The internal and marginal adaptation of lithium disilicate endocrowns fabricated using intra and extraoral scanners: An in-vitro study.

OBJECTIVES: The impression technique highly influences the adaptation of ceramic restorations. Not enough information is available to compare the marginal (MF) and internal fit (IF) of endocrowns fabricated with various digitization techniques. Therefore, this in-vitro study aimed to compare the MF and IF of lithium disilicate (LDS) endocrowns fabricated through direct and indirect digital scanning methods. MATERIALS AND METHODS: One extracted maxillary molar was used to fabricate endocrowns. The digitization of the model was performed with (G1) direct scanning (n = 10) utilizing an intraoral scanner (IOS), (G2) indirectly scanning the conventional impression taken from the model using the same IOS (n = 10), (G3) indirectly digitalizing the obtained impression using an extraoral scanner (EOS) (n = 10), and (G4) scanning the poured cast using the same EOS (n = 10). The MF and IF of the endocrowns were measured using the replica method and a digital stereomicroscope. The Kruskal-Wallis test was used to analyze data. RESULTS: The studied groups differed significantly (p<0.001). G2 (130.31±7.87 µm) and G3 (48.43±19.14 µm) showed the largest and smallest mean vertical marginal gap, respectively. G2 and G3 led to the highest and lowest internal gaps in all regions, respectively. With significant differences among the internal regions (p<0.001), the pulpal area demonstrated the most considerable misfit in all groups. CONCLUSIONS: Scanning the impression using an extraoral scanner showed smaller marginal and internal gaps.

Influence of screw channel angulation on reverse torque value and fracture resistance in monolithic zirconia restorations after thermomechanical cycling: an in-vitro study.

BACKGROUND: While the concept of angled screw channels has gained popularity, there remains a scarcity of research concerning the torque loss and fracture strength of monolithic zirconia restorations with various screw channel angulations when exposed to thermomechanical cycling. This in-vitro study aimed to evaluate the reverse torque value and fracture resistance of one-piece screw-retained hybrid monolithic zirconia restorations with angulated screw channels after thermomechanical cycling. METHODS: One-piece monolithic zirconia restorations, with angulated screw channels set at 0°, 15°, and 25° (n = 6 per angulation) were fabricated and bonded to titanium inserts using a dual-cure adhesive resin cement. These assemblies were then screwed to implant fixtures embedded in acrylic resin using an omnigrip screwdriver, and reverse torque values were recorded before and after thermomechanical cycles. Additionally, fracture modes were assessed subsequent to the application of compressive load. One-way ANOVA and Bonferroni post hoc test were used to compare the groups (α = 0.05). RESULTS: The study groups were significantly different regarding the fracture resistance (P = 0.0015), but only insignificantly different in the mean percentage torque loss (P = 0.4400). Specifically, the fracture resistance of the 15° group was insignificantly higher compared to the 0° group (P = 0.9037), but significantly higher compared to the 25° group (P = 0.0051). Furthermore, the fracture resistance of the 0° group was significantly higher than that of the 25° group (P = 0.0114). CONCLUSIONS: One-piece hybrid monolithic zirconia restorations with angulated screw channels can be considered an acceptable choice for angulated implants in esthetic areas, providing satisfactory fracture strength and torque loss.

Vertical marginal fit of zirconia copings fabricated with one direct and three indirect digital scanning techniques.

STATEMENT OF PROBLEM: Direct digitization of the impression by using an intraoral scanner is a newly introduced technique, but studies comparing the method with other digitization techniques are lacking. PURPOSE: The purpose of this in vitro study was to compare the marginal accuracy of zirconia copings fabricated with 4 different scanning methods. MATERIAL AND METHODS: Scans (n=10) of a brass master die were made with a direct method (ISD) in which the die was directly digitized by using an intraoral scanner (IOS) and 3 indirect scanning methods, a conventional impression with polyvinyl siloxane material digitized with either the same IOS (ISI) or with a laboratory extraoral scanner (ESI), or a cast from the impression was scanned by using a laboratory extraoral scanner (ESC). Forty zirconia copings were milled from presintered zirconia blanks and sintered. The vertical marginal gap was measured at 12 points on the master die by using a digital microscope. One-way ANOVA was used to compare the mean differences among the 4 groups, and post hoc analysis was used for pairwise comparison of the groups (α=.05). RESULTS: The mean ±standard deviation of the vertical marginal gap was 73 ±7 µm in the ISD group, 109 ±36 µm in the ISI group, 42 ±4 µm in the ESI group, and 97 ±5 µm in the ESC group. The lowest marginal gap was seen in the ESI group, which was significantly different from the 3 other groups (P≤.001). The copings in the ISD group had a significantly lower marginal gap than those in the ISI (P=.04) and ESC (P<.001) groups. However, the ISI and ESC groups were not significantly different (P=.69). CONCLUSIONS: Marginal adaptation of all zirconia copings fabricated with these 4 scanning techniques was within a clinically acceptable range. However, ESI was the best method of digitization and yielded copings with minimum vertical marginal gap.

Evaluation of the marginal fit of metal copings fabricated by using 3 different CAD-CAM techniques: Milling, stereolithography, and 3D wax printer.

