Crown Copings Research Articles

A comparative study of marginal fit of copings prepared with various techniques on different implant abutments.

This study evaluated fabrication techniques of recently introduced all-ceramic copings' marginal adaptation on two different implant abutments with different finish lines. Five different copings were prepared (Casted chrome-cobalt metal coping, Zirkonzahn, Cercon, In Ceram Alumina and IPS e.max Press) on two cementable implant abutments with two marginal designs. Ten samples for each coping group were prepared (totally 100 samples). Copings were cemented to implant abutments and marginal gap measurements were done from 24 points with stereomicroscope and the datas were analyzed by the Kruskal-Wallis test before cementation. Cercon copings showed the lowest marginal fit scores and metal copings showed the highest scores. After cementation, all marginal gap values have been increased. All marginal gap values obtained from crown copings can be considered in clinically acceptable limits (<120 µm) except metal copings after cementation on abutment with 135 degrees shoulder group (123 µm).

CBCT AND SEM EVALUATION OF MARGINAL AND INTERNAL FIT OF LITHIUM DISILICATE COPINGS FABRICATED WITH 2 DIFFERENT TECHNIQUES (IN VITRO STUDY)

Statement of the problem. The marginal & internal adaptation of all-ceramic crowns may vary depending on the technique of fabrication and the methodology of measurement. Purpose. To evaluate the marginal and internal adaptation of lithium disilicate crown copings fabricated with two techniques using two measuring modalities, cone beam computed tomography CBCT and scanning electron microscopy (SEM). Materials and methods. The materials selected were Emax CAD for CEREC inlab (Group 1) and Emax Press for the lost wax and heat pressing (Group2). 10 copings were designated for each group. The copings were cemented on standardized epoxy resin dies with self-adhesive resin cement RelyX Uncem. A CBCT system was used to obtain measurements from reconstructed slices in two orthogonal planes for each sample, and a scanning electron microscope (SEM) was used for cross-sectioned samples. Mean, SD, median, range and 95% confidence level were calculated using Kolmogrov-Smirnov test and comparison of the ceramic types and methodology of testing were accomplished with Mann-Whitney U test and Spearman correlation tests respectively the significance level being set at P<0.05. Results. Heat pressing exhibited lower vertical gap margin in SEM 22.7um (±5.3) than Emax CAD 47um (±16.1). The overall values of misfit of Emax CAD were significantly higher than Emax press in SEM & CBCT measurements. There was statistical correlation between the two measuring methods in both the mesial, lingual and total measurements according to Spearman correlation test. Conclusion. The Emax press has a better overall marginal and internal fit than the Emax CAD, still the values are clinically acceptable for both.

Effect of Circumferential and Axial Grooves on the Retention of Provisionally Cement-Retained Implant-Supported Crowns (In Vitro Study)

Introduction: Retention of provisionallycemented implant-supported restorations plays an important role in success of the treatment. Uncementedrestorations may cause several problems such as restoration swallowing, increased bone loss and prosthesis failure. Therefore, suitable cementcompromising between retention and retrievability as well as extra-means of abutment retention might be preferable for durable implantrestorations.Objectives: This study introduced circumferential and axial grooves on implant abutments as retentive promoters and evaluated their effect on theretention of crown copings cemented to implant abutments with provisional luting cements.Materials and methods: Twenty straight titanium implant abutments were divided into 4 groups (n=5): without grooves, with 1 axial groove, with2 circumferential grooves, and with 3 circumferential grooves. Twenty nickel chromium crown copings were fabricated to fit all 20 abutments. Thecopings were cemented to each group of abutments with non-eugenol provisional cement. After storage for 24 hours in 100% humidity at 37°C,specimens were subjected to 500 cycles of thermal cycling, then 5000 cycles of compressive load to simulate the thermal and mechanical stresses inthe oral environment. Tensile strengths were conducted with a universal testing machine and tensile stresses were recorded in Newton. After theretention test, the copings and abutments were evaluated for cement failure mode with Stereoscopic Microscope. Collected data were analyzedusing one-way analysis of variance (ANOVA) and Tukey’s HSD tests (α=.05).Results: The mean tensile strength of the three circumferential grooves was significantly higher than the other groups (P<0.001) while there was nosignificant difference between the axial groove and control group (P=0.999). The cement failure mode was generally adhesive in nature, althoughsome mixed failures were observed.Conclusion The surface modification of an implant abutment by means of circumferential grooves was found to be an effective method ofimproving the retention of crown copings cemented with non-eugenol cement.

