Marginal and Internal Fit of CAD/CAM Crowns Fabricated Over Reverse Tapered Preparations.

PURPOSE: Intraoral digital scanning can accurately record single abutment tooth preparations despite their geometry, and the algorithms of the CAD software can be set to manage different abutment forms. The purpose of this in vitro study was to evaluate the marginal and internal fit of CAD/CAM zirconia crowns fabricated over conventional and reverse-tapered preparations. MATERIALS AND METHODS: Crown preparations with known total occlusal convergence (TOC) angles (-8°, -4°, 0°, 8°, 12°, 16°, and 22°) were digitally created from a maxillary left central incisor and printed in acrylic resin. Next, casts were scanned with a TRIOS intraoral scanner, and crowns were designed with KaVo multiCAD software using default parameters (50 µm cement space) on abutments with positive TOC angles, whereas reverse-tapered abutments (negative TOC angles) were digitally blocked out at 0° and had an extra mesiodistal gap set to 50 µm. Then, zirconia crowns were fabricated, and their marginal and internal discrepancies were recorded with the silicone replica technique. All replicas were examined under a stereomicroscope at 50× magnification. Collected data were analyzed with one-way ANOVA and post hoc Tukey test for marginal fit. For the axial and incisal fit, measured values did not follow a normal distribution; therefore, the Kruskal-Wallis and the Dunn/Bonferroni multiple comparison tests were applied (p = 0.05). RESULTS: The mean marginal fit of -8° crowns (58.2 ± 6.0 µm) was statistically different (p < 0.0001) from all the remaining crowns (range 42.1-47.3 µm). Also, the internal fit was statistically significant when comparing crowns fabricated over abutments with positive and negative TOC angles (p < 0.0001). The largest median axial discrepancies were found in the -8° (165.5 µm) and -4° (130.8 µm) groups; however, when evaluating the incisal fit, they showed the smallest discrepancies (67.3 and 81.8 µm, respectively). CONCLUSIONS: Under the conditions of this study, the marginal and internal fit of zirconia crowns fabricated over inverse-tapered preparations is within clinically accepted values.

Influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions.

STATEMENT OF PROBLEM: Direct (intraoral) and indirect (desktop) digital scanning can record abutment tooth preparations despite their geometry. However, little peer-reviewed information is available regarding the influence of abutment tooth geometry on the accuracy of digital methods of obtaining dental impressions. PURPOSE: The purpose of this in vitro study was to evaluate the influence of abutment tooth geometry on the accuracy of conventional and digital methods of obtaining dental impressions in terms of trueness and precision. MATERIAL AND METHODS: Crown preparations with known total occlusal convergence (TOC) angles (-8, -6, -4, 0, 4, 8, 12, 16, and 22 degrees) were digitally created from a maxillary left central incisor and printed in acrylic resin. Each of these 9 reference models was scanned with a highly accurate reference scanner and saved in standard tessellation language (STL) format. Then, 5 conventional polyvinyl siloxane (PVS) impressions were made from each reference model, which was poured with Type IV dental stone scanned using both the reference scanner (group PVS) and the desktop scanner and exported as STL files. Additionally, direct digital impressions (intraoral group) of the reference models were made, and the STL files were exported. The STL files from the impressions obtained were compared with the original geometry of the reference model (trueness) and within each test group (precision). Data were analyzed using 2-way ANOVA with the post hoc least significant difference test (α=.05). RESULTS: Overall trueness values were 19.1 µm (intraoral scanner group), 23.5 µm (desktop group), and 26.2 µm (PVS group), whereas overall precision values were 11.9 µm (intraoral), 18.0 µm (PVS), and 20.7 µm (desktop). Simple main effects analysis showed that impressions made with the intraoral scanner were significantly more accurate than those of the PVS and desktop groups when the TOC angle was less than 8 degrees (P<.05). Also, a statistically significant interaction was found between the effects of the type of impression and the TOC angle on the precision of single-tooth dental impressions (F=2.43, P=.002). Visual analysis revealed that the intraoral scanner group showed a homogeneous deviation pattern across all TOC angles tested, whereas scans from the PVS and desktop scanner groups showed marked local deviations when undercuts (negative angles) were present. CONCLUSIONS: Conventional dental impressions alone or those further digitized with an extraoral digital scanner cannot reliably reproduce abutment tooth preparations when the TOC angle is close to 0 degrees. In contrast, digital impressions made with intraoral scanning can accurately record abutment tooth preparations independently of their geometry.

Fracture strength and behavior of resin-faced CAD/CAM anterior crowns.