STATEMENT OF PROBLEM: Digital systems have been developed as substitutes for the traditional fabrication of wax patterns and definitive restorations, but the accuracy of these systems is unclear. PURPOSE: The purpose of this in vitro study was to evaluate and compare the marginal fit of metal copings fabricated from milling, stereolithography (SLA), and 3D wax printer (PolyJet) patterns. MATERIAL AND METHODS: A standard machined brass die model was designed and prepared. To fabricate metal copings of standardized shape and dimensions, the die was scanned and nickel-chromium (Ni-Cr) copings were designed by using a dental designer software program. By using the same CAD data to fabricate 30 patterns through milling, SLA, and PolyJet techniques, 10 resin patterns were milled by using a 5-axis milling machine, 10 resin patterns were produced by using a NextDent 5100 3D Printer, and 10 wax patterns were printed by using a Solidscape 3D printer. The patterns were invested and cast in Ni-Cr alloy. Each coping was evaluated by using a digital microscope on 16 points around the finish line on the metal die at ×230 magnification. The mean marginal discrepancy was calculated. The mean differences among the groups were compared by using 1-way ANOVA, and post hoc analysis was used for pairwise comparison of the groups (α=.05). RESULTS: The mean value of marginal discrepancy was 93.1 ±25 µm in the milling, 71.1 ±25 µm in the SLA, and 41.3 ±6 µm in the PolyJet group. No statistically significant difference was found between the milling and SLA groups (P=.158), while the PolyJet group showed significantly lower mean marginal discrepancy than the 2 other groups (P<.05). CONCLUSIONS: The PolyJet method produces metal copings with better marginal fit than the SLA or milling techniques. However, the marginal fit was clinically acceptable in metal copings made with all the 3 methods.

Comparison of the Water Sorption and Solubility of Four Reline Acrylic Resins after Immersion in Food-Simulating Agents.

OBJECTIVES: Water sorption and water solubility adversely affect the mechanical properties and biocompatibility of the denture material. This study aimed to evaluate the water sorption and solubility of three direct hard reline acrylic resins and a heat-curing one after immersion in food-simulating agents. MATERIALS AND METHODS: This study was performed on four groups of samples (n = 10 per group). The samples were made of three direct hard reline acrylic resins (TDV Cold Liner Rebase, Tokuyama Rebase II Fast, GC Reline Hard) and a heat-curing one (Meliodent). Each group was divided into four subgroups (n = 10) to undergo 7-day immersion in distilled water, 75% ethanol/water, 0.02 N citric acid, and heptane. Water sorption and solubility were calculated according to Oysaed and Ruyter formula. The statistical analyses were done by using SPSS software (version 22). Kruskal-Wallis H Test and Dunn's test were used to detect any significant difference among the groups (P < 0.05). RESULTS: The median range of water solubility and water sorption values were -0.87-4.92 and 3.75-27.25 µg/mm3, respectively. The median solubility and sorption values of different resins differed significantly in the same solution (P < 0.05). Besides, immersion in different solutions caused significant differences in the median solubility and sorption values of each reline material (P < 0.05), except for Meliodent whose solubility was not significantly affected by different solutions (P = 0.16). CONCLUSIONS: Water sorption and solubility values of the tested hard reline resins were within the range of International Standards Organization 1567:1999. Given the low sorption and solubility values, these hard reline materials can be safely used in clinical situations.

Evaluation of Bond Strength of Pressed and Layered Veneering Ceramics to Nickel-Chromium Alloy.

STATEMENT OF THE PROBLEM: The success of metal- ceramic- restorations (MCR) depends on the presence of strong bond between porcelain and metal substructure. PURPOSE: The purpose of this study was to evaluate the effect of hot pressing technique on the bond strength of a metal-porcelain composite in comparison to layering technique. MATERIALS AND METHOD: Thirty Nickel-Chromium specimens were produced by two methods; conventional porcelain layering on metal and hot pressing (n=15). Bond strengths of all specimens were assessed by the means of three-point bending test according to ISO 9693: 1999 (E) instructions. The data were analyzed using Students t-test (p< 0.001). RESULTS: The mean ± SD bond strength of conventional and hot pressing technique was 48.29 ± 6.02 and 56.52 ± 4.97, respectively. Therefore, the conventional layering technique yielded significantly lower mean bond strength values than hot pressing technique (p< 0.001). CONCLUSION: This study showed that it is possible to improve metal-porcelain bond strength significantly by applying an overpressure during porcelain firing.

Evaluation of the effect of a home-bleaching agent on the surface characteristics of indirect esthetic restorative materials: part I--roughness.

BACKGROUND: Pressing esthetic demands of good looking make people undergo bleaching procedures. However, the effect of bleaching agents on esthetic restorative materials with different surface preparations has been poorly studied. AIM: The aim of this study was to examine the effect of a homebleaching agent (carbamide peroxide: CP 38%) on the surface roughness of the polished fiber reinforced composite (FRC), overglazed, autoglazed, or polished ceramic samples. MATERIALS AND METHODS: Twenty standardized cylindrical specimens were made of each of the following groups: over-glazed, autoglazed, polished porcelain and also FRC. The test specimens exposed to the CP 38%, 15 minutes, twice a day for 2 weeks according to the manufacturer's recommendation. Six samples from each group were selected randomly to form negative controls. Surface roughness measurements (Ra, micrometer) for baseline, test and control specimens were performed by use of a profilometer. Paired t-test, Mann-Whitney test, and Kruskal-Wallis test were used for statistical analyses. RESULTS: The data showed that bleaching with CP 38% significantly increased the surface roughness of all the test samples (p < 0.05). The type of surface preparation caused significant differences between the susceptibility of porcelain subgroups to bleaching (p < 0.05). The polished porcelain specimens showed the highest changes after bleaching. CONCLUSION: CP 38% significantly increases the surface roughness of the porcelains and FRC. The type of surface condition affects the amenability of the porcelain surface to the bleaching agent. Glazed porcelains were more resistant to roughness than the polished porcelains and also the composite. CLINICAL SIGNIFICANCE: Roughening of porcelain and FRC occur following bleaching procedure. No special surface preparation of indirect esthetic restorative materials can completely preserve these materials from adverse effects of bleaching agents.