Lithium Disilicate Crowns Research Articles

Effects of the Cervical Marginal Relocation Technique on the Marginal Adaptation of Lithium Disilicate CAD/CAM Ceramic Crowns on Premolars

To evaluate the effect of cervical margin relocation (CMR) for crowns designed using CAD/CAM technology and fabricated from lithium disilicate (e.max, CAD) before and after aging; and to compare the fracture forces and failure type of the tested crowns. Mesio-occluso-distal(MOD) cavities 1 mm above the cementoenamel junction(CEJ) were prepared on 40 maxillary first premolars. The teeth were divided into four groups. In group A, all cervical margins (CM) were located 1 mm above the CEJ. However, in both mesial and distal proximal boxes of groups B, C, and D, in addition to the MOD cavities, the CMs were extended 2 mm on both sides below the CEJ apically to simulate the CMR technique. In group B, the mesial and distal proximal boxes were filled with flowable composite, while for group C and group D, specimens were filled with composite resin fillings. To simulate the CMR technique, the cavities were filled with composite layers of 3 mm in two increments. Using the CAD/CAM system, 40 standard crowns were prepared on premolars, then cemented using a dual-curing adhesive cement. Assessments of the marginal integrity of interfaces of the proximal boxes of the cemented crowned teeth were recorded. Statistical differences between groups were analyzed using the ANOVA and Bonferroni's posthoc test. The first null hypothesis was accepted since no statistically significant differences were found in marginal integrity before and after aging (p>0.05), while the second was partially rejected since different fractured force values were recorded and a significant difference was detected between group D and group B. The third hypothesis was rejected because the catastrophic fracture rate was the highest among the four groups. The implementation of CMR before and after aging had a good effect on the marginal integrity of CM relocation. The CMR technique with resin luting cement of lithium disilicate crowns is effective and recommended for the restoration in deep proximal boxes of premolars or posterior teeth. CAD/CAM-generated e. max all-ceramic crowns with composite as the CMR enable the reconstruction of severely destroyed teeth irrespective of the position of the cavity margins.

The influence of a composite resin adhesive on microleakage into the implant screw access chamber.

The sealing of implant screw access chambers can influence prosthesis success, peri-implant health and patient comfort. The aim of this study was to compare the microleakage of single implant crown screw access chambers sealed with and without a composite resin adhesive. Twenty milled lithium disilicate crowns were luted to titanium-base abutments, attached to implants and randomly assigned to one of two groups. The first group had the screw access chamber sealed with polytetrafluoroethylene tape, a 10-methacryloyloxydecyl dihydrogen phosphate containing adhesive, and composite resin. The second group used the same procedure but without an adhesive. All samples were sealed at the implant-abutment interface and subjected to thermocycling then immersed in a ferrous sulphate contrast media for 48 h. Infiltration and microleakage was measured using a microcomputed tomography scanner. All samples demonstrated high resistance to microleakage with no significant contrast media diffusion in either of the two groups. Within the limitations of this invitro study, the use of a composite resin adhesive for sealing the screw access chamber for single implant crowns provided no additional sealing capacity compared with composite resin alone. Composite resin restorations placed in the traditional manner without sealing provided excellent resistance to microleakage.

Fracture strength of zirconia and lithium disilicate restorations following endodontic access.

To compare the fracture load of zirconia and lithium disilicate crowns prepared with endodontic access with fine and coarse diamond instruments. 0.8 mm (3Y zirconia) or 1 mm (lithium disilicate) crowns were luted to resin composite dies with resin-modified glass ionomer (zirconia) or self-adhesive resin (lithium disilicate) cement. A 2.5 mm endodontic access hole was placed in each crown with fine (8369DF.31.025FOOTBALL) or coarse (6379 DC.31.023FOOTBALL) diamond instruments and restored with composite. A control group was prepared without access holes. Crowns were thermocycled for 10,000 cycles (5-55°C) and tested in compression with a steel indenter until failure (n=8/group). A one-way ANOVA and Dunnett 2-sided test (alpha=0.05) compared differences in fracture load between groups. For zirconia, there was no statistical difference between the control group (2335 ± 160 N) and coarse diamond group (2345 ± 246 N); however, the fine diamond group (2077 ± 216 N) was significantly lower. For lithium disilicate, there was no statistical difference between the control group (2113 ± 183 N) and the fine (2049 ± 105 N) or coarse (2240 ± 118 N) groups. 3Y zirconia crowns became weaker when accessed with a fine diamond instrument. There was no negative effect of the endodontic access with bonded lithium disilicate crowns. Conservative endodontic access openings in high-strength ceramic restorations do not have a negative effect on their static fracture load. The coarse zirconia-cutting diamond rotary instrument is more efficient and has a less detrimental effect on the strength of the crowns than a fine diamond rotary instrument.