The fracture strength and behavior of a novel resin-faced computer-aided design/computer-aided manufacturing (CAD/CAM) crown were investigated to evaluate application to the anterior teeth. Resin-faced CAD/CAM crowns were fabricated by arranging a resin composite on a frame prepared from a CAD/CAM resin block. The fracture strength was evaluated after 24 h of complete polymerization (day 0) and after water immersion for 30 days (day 30). Uniaxial loading was applied to the center point between the incisal edge and cingulum (loading point 1) or at 1.5 mm from the incisal edge (loading point 2). There was no significant difference in the fracture strength of the resin-faced CAD/CAM crowns between day 0 and 30 at loading point 1. At loading point 2, they exhibited decreased fracture strength after water immersion; however, the mean strength was still >1 kN. This novel crown showed good mechanical properties to serve as a prosthesis for the anterior teeth.

Fluorescence and physical properties of thulium and erbium co-doped dental zirconia.

The fluorescence and physical properties of thulium and erbium co-doped dental zirconia were investigated. The high-translucency yttria-stabilized dental zirconia specimens co-doped with Tm2O3 powder 0.8 wt% and Er2O3 powder at proportions from 0.1 to 0.8 wt% were used. The specimens co-doped with Tm2O3 powder 0.8 wt% and Er2O3 from 0.3 to 0.5 wt% exhibited the fluorescence similar to that of natural tooth. All the specimens had a tetragonal peak and no major change in the Vickers hardness, fracture toughness and bending strength on addition of Tm2O3 and Er2O3. These results suggest that the method of co-doping trace amounts of Tm2O3 and Er2O3 into high-translucency dental zirconia powder can effectively improve the esthetics of zirconia monolithic fixed dental prothesis.

In vitro investigation of fracture load and aging resistance of high-speed sintered monolithic tooth-borne zirconia crowns.

OBJECTIVES: The purpose of this study was to evaluate the fit, fracture load and aging resistance of the monolithic zirconia tooth-borne crowns with conventional and high-speed sintering. METHODS: The Y-TZP block was machined and sintered with conventional and high-speed sintering furnace. The marginal and internal gap between the crown and abutment was measured using a microscope and a fit checking material. A total of 28 crowns were further divided into an undegraded and a degraded group. An accelerated aging test was carried out on the degraded group. The crown was cemented and a fracture resistance was tested. X-ray diffraction was used to evaluate the crystalline structure. The data were analyzed with Student's t-test, and a one-way ANOVA and Tukey's multiple comparison test. RESULTS: There was no significant difference in mean marginal gap between the two groups. The mean internal gap was significantly greater in the speed sintering than in the conventional sintering (P <0.001). The mean fracture load of the conventional sintering crowns was not significantly different from that of speed sintering crowns after aging. The occurrence of monoclinic crystals of degraded crown was significantly higher than that of undegraded crown both in the conventional (P <0.001) and speed-sintering group (P <0.001). CONCLUSIONS: It was concluded that the monolithic zirconia crowns produced by high-speed sintering showed no significant difference in the marginal gap and the fracture load after aging compared to conventional sintering. Therefore, the high-speed sintering seems a valid method of producing tooth-borne monolithic zirconia crowns.

CAD/CAM fabrication and clinical application of surgical template and bone model in oral implant surgery.

OBJECTIVES: A novel implant surgery support system with computer simulation for implant insertion and fabrication of a surgical template that helps in drilling bone was developed. A virtual reality haptic device that gives the sense of touch was used for simulation and a surgical template was fabricated by CAD/CAM method. Surgical guides were applied for two clinical cases. MATERIAL AND METHODS: Three-dimensional (3D) jaw bone images transferred from DICOM data filmed by CT scanner were fed to the software and manipulated using the haptic device. The site for implant insertion was determined after evaluating the quality of bone and position of the mandibular canal. The surgical template was designed with ease using the free design CAD function of haptic device. The surgical template and bone model were fabricated by a fused deposit modeling machine. Two clinical cases were applied using the present system. RESULTS: Simulation to determine the site of implant insertion and fabrication of the surgical bone templates were successfully done in two clinical cases, one for three implant insertion in lower right jaw and the other is for seven implant insertion in lower edentulous jaw, respectively. During surgery, the templates could be firmly adapted on the bone and drilling was successfully performed in both cases. CONCLUSION: The present simulation and drilling support using the surgical template may help to perform safe and accurate implant surgery.

Large-area electron beam irradiation for surface polishing of cast titanium.

