Evaluation of the effect of different core substrates on the accuracy of intraoral scanners.

BACKGROUND: The aim of this study was to determine if different types of core substrates have any effect on the trueness and precision of digital intraoral impressions. MATERIAL AND METHODS: A customized typodont with four similar cores of natural dentine, composite, metal (Ni-Cr), and zirconia in the position of premolars was fabricated. The study model was scanned five times with two types of intraoral scanners (Carestream 3600 and 3Shape Trios 3), and a reference standard scan was obtained using a laboratory scanner (3shape D1000). A metrology software (Geomagic X) was used to align the data of experimental scans and the reference scan to determine deviation values (trueness). Precision values were calculated with random superimposition in each intraoral scanner group. The Kruskal-Wallis test was used to compare differences between different substrates, and the Mann-Whitney test was used to compare the average values between the two scanners. RESULTS: Trios 3 was found to be significantly truer and more precise than Carestream 3600 (p value = .005, <0.001). There were no significant differences in the trueness of different substrates when they were scanned by Trios 3, while different materials showed significantly different trueness values in the Carestream 3600 group (p value = .003). Dentin showed the best trueness, and zirconia performed worse than other substrates. Regarding the precision of the scanners, neither of the scanners was affected by the type of scanning substrate. CONCLUSION: For Carestream 3600, substrate type did impact the trueness of intraoral scans, with dentin and zirconia showing the highest and lowest accuracy, respectively, while Trios 3 was similarly accurate across all substrates. Trios 3 had both higher trueness and precision than Carestream 3600.

Co-Infection Rates between SARS-CoV-2 and RSV in Oropharyngeal, Nasopharyngeal Aspirate and Saliva Samples of COVID-19 Patients, Shiraz, South of Iran.

Statement of the Problem: Determining the prevalence of respiratory viruses' coinfection with coronavirus disease 2019 (COVID-19) is essential to defining its true clinical influence. Purpose: This study aimed to evaluate co-infection rates between severe acute respiratory syndrome-related coronavirus 2 (SARS-CoV-2) and respiratory syncytial virus (RSV) in infected patients in Shiraz, south of Iran. Materials and Method: In a cross-sectional descriptive study, oropharyngeal, nasopharyngeal aspirate (NPA), and saliva samples of 50 COVID-19 patients who were referred to Ali-Asghar hospital (Shiraz, Iran) from March to August 2020, were collected. A control group consisted of age and sex-matched healthy participants. The nasopharyngeal and oropharyngeal aspirates were collected by sterile swabs. All cases were hospitalized, and all SARS-CoV-2 patients had a fever and respiratory symptoms. The samples were packed in a vial with 1 mL of transport medium and transported to the Valfagre specialty laboratory, where they were tested for RSV using a real-time polymerase chain reaction (PCR). Results: 100 nasopharyngeal/oropharyngeal aspirates and saliva samples including 50 healthy controls (24 females, 26 males) and 50 COVID-19 patients' samples (27 males and 23 females) were studied. There was no significant difference regarding age as well as gender between both groups (p> 0.05). None of the healthy subjects was infected with RSV; however, 5(10%) patients from COVID-19 group were infected with the RSV virus. Chi-square test did not show a significant difference between RSV infection in COVID-19 patients and healthy subjects. Conclusion: The outcome of present research showed that concurrent RSV with COVID 19 infection might be seen in hospitalized patients in Shiraz Southwest of Iran. For more reliable findings, further research on bigger populations, including more pathogens in several places around the country, and considering the severity of symptoms is required.

Comparing Castability of Nickel-Chromium, Cobalt-Chromium, and Non-Precious Gold Color Alloys, Using two Different Casting Techniques.

