Effect of different resin luting cements on the marginal fit of lithium disilicate pressed crowns

Statement of problem

The vertical marginal discrepancy of restorations can increase upon cementation, and poor marginal fit can lead to cement dissolution, marginal discoloration, microleakage, and secondary caries. The amount of increase is related to the type of luting cement used, but how lithium disilicate pressed crowns are affected by different resin cements is unclear.

Effect of cement types on the tensile strength of metallic crowns submitted to thermocycling

The relationship between metallic cast crowns and tensile strength according to cement types submitted to thermocycling was studied. Seventy-two metallic crowns were cast with Verabond II Ni-Cr alloy and cemented in standardized preparations with 10 masculine tapering. Three types of finishing line (45-degree chamfered, 20-degree bevel shoulder and right shoulder) were made with diamond burs on bovine teeth. Twenty-four metallic crowns in each group were randomly subdivided into three subgroups of 8 samples each according to the cement used: SS White zinc phosphate cement, Vitremer resin-modified glass ionomer cement, and Rely X resin cement and were submitted to thermocycling. Retention was evaluated according to tensile load required to displace the metallic cast crowns from tooth preparations with an Instron testing machine. ANOVA and Tukey's test showed a statistically significant difference among luting materials, with greater results for Rely X resin cement (24.9 kgf) followed by SS White zinc phosphate cement (13.3 kgf) and Vitremer resin-modified glass ionomer cement (10.1 kgf). The finishing line types did not influence the tensile resistance of the crowns fixed with the three cements. Increased tensile resistance of metallic crowns fixed on bovine teeth was obtained with resin cement, independent of the finishing line types.     

Marginal Adaptation of Implant-Supported Metal-Ceramic Crowns Fabricated With Gold Cylinders

Purpose The purpose of this investigation was to determine the mean marginal discrepancy of metal-ceramic crowns fabricated with gold cylinders and cemented on implant abutments. These discrepancies were then compared with those measured previously for implant-supported ceramic crowns. Materials and Methods Fifteen Nobel BioCare CeraOne abutments were connected to implant fixtures embedded in acrylic resin blocks. Marginal discrepancies were determined for gold cylinders, gold cylinders plus ceramic alloy (metal frameworks), completed metal-ceramic crowns, and cemented metal-ceramic crowns using a stereomicroscope equipped with a video camera linked to a computer. A Hotelling's T² test (p .05) was used to evaluate potential differences in mean marginal discrepancies among groups. Results The mean marginal discrepancies were: 1) gold cylinders, 7.56 ± 2.73 μm; 2) metal frameworks, 6.21 ± 1.34 μm; 3) metal-ceramic crowns, 11.06 ± 3.21 μm; and 4) zinc-phosphate cemented crowns, 31.47 ± 6.65 μm. No significant difference between gold cylinders and metal frameworks was found. Mean marginal discrepancies for metal-ceramic crowns were significantly greater than discrepancies for cast gold cylinders. Cemented-crown mean marginal discrepancy was significantly greater than all other means. Conclusions Cemented metal-ceramic crowns fabricated using proprietary gold cylinders exhibited well-fitting margins (31.47 μm).     

Influence of marginal fit and cement types on microleakage of all-ceramic crown systems

The purpose of this study was to evaluate the effects of both marginal fit and cementing with different luting agents on the microleakage of all-ceramic crown systems. Thirty-six extracted upper central incisors were prepared for full-coverage crowns and were divided into three groups. Group 1: CAD/CAM-fabricated ZrO2, Group 2: Heat-pressed lithium-disilicate, and Group 3: Cast Cr-Co copings as the control group. Copings were made following standard techniques, and groups were assigned cementation with either self-adhesive resin cement (A) or glass-ionomer luting cement (B). The specimens were subjected to thermocycling, immersed in basic fuchsin solution, sectioned mesiodistally and buccolingually. The surface of each section was digitally photographed under a stereomicroscope. Microleakage was scored using a five-point scale, and the marginal gap was measured using image analysis software. Data were statistically analyzed using 2-way ANOVA, Kruskal-Wallis, and Mann-Whitney U tests (α: 0.05). The marginal discrepancy of each group was 82.7 ± 7 μm, 92.6 ± 4 μm and 96.5 ± 7 μm respectively. Group 1 showed significantly smaller gaps than Group 3 (P = 0.042). Self-adhesive resin cement (A) showed a lower level of microleakage than glass-ionomer luting cement (B) in all groups (P = 0.029). Microleakage scores of '0' were 83% for 1A, 50% for 1B, 50% for 2A, 16% for 2B, 33% for 3A and none for 3B. Marginal discrepancy and cement type both had significant effects on microleakage. Lower levels of microleakage were recorded with self-adhesive resin cement, while CAD/CAM-fabricated ZrO2 copings showed smaller marginal discrepancy and less microleakage in comparison to cast Cr-Co.     