An Investigation into the Integrity of Fit of Provisional Crowns made over a Dental Implant Analogue Using Two Current Proprietary Provisional Crown Materials Compared to a Proprietary 'Snap On' Provisional Core

The aim of this study was to investigate the marginal accuracy of three currently used methods of direct provisional crown construction for the accuracy of marginal fit during in vitro fabrication. Three groups of provisional crowns were investigated using a modified McLean and von Fraunhofer technique. Proprietary provisional crown coping modified to tooth shape with a Bis-acryl temporary material was compared to Bis-GMA and isobutyl methacrylate resin provisional crowns with modified margins using a light cured flowable composite and ‘bead on’ isobutyl methacrylate respectively. Data was analyzed with the Mann Whitney test (a = 0 .05). Reliability was determined using the Bland Altman test. Bis-GMA acryl resin composite relined with flowable composite produced significantly better fitting restorations compared to the two other groups.

Evaluation of marginal and internal fit of ceramic and metallic crown copings using x-ray microtomography (micro-CT) technology

Statement of problemProsthetic crown fit to the walls of the tooth preparation may vary depending on the material used for crown fabrication. PurposeThe purpose of this study was to compare the marginal and internal fit of crown copings fabricated from 3 different materials. Material and methodsThe selected materials were zirconia (ZirkonZahn system, group Y-TZP), lithium disilicate (IPS e.max Press system, group LSZ), and nickel-chromium alloy (lost-wax casting, group NiCr). Five specimens of each material were seated on standard dies. An x-ray microtomography (micro-CT) device was used to obtain volumetric reconstructions of each specimen. Points for fit measurement were located in Adobe Photoshop, and measurements were obtained in the CTAn SkyScan software environment. Marginal fit was measured at 4 points and internal fit at 9 points in each coping. Mean measurements from the 3 groups were compared by analysis of variance (ANOVA) at the 5% significance level, and between-group differences were assessed with the Tukey range test. ResultsThe nickel-chromium alloy exhibited the best marginal fit overall, comparable with zirconia and significantly different from lithium disilicate. Lithium disilicate exhibited the lowest mean values for internal fit, similar to zirconia and significantly different from the nickel-chrome alloy. ConclusionsThe marginal and internal fit parameters of the 3 tested materials were within clinically acceptable range.

Retention properties of six different luting cements on titanium surface

800x600 Objectives: With conflicting results in the literature and various manufacturer recommendations, implant restorative cements can provide inadequate retention on implants, especially short or single implants. The aim of this study was to evaluate and compare the retentive properties of six different implant restorative cements on titanium surface. Materials and Methods: A total of 120 titanium rods of specimens (10 mm in length and 12 mm in diameter) were divided into 6 experimental groups (n=20) and six different cements were compared: Adhesor (A), Adhesor Carbofine (AC), Cavitan Cem (CC), Meron (M), Implacem (IM), and MIS Crown Set (MIS). Specimens were subjected to shear bond strength test by a universal testing machine with a crosshead speed of 1 mm/min. The data were analyzed with Kruskal-Wallis and Mann-Whitney U tests (α=0.05). Results: The highest mean bond strength was observed in specimens of group MIS, and followed by specimens of group AC. The adhesive failure mode was predominantly observed in all groups. Conclusions: Different cements on titanium surfaces provide different retention levels. Resin cement is the cement of choice for the definitive non-retrievable cementation of crown copings to implant abutments. Normal 0 21 false false false TR X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Normal Tablo; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin:0cm; mso-para-margin-bottom:.0001pt; mso-pagination:widow-orphan; font-size:10.0pt; font-family:Calibri,sans-serif;}

