A clinical technique for virtual articulator mounting with natural head position by using calibrated stereophotogrammetry.

Accurate articulator-mounted casts are essential for occlusion analysis and for fabrication of dental prostheses. Although the axis orbital plane has been commonly used as the reference horizontal plane, some clinicians prefer to register the horizontal plane with a spirit level when the patient is in the natural head position (NHP) to avoid anatomic landmark variations. This article presents a digital workflow for registering the patient's horizontal plane in NHP on a virtual articulator. An orientation reference board is used to calibrate a stereophotogrammetry device and a 3-dimensional facial photograph with the patient in NHP. The horizontal plane can then be automatically registered to the patient's virtual model and aligned to the virtual articulator at the transverse horizontal axis level. This technique showed good repeatability with positional differences of less than 1 degree and 1 mm in 5 repeated measurements in 1 patient.

A 2-part facebow for CAD-CAM dentistry.

Converting the patient's clinical information into the virtual world is a prerequisite for the computer-aided design and computer-aided manufacturing (CAD-CAM) of dental restorations. This article describes a modified facebow which facilitates the radiation-free registration of the maxillary teeth to a 3-dimensional face image for transfer to a virtual articulator. This facebow can be easily fabricated with minimal materials and adjusted to fit different patients; its error in tooth registration was demonstrated to be less than 1 mm.

The adverse effects of tungsten carbide grinding on the strength of dental zirconia.

OBJECTIVES: This study investigated the effects of tungsten-carbide grinding on the surface characteristics and mechanical strength of dental 3 mol% yttria-stabilized zirconia (3Y-TZP). METHODS: Two types of tungsten-carbide burs (TC), 6-blade (TC1) and 8-blade (TC2) were used to grind 3Y-TZP, in a dental air-turbine handpiece with water-cooling and were also subjected to air-particle abrasion (APA): TC1 + APA and TC2 + APA; and rubber polishing (RP): TC1 + RP and TC2 + RP; one group received only rubber-polishing RP. The control group received no treatment. Surface characterization was examined by surface roughness (Ra) and atomic force microscopy. Specimens were also observed with scanning electron microscopy (SEM) and X-ray-diffraction (XRD) for microstructure and crystalline phases. A piston-on-three-balls biaxial-flexural strength (BFS) test was performed with 15 samples-per-group and the broken specimen were observed under SEM to investigate the fracture origin pattern. One-way ANOVA, Kruskal-Wallis test and Weibull analysis were performed at α = 0.05. RESULTS: Groups TC1 and TC2 had the lowest mean BFS (p < 0.05) with up to 74 % reduction in strength. APA and RP both significantly increased the mean BFS after tungsten-carbide grinding but was still less than the control (p < 0.05). Compared to the control, the mean BFS was significantly reduced for all groups except for the RP group (p < 0.05). APA and rubber-polishing following TC2 grinding had significant higher mean BFS than those following TC1 grinding respectively (p < 0.05). SEM revealed distinct micro-cracks after tungsten-carbide grinding. SIGNIFICANCE: Tungsten-carbide burs (6- and 8-blade) are not recommended for zirconia grinding due to the significant reduction of biaxial-flexural strength and observed micro-structural surface and subsurface damage.

Effect of burn-out temperature on magnetic keepers in cast noble metal alloy posts.

STATEMENT OF PROBLEM: Early loss of magnetic keepers in cast gold posts is seen in clinical practice. PURPOSE: The purpose of this vitro study was to investigate the effect of burn-out temperature on the retention of magnetic keepers in cast gold alloy and on the thickness of the oxide layer at the keeper-alloy interface. MATERIAL AND METHODS: One hundred and five keepers (Magfit EX) were cast with gold alloy (Protor 3) at 3 different burn-out temperatures: 500 degrees C, 600 degrees C, or 700 degrees C (n=35). To test for retention of keepers, 60 specimens (n=20) were tested to failure in tension mode in a universal testing machine (UTM); 45 specimens (n=15) were sectioned, and the interface was evaluated under a scanning electron microscope (SEM). Tensile force data were analyzed with 1-way ANOVA, and SEM data were analyzed with 1-way repeated measures ANOVA. Bonferroni multiple comparisons were performed for post hoc analysis (alpha=.05). RESULTS: Retention of the keepers was significantly higher at lower burn-out temperatures (P<.001). The retention of the keepers became unpredictable and immediate failures were observed at a burn-out temperature of 700 degrees C. Oxide formation at the keeper-alloy interface was significantly less at lower burn-out temperatures (P<.001). CONCLUSIONS: A burn-out temperature of 500 degrees C for casting the Magfit EX keeper with type IV high-strength gold alloy is recommended.

Precision surgical template for implant placement: a new systematic approach.

The importance of a precise surgical template for implant placement cannot be overstated. The radiographic template carries both clinical and radiographic information for the planning of fixture angulation and location. This article describes a systematic approach to the fabrication of a dual-purpose radiographic surgical template. The simple steps result in the accurate transfer of radiographic information to the surgical template with no need for complex equipment or maneuvers. key words: dental implants, implant placement, radiographic template, surgical template

A quality assessment of the casting process on magnetic keepers.