Statement of the Problem: The castability of nonprecious gold color alloy using torch/ centrifugal and induction/vacuum-pressure casting techniques has not been studied yet. Purpose: This study was conducted to compare the castability of nickel chromium, cobalt-chromium and nonprecious gold color alloy using torch/centrifugal and induction/ vacuum-pressure casting techniques. Materials and Method: In this in vitro study, a total number of 54 identical acrylic wax meshes were prepared and divided into 6 different groups of 9 each. Group 1: nickel-chromium alloy, which was casted with induction technique. Group 2: nickel-chromium alloy was casted with centrifugal technique. Group 3: cobalt-chromium alloy was casted with induction technique. Group 4: cobalt-chromium alloy was casted with centrifugal technique. Group 5: nonprecious gold color alloy was casted with induction technique. Group 6: nonprecious gold color alloy was casted with centrifugal technique. Then castability of specimens was measured using modified Whitlock's method. The results were analyzed using two way ANOVA and post hoc tests. Results: ANOVA test revealed no statistically significant difference between different alloys with a p Value of 0.313. Moreover, it represented no significant differences within the groups regarding alloy types and casting techniques with a p Value of 0.511 and 0.682, respectively. Conclusion: No significant difference was found in the castability value of nickel-chromium, cobalt-chromium, and nonprecious gold color alloys. In addition, the castability value of three alloys tested in this study was not different by using torch/centrifugal or induction/vacuum-pressure casting machines.

Effect of external staining on the optical properties and surface roughness of monolithic zirconia of different thicknesses.

STATEMENT OF PROBLEM: The effect of the number of external staining applications on the color, translucency, and surface roughness of monolithic zirconia is not known. PURPOSE: The purpose of this in vitro study was to investigate the effect of glazing and repeated external staining on the color, translucency, and surface roughness of 2 types of monolithic zirconia with 2 different thicknesses. MATERIAL AND METHODS: Thirty-two 10×10-mm monolithic zirconia blocks, of which 16 were super high translucent and 16 were high translucent, were milled. The thickness of half the blocks was 0.7 mm, and the others were 1.2-mm thick. Four specimens in each subgroup were colored with value stain, and the other half with yellow stain. Staining was repeated twice for each specimen. The color and translucency of each specimen were measured before overglazing as the baseline, after overglazing, and after each staining process by using a spectrophotometer. The surface roughness (Ra) was measured after overglazing, the first staining, and the second staining. Data were analyzed by using repeated measures ANOVA and the independent t test (α=.05). RESULTS: The number of coloring, type of coloring, and thickness changed the optical properties of monolithic zirconia. a∗, b∗, L∗ Value and ΔE were affected by the stage of firing (glazing, first staining, and second staining) (P<.001). ΔE Increased after the first and second staining in each type and thickness of monolithic zirconia and each type of stain. The amount of ΔE change was more for the first staining stage. The color difference (ΔE) after overglazing ranged from 3.08 to 3.75. The highest changes in color were related to the first staining with yellow colorant of super-high-translucent monolithic zirconia (cubeX²) with 0.7-mm thickness (ΔE=17.62). The surface roughness of the specimens was affected by the number of colorings (P<.001). In all groups, the first staining increased the surface roughness, and the second staining decreased the surface roughness. The type of monolithic zirconia (P=.756), thickness (P=.735), and type of stain (P=.239) had no significant effect on the surface roughness. CONCLUSIONS: The first stage of staining with both stains caused more color change in all types and thicknesses of monolithic zirconia. Translucency increased after glazing and decreased after the first and second staining. The first staining increased the surface roughness, and the second decreased the surface roughness.

A Special Design to Facilitate Retrieval of Cement-Retained Zirconia-Based Implant-Supported Restorations.