Evaluation of marginal and internal fit of ceramic crown copings

Objectives(1) To measure the marginal and internal adaptation of different prosthetic crowns infrastructures (IS); (2) to analyze two types of methodologies (replica and weight technique) used to evaluate the adaptation of indirect restorations. MethodsCeramic IS were fabricated using CAD/CAM technology and slip-casting technique, and metal IS were produced by casting (n=10). For each experimental group, the adaptation was evaluated with the replica (RT) and the weight technique (WT), using an impression material (low viscosity silicon) to simulate the luting agent. Cross-sectional images of the silicon replica were obtained and analyzed with Image J software to measure the low viscosity silicon layer thickness at pre-determined points. The silicon layer was also weighted. Results were statistically analyzed with ANOVA and Tukey's test (α=0.05). Pearson correlation was used to analyze the relation between the two types of evaluation methods. ResultsAll IS evaluated showed clinically acceptable internal and marginal adaptation. Metal IS showed the best adaptation, irrespective of the measuring technique (RT and WT). The IS produced by CAD–CAM showed greater gap values at the occlusal area than at other evaluated regions. The IS produced by the dental laboratory technician showed similar gap values at all evaluated regions. There is no correlation between RT and WT (p>0.05). SignificanceDifferent levels of adaptation were found for the different experimental groups and for the different evaluation methods. However, all IS evaluated showed clinically acceptable values of marginal and internal adaptation.

Misfit and microleakage of implant-supported crown copings obtained by laser sintering and casting techniques, luted with glass-ionomer, resin cements and acrylic/urethane-based agents

ObjectivesThis study evaluated the marginal misfit and microleakage of cement-retained implant-supported crown copings. MethodsSingle crown structures were constructed with: (1) laser-sintered Co–Cr (LS); (2) vacuum-cast Co–Cr (CC) and (3) vacuum-cast Ni–Cr–Ti (CN). Samples of each alloy group were randomly luted in standard fashion onto machined titanium abutments using: (1) GC Fuji PLUS (FP); (2) Clearfil Esthetic Cement (CEC); (3) RelyX Unicem 2 Automix (RXU) and (4) DentoTemp (DT) (n=15 each). After 60 days of water ageing, vertical discrepancy was SEM-measured and cement microleakage was scored using a digital microscope. Misfit data were subjected to two-way ANOVA and Student–Newman–Keuls multiple comparisons tests. Kruskal–Wallis and Dunn's tests were run for microleakage analysis (α=0.05). ResultsRegardless of the cement type, LS samples exhibited the best fit, whilst CC and CN performed equally well. Despite the framework alloy and manufacturing technique, FP and DT provide comparably better fit and greater microleakage scores than did CEC and RXU, which showed no differences. ConclusionsDMLS of Co–Cr may be a reliable alternative to the casting of base metal alloys to obtain well-fitted implant-supported crowns, although all the groups tested were within the clinically acceptable range of vertical discrepancy. No strong correlations were found between misfit and microleakage. Notwithstanding the framework alloy, definitive resin-modified glass-ionomer (FP) and temporary acrylic/urethane-based (DT) cements demonstrated comparably better marginal fit and greater microleakage scores than did 10-methacryloxydecyl-dihydrogen phosphate-based (CEC) and self-adhesive (RXU) dual-cure resin agents.

