Limitations of Surface Mapping Technology in Accurately Identifying Critical Errors in Dental Students' Crown Preparations.

The aim of this study was to evaluate the effectiveness of the Planmeca Compare software in identifying and quantifying a common critical error in dental students' crown preparations. In 2014-17, a study was conducted at one U.S. dental school that evaluated an ideal crown prep made by a faculty member on a dentoform to modified preps. Two types of preparation errors were created by the addition of flowable composite to the occlusal surface of identical dies of the preparations to represent the underreduction of the distolingual cusp. The error was divided into two classes: the minor class allowed for 1 mm of occlusal clearance, and the major class allowed for no occlusal clearance. The preparations were then digitally evaluated against the ideal preparation using Planmeca Compare. Percent comparison values were obtained from each trial and averaged together. False positives and false negatives were also identified and used to determine the accuracy of the evaluation. Critical errors that did not involve a substantial change in the surface area of the preparation were inconsistently identified. Within the limitations of this study, the authors concluded that the Compare software was unable to consistently identify common critical errors within an acceptable degree of error.

Resistance to abrasion of extrinsic porcelain esthetic characterization techniques.

STATEMENT OF PROBLEM: A novel esthetic porcelain characterization technique involves mixing an appropriate amount of ceramic colorants with clear, low-fusing porcelain (LFP), applying the mixture on the external surfaces, and firing the combined components onto the surface of restorations in a porcelain oven. This method may provide better esthetic qualities and toothbrush abrasion resistance compared to the conventional techniques of applying color-corrective porcelain colorants alone, or applying a clear glaze layer over the colorants. However, there is no scientific literature to support this claim. PURPOSE: This research evaluated toothbrush abrasion resistance of a novel porcelain esthetic characterization technique by subjecting specimens to various durations of simulated toothbrush abrasion. The results were compared to those obtained using the conventional characterization techniques of colorant application only or colorant followed by placement of a clear over-glaze. METHOD AND MATERIALS: Four experimental groups, all of which were a leucite reinforced ceramic of E TC1 (Vita A1) shade, were prepared and fired in a porcelain oven according to the manufacturer's instructions. Group S (stain only) was characterized by application of surface colorants to provide a definitive shade of Vita A3.5. Group GS (glaze over stain) was characterized by application of a layer of glaze over the existing colorant layer as used for Group S. Group SL (stain+LFP) was characterized by application of a mixture of colorants and clear low-fusing add-on porcelain to provide a definitive shade of Vita A3.5. Group C (Control) was used as a control without any surface characterization. The 4 groups were subjected to mechanical toothbrushing using a 1:1 water-to-toothpaste solution for a simulated duration of 32 years of clinical use. The amount of wear was measured at time intervals simulating every 4 years of toothbrushing. These parameters were evaluated longitudinally for all groups as well as compared at similar time points among groups. RESULTS: In this study, the novel external characterization technique (stain+LFP: Group SL) did not significantly enhance the wear resistance against toothbrush abrasion. Instead, the average wear of the applied extrinsic porcelain was 2 to 3 times more than Group S (stain only) and Group GS (glaze over stain). Application of a glaze layer over the colorants (Group GS) showed a significant improvement on wear resistance. Despite its superior physical properties, the leucite reinforced ceramic core (Group C) showed 2 to 4 times more wear when compared with other test groups. CONCLUSION: A conventional external esthetic characterization technique of applying a glaze layer over the colorants (Group GS) significantly enhanced the surface wear resistance to toothbrush abrasion when compared with other techniques involving application of colorants only (Group S) or mixture of colorant and LFP (Group SL). The underlying core ceramic had significantly less wear resistance compared with all externally characterized specimens. The novel esthetic characterization technique showed more wear and less color stability, and is thus not advocated as the "best" method for surface characterization. CLINICAL IMPLICATIONS: Application of a glaze layer provides a more wear-resistant surface from toothbrush abrasion when adjusting or extrinsically characterizing leucite reinforced ceramic restorations. Without the glaze layer, the restoration is subjected to a 2 to 4 times faster rate and amount of wear leading to possible shade mismatch.

Randomized controlled trial demonstrates that exposure to mercury from dental amalgam does not adversely affect neurological development in children.