Cast titanium is a known hard-to-polish material, and its final polishing step is a perpetual challenge. The best way to tackle this challenge lies in automatic and non-mechanical polishing methods. Against this background, the suitability of large-area electron beam (EB) irradiation was examined in this study. In parallel, the optimum condition for efficient surface polishing was investigated. Cast titanium specimens were prepared, whereby their surface glossiness, surface roughness, and corrosion resistance were measured before and after EB irradiation. After EB irradiation, favorable results were observed: the cast titanium surface became smooth, the glossiness increased, and corrosion resistance was enhanced. These results were attributed to the low heat conductivity of titanium. With mechanical polishing, this property results in temperature rise and burnout reaction of the titanium surface with oxygen and the abrasives. However, during EB irradiation, the low heat conductivity of titanium was an advantage in raising the surface temperature to the melting point, such that a smooth surface was yielded after solidification. Based on the results obtained, automatic polishing by EB seemed to be a suitable polishing method for metal frameworks of removable dentures, and an efficient one too by saving time and effort.

Application of haptic device to implant dentistry--accuracy verification of drilling into a pig bone.

To enable accurate implant placement and precise drilling following preoperative simulation, we developed the BoneNavi system. To realize more precise drilling when the holes are upsized, two methods of surgical guiding were attempted in the present study. One involved using multiple surgical guides with titanium tubes of different diameters, and the other involved using a single surgical guide but employing titanium drill guide tubes with different diameters. Drilling accuracy of the two newly developed methods was examined and compared with the results of drilling into a pig bone using only the initial surgical guide. Deviations of the position and angle with the two novel methods were similar: 0.17 mm and 1 degree respectively. As for the control group whereby drilling was done using only the initial surgical guide, the deviations were 0.25 mm and 3.50 degrees--which were significantly larger than those achieved with the two novel methods. In light of the results obtained, our newly developed BoneNavi system is especially applicable for severe clinical cases that require precise implant placement.

Fracture Resistance of Computer-Aided Design/Computer-Assisted Manufactured Glass-Ceramic Incisor Crowns Fabricated on Reverse-Tapered Preparations.

PURPOSE: To evaluate the fracture resistance of computer-aided design/computer-assisted manufactured (CAD/CAM) glass-ceramic incisor crowns fabricated on reverse-tapered preparations. MATERIALS AND METHODS: Resin abutments with known total occlusal convergence angles (-8, 0, and 12 degrees; n = 20) were scanned with the TRIOS scanner, and glass-ceramic crowns were fabricated with KaVo Arctica, stored in water for either 1 or 30 days, and submitted to fracture test. RESULTS: The fracture resistance of crowns made on reverse-tapered preparations was not significantly different (P = .471) from crowns fabricated over abutments with 0- or 12-degree tapers, irrespective of the water storage period. CONCLUSION: The fracture resistance of anterior glass-ceramic crowns fabricated over reverse-tapered preparations was similar to that of crowns made on abutments with 0- or 12-degree tapers.

Fluorescence of thulium-doped translucent zirconia.

The fluorescence and physical properties of thulium-doped zirconia were investigated. A standard grade of zirconia (TZ-3Y-E) and two translucent dental zirconia materials (Zpex and Zpex Smile) were examined. The specimens were prepared by addition of 0-1.5 wt% Tm2O3 and then sintering. When exposed to UV light, the Tm2O3-doped zirconia exhibited blue fluorescence with a peak wavelength of 460 nm. The fluorescence intensity of Zpex and Zpex Smile was higher than that of TZ-3Y-E, with Zpex being more intense than Zpex Smile. Zpex exhibited maximum fluorescence intensity when doped with 0.8 wt% Tm2O3. XRD analysis revealed that TZ-3Y-E and Zpex contained primarily tetragonal zirconia, while Zpex Smile contained largely cubic phase zirconia. There were no changes observed in the microstructure or physical properties of the zirconia specimens when doped with Tm2O3.

Analysis of internal defects in all-ceramic crowns using micro-focus X-ray computed tomography.

The purpose of this study was to examine all-ceramic crowns for internal defects using micro-focus X-ray computed tomography (microCT). Mono-ceramic CAD/CAM crowns, ceramic-core CAD/CAM crowns, and heat-pressed ceramic crowns were used as specimens. The microCT images of the specimens were obtained and analyzed using a 3D volume rendering software. Mono-ceramic CAD/CAM crowns contained almost no pores, while ceramic-core CAD/CAM crowns contained pores only in the porcelain veneer area. Heat-pressed ceramic crowns had lots of pores in the cusps and the area around them. Results of this study indicated that internal pores in all-ceramic crowns could be observed non-destructively using microCT, and that CAD/CAM all-ceramic crowns contained fewer pores than crowns fabricated using manual techniques.

Electrophoretic deposition behavior of ceria-stabilized zirconia/alumina powder.