Digital assessment of marginal accuracy in ceramic crowns fabricated with different marginal finish line configurations.

To digitally evaluate the marginal adaptation of different types of all-ceramic crowns on two different marginal configurations such as Shoulder and Chamfer of the prepared teeth. The marginal fit of the crown specimens on the model of the prepared teeth were assessed using the dual scan replica technique. Two plastic teeth were prepared; one having a Shoulder finishing line (A), while the other with a Chamfer finish line (B). Three types of different all-ceramic crowns, layered zirconia, monolithic zirconia, and pressed lithium disilicate, were fabricated for each prepared tooth (A) and (B). Measurements for assessing the marginal discrepancy were done on four designated points on the mesial, distal, buccal, and palatal surface of the superimposed image. The average marginal gap was the largest in the layered zirconia group, followed by monolithic zirconia, while pressed lithium disilicate crowns showed the least marginal gaps. Crowns with Chamfer finish lines were better fitting (less marginal gap) than the shoulder finish lines across all the groups. Chamfer finish lines produced better fitting restorations, and heat-pressed lithium disilicate crowns showed better adaptation at the margins than both layered zirconia and monolithic zirconia.

The impact of different dental restorations on detection of proximal caries by cone beam computed tomography.

To evaluate the effects of different types of restorations on observer ability to detect proximal caries in CBCT images. Forty human premolars and molars with artificial proximal caries were placed proximal and distal to 5 molars having different restorations (amalgam, composite, resin-modified glass ionomer cement (RMGIC) fillings, zirconia, and lithium disilicate crowns) and a non-restored molar. CBCT scans were obtained using i-CAT®Next Generation. Images were rated twice by 2 observers. The exact depth of artificial caries was histologically established. Sensitivity, specificity, and area under the receiver operating characteristic curve (Az) values were calculated. Caries detection in teeth surfaces mesial and distal to amalgam showed compromised specificity and accuracy. Moreover, caries detection in teeth surfaces mesial to zirconia crown showed low sensitivity, specificity, and accuracy. Capability of CBCT in detection of proximal caries in teeth adjacent to composite, RMGIC, and lithium disilicate was comparable to those adjacent to non-restored molar. CBCT scans performed for tasks other than caries detection should be assessed for proximal caries in absence of any restorations as well as in presence of composite, RMGIC fillings, and lithium disilicate crowns. However, CBCT should not be used for proximal caries detection in teeth adjacent to amalgam and teeth surfaces mesial to zirconia crowns. It is important to investigate the influence of artifacts produced by various restorations on CBCT-based caries detection to optimize CBCT benefits, caries diagnosis and avoid unnecessary treatment of sound surfaces.

Cemented versus screw-retained posterior implant-supported single crowns: A 24-month randomized controlled clinical trial.

To compare the incidence of biological and technical complications of cemented and screw-retained monolithic lithium-disilicate implant-supported posterior single crowns. Forty-one subjects with a total of 56 implants received randomly allocated 28 cemented and 28 screw-retained crowns. In the screw-retained group, monolithic lithium-disilicate restorations were luted to titanium bases extraorally. In the cemented group, monolithic lithium-disilicate crowns were cemented on individualized titanium abutments intraorally. All restorations were examined according to modified FDI criteria within 2weeks of inserting the crowns (baseline) and after 12 (n=46) and 24 (n=43) months. Bone loss was evaluated by standardized radiographs at baseline and 12months. After 12months, the incidence of mucositis (positive bleeding on probing) was 14.2% (screw-retained) and 17.9% (cement-retained). The gingival and plaque index and a mean marginal bone loss between 0.03-0.15mm showed no significant difference between the groups. In the cemented group, cement residues were detected at baseline at two restorations (6.9%) by radiographic examination. A complete digital workflow was realized in most cases (85.7%). At 24months, no restoration had failed, and no chipping of the ceramic had occurred. In the screw-retained group, screw loosening occurred in one implant. In both groups, there was obvious deterioration in the quality of 32% of the occlusal and of 18% of the proximal contact points. The type of retention mode of monolithic implant-retained lithium-disilicate posterior crowns had no influence on the biological and technical complication rate.