Influence of scanner,powder application,and adjustments on CAD-CAM crown misfit

Statement of problem

The manufacturers of computer-aided design and computer-aided manufacturing (CAD-CAM) systems emphasize that new technologies can improve the marginal fit of dental crowns. However, data supporting this claim are limited.

CORRELATION BETWEEN MARGIN FIT AND MICROLEAKAGE IN COMPLETE CROWNS CEMENTED WITH THREE LUTING AGENTS

Microleakage can be related to margin misfit. Also, traditional microleakage techniques are time-consuming. This study evaluated the existence of correlation between in vitro margin fit and a new microleakage technique for complete crowns cemented with 3 different luting agents. Thirty human premolars were prepared for full-coverage crowns with a convergence angle of 6 degrees, chamfer margin of 1.2 mm circumferentially, and occlusal reduction of 1.5 mm. Ni-Cr cast crowns were cemented with either zinc phosphate (ZP) (S.S. White), resin-modified glass-ionomer (RMGI) (Rely X Luting Cement) or a resin-based luting agent (RC) (Enforce). Margin fit (seating discrepancy and margin gap) was evaluated according to criteria in the literature under microscope with 0.001 mm accuracy. After thermal cycling, crowns were longitudinally sectioned and microleakage scores at tooth-cement interface were obtained and recorded at x100 magnification. Margin fit parameters were compared with the one-way ANOVA test and microleakage scores with Kruskal-Wallis and Dunn''s tests (α=0.05). Correlation between margin fit and microleakage was analyzed with the Spearman''s test (α=0.05). Seating discrepancy and marginal gap values ranged from 81.82 μm to 137.22 μm (p=0.117), and from 75.42 μm to 78.49 μm (p=0.940), respectively. Marginal microleakage scores were ZP=3.02, RMGI=0.35 and RC=0.12 (p<0.001), with no differences between RMGI and RC scores. The correlation coefficient values ranged from -0.27 to 0.30 (p>0.05). Conclusion: Margin fit parameters and microleakage showed no strong correlations; cast crowns cemented with RMGI and RC had lower microleakage scores than ZP cement.     

Evaluation of the marginal fit of three margin designs of resin composite crowns using CAD/CAM

OBJECTIVES: To examine the marginal fit of resin composite crowns manufactured with the CEREC 3 system employing three different margin designs; bevel, chamfer and shoulder, by means of a replica technique and a luting agent. METHODS: Three master casts were fabricated from an impression of a typodont molar tooth and a full-coverage crown prepared with a marginal finish of a bevel, a chamfer and a shoulder. Each cast was replicated 10 times (n=10). Scanning of the replicas and crown designing was performed using the CEREC Scan system. The crowns were milled from Paradigm MZ100 composite resin blocks. The marginal fit of the crowns was evaluated with a replica technique (Aquasil LV, Dentsply), and with a resin composite cement (RelyX Unicem, Aplicap) and measured with a travelling microscope. Statistical analysis was performed using two-way ANOVA. RESULTS: For the replica technique the average marginal gaps recorded were: Bevel Group 105+/-34 microm, Chamfer Group 94+/-27 microm and Shoulder Group 91+/-22 microm. For the resin composite cement the average marginal gaps were: Bevel Group 102+/-28 microm, Chamfer Group 91+/-11 microm and Shoulder Group 77+/-8 microm. Two-way ANOVA analysis showed that there was no statistically significant difference between the three groups of finishing lines regardless of the cementation technique used. CONCLUSIONS: The marginal gap of resin composite crowns manufactured with the CEREC 3 system is within the range of clinical acceptance, regardless of the finishing line prepared or the cementation technique used.     