The aim of this study was to evaluate the electrophoretic deposition (EPD) behavior of ceria-stabilized zirconia/alumina (Ce-TZP/Al2O3) granulated powder. Two types of slurry with powder-to-solvent ratios of 10 wt% and 20 wt% were used. Zeta potential of the slurries was measured using a spectrometer at different pH levels. Then, EPD was performed to measure the weight of the deposited particles at varying pH levels and at two voltages (50 V and 100 V). The isoelectric point of Ce-TZP/Al2O3 mixed powder was approximately at pH 8.5. When EPD was performed, deposition of ceramic particles was typically observed in the range of pH 3 to pH 7, with the greatest deposition found at around pH 7. Moreover, the deposition of ceramic particles increased with increase in slurry concentration and voltage.

Fracture resistance of inlay-retained fixed partial dentures reinforced with fiber-reinforced composite.

In this study, the effect on the fracture load of inlay-retained composite fixed partial dentures (FPDs) caused by reinforcing them with fiber-reinforced composite (FRC) in different positions was examined. Experimental FPDs were fabricated using Estenia/EG Fiber (Kuraray Medical). Pontic reinforcement was then performed in one of the following three ways: reinforced the central area in a single line or in double straight lines, or reinforced the bottom in a curved line. The finding was that, when the area ranging from the connector to the bottom of the pontic was reinforced with FRC in a curved line, the fracture load of the FPDs tended to become higher. In addition, the FPDs fractured mainly at the veneering composite of the connector area. Based on the results of this study, it was concluded that reinforcement using FRC is effective, and that the veneering composite in the connector area needs to have sufficient strength to prevent the fractures.

Stress analysis of endodontically treated anterior teeth restored with different types of post material.

Finite element analysis was performed to evaluate stress distribution in maxillary central incisors treated endodontically and restored with a post and an all-ceramic crown. Tensile stress at tooth root was analyzed using two-dimensional finite element models with different post diameters and lengths. One post length was 1/3 of the root (short), while the other was 2/3 of the root (long); one post diameter was 1/3 of the root (narrow), while the other was 2/3 of the root (wide). The following combinations were used for posts and cores: gold alloy cast post and core, commercial stainless steel post and resin core, and fiber post and resin core. Results showed that the fiber post produced less stress on the root dentin around the post tip than did the metal posts. This finding thus suggested that to reduce the stresses that cause root fracture, a long, thin fiber post should be used.

Marginal Bone Loss Around Dental Implants Inserted with Static Computer Assistance in Healed Sites: A Systematic Review and Meta-analysis.

PURPOSE: The radiologic outcomes of implants placed using static computer-guided surgery have not yet been systematically investigated. The purpose of this study was to evaluate the marginal bone loss (MBL) around dental implants inserted with static computer assistance in healed sites. MATERIALS AND METHODS: An electronic search of publications in English from three databases (from 2000 to March 2015), including PubMed, Web of Science, and Cochrane Oral Health Group Trials Register, and a hand search of peerreviewed journals for relevant articles were performed. Only clinical human studies, either randomized or nonrandomized, with at least 10 cases and a minimum follow-up time of 12 months, reporting on MBL were included. RESULTS: The search strategy resulted in 18 publications, with 2,675 implants inserted with static computer assistance in healed sites. The pooled mean MBL at 1-year follow-up was 1.06 mm (95% CI: 0.83 to 1.30 mm; heterogeneity: random-effects model, I² = 99.38%; P < .01). Moreover, when considering studies with a 3-year follow-up only (n = 5; 748 implants), the pooled MBL was 1.48 mm (95% CI: 0.81 to 2.15 mm; heterogeneity: random-effects model, I² = 99%; P < .01). CONCLUSION: Within the limitations of this review, the MBL around dental implants placed in healed sites with computer-guided surgery seems to be a well-functioning one-stage alternative to extended two-stage conventional procedures if patients are appropriately selected and an appropriate width of bone is available for implant placement. However, current evidence is limited by the quality of available studies and the lack of comparative long-term clinical trials.

Translucency and low-temperature degradation of silica-doped zirconia: A pilot study.

The purpose of this study was to examine the translucency and low-temperature degradation of silica-doped experimental Y-TZP (Yttria-stabilized tetragonal zirconia polycrystal) containing almost no alumina. The experimental Y-TZP samples were sintered at either 1,450 or 1,500°C. The samples of commercially available translucent Y-TZP and conventional Y-TZP were used as controls. The contrast ratio (CR) and translucency parameter (TP) were obtained to compare the translucencies. In addition, the specimens were also subjected to an accelerated aging test. The results showed that the experimental Y-TZP sintered at 1,500°C and translucent Y-TZP exhibited almost the same level of translucency. During the accelerated aging test, the translucent Y-TZP underwent a substantial increase in monoclinic content, an index of degradation after the aging test. However, neither the experimental Y-TZP nor the conventional Y-TZP exhibited any appreciable change. It was concluded that the silica-doped Y-TZP will develop translucency and resistance to degradation when sintered at 1,500°C.