STATEMENT OF THE PROBLEM: Retrieval of cement-retained implant-supported restorations is challenging in cases of screw loosening or periodontal problems. PURPOSE: The purpose of this study was to evaluate the effect of the screw access hole on the fracture resistance of zirconia-based cement-retained restorations with and without an access opening. MATERIALS AND METHOD: In this in vitro study thirty-three cement-retained implant-supported zirconia-based molar crowns were fabricated and divided into 3 groups (n=11). As the control group, group 1 consisted of conventional cement-retained crowns. Group 2 comprised conventional cement-retained crowns in which a hole was created in the location of the screw. Group 3 consisted of cement-retained crowns in which a ledge was created in the location of the screw access channel. The specimens were cemented to their abutments and their access openings were filled with composite resin. A compressive load was applied to the specimens using a universal testing machine until they fractured. The mean fracture resistance values of the samples were compared by using the one-way ANOVA and Tamhane post-hoc test (a=0.05). RESULTS: The mean fracture resistance values were 1270.18± 12.67 N in group 1 (the control group), 960.09±210.67 N in group 2 (conventional), and 1357.81±361.68 N in group 3 (the special design). The fracture resistance value was higher in the special design group than that of the conventional design (p= 0.018) and the fracture resistance value of the conventional design group was less than that of the control group (p= 0.042). No statistically significant difference was detected between the control group and the special design group in fracture resistance values. CONCLUSION: Preparing a screw access hole in cement-retained implant-supported zirconia-based crowns decreased the fracture resistance of the restoration. Designing a ledge in the zirconia framework around the access hole may increase the fracture resistance of the restoration.

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.

Effect of Core Materials on the Dimensional Accuracy of Casts Made of Two Different Silicone Impression Materials: An Experimental Study.

BACKGROUND AND AIM: Dimensional accuracy of impressions is crucial to the quality of fixed restorations. This accuracy can be highly affected by the type of core and impression materials. This study aimed to assess the effect of different core materials on dimensional accuracy of two silicone impression materials. MATERIALS AND METHODS: In this in vitro study, three master core models were fabricated of amalgam, composite resin, and nickel-chromium. Of each model, 30 impressions were taken; 15 with additional and 15 with condensational silicone impression material. The accuracy of impressions of the three core materials was assessed by measuring two linear dimensions and one vertical dimension on the stone casts. STATISTICAL ANALYSIS: Data were analyzed with two-way analysis of variance and Scheffe post hoc test (α = 0.05). RESULTS: Accuracy of the three measured dimensions was significantly affected by both the impression and core materials (P < 0.05). Additional silicone was significantly more accurate in linear dimensions, and impressions of the amalgam core were significantly more accurate than other cores in linear dimensions. CONCLUSION: Additional silicone impression material had more detail reproduction, and the impressions of amalgam core were more accurate than the composite and nickel-chromium core materials.

PCR Detection of HHV8 DNA in the Saliva of Removable Denture Wearers Compared to Dentate Cases in Shiraz, South of Iran.

RESULT: In the denture wearers, HHV8 DNA was detected in 11 cases. Two of the controls amplified HHV8 DNA. Fisher's exact test demonstrates a significant difference between virus infection and using removable dentures (p = 0.015). CONCLUSION: Our findings suggested that HHV8 detection could be associated with use of denture.

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.

Effects of number of firings and veneer thickness on the color and translucency of 2 different zirconia-based ceramic systems.

STATEMENT OF PROBLEM: Color matching between a zirconia-based ceramic restoration and a natural tooth is a common clinical challenge. PURPOSE: The purpose of this in vitro study was to investigate the effects of the number of firings, veneer thickness, and zirconia core translucency on the final color and translucency of zirconia-based ceramic restorations. MATERIAL AND METHODS: Sixty disk-shaped zirconia specimens (10×1 mm) were prepared in high and low translucencies (n=30 per translucency group). Each zirconia group was subdivided into 3 subgroups (n=10) veneered with ceramic layers of 0.5-, 1-, and 1.5-mm thicknesses. The specimens were then subjected to 1, 3, 5, and 7 firing times. Color and translucency parameters were measured by using a spectrophotometer. Data were analyzed by using multifactorial repeated measures ANOVA and subgroup analysis (α=.05). RESULTS: The number of firings affected the L*, a*, and b* values and translucency parameter in both zirconia-based systems with different veneer thicknesses (P<.05). Increasing the veneer thickness decreased the translucency parameter and increased the L*, a*, and b* values in both zirconia-based systems (P<.05). The color difference values (ΔE) were the highest between 1 and 3, 1 and 5, and 1 and 7 firings, and the lowest between 5 and 7 firings in both zirconia systems. CONCLUSIONS: Repeated firings and porcelain veneer thickness affected the final color and translucency of both zirconia systems and, consequently, adversely influenced the esthetic outcomes. Thus, these factors should be considered in the tooth preparation and laboratory phase.