PURPOSE: The objective of this study was to qualitatively investigate the effect of the burn-out (mold) temperature, investment material, and casting alloy on the surface integrity of the Magfit EX keeper. MATERIALS AND METHODS: Forty-two Magfit EX keepers were waxed-up, invested in five investment materials (Beauty-Cast, Cristobalite, CM-10, Cera-Fina, Castorit-super), and subjected to burn-out temperatures ranging from 450 to 700 degrees C at intervals of 50 degrees C. The keeper samples were then cast into copings with three alloys (Castwell, Protor 3, Optimum) under standard conditions. The keeper surfaces were then examined under a microscope, and the compositions were assessed by an X-ray micro-analyzer in a scanning electron microscope (SEM). A new keeper served as control. RESULTS: At a burn-out temperature of 550 degrees C, the keeper surface started to disintegrate. X-ray micro-analysis showed an increase in oxygen content with increasing temperature. At 700 degrees C, the keeper surface disintegrated, and the composition differed markedly from that of the new keeper. The keeper surfaces were intact with all investments except those with Beauty-Cast. The keeper surfaces were found to be damaged when the casting alloy was Optimum. CONCLUSIONS: Beauty-Cast investment with a burn-out temperature of 700 degrees C is unsuitable for casting the Magfit EX keeper-coping unit. Also, high fusing alloys are not recommended for casting Magfit EX keepers.

Comparison of the retentive characteristics of cobalt-chromium and commercially pure titanium clasps using a novel method.

This study aimed to compare the retentive forces of cast cobalt-chromium (Co-Cr) and commercially pure titanium (cpTi) clasps. A clasp assembly comprising a pair of symmetrical clasps was made to fit the opposite halves of a hardened stainless-steel sphere. This twin clasp was designed to counterbalance the tipping forces when the clasp assembly was drawn from the sphere. A total of 120 clasp assemblies were fabricated in cast Co-Cr and cpTi and placed at undercut depths of 0.25 mm, 0.50 mm, and 0.75 mm (n = 20 for each). For Co-Cr clasps, the retentive forces at these undercuts depths were 2.34 +/- 0.23 N, 4.65 +/- 0.35 N, and 7.56 +/- 0.50 N, respectively. The corresponding retentive forces for cpTi clasps were 1.24 +/- 0.13 N, 2.34 +/- 0.23 N, and 3.70 +/- 0.27 N. The retentive force of cpTi clasps was approximately half that of Co-Cr clasps for the same undercut depth.

A preliminary report on patient acceptance of a novel intra-oral lubricating device for the management of radiotherapy-related xerostomia.

Management of radiotherapy-related xerostomia is difficult. Saliva substitutes are helpful but the effects are short-lived. The purpose of the study was to develop a prototype intra-oral lubricating device for the management of radiotherapy-related xerostomia and to evaluate patient acceptance. An intra-oral lubricating device was fabricated that incorporated a reservoir in the palatal vault and permitted slow release of saliva substitute by the patient. Preliminary clinical testing was done in five patients with radiotherapy-related xerostomia. A measure incorporating seven questions was used to explore patient acceptance. The device was simple to fabricate using materials available in a technical laboratory. All patients were able to wear the device for at least 4 h per day throughout the test period. The device was considered easy to use and clean. Some impairment of speech and chewing was noted although this appeared to be related to the bulkiness of the reservoir. General oral comfort was improved due to the lubricating effect. The bulk of the reservoir was reduced as a consequence of patient feedback. The design addressed key problems associated with previous lubricating systems. Patient reports on oral functioning with the device in situ provided pivotal information on the device's utility.

A double casting technique to minimize distortion when constructing fixed partial dentures on implants.

The casting of extensive implant superstructures that have a good passive fit may be technically demanding. A simple, 2-stage casting technique is presented that avoids common problems associated with the casting of large superstructures. The fabrication of a 7-unit metal-ceramic screw-retained fixed partial denture supported by 5 implants is used to illustrate this technique.

Threshold contrast ratio and masking ability of porcelain veneers with high-density alumina cores.

PURPOSE: The purposes of this study were to: (1) investigate the correlation between the color difference of bilayer porcelain veneers over white and black backgrounds (deltaE1) and their opacity (contrast ratios); (2) determine whether there is a recommendable threshold contrast ratio above which the color difference is clinically acceptable (when deltaE < or = 5); and (3) compare the ability of porcelain veneers to mask a color change from white to black backgrounds (deltaE1) and their ability to mask a color change from white to clinically discolored teeth (deltaE2). MATERIALS AND METHODS: Forty-four maxillary anterior teeth of eight patients with severe tetracycline discoloration were prepared for bilayer porcelain veneers in shade A2 porcelain. The cores were 0.25 mm thick. The color (CIE L*a*b*) and reflectance (Y) of the midbuccal region of each veneer were measured over white and black backgrounds using a colorimeter under artificial daylight. The veneers were bonded to discolored teeth, and their color was measured after 1 week. RESULTS: The mean color difference deltaE1 was 10.6 (SD 2.6). The mean contrast ratio was 0.75 (SD 0.1). There was a close and statistically significant correlation between deltaE1 and contrast ratio. The determined threshold contrast ratio was 0.91. The mean color difference deltaE2 was 11.6 (SD 5.5). A paired t test showed no difference between deltaE1 and deltaE2. CONCLUSION: There was a significant correlation between the masking ability of veneers (deltaE1) and their opacity (contrast ratio). There was no significant difference in the ability of the porcelain veneers in masking a color change from white to black backgrounds compared to their ability to mask the color change from white to the discolored teeth.