Fabrication of a radiopaque fit-testing material to evaluate the three-dimensional accuracy of dental prostheses.

OBJECTIVES: The objective of this study was to fabricate a radiopaque prosthetic fit-testing material, and to develop methodology to evaluate the fitting accuracy of prostheses three-dimensionally (3D) using a combination of the silicone replica technique and micro-computed tomography (µCT). METHODS: Eight types of prototype specimens of fit-testing materials were prepared by adding contrast agents (zirconia, alumina, and barium-glass) to a commercially available fit-testing material. These specimens were evaluated on their mechanical properties, X-ray absorption coefficients, reproducibility of cement space, and suitability for 3D analysis by µCT. Then, silicone replicas made from prototype specimens were assessed for accurate 3D morphology. Subsequently, color-mapping analyses of the silicone replicas were performed according to replica thickness, and the results were compared with stereomicroscopic images. RESULTS: The mechanical properties, X-ray absorption coefficients, and reproducibility of the cement space demonstrated that prototypes containing 20wt% zirconia (Zr-20) or barium glass (diameter 2µm; Ba2-20) were useful as fit-testing materials. However, the morphology of the Ba2-20 silicone replica was unable to be accurately described using µCT because of its low X-ray absorption threshold. Zr-20, however, could be clearly observed on µCT imaging. Furthermore, color-mapping analysis of the µCT images demonstrated that Zr-20 was the most suitable for 3D observation of prosthetic fit. SIGNIFICANCE: This method could allow any professional to evaluate the fit of any type of dental prosthesis, such as inlays, crowns, and fixed and removable dentures. This study demonstrated that the technique presented in the current study is able to accurately describe the abutment-crown prosthetic discrepancy based on silicone replicas.

Finite element analysis of fiber-reinforced fixed partial dentures.

Two-dimensional finite element models were created for a three-unit posterior fixed partial denture. An experimental resin-impregnated glass fiber was used as the fiber-reinforced composite (FRC) for the framework. The FRC was evaluated using varying combinations of position and thickness, alongside with two types of veneering composite. A load of 50 N simulating bite force was applied at the pontic in a vertical direction. Tensile stress was examined using a finite element analysis program. Model without FRC showed tensile stress concentrations within the veneering composite on the cervical side of the pontic--from the connector area to the bottom of the pontic. Model with FRC at the top of the pontic had almost the same stress distribution as the model without FRC. Models with 0.4-0.8 mm thick FRC positioned at the bottom of the pontic showed maximum tensile stresses reduced by 4-19% within the veneering composite.

New evaluation method by microfocus radiograph CT for 3D assessment of internal adaptation of all-ceramic crowns.

The aim of this study was to evaluate the effectiveness of the microfocus radiograph CT system in examining the adaptation of all-ceramic crowns three-dimensionally and non-destructively. The computed tomograms of the crown and abutment model were filmed by microfocus radiograph CT. Using a volumetric rendering software, images of gaps were extracted and reconstructed three-dimensionally, and their volume data analyzed. In order to compare this method with the conventional method, fitness test silicone paste was sandwiched between the abutment and all-ceramic crown. Adaptation of the crown on the abutment model was then observed non-destructively and three-dimensionally. Furthermore, the gaps could be analyzed in any arbitrary position. Concerning mean gap thickness, there was significant differences between the two measurement methods. However, it was very slight. We therefore concluded that the microfocus radiograph CT system is well positioned to be an extremely effective method in examining the adaptation of all-ceramic crowns.

In vitro study on marginal and internal fit of CAD/CAM all-ceramic crowns.

The purpose of this study was to evaluate the marginal and internal fit of all-ceramic crowns fabricated using the Decsy CAD/CAM system. A master die of maxillary first premolar was measured, and experimental crowns were fabricated. Four conditions were established by combining two convergence angles (4 degrees and 12 degrees) of the abutment with two different luting space settings (15 microm and 55 microm). The results showed that the experimental crowns had a marginal gap of 42 to 56 microm. When the luting space setting was 15 microm, the internal gap was 85-88 microm; when the setting was 55 microm, the internal gap was 126-138 microm. The marginal gap for each experimental crown met the clinically acceptable criterion.