Evaluation of biocompatibility of veneered Bio HPP and veneered lithium disilicate crowns in anterior zone (Randomized controlled clinical trial)

Objective: Evaluation of the biocompatibility of Bio-High Performance Polymer (Bio HPP) crowns veneered with Visio-Ling versus e.max crowns veneered with e.max veneering system. Material and Methods: 42 full-coverage crowns were fabricated for maxillary anterior teeth. A swap was obtained using a sterile paper cone to determine bacterial count and type. Pocket depth (PD) was determined using a William Periodontal probe. Measurements were repeated after 3, 6, 9 and 12 months respectively. Patients were randomly divided into: Group A fabricated from IPS e.max crowns and Group B fabricated from Bio HPP crowns. The preparations were standardized with an equi-gingival, finish line. Fisher’s test was used to compare between the two groups. The significance level was set at P 0.05. Statistical analysis was performed with Windows, Version 23.0. (IBM SPSS Statistics) Armonk, NY: IBM Corp. Results: Bio HPP and e.max showed no statistically significant difference in bleeding on probing and PD except after 9and 12 months; Bio HPP showed statistically significantly higher PD than e.max (P-value = 0.027, Effect size = 0.245) and (P-value = 0.011, Effect size = 0.310), respectively. Fisher’s test showed there was no statistically significant difference between total bacterial counts and the type of the two materials. Conclusion: Both e.max and Bio HPP crowns revealed successful biological behavior. No significant difference between the materials regarding the bacterial count and type as well as the pocket depth, however after 9 and 12 months, Bio HPP showed a higher significant difference PD than e.max. Keywords Biocompatibility; Bio HPP; e.max; Pocket depth; Bacteria.

Assessment of the survival and success rates of lithium disilicate crowns after different surface finishing procedures: An in vitro study

Statement of problemEvidence is limited for the impact of clinical adjustments and polishing on the longevity of glazed lithium disilicate restorations. PurposeThe purpose of this in vitro study was to evaluate the influence of surface finishing on the survival and success rates of lithium disilicate restorations based on fatigue resistance and failure mode. Material and methodsLithium disilicate (IPS e.max CAD) maxillary premolar crowns (N=54) were cemented on a dentin analog. The restorations were divided into 3 groups: overglaze (OG), abrasion (GA), and abrasion and polishing (AP). The crowns were submitted to cyclic fatigue in 37 oC water at 100 N and 2 Hz in 2 lifetimes. The load was applied to the occlusal surface by using anatomic pistons to simulate a clinical tripod occlusal contact. After cycling, the crowns were examined for failure (cracking, chipping, or catastrophic fractures) under optical and scanning electron microscopy. Cracking was considered either a structural failure (success analysis) or a survival (clinical criteria - survival analysis). Data were analyzed by using the log rank Kaplan-Meier and Holm-Sidak tests (α=.05). ResultsSurface finishing had no influence on the structural integrity of lithium disilicate, with similar success rates (P=.720). The calculated survival rate was higher for AP than that for other groups (P=.028). Cracking was found for GA and AP crowns, mostly initiating from the external surface. Chipping occurred in all experimental groups, and AP crowns did not show catastrophic failures. ConclusionsAlthough surface treatments had no influence on the success of lithium disilicate, polishing showed a positive effect on the survival rate of the crowns based on the clinical implications of cracking (no need for replacement).

Comparison of the residual cement on custom computer-aided design and computer-aided manufacturing titanium and zirconia abutments: A preliminary cohort study