SUBJECTS: The study subjects were 507 children (228 females and 279 males) from the Casa Pia school system in Lisbon, Portugal; the same cohort of children used in a previous study. The children were 8 to 12 years old at the time of enrollment in the study, which began in January of 1997. To be included in the study, a child must have had at baseline at least one carious lesion in a permanent tooth, no previous exposure to amalgam treatments, urinary mercury level less than 10 mg/L, blood lead level less than 15 mg/dL, IQ greater than or equal to 67 as obtained with the Comprehensive Test of Nonverbal Intelligence, and no interfering health condition. KEY EXPOSURE/STUDY FACTOR: Study subjects were randomly assigned to receive dental treatment with either amalgam or resin-based composite for posterior restorations (and composite for all anterior restorations). By the end of year 1, the participants had a mean of 8.3 surfaces restored with amalgam, and the mean number of surfaces restored varied between 7.7 and 10.7 during the 7 years of follow-up. MAIN OUTCOME MEASURE: The primary outcome measures for the study were the presence of neurological hard signs (NHSs), such as tremor, and the presence and severity of neurological soft signs (NSSs), such as synkinesias. NHSs were scored as present or absent. Tremor was reported separately from other NHSs because of its relationship to mercury toxicity. Observation of NSSs did not begin until follow-up year 2, so there were no baseline data for NSSs. NSSs were evaluated using the examination described by Peters et al, and each item was scored from 0 (absent) to 3 (maximum deviation). MAIN RESULTS: Over the 7-year duration of the study, there were slight differences in the percentages of subjects exhibiting any NHSs (including tremor, which was reported separately) between the 2 treatment groups, but the directions of the differences were not consistent from year to year, and the differences were not statistically significant in any year. No statistically significant difference in NSSs--either percentage of subjects exhibiting NSSs or NSS severity scores--was observed between the 2 groups in any year. CONCLUSIONS: The results of this study demonstrate that exposure to mercury from dental amalgam does not adversely affect neurological development in children.

Isothermal phase transformations of a dental porcelain.

OBJECTIVES: The purpose of this investigation was to determine if the change in the leucite weight fraction during an isothermal heat treatment could be estimated by observing the deformation of PFM strips in a high-heating-rate, computer-controlled bending beam viscometer (BBV). METHODS: Specimens of a commercial body porcelain were fired according to the manufacturer's instructions-50 disk specimens for quantitative X-ray diffraction (XRD) and 100 bimaterial strip specimens for BBV. The XRD specimens were annealed at temperatures between 650 and 1000 degrees C, and leucite weight fraction was measured using an alumina internal standard. The BBV specimens were annealed in the BBV using time-temperature schedules designed to elucidate the leucite crystallization behavior between 700 and 1000 degrees C. Timoshenko's equation for a bimaterial thermostat was used to estimate the change in the thermal expansion of the porcelain near room temperature. Changes in leucite weight fraction were determined from these thermal expansion changes. RESULTS: The means and SDs were compared to values obtained by quantitative XRD. Good agreement was obtained between values of leucite weight fraction derived from beam deformation and those determined by quantitative XRD (p> or =0.45). The anneal sequence showed that the increase in leucite weight fraction at 800 or 900 degrees C is reversible by an anneal at 1000 degrees C. SIGNIFICANCE: The BBV technique yields comparable results to quantitative XRD and provides the opportunity to efficiently monitor porcelain leucite changes nondestructively over multiple heat treatments. This technique could prove useful for testing firing schedules designed to stabilize the leucite content in dental porcelain.

Are there acceptable alternatives to amalgam?

Amalgam has been the material of choice for restoring posterior teeth for more than 100 years. The past 25 years have witnessed significant advances in restorative materials themselves and in the bonding systems for retaining a restoration in the prepared tooth. As a result, there has been a shift toward resin composite materials during this same period because of concerns about the esthetics and biocompatibility of dental amalgam. In addition, other materials such as glass ionomer cements, ceramic inlays and onlays, and gold alloys have been used as alternatives to amalgam. This article will review recent studies on the longevity and biocompatibility of these alternatives to dental amalgam.

Effects of local cooling rate and processing variables on leucite in dental porcelain.

PURPOSE: This research determined whether there is a measurable effect of local geometry factors on leucite content of dental porcelain in fixed partial dentures (FPD). MATERIALS AND METHODS: Four-unit FPD frameworks (n = 36) were fabricated using a nickel-chromium alloy (Rexillium III). Body porcelain (Crystar, shade A2) was applied in one increment and subjected to two simulated body firings, followed by a simulated glaze firing to achieve a thickness of 1.5 to 2.0 mm. The completed FPD specimens were randomly assigned to three groups of 12 specimens each: (1) simulated post-soldering, (2) multiple firing, and (3) control. The FPDs from each test group were sectioned into individual units: canine retainer, premolar pontic, molar pontic, and molar retainer. The porcelain was removed from each unit, and the leucite content was measured via quantitative x-ray diffraction. RESULTS: Porcelain cracking indicated that the soldering simulation had successfully reproduced conditions in the dental laboratory that result in porcelain cracking during soldering. The leucite content was not significantly different between the retainer and pontic units for either the soldering simulation or control FPDs, although the canine retainer units did have a slightly lower leucite content than the pooled values of the other units. Comparison of the pooled data for the three groups indicated statistically significant differences among the leucite contents. CONCLUSION: Compared to the control, the simulated post-soldering procedure produced a significant increase in leucite, and the multiple firing group exhibited a significant decrease in leucite. Increases in leucite weight fraction during post-soldering operations-and the larger thermally induced stresses that accompany these leucite increases-are responsible for the cracking that occurs.