The Effect of Different Fabrication Steps on the Marginal Adaptation of Two Types of Glass‐Infiltrated Ceramic Crown Copings Fabricated by CAD/CAM Technology

Marginal adaptation is an important factor affecting the longevity of all-ceramic restorations, although the effects of different fabrication steps on marginal adaptation at various stages of fabrication are not fully understood. The purpose of this study was to assess with an in vitro model whether In-Ceram alumina (IA) or In-Ceram zirconia (IZ) copings produced by the CAD/CAM method would be clinically acceptable, and to evaluate the effect of each fabrication step (post-milling, post-trimming, and post-glass infiltration) on the marginal discrepancy of the coping. A melamine tooth was prepared, duplicated, poured with inlay wax, and then cast with metal to fabricate a master die. An InLab 3D system was used to scan the master die and to design and mill the copings. Thirty IA and IZ copings each were developed with thicknesses of 0.6 mm and a 30-μm thick computer luting space. Epoxy resin replicas of the master die were fabricated, and the vertical and horizontal marginal discrepancies were measured using a Micro-Vu optical microscope at three stages of the fabrication (post-milling, post-trimming, post-infiltration). One-way ANOVA was used to analyze the data between the three stages of fabrication for each marginal discrepancy, and a t-test was used to compare vertical and horizontal marginal discrepancies (after glass infiltration) between IZ and IA copings There were no significant differences (p > 0.05) in the vertical marginal discrepancies (μm) between IA (36 ± 14) and IZ (40 ± 14) copings after glass infiltration. ANOVA (comparing three stages within horizontal marginal discrepancy for IZ copings) showed that post-milling (40 ± 26) > post-trimming (23 ± 11) = post-infiltration (19 ± 13). ANOVA (comparing three stages within vertical marginal discrepancy for IZ copings) showed that post-milling (53 ± 12) = post-trimming (47 ± 13) > post-infiltration (36 ± 14). ANOVA (comparing three stages within horizontal marginal discrepancy for IA copings) showed that post-milling (52 ± 28) > post-trimming (30 ± 16) > post-infiltration (30 ± 16). ANOVA (comparing three stages within vertical marginal discrepancy for IA copings) showed that post-milling (54 ± 13) = post-trimming (56 ± 26) > post-infiltration (40 ± 14). There was no significant difference in the marginal adaptation of both material copings. After the trimming process, the glass infiltration firing cycle improved the vertical marginal discrepancy for both IZ and IA copings. Clinical implications. IA and IZ copings fabricated by CAD/CAM technology have an acceptable marginal fit as documented in the literature, and the glass infiltration process improves the marginal fit after machining.

Vertical misfit of laser-sintered and vacuum-cast implant-supported crown copings luted with definitive and temporary luting agents

Objectives. This study aimed to evaluate the vertical discrepancy of implant-supported crown structures constructed with vacuum-casting and Direct Metal Laser Sintering (DMLS) technologies, and luted with different cement types. Study Design. Crown copings were fabricated using: (1) direct metal laser sintered Co-Cr (LS); (2) vacuum-cast Co-Cr (CC); and (3) vacuum-cast Ti (CT). Frameworks were luted onto machined implant abutments under constant seating pressure. Each alloy group was randomly divided into 5 subgroups (n = 10 each) according to the cement system utilized: Subgroup 1 (KC) used resin-modified glass-ionomer Ketac Cem Plus; Subgroup 2 (PF) used Panavia F 2.0 dual-cure resin cement; Subgroup 3 (RXU) used RelyX Unicem 2 Automix self-adhesive dual-cure resin cement; Subgroup 4 (PIC) used acrylic/urethane-based temporary Premier Implant Cement; and Subgroup 5 (DT) used acrylic/urethane-based temporary DentoTemp cement. Vertical misfit was measured by scanning electron microscopy (SEM). Two-way ANOVA and Student-Newman-Keuls tests were run to investigate the effect of alloy/fabrication technique, and cement type on vertical misfit. The statistical significance was set at α = 0.05. Results. The alloy/manufacturing technique and the luting cement affected the vertical discrepancy (p < 0.001). For each cement type, LS samples exhibited the best fit (p < 0.01) whereas CC and CT frames were statistically similar. Within each alloy group, PF and RXU provided comparably greater discrepancies than KC, PIC, and DT, which showed no differences. Conclusions. Laser sintering may be an alternative to vacuum-casting of base metals to obtain passive-fitting implant-supported crown copings. The best marginal adaptation corresponded to laser sintered structures luted with glass-ionomer KC, or temporary PIC or DT cements. The highest discrepancies were recorded for Co-Cr and Ti cast frameworks bonded with PF or RXU resinous agents. All groups were within the clinically acceptable misfit range. Key words:Dental alloy, laser sintering, implant-supported prostheses, vertical discrepancy, vertical misfit.