Statement of problemClinical studies comparing the occurrence and quality of residual cement between custom zirconia and custom titanium abutments with subgingival margins are scarce. PurposeThe purpose of this clinical study was to assess the difference in the amount of residual cement between custom zirconia and titanium abutments with a 1-mm subgingival margin. Material and methodsEighty participants were randomized to receive either a custom zirconia abutment with a bonded titanium insert or a custom titanium abutment with a 1-mm subgingival margin on a posterior bone-level implant. Monolithic lithium disilicate crowns with a screw-access hole were cemented to abutments randomly with either a resin-modified glass ionomer cement or a resin cement. After cementation, the crown-abutment assemblies were removed and photographed from the mesial, buccal, distal, and lingual of the specimen to record the residual cement. The length along the abutment margin of each aspect of the assembly was measured. The surface area of the residual cement (SA) and the surface area of the residual cement per unit length of margin (SA_P) were calculated. Results for the groups were compared with the Fisher exact test, the Friedman test, and the Mann-Whitney U test (α=.05). ResultsThe median (lower quartile, upper quartile) of SA and SA_P for the custom zirconia abutment with a bonded titanium insert was 1.9 (0.5, 3.9) mm2 and 0.086 (0.032, 0.02) mm2, respectively, and for the custom titanium abutment, the values were 2.9 (1.3, 5.1) mm2 and 0.138 (0.062, 0.239) mm2, respectively. No significant difference was found between the custom zirconia abutments with bonded titanium inserts and titanium abutments for SA (P=.075) and SA_P (P=.083) with the Mann-Whitney U test. No significant difference was found in residual cement between the 4 aspects of the abutment (SA: P=.852; SA_P: P=.954) with the Friedman test and between the 2 types of cement (SA: P=.447; SA_P: P=.878) with the Mann-Whitney U test. ConclusionsA similar amount of subgingival residual cement was recorded around the abutment-crown assembly, regardless of the abutment material or cement type used.

Residual stresses explaining clinical fractures of bilayer zirconia and lithium disilicate crowns: A VFEM study

ObjectiveTo understand the stress development in porcelain-veneered zirconia (PVZ) and porcelain-veneered lithium disilicate (PVLD) crowns with different veneer/core thickness ratios and cooling rates. To provide design guidelines for better performing bilayer restorations with the aid of Viscoelastic Finite Element Method (VFEM). MethodsThe VFEM was validated by comparing the predicted residual stresses with experimental measurements. Then, the model was used to predict transient and residual stresses in the two bilayer systems. Models with two different veneer/core thickness ratios were prepared (2:1 and 1:1) and two cooling protocols were simulated (Fast: ∼300 °C/min, Slow: ∼30 °C/min) using the heat transfer module, followed by stress analysis in ABAQUS. The physical properties of zirconia, lithium disilicate, and the porcelains used for the simulations were determined as a function of temperature. ResultsPVLD showed lower residual stresses than PVZ. The maximum tensile stresses in PVZ were observed in the cusp area, whereas those in PVLD were located in the central fossa. The 1:1 thickness ratio decreased stresses in both layers of PVZ. Slow cooling slightly decreased residual stresses in both systems. However, the cooling rate effect was more evident in transient stresses. SignificanceSlow cooling is preferable for both systems. A thinner porcelain layer over zirconia lowers stresses throughout the restoration. The different stress distributions between PVZ and PVLD may affect their failure modes. Smaller mismatches in modulus, CTE, and specific heat between the constituents, and the use of low Tg porcelains can effectively reduce the deleterious transient and residual tensile stresses in bilayer restorations.

Mechanical Performance of Chairside Ceramic CAD/CAM Restorations and Zirconia Abutments with Different Internal Implant Connections: In Vitro Study and Finite Element Analysis.

(computer-aided design-computer-aided manufacturing) CAD/CAM monolithic restorations connected to zirconia abutments manufactured with a chairside workflow are becoming a more common restorative option. However, their mechanical performance is still uncertain. The aim of this study was to evaluate the mechanical behavior of a combination of a zirconia abutment and monolithic all-ceramic zirconia and lithium disilicate crown manufactured with a chairside workflow, connected to titanium implants with two types of internal connection—tube in tube connection and conical connection with platform switching. They were thermally cycled from 5 °C to 55 °C and were subjected to a static and fatigue test following ISO 14801. The fractured specimens from the fatigue test were examined by SEM (scanning electron microscopy). Simulations of the stress distribution over the different parts of the restorative complex during the mechanical tests were evaluated by means of (finite element analysis) FEA. The mechanical performance of the zirconia abutment with an internal conical connection was higher than that of the tube in tube connection. Additionally, the use of disilicate or zirconia all-ceramic chairside CAD/CAM monolithic restorations has similar results in terms of mechanical fracture and fatigue resistance. Stress distribution affects the implant/restoration complex depending on the connection design. Zirconia abutments and monolithic restorations seem to be highly reliable in terms of mechanical resistance.