Randomized controlled within-subject evaluation of digital and conventional workflows for the fabrication of lithium disilicate single crowns. Part III: marginal and internal fit

Statement of problem

Trials comparing the overall performance of digital with that of conventional workflows in restorative dentistry are needed.

Relationship of Margin Design for Fiber‐Reinforced Composite Crowns to Compressive Fracture Resistance

Purpose: Fiber‐reinforced composite restorations provide excellent esthetics; however, little is known regarding the influence of margin design on marginal fit and fracture resistance for this type of crown. This study evaluated the effect of variations in tooth‐preparation design on the marginal fit and compressive fracture resistance of fiber‐reinforced composite crowns. Materials and Methods: Three metal dies with a total convergence of 5° and different margin designs (0.5‐mm light chamfer, 1.0‐mm deep chamfer, and 1.0‐mm shoulder) were prepared. Sixty standardized crowns (FibreKor) were made on duplicated base metal alloy dies (n = 20 for each margin design). Marginal fit was stereoscopically evaluated by measuring the distances between each of the four pairs of indentations on the crowns and on the dies. The specimens were then subjected to a compressive fracture‐loading test using a universal testing machine. The data were analyzed with one‐way analysis of variance (ANOVA) followed by Ryan‐Einot‐Gabriel‐Welsch multiple‐range test (α= 0.05). Results: Analysis of marginal fit and fracture resistance disclosed a statistically significant difference for tooth‐preparation design (p < 0.001). The marginal adaptation of preparations with the 0.5‐mm light chamfer (66.2 μm) and 1.0‐mm deep chamfer (69.7 μm) was significantly better than preparations with a shoulder finish line (92.8 μm) (p < 0.001). The fracture strength of the preparations with the 0.5‐mm light chamfer (15.8 MPa) and 1.0‐mm deep chamfer (15.1 MPa) was significantly greater than those of the preparations with the 1.0‐mm shoulder (13.7 MPa) (p < 0.001). Conclusions: Marginal fit of fiber‐reinforced crowns was adversely affected by tooth‐preparation design. The marginal gaps were greater for the shoulder margin specimens than in the light or deep chamfer margin specimens; however, the fracture strength of the chamfer margin specimens was greater than that of the shoulder margin specimens.     

Microleakage of core materials for complete cast gold crowns

This study evaluated the microleakage patterns of complete cast-gold crowns cemented onto teeth rebuilt with pin-retained cores made from cast gold, amalgam, composite resin, and silver-reinforced glass ionomer by using three different cements (zinc phosphate, glass ionomer, and resin cement). Crowns cemented onto unrebuilt tooth preparations with zinc phosphate cement served as controls. The cemented specimens were thermocycled between 4 degrees and 50 degrees C in waterbaths. They were then embedded in epoxy resin and sectioned. The extent of marginal microleakage was evaluated with a stereomicroscope and scored. The findings indicated that the type of luting agent used appeared to affect microleakage more than the core material. No significant differences in the degree of microleakage were found under crowns cemented onto teeth rebuilt with the four core materials when cemented with the same luting cement.     

Effect of Porcelain‐Firing Cycles and Surface Finishing on the Marginal Discrepancy of Titanium Copings