Bonding quality of contemporary dental cements to sandblasted esthetic crown copings*

To evaluate the shear bond strength of current luting cements to sandblasted crown-coping substrates. Specimens of nickel-chromium, pressable glass ceramic, and zirconia crown-coping substrates were sandblasted in three groups (n = 30 each) with 50 (group 1), 110 (group 2), and 250 μm (group 3) alumina particles at a pressure of 250 kPa. Cylinders of glass ionomer, universal resin, and self-adhesive resin cements were then built up on the sandblasted substrate surfaces of each group (n = 10). All bonded specimens were stressed to evaluate the cement-substrate shear bond strength. Both the mode and incidence of bond failure were also considered. No difference was noticed between all test groups in terms of cement-substrate bond strength. In comparison to self-adhesive type, the universal resin cement provided lower bond strengths to both metal and glass-ceramic substrates in group 1. The self-adhesive resin cement provided the highest bond strengths to the zirconia substrates in groups 2 and 3. The adhesive type of bond failure was common in the metal and zirconia substrates in all groups. Cement-substrate bonding quality is not affected by the size of sandblasting particles. Resin cements bond better to different coping substrates. Self-adhesive resin cement is the best choice to bond zirconia-based substrates.

An Analysis of the Effect of a Vent Hole on Excess Cement Expressed at the Crown–Abutment Margin for Cement‐Retained Implant Crowns

The labial margins of anterior implant-retained crowns are often positioned subgingivally for a superior esthetic appearance. One of the consequences of subgingival margins is the increased risk of leaving excess cement behind following cementation. This can lead to potential problems, including peri-implant inflammation, soft tissue swelling, soreness, bleeding or suppuration on probing, and bone loss. The purpose of this laboratory study was to investigate the effect of placement, location, and diameter of a vent hole on the amount of cement being expressed at the margin of an anterior implant abutment-retained crown. Three implant crown copings were fabricated to fit on the same custom abutment. Three vent diameters (0.75, 1.25, and 1.65 mm) and three locations on the palatal surface of the coping (cervico-palatally, mid-palatally, inciso-palatally) were chosen for vent hole placement. For each test, the coping was cemented onto the abutment under standardized conditions. A preweighed thin coating of cement was applied to the fit surface of the coping. The amount of cement expressed at the margin and vent hole was measured by weight and calculated as a proportion of the amount of cement placed in the coping before seating. The procedure was completed 15 times for each variable. The results were statistically analyzed using univariate ANOVA with post hoc Bonferroni-adjusted independent samples t-tests. The presence of a vent hole influenced the proportion of cement expressed at the coping margin (p < 0.05). The location of a vent hole influenced the proportion of cement expressed at the coping margin (p < 0.05), with the exception of the mid-palatal and inciso-palatal positioning where there was no significant difference (p= 0.61) between groups. The diameter of the vent hole did not significantly influence the proportion of cement expressed at the coping margin (p= 0.096). When using anterior cement-retained implant crowns, the use of a 0.75-mm mid-palatal or inciso-palatal vent hole to minimize the amount of cement expressed at the margin during cementation should be considered.