Evaluation of the marginal and internal gaps of partially crystallized versus fully crystallized zirconia-reinforced lithium silicate CAD-CAM crowns: An in vitro comparison of the silicone replica technique, direct view, and 3-dimensional superimposition analysis

Statement of problemThe crystallization process of lithium disilicate crowns has been reported to cause dimensional change, but whether the fit of chairside computer-aided design and computer-aided manufactured (CAD-CAM) zirconia-reinforced lithium silicate crowns is affected is unclear. PurposeThe purpose of this in vitro study was to evaluate with a 3-dimensional superimposition analysis technique the marginal and internal adaptation of fully crystallized versus partially crystallized zirconia-reinforced lithium silicate ceramic CAD-CAM fabricated crowns. Additionally, the silicone replica technique and direct viewing of marginal gap values were compared with the 3-dimensional superimposition analysis technique. Material and methodsThe marginal and internal adaptation of a fully crystallized zirconia-reinforced lithium silicate (CELTRA DUO) were compared with those of a partially crystallized zirconia-reinforced lithium silicate (VITA SUPRINITY) after crystallization. Sixteen crowns (n=8) were fabricated with a chairside CAD-CAM system. The crowns and die and crown assembly were scanned with an optical scanner for the 3-dimensional superimposition analysis. Four hundred sixty-eight measurements were made for each crown, 78 in each 2-dimensional section. Marginal discrepancy was measured by using the direct viewing technique. The internal adaptation of the shoulder area, axial space, and occlusal space was measured by using the silicone replica technique. Both gap values were compared with the 3-dimensional superimposition analysis results by using the independent t test. The 2-way ANOVA was used to detect the effect of each variable (group and site) (α=.05). ResultsThe VITA SUPRINITY crowns showed statistically higher marginal discrepancy values than the CELTRA DUO crowns in both 3-dimensional superimposition analysis and the direct viewing method, and the lingual aspect recorded the highest marginal discrepancy mean value when compared with other aspects. The 3-dimensional superimposition analysis and the direct viewing method were statistically similar (P=.076). The VITA SUPRINTY crowns showed higher internal gap mean values than the CELTRA DUO crowns in both 3-dimensional superimposition analysis and silicone replica techniques. The occlusal space recorded the highest mean value in both groups. Assessment by 3-dimensional superimposition analysis and silicone replica techniques showed statistical difference in internal gap values (P=.04). ConclusionsCELTRA DUO showed better precision fit values than VITA SUPRINITY. Three-dimensional superimposition analysis is a reliable method of evaluating marginal and internal adaptation.

Fracture Resistance of Single-Unit Implant-Supported Crowns: Effects of Prosthetic Design and Restorative Material.

To evaluate the fracture resistance and fracture patterns of single implant-supported crowns with different prosthetic designs and materials. One hundred and forty-four identical crowns were fabricated from zirconia-reinforced lithium silicate (ZLS), leucite-based (LGC), and lithium disilicate (LDS) glass-ceramics, reinforced composite (RC), translucent zirconia (ZR), and ceramic-reinforced polyetheretherketone (P). These crowns were divided into 3 subgroups according to restoration design: cementable crowns on a prefabricated titanium abutment, cement-retained crown on a zirconia-titanium base abutment, and screw-cement crown (n = 8). After adhesive cementation, restorations were subjected to thermal-cycling and loaded until fracture. The fracture patterns were evaluated under a stereomicroscope. Statistical analysis was performed by using 2-way ANOVA/Bonferroni multiple comparison post hoc test (α = 0.05). For each prosthetic design, ZR presented the highest fracture resistance (p ≤ 0.005). Other than the differences with ZLS and RC for screw-cement crowns (p > 0.05) and RC for crowns on zirconia-titanium base abutments (p > 0.05), LGC showed the lowest fracture resistance. P endured higher loads than LDS (p < 0.001), except for the crowns on zirconia-titanium base abutments (p > 0.05). Cementable crowns presented the highest fracture resistance (p < 0.001), other than LGC and LDS. The differences between LGC crowns (p > 0.05) or LDS crowns on prefabricated titanium and zirconia-titanium abutments were nonsignificant (p = 0.133). Fragmented crown fracture was predominant in most of the restorations. Screw and abutment fractures were observed in ZR screw-cement crowns, and all P crowns were separated from the abutments. Restorative material and restoration design affect the fracture resistance and fracture pattern of implant-supported single-unit restorations. Clinicians may restore single-unit implants in premolar sites with the materials and prosthetic designs tested in the present study.