Tensile Bond Strength of Three Custom-made Tooth-Colored Implant Superstructures to Titanium Inserts.

STATEMENT OF THE PROBLEM: Hybrid abutments are made of a titanium implant insert and a ceramic component. The tensile bond strength between the titanium implant insert and the ceramic component is not still clearly known. PURPOSE: This in vitro study aimed to compare the tensile bond strength of the titanium insert to ceramic components made of milled lithium disilicate, milled zirconia, and pressable lithium disilicate. MATERIALS AND METHOD: To standardize the shape and dimension of the ceramic components, a single computer-aided design/computer assisted manufacturing (CAD/CAM) superstructure was designed with Dental Designer software. Based on this model, CAD milled zirconia (Zr), CAD milled Lithium disilicate (CAD-LD) and heat-pressed lithium-disilicate (H-LD) superstructures were fabricated (n=10 per group). They were bonded to the titanium inserts by using self-adhesive resin cement. The prepared superstructure-titanium insert complexes (hybrid abutments) were screwed into the implants with 35 Ncm torque. The tensile bond strength of the ceramic superstructures to the titanium inserts were recorded by the universal testing machine with a crosshead speed of 2 mm/min. The data were analyzed by using one-way ANOVA and Tamhane post-hoc test (p< 0.05). RESULTS: The mean±SD of tensile bond strength was 328.50±30.4 N in CAD-LD, 257.30±23.8 N in H-LD, and 242.20±21.2 N in Zr groups. One-way ANOVA revealed the groups significantly different in terms of the tensile bond strength (p< 0.001). Tamhane post-hoc test showed a significant difference between the CAD-LD and the two other groups (p<0.001); however, no statistically significant difference was detected between the H-LD and Zr groups (p= 0.39). CONCLUSION: It can be concluded that the CAD-LD has higher bond strength to titanium insert compared with the H-LD and Zr groups. Therefore, they might be clinically more beneficial in high-esthetic areas.

Influence of Age on Perception of Best Esthetical Profile.

STATEMENT OF THE PROBLEM: As an important determinant of social acceptability, facial attractiveness can be influenced by decisions of orthodontists through treatment. The perception of facial attractiveness is influenced by several factors. PURPOSE: The purpose of this study was to determine the difference in perception of facial attractiveness among male and female raters in different age groups through comparison of silhouettes. MATERIALS AND METHOD: 208 subjects (99 male, 109 female) aged between 18 to 70 years were assigned to three groups of young adults, middle-aged and senior and rated pre-designed silhouettes on a 7-level rating scale (1= the least esthetic, 7=the most esthetic). Two series of 7 silhouettes were prepared for men and women in which the mandible was protruded or retruded in 2-mm increments from the average. The evaluators were asked to grade the profiles separately for male and female. T-test and one-way ANOVA were used for the statistical analysis with α = 0.05. RESULTS: The first and third age groups, favored female profile 2 among class II profiles. However, the second age group preferred the average profile. Male class II profiles were rated exactly as female ones among the three age groups. The average female class III silhouette was preferred in all age groups. The first and second age group preferred the average male profile in male class III silhouettes. The oldest group however, preferred profile 2. For both sexes, the least acceptable profile in each set of silhouettes was the most protruded or retruded. CONCLUSION: There were few differences in perception of facial attractiveness between different age groups. In all groups, even small mandibular protrusion was unacceptable and mild retrusion was considered attractive.