Retention of cast crown copings cemented to implant abutments

The cementation of crowns to dental implant abutments is an accepted form of crown retention that requires consideration of the properties of available cements within the applied clinical context. Dental luting agents are exposed to a number of stressors that may reduce crown retention in vivo, not the least of which is occlusal loading. This study investigated the influence of compressive cyclic loading on the physical retention of cast crown copings cemented to implant abutments. Cast crown copings were cemented to Straumann synOcta titanium implant abutments with three different readily used and available cements. Specimens were placed in a humidifier, thermocycled and subjected to one of four quantities of compressive cyclic loading. The uniaxial tensile force required to remove the cast crown copings was then recorded. The mean retention values for crown copings cemented with Panavia-F cement were statistically significantly greater than both KetacCem and TempBond non-eugenol cements at each compressive cyclic loading quantity. KetacCem and TempBond non-eugenol cements produced relatively low mean retention values that were not statistically significantly different at each quantity of compressive cyclic loading. Compressive cyclic loading had a statistically significant effect on Panavia-F specimens alone, but increased loading quantities produced no further statistically significant difference in mean retention. Within the limitations of the current in vitro conditions employed in this study, the retention of cast crown copings cemented to Straumann synOcta implant abutments with a resin, glass ionomer and temporary cement was significantly affected by cement type but not compressive cyclic loading. Resin cement is the cement of choice for the definitive non-retrievable cementation of cast crown copings to Straumann synOcta implant abutments out of the three cements tested.

High-strength CAD/CAM-fabricated veneering material sintered to zirconia copings — A new fabrication mode for all-ceramic restorations

Objectives With this in vitro study the fracture strength of zirconia-based crown copings being veneered with a CAD/CAM generated high-strength ceramic cap by sintering is compared with anatomically identical zirconia-based crowns, which were either overpressed or veneered by the layering technique for completion. Methods A 1.2 mm, 360° chamfer preparation was performed on a second maxillary molar and was dublicated 15 times in a cobalt–chromium-alloy. A sample of 45 zirconia copings was produced and divided into three groups. In the first group (VT) zirconia copings received conventional veneering in layering technique, in the second group the veneering porcelain was pressed over the zirconia coping (PT), and for the third group (ST) a CAD/CAM-fabricated high-strength anatomically shaped veneering cap was sintered onto the zirconia coping. All crowns were cemented conventionally onto their dies and tested in the universal testing machine until clinical failure. The fracture load data were compared by a one-way analysis of variance and a multiple comparison posthoc test ( α < 0.05). Results Specimens from group VT showed a mean (S.D.) fracture load of 3700.39 (1238.72) N, group OT 3523.73 (1181.11) N and group ST 6262.67 (2257.42) N. The difference between groups VT/OT and ST were statistically significant ( P < 0.001). Significance The new CAD/CAM-fabricated bilayered restorations (ST) were superior to the present techniques (VT and OT) in terms of fracture load and offer the possibility to produce cost-effective crowns and fixed partial dentures with a potential lower risk of chippings.

Fracture Strength and Fatigue Resistance of All‐Ceramic Molar Crowns Manufactured with CAD/CAM Technology