Comparison of the Fracture Resistance and Fracture Mode of Contemporary Restorative Materials to Overcome the Offset of Mandibular Implant-Supported, Cement-Retained Crowns

Background: The purpose was to compare the fracture resistance and the mode of failure of different contemporary restorative materials to restore implant supported, cement-retained mandibular molars. Methods: Two 5 × 10 mm titanium dental implants were mounted in resin blocks and prefabricated titanium and zirconia abutments were connected to each implant. Each implant received forty crowns resembling mandibular first molars. The specimens were divided into four groups (n = 10/group) for each abutment according to the type of material; Group A: porcelain fused to metal crowns; Group B: monolithic zirconia crowns; Group C: zirconia coping with ceramic veneer; Group D: all ceramic lithium disilicate crowns. Specimens were cemented to the abutments, mounted into a universal testing machine, and vertical static load was applied at a speed of 1 mm/min. The test stopped at signs of visual/audible fracture/chipping. Fracture resistance values were analyzed using ANOVA and Tukey’s tests (α ≤ 0.05). The modes of failure were visually observed. Results: A statistically significant difference (p < 0.001) of the fracture resistance values among tested groups was found. The group that showed the highest fracture resistance was Group A for both the titanium and the zirconia abutments (3.029 + 0.248 and 2.59 ± 0.39, respectively) while Group D for both abutments (1.134 + 0.289 and 1.68 ± 0.13) exhibited the least resistance. Conclusions: Fracture resistance and fracture mode varied depending on type of restorative material. For both titanium and zirconia abutments, porcelain fused to metal showed the highest fracture resistance values followed by monolithic zirconia.

Finite Element Analysis of Stress in Anterior Prosthetic Rehabilitation with Zirconia Implants with and without Cantilever.

Objectives The aim of this study was to evaluate using finite element analysis (FEA), the stress distribution in prostheses (lithium disilicate crowns) on monotype zirconia implants with and without cantilever in the anterior region of the maxilla. Materials and Methods From a virtual reconstruction of bone model of the toothed maxilla from a computed tomography, three models (groups) were created: Zr (11–21)—implants placed in the area of 11 and 21 with cantilever; Zr (12–22)—implants placed in the area of 12 and 22 without cantilever; and Zr (11–22)—implants intercalated placed in the area of 11 and 22. In all models, monotype zirconia implant (4.1 × 12.0 mm) was used in single-body configuration. Lithium disilicate crowns were designed on the implants and pontics for all groups. A 150-N load was applied to the prostheses. The materials used were considered isotropic, homogeneous, and linearly elastic. FEA was performed to evaluate the maximum (tensile) and minimum (compressive) principal stresses in the implant, crowns, and bone tissue. Data were analyzed qualitatively and quantitatively. Results For all groups, the highest maximum principal stress occurred in the palatal cervical area of the implant, with the high values for the Zr (12–22) group and the low values for the Zr (11–21) group. The maximum principal stress was concentrated in the cervical palatal area of the crown, with the Zr (11–21) group presented the highest values and the Zr (12–22) group showed the lowest values. In the bone tissue all the groups presented similar values of maximum and minimal principal stress, with the palatal (maximum principal) and vestibular (minimum principal) close to the cervical of the implants the area with the highest concentration of stresses. Conclusions The position of monotype zirconia implant did not interfere in the bone tissue stress, and the implants placed in the 11–21 present lower stress in implants and higher in the crown. The cantilever does not increase the stress in the implants, crown, and bone tissue.

Comparison of Fracture Resistance for Chairside CAD/CAM Lithium Disilicate Crowns and Overlays with Different Designs.