Observational Molecular Case-Control Study of Genetic Polymorphisms 1 in Programmed Cell Death Protein-1 in Patients with Oral Lichen Planus

Background: The association between programmed cell death protein 1 (PD-1) variations and susceptibility to autoimmune diseases has been recurrently reported. However, there is no report about its relationship with oral lichen planus (OLP) as one of autoimmune diseases. Methods: We investigated the association between genetic predisposition to OLP and two single nucleotide polymorphisms in PD-1. Results: GG, GA, and AA genotypes at position +7146 were found in 59 (80.8 %), 10 (13.7 %), and 4 (5.5 %) patients, and in 132 (77 %), 34 (20 %), and 5 (3 %) healthy participants. CC, CT, and TT genotypes at position +7785 were found in 32 (43.8 %), 35 (47.9 %), and 6 (8.2 %) patients and in 99 (58 %), 66 (39 %), and 6 (3 %) controls. Analysis indicated that patients' genotypes were not statistically different from controls' genotypes at both positions +7146 (P = 0.35 and P = 0.98) and +7785 (P = 0.07 and P = 0.06). Conclusion: The findings indicated that PD-1 SNPs at +7146 [PD-1.3] G/A and +7785 [PD-1.5] C/T was not associated with susceptibility to OLP. However, further research with higher sample size and in different geographical regions is needed in order to achieve the generalizability of the findings.

The Effect of Sintering Time on the Marginal Fit of Zirconia Copings.

PURPOSE: This study analyzed the effect of different sintering times on the marginal adaptation of zirconia ceramic copings. MATERIALS AND METHODS: Thirty copings of pre-sintered 3Y-TZP blanks were milled and subsequently sintered in a special furnace for three different durations (n = 10 per group). The sintering time was 1 hour, 15 minutes for IPS e.max ZirCAD, 4 hours 20 minutes for Speed ZrO2, and 7 hours 20 minutes for the conventional ZrO2 group. The marginal gaps of specimens were measured at 18 spots on the master die by employing a digital microscope. One-way ANOVA test compared the mean differences among the 3 groups (α = 0.05). RESULTS: The mean values for the marginal gap of the IPS e.max ZirCAD, Speed ZrO2 , and conventional ZrO2 groups were 41.06 ± 14.03, 43.03 ± 11.67, and 39.88 ± 15.23 µm, respectively. There was no statistically significant difference among the groups regarding the marginal gap. CONCLUSION: Different sintering times did not alter the marginal fit of the zirconia copings. The mean values of the marginal gap were within the clinically acceptable level for all three groups.

Prevalence of Chronic Orofacial Pain in Elderly Patients Referred to Shiraz Dental School From 2005 to 2017.

BACKGROUND: Orofacial pain is presented mainly as a pain in the front part of the head, including the oral cavity. Adult and elderly patients are important target groups affected by this chronic pain, which may cause several problems. METHODS: In this cross-sectional study, the characteristics of patients who suffered from orofacial pain were evaluated. The data such as patients' gender, age, pain diagnoses, treatments, responses to treatments, duration and severity of pain, and disability were collected from their records. The diagnoses were provided based on the characteristics of pain, X-ray findings, physiological findings, lab tests, nerve block injection, and a drug challenge test. RESULTS: Among all the 5,221 patients referred to Oral and Maxillofacial Medicine and Prosthodontics Department, 583 (11.16%) reported orofacial pain as the chief complaint. The frequency of orofacial pain was more prevalent among women than men. Burning mouth syndrome was the most prevalent cause of orofacial pain in the elderly, followed by temporomandibular disorder and trigeminal neuralgia. CONCLUSIONS: Most cases of orofacial pain in elderly patients were burning mouth syndrome, temporomandibular disorder, and neuralgia. It was more observed in females than males with the mean age of 55.3 ± 10.7 years old.

Comparison of temporomandibular disorders between menopausal and non-menopausal women.