All-ceramic crowns are subject to fracture during function, especially in the posterior area. The use of yttrium-stabilized zirconium-oxide ceramic as a substructure for all-ceramic crowns to improve fracture resistance is unproven. The aim of this study was to compare fracture strength and fatigue resistance of new zirconium-oxide and feldspathic all-ceramic crowns made with computer-aided design/computer-aided manufacturing (CAD/CAM). An ivorine molar was prepared to receive an all-ceramic crown. Using epoxy resin, 40 replication dies were made of the prepared tooth. Twenty feldspathic all-ceramic crowns (Vita Mark II) (VMII) and 20 zirconium-oxide crown copings (In-Ceram YZ) (YZ) were made using CAD/CAM technique (CEREC-3D). The YZ copings were sintered and veneered manually with a fine-particle ceramic (VM9). All crowns were cemented to their respective dies using resin cement (Panavia F 2.0). Ten crowns in each group were subjected to compressive fatigue loading in a universal testing machine (instron). The other ten crowns from each group were loaded to fracture at a crosshead speed of 1 mm/min. Data were statistically analyzed using independent t-test and Fisher's exact test at alpha= 0.05. There was a significant difference between the survival rates of the two materials during the fatigue test (p < 0.001). All VMII crowns survived without any crack formation, while all YZ crowns fractured (40%) or developed cracks (60%). All the YZ crown fractures occurred within the veneering layer during the fatigue test. There was no significant difference in mean fracture load between the two materials (p= 0.268). Mean fracture loads (standard deviation) in N were: 1459 (492) for YZ crowns and 1272 (109) for VMII crowns. The performance of VMII crowns was superior to YZ crowns in the fatigue test. The premature fractures and cracks of the YZ crowns were attributed to weakness in the YZ veneer layer or in the core/veneer bond.

The effect of finish line preparation and layer thickness on the failure load and fractography of ZrO 2 copings

To prevent tooth weakening or pulp irritation, there is a need for a minimally invasive method of preparing single anterior crowns. Restoration dimensions for reduced coping thicknesses or less invasive finish line preparations are required. The purposed of this in vitro was to study investigate the fracture performance of high-strength zirconia copings, compare knife-edge margins with chamfer finish lines, and examine the effect of reducing the layer thickness from 0.5 mm to 0.3 mm. Y-TZP zirconia copings were manufactured on brass dies of a maxillary central incisor. Forty copings, with 2 layer thicknesses (0.5 and 0.3 mm), and 2 finish line preparations (knife edge and chamfer; n=10) were cemented using a conventional glass ionomer cement and stored in distilled water at 37 degrees C for 24 hours. The copings were vertically loaded until fracture using a universal testing machine. Data were analyzed by 2-way ANOVA (alpha=.05). Fractographic examination was performed using scanning electron microscopy and confocal laser scanning microscopy. A significantly higher mean failure load was measured for knife-edge (0.5 mm, 1110 +/-175 N; 0.3 mm, 730 +/-160 N) versus chamfer (0.5 mm, 697 +/-126 N; 0.3 mm, 455 +/-79 N) preparations (P<.001), and for 0.5-mm versus 0.3-mm thickness layers (P<.001). Knife-edge preparations present a promising alternative to chamfer finish lines; the fracture load required for knife-edge preparations was 38% greater than that required for chamfer preparations, regardless of coping thickness. Reducing the thickness of a single crown coping from 0.5 to 0.3 mm resulted in a 35% reduction in fracture load required for either preparation type.

Effect of Preparation Design on the Fracture Resistance of Zirconia Crown Copings

The aim of this in vitro study was to evaluate the effects of different preparation designs on the fracture resistance of single-crown zirconia frameworks. To this end, maxillary molar dies of CrCo alloy were fabricated with five different preparation designs: shoulderless, slight and pronounced deep chamfer, beveled and non-beveled shoulder. Ten zirconia copings with a wall thickness of 0.4 mm were fabricated for each type of preparation. After cementation by glass ionomer cement, they were loaded until fracture. There were significant differences in the breaking load of the experimental groups (ANOVA, p < 0.01). The shoulder preparation had a mean breaking load of 2286 N, the shoulderless preparation 2041 N, the beveled shoulder 1722 N, the pronounced deep chamfer 1752 N, and the slight chamfer 1624 N. Based on the results of this study, a shoulder preparation is highly recommended whenever possible. Moreover, for endodontically treated teeth that are structurally compromised or which have anatomically limited areas, the slight chamfer preparation is an optimal recommendation.