To determine the fracture resistance of chairside computer-aided design and computer-assisted manufacturing (CAD/CAM) lithium disilicate full coverage crowns and two different designs of overlay restorations for premolars. CAD/CAM lithium disilicate (IPS e.max CAD for CEREC/HT A1 C14, Ivoclar Vivadent) restorations (15 specimens/group) with 1.5 mm occlusal thickness and 1.0 mm chamfer were designed and fabricated with a chairside CAD/CAM system (CEREC, Dentsply Sirona). The restorations were prepared in three different designs: (1) full coverage crowns, (2) overlays with the margin located 2 mm above the gingiva, and (3) overlays with the margin located 4 mm above the gingiva. Restorations were cemented using conventional resin luting cement (Multilink, Ivoclar Vivadent) with primer system (Monobond Plus, Ivoclar Vivadent) to resin printed dies, load cycled (2,000,000 load cycles at 1 Hz with 275 N force), and then finally loaded with a steel indenter until failure. Scanning electron microscopy observations of fractured surfaces were also conducted. Group results were analyzed with one-way analysis of variance, and the medians were evaluated independently with Kruskal-Wallis. The fracture force of CAD/CAM lithium disilicate restorations was significantly different (p < 0.001) depending on the design of the restoration. Full coverage crowns showed significantly higher force to fracture (1018.8 N) than both types of overlays (p = 0.002 for overlay 2.0 mm and p < 0.001 for overlay 4.0 mm above gingiva). Among the two overlays, the restoration with the margin located 2 mm above the gingiva showed significantly higher fracture force (813.8 N) than the overlay with margin at 4 mm above the gingiva (436.1 N; p < 0.001). The fracture appearance of the crowns was much more complex than that of the overlay restorations. Full coverage chairside CAD/CAM lithium disilicate premolar crown showed higher fracture resistance than overlay restorations. Overlays with the margin located 2 mm above the margin demonstrated higher resistance than those with the margin located 4 mm above the gingiva.

Metal-ceramic and porcelain-veneered lithium disilicate crowns: a stress profile comparison using a viscoelastic finite element model

Metal-ceramics (MC) are one of the oldest dental restorative systems, which are considered to be the gold standard for full crown restoration. Porcelain-veneered lithium disilicate (PVLD), on the other hand, are newer material systems that have shown high survival rate in clinical follow-ups but needs to be studied more. This study compares the stresses developed in the single crowns made from newer PVLD system against those with MC configuration. For this comparison, influence of the layer thickness and cooling rates is also taken into consideration. An experimentally validated viscoelastic finite element model (VFEM) has been developed to predict the stress profile in these systems. Three-dimensional rotationally symmetric crowns were analyzed using this validated model for both material systems, three veneer to core thickness ratios (2:1, 1:1, 1:2), and two cooling rates: slow cooling at 1.74E-5 W/mm2 (∼30 K/min) and fast cooling at 1.74E-4 W/mm2 (∼300 K/min). PVLD showed lower values of transient and residual stresses than MC. The maximum tensile residual stresses in MC systems were observed in the cusp area, whereas those in PVLD were located in the central fossa. With the reduction in veneer layer, there was reduction in residual stress in MC; however, the veneer thickness had little to no effect in PVLD. The effect of cooling rate was also evident as slow cooling resulted in lower residual and tensile stresses for both material systems.

Microcomputed tomography void analysis after cement cleanup methods

Statement of problemCement at the restorative margin after implementation of the manufacturer’s recommended procedure for cement cleanup has not been extensively evaluated. PurposeThe purpose of this in vitro study was to use microcomputed tomography (μCT) to evaluate the number, volume, and width of voids left at the margin when dental cement is cleaned during the cementation process as per 2 manufacturer-prescribed methods for cement cleanup by evaluating a self-adhesive resin cement and a resin-modified glass ionomer cement. Material and methodsTwenty molar Ivorine Typodont prepared teeth were scanned, and lithium disilicate crowns were milled and cemented to the prepared teeth as per the manufacturer-recommended cementation process. Two methods of cement cleanup were performed: the excess cement was tack-polymerized with a polymerization light, or a delay of 3 minutes was used for the excess cement to reach partial polymerization, and the excess cement was removed with a sharp sickle scaler. These 2 methods of cement cleanup were performed with 2 cements: a resin-modified glass ionomer cement and a self-adhesive resin. Each specimen was then scanned with a μCT machine. The buccal margin of each specimen, from mesiobuccal line angle to distobuccal line angle, was examined, and the number, volume, and width of any voids remaining at the margin open to the environment were measured. For statistical analysis, the specimens were first grouped by method of cement cleanup and then grouped by cement type. The Wilcoxon ranked sum test was implemented because of the lack of a normal distribution and the heterogeneity of the data (α=.05). ResultsNo statistically significant difference was found in the median number of voids, median volume of voids, or median width per void between the 2 cleanup methods, irrespective of cement type (P>.05). Significant differences were found in the median number of voids, median volume of voids, and median width per void when comparing cement types, irrespective of cement cleanup methods (P<.05). ConclusionsThe choice of cement may be more important than the method of cement cleanup when considering voids left at the margin. Microcomputed tomography is an excellent nondestructive tool for volumetric measurements of voids at the margin.

