The setting and thermal expansion curves of the investment were recorded according to the requirements of ISO7490 and expansion rate calculated, respectively. Plate-like wax-patterns with pre-marked points were invested in casting rings (lined with ceramic fiber liners with thickness of 0.45 mm). Castwell M.C dental alloy were melted with gas flame and cast with a centrifugal casting machine. The differences between the castings and the wax patterns at corresponding points were calculated to represent the dimensional changes of the investment.</p><p><b>RESULTS</b>The setting, thermal and total expansion rates were 0.68, 1.18, 1.86 for Cristo balite Micro II, and 0.94, 1.03, 1.97 for Cristo Quick. The setting and thermal expansion rates were statistically significant between two kinds of investment (P < 0.05), while not for the total expansion rates (P > 0.05). Effect of different investment on the precision of castings was not statistically significant (P > 0.05).</p><p><b>CONCLUSION</b>Within the scope of this study, gypsum-bonded investment with similar total expansion rates but different expansion characteristics has no effect on the casting precision of Ag-Pd dental alloys.</p>

The expansion characters of a quick-heating gypsum-bonded investment under different heating methods / 华西口腔医学杂志

<p><b>OBJECTIVE</b>To study the effects of different heating timings after mixing the materials and different heating methods on the expansion characters of a quick-heating gypsum-bonded investment.</p><p><b>METHODS</b>The setting expansion rate of GC Cristoquick II gypsum-bonded investment was measured with a setting expansion tester 30, 60, 120 min after mixing the material respectively. The thermal expansion rates of the material at different setting time (30, 60, 120 min after mixing) and under different heating treatment methods (quick and conventional heating techniques) were also determined using a thermal dilatometer for dental investments, the total linear expansion rate were calculated. The effects of heating time and heating methods on the expansion of the investments were statistically analyzed with SPSS 11.0 software, using ANOVA multiple comparison (alpha=0.05).</p><p><b>RESULTS</b>Statistical differences were found among the setting and thermal expansion rates of the investment at different heating timings after mixing the material (P<0.01). Setting expansion, thermal expansion and total expansion rates increased with the setting time before heat treatment, while thermal expansion rates under different (quick and conventional) heating methods were not statistically significant (P>0.05).</p><p><b>CONCLUSION</b>The expansion characters of the quick-heating gypsum-bonded investment are influenced by different heating timings after mixing the material. The heat treatment technique of this quick-heating investment recommended needs to be modified to ensure casting precision.</p>

Wear behavior of enamel and veneering ceramics / 华西口腔医学杂志

<p><b>OBJECTIVE</b>To compare the wear between the enamel and two types of dental decoration porcelains for all-ceramic restorations (Vita-alpha, Vintage AL).</p><p><b>METHODS</b>Friction coefficients, wear scar width, element concentrations and wear surface evolution were considered relatively to the tribology of that in vivo situation. The wear scars of the samples were characterized by means of dynamic atomic force microscopy (DFM). The different element concentrations of the surface before/after the wear test were determined with energy dispersion spectrometry (EDS).</p><p><b>RESULTS</b>The friction coefficient varied from time in each kind of material. The statistical differences between materials were observed in wear scar width and properties of materials (P<0.05). DFM results showed wear surface of natural tooth full of abrasive particles and denaturation of dental texture. Wear surface of veneering ceramics consisted mainly of abrasive particles, plough and microcracking. EDS results showed that the element concentration of Fe was obviously found on the samples after wear.</p><p><b>CONCLUSION</b>The main underlying mechanisms of natural teeth wear are abrasive, and denaturation of dental texture. Abrasive wear, adhesion and fatigue of veneering ceramics characterize the wear patterns which plays different role in Vita-alpha and Vintage AL. The wear patterns of veneering ceramics can be described as mild wear.</p>