Effect of preparation angles on the precision of zirconia crown copings fabricated by CAD/CAM system

The aim of this in vitro study was to evaluate the effects of different preparation angles on the precision of fit of zirconia crown frameworks. Dies were fabricated with three different preparation angles: 4, 8, and 12 degrees total taper. Ten copings were fabricated for each angle by a laboratory and a milling-center CAD/CAM system. After cementation, cross-sections were obtained and cement gaps were measured. Preparation angle (ANOVA, p < 0.01) and measurement location (ANOVA, p < 0.01) exhibited statistically significant influence on the precision of fit. On the other hand, no statistically significant influences were detected between copings prepared using the laboratory and milling-center CAD/CAM systems (ANOVA, p = 0.92). All groups showed marginal openings ranging between 36.6 and 45.5 microm. In light of the results obtained in this study, a preparation angle of 12 degrees is hence recommended with the confidence that the marginal gap will be consistently less than 50 microm.

Effect of Sintering on the Marginal and Internal Fit of CAD/CAM-fabricated Zirconia Frameworks

The aim of this study was to investigate the effect of post-machining sintering on marginal and internal fit of CAD/CAM-fabricated zirconia frameworks. Single crown copings (A: abutment), three-unit bridge frameworks (APA, P: pontic), four-unit bridge frameworks (APPA), and five-unit bridge frameworks (A1P1A2P2A3) were fabricated with raw-stage zirconia blanks using a commercial CAD/CAM system (KATANA, Noritake Dental Supply Co. Ltd., Aichi, Japan). Crown copings and frameworks were cemented to their respective master abutment models, and thickness of the cement layer was measured at specific measuring points. Marginal and internal fit of both APA and APPA were within clinical acceptance. However, the marginal gap and thickness of the cement layer on the axial surface of the pontic side of APA and APPA were slightly higher than those of the non-pontic side. As for the marginal gap of A1P1A2P2A3 framework, it was superior to those of APA and APPA because the center abutment supported the framework to prevent distortion.

Thin‐wall ceramic CAD/CAM crown copings: strength and fracture pattern

The strength and fracture pattern of posterior CAD/CAM-generated crown copings with 0.4 mm wall thickness were evaluated in vitro hypothesizing that fracture resistance of YTZP-zirconia copings might be independent of mode of cementation whether resin-bonded or cemented because of the high strength of YTZP-zirconia. Two sets of copings (n = 15) each were fabricated using CEREC inLab CAD/CAM from (i) lithiumdisilicate glass-ceramic, (ii) infiltration ceramic as controls and (iii) YTZP-zirconia. Copings (n = 15) of ceramics (i), (ii) and (iii) each were (a) zinc-phosphate cemented, (b) adhesively seated on resin-based composite dies and loaded until fracture. Load (N) data was analysed using anova and Scheffé tests. Crack pattern was evaluated on additional three sample cross-sections for each group at fracture-start. Radial cracks originated early at the cementation interfaces and cone cracks were observed finally at the loading sites. Mean load (N) values (+/-s.d.) of A-copings at fracture-start/-end (i) 804 +/- 195/862 +/- 162, (ii) 923 +/- 180/975 +/- 147, (iii) 697 +/- 110/1607 +/- 145, were all significantly (P < 0.01) lower when compared with their B-crown coping analogs (i) 1183 +/- 318/1919 +/- 326, (ii) 1621 +/- 165/1820 +/- 211, (iii) 731 +/- 115/1973 +/- 287 except for A3 and B3 at fracture-start. This confirmed our hypothesis at fracture-start (P > 0.05) but rejected it at fracture-end (P < 0.01). The A3 fracture-end data, even if significantly (P < 0.01) lower, came close to the B3 values by 18%. A3 was significantly (P < 0.001) stronger by 86/74% than A1/A2 at fracture-end. The data indicates that YTZP-zirconia copings have the potential to provide support for all-ceramic core crowns, which may be adequate for non-adhesive cementation.