FRACTURE RESISTANCE OF ENDODONTICALLY TREATED TEETH RESTORED WITH LITHIUM DISILICATE CROWNS RETAINED WITH FIBER POSTS COMPARED TO CERASMART AND CELTRA DUO ENDOCROWNS (IN VITRO STUDY)

Aim: The purpose of this study wasto evaluate the fracture resistance of cerasmart and Celtra Duo endocrowns compared to endodontically treated teeth restored with glass fiber post and lithium disilicate crowns. Methodology: Eighteen sound maxillary premolars were endodontically treated, then divided into three groups: Group (GE) received fiber post, resin core and lithium disilicate (IPS E.max CAD, Ivoclar-Vivadent) crown. Group (GC) received endocrown with butt joint finish line design fabricated from hybrid ceramic (CERASMART, GC Dental USA). Group (GD) received endocrown with butt joint finish line design fabricated from zirconia reinforced lithium disilicate (CELTRA DUO, Dentsply Sirona). All restorations were fabricated using (CEREC MC XL SW 4.0) milling machineand cemented with self-adhesive dual cureresincement. Thermo-mechanical fatigue was applied on all samples and fracture test was then done. Scanning electron microscope was used for examining the fractured samples. Results: Fracture resistance test showed that there was statistically significant difference between the three groups (P-value = 0.001, Effect size = 0.603).GE showed statistically significant highest median fracture value (1866 +/- 399.9).Meanwhile, there was no statistical difference between mean fracture resistance values of GC of (1045.1 +/- 122.1) and GD (1377 +/-307) where both showed statistically significant lowest mean fracture resistance values. Conclusion: Conventional post, core andcrown treatment option remains the gold standard for treating endodontically treated maxillary premolar, nevertheless endocrown restoration on premolars needs further research to become a viable restoration in this condition.

Effect of Thermo-Mechanical Aging and Repressing on Fracture Resistance of Lithium Disilicate Crowns

Objective: The recycling of heat pressed lithium disilicate glass-ceramic leftover material has been reported to be done by dental laboratories. The effect of this procedure on the fracture resistance of single crowns is unknown, especially when it is functioning inside the oral cavity with subsequent exposure to temperature changes and cycles of mastication. Material and Methods: A total of 28 lithium disilicate glass-ceramic crowns (IPS emax Press) were constructed and randomly assigned into two groups (n = 14); Group (P): Included crowns fabricated from new e.max ingots. Group (R): Included crowns fabricated from repressed e.max buttons. Specimens of each group were divided into two equal subgroups (n = 7) according to whether the aging of specimens will be performed or not before fracture resistance testing. Subgroup (N), samples were subjected to fracture resistance without thermo-mechanical aging, while subgroup (A), samples were subjected to thermo-cycling and cyclic loading before being subjected to fracture strength testing. Different methods; SEM, XRD, EDAX were used to characterize the properties of lithium disilicate glass-ceramics before and after repressing. Results: The highest statistically significant fracture resistance value was recorded for the subgroup (RN) repressed/non-aged, followed by the subgroup repressed/aged (RA), while the lowest statistically significant mean value was recorded for the subgroup pressed/aged (PA). There was no significant difference between pressed/non-aged (PN) and repressed/aged (RA) subgroups. Conclusion: Repressing of leftover buttons may increase the fracture resistance of IPS emax Press crowns. Thermo-mechanical aging may negatively affect the fracture resistance of IPS emax Press crowns, yet Repressing may decrease this effect. Clinical implications: This is a novel approach that targets a point of research that has not been investigated before. It elaborates how repressing may decrease the effect of aging and increase the fracture resistance of lithium disilicate crowns. Thus, recycling of lithium disilicate glass ceramics might decrease its failure and prolong their serviceability. Keywords Fracture resistance; Heat pressed; Lithium disilicate; Recycling; Repressing; Thermo-mechanical aging.