OBJECTIVES: Hormonal changes during menopause alter a woman's susceptibility to some disorders. Information regarding the prevalence of temporomandibular disorder (TMD) in menopausal women is limited in the literature. In this study, the prevalence and severity of TMDs were compared between menopausal and non-menopausal women. MATERIALS AND METHODS: The study included 140 women (69 premenopausal and 71 postmenopausal) 45 to 55 years of age that were examined in Shiraz Dental School, Shiraz in Iran. The Helkimo clinical dysfunction index (Di) was used to evaluate temporomandibular joint (TMJ) dysfunction. The data were analyzed using chi-square and Fisher's exact tests. RESULTS: Occurrence of TMD was significantly higher in menopausal than non-menopausal women (P<0.001). All the TMD criteria based on Helkimo Di except range of mandibular movement were significantly more common in menopausal women. The range of mandibular movement was not significantly different between menopausal and non-menopausal women (P=0.178). CONCLUSION: The results from this study show that TMD can be considered more common and severe in menopausal than non-menopausal women. This finding indicates that, similar to other conditions in menopausal women such as arthritis and osteoporosis, TMD should be taken into consideration by dental and medical professionals.

The Effect of Sintering Program on the Compressive Strength of Zirconia Copings.

STATEMENT OF THE PROBLEM: Considering the limitations of conventional sintering of zirconium oxide (ZrO2) copings, shortening the sintering time can be proposed as an alternative method for making the copings. PURPOSE: This study aimed to assess the effects of sintering time on compressive strength of Yttria Tetragonal Zirconia Polycrystal (Y-TZP) copings. MATERIALS AND METHOD: Thirty copings of pre-sintered 3Y-TZP blanks were milled and sintered in a special furnace for three different durations (n=10 per group). The sintering time was 1 h 15 min for IPS e.max ZirCAD, 4 h 20 min for Speed ZrO2, and 7 h 20 min for the conventional ZrO2 group. The specimens were cemented on the brass dies by using conventional glass ionomer cement. The copings were vertically loaded until fracture by using a universal testing machine. The data were analyzed through one-way analysis of variance (ANOVA) and post hoc test to compare the mean differences of compressive strength yielded in three study groups (α=0.05). RESULTS: The mean ± SD of compressive strength value was (3617 ± 543.54) N for IPS-e.maxZirCAD group, (2663 ± 508.11) N for Speed ZrO2 group, and (1662± 466.71 N) for conventional ZrO2 group. There were statistically significant differences among compressive strength values of the tested groups (p< 0.05). The highest compressive strength values were obtained from the IPS e.max ZirCAD group. CONCLUSION: Within the limitations of this in vitro study, it can be concluded that compressive strength of the zirconia copings is affected by the sintering time. High compressive strength of zirconia copings can be obtained by shortening the sintering time.

The Effect of Multiple Firings on the Shear Bond Strength of Porcelain to a New Millable Alloy and a Conventional Casting Alloy.

This study compared the effect of multiple firings on the shear bond strength (SBS) of porcelain to the new millable alloy (Ceramill Sintron) and a conventional casting alloy (4-all). Thirty-six cylindrical cores (6.8 × 9 mm) were made of millable and castable alloy through CAD/CAM and casting techniques, respectively (n = 18). In the center of each bar, a 4 × 4 × 2-mm shot of porcelain was fused. Having divided each group into 3 subgroups based on the number of firing cycles (3, 5, 7), the specimens were fixed in a universal testing machine and underwent a shear force test (1.5 mm/min crosshead speed) until fractured. Then the SBS values (MPa) were calculated, and the failure patterns were microscopically characterized as adhesive, cohesive, or mixed. Two-way ANOVA statistical test revealed that the number of porcelain firings had no significant effect on the SBS of any of the metal groups (p = 0.1); however, it was statistically higher in the millable group than the castable group (p < 0.05). Moreover, detecting the mixed failure pattern in all the specimens implied that the multiple firings had no significant effect on the failure pattern. The multiple porcelain firings had no significant effect on the SBS of porcelain to neither the millable nor castable alloys.

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.