Purpose : The aim of the study was to evaluate the effect of simulated porcelain firing cycles and surface finishing on the marginal fit of commercially pure titanium (Cp Ti) copings. Materials and Methods: A machined stainless steel die system with standard 0.5‐mm copings was fabricated. Wax patterns were prepared by pouring the molten wax on a two‐part stainless steel die. Thirty specimens were cast in Cp Ti. These were divided into three groups with ten specimens in each group. Group 1 was treated with conventional cold working and later oxidized. Group 2 specimens were oxidized initially and then cold worked. Group 3 was heat treated in its original investment and later treated as in group 1. All specimens were later subjected to sequential simulated porcelain firing cycles, that is, oxidation, bonder, opaque, body, and glaze firing. Following the completion of each firing cycle, marginal discrepancy was measured in μm using a traveling microscope. The obtained data were subjected to one‐way analysis of variance (ANOVA) and Student's t‐test. The statistical level of significance was set at 1%. Results: The results showed that the mean and SD values (in μm) were 55 ± 2.6, 43 ± 3.0, and 68 ± 4.0 after oxidation for groups 1, 2, and 3, respectively. Mean and SD values (in μm) after glaze firing were 76 ± 3.9, 64 ± 4.1, and 89 ± 4.3 for groups 1, 2, and 3, respectively. The mean marginal opening was largest for group 3 specimens. One‐way ANOVA showed the difference within the three groups was highly significant after oxidation (F‐value 149.37 at p‐value 0.0000) and glaze firing (F‐value 82.43 at p‐value 0.0000). Conclusions: (1) The Student's t‐test values demonstrated that increased marginal openings of the specimens resulted after the sequential simulated porcelain firing cycles. (2) Marginal discrepancy values improved when the specimens were thermocycled prior to cold working.     

Influence of luting space of zirconia abutment on marginal discrepancy and tensile strength after dynamic loading

Statement of problemAlthough luting space is related to the marginal fit of indirect restorations, information on its influence on the marginal fit and tensile strength of zirconia abutments bonded to titanium bases is lacking.PurposeThe purpose of this in vitro study was to evaluate the influence of luting space on the marginal discrepancy and tensile strength of zirconia abutments bonded to a titanium base after dynamic loading.Material and methodsNinety implant replicas were embedded in resin blocks to attach 4-mm-high titanium luting bases. Zirconia abutments (Ceramill Zolid FX) were fabricated with different internal luting spaces: 25 μm (G25), 50 μm (G50), or 75 μm (G75). The zirconia abutments were cemented on the titanium bases by using a resin cement (Panavia F 2.0) under a constant load of 20 N. The marginal discrepancy and internal fit of 10 random specimens from each group were evaluated with a stereoscopic microscope at ×50 magnification. The remaining specimens were submitted to the tensile strength test in which half were evaluated after dynamic loading (1.2 million cycles of 200 N at 3.8 Hz) in a mechanical fatigue machine. The tensile strength test was performed using a pullout apparatus coupled to a universal testing machine at a crosshead speed of 0.5 mm/min. The mode of failure was determined by observation at ×50 magnification under a stereomicroscope and classified into adhesive or mixed. The groups were compared by using 2-way ANOVA and the Tukey HSD test (α=.05).ResultsIncrease in the luting space did not influence the marginal discrepancy (P>.05). All zirconia abutments exhibited lower pullout strength after fatigue simulation (P<.05). G75 demonstrated lower tensile strength than G25 and G50 before and after loading (P<.05). Most failures were adhesive at the zirconia-cement interface.ConclusionsThe increase of the luting space to 75 μm did not influence marginal discrepancy; however, it reduced the tensile strength of a zirconia abutment bonded to a titanium base.     

The effect of different fabrication steps on the marginal adaptation of two types of glass-infiltrated ceramic crown copings fabricated by CAD/CAM technology

Purpose: 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. Materials and Methods: 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 Results: 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). Conclusion: 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.     

Surface roughness and wettability of enamel and dentine surfaces prepared with different dental burs

The aim of dental adhesive restorations is to produce a long lasting union between the restoration and the tooth structure. This bond depends on many variables including the geometry of the preparation and the type of bonding agent or luting cement. It is therefore suggested that the topography of the tooth surface may influence the wettability and the bonding quality of adhesive systems. This study measured the surface roughness and wettability of enamel and dentine after preparation with different dental burs. The mesial and distal surfaces of 15 extracted sound human premolar teeth were prepared with a tungsten carbide crown bur, a diamond bur and a tungsten carbide finishing bur and finished in enamel or dentin, respectively. The prepared surfaces were analysed with a surface profilometer and scanning electron microscopy (SEM). The contact angle of distilled water on each of the prepared surfaces was used as the measure of wettability. The differences in average surface roughness (Ra) were significant between the rotary instrument groups, as revealed by a two-way ANOVA test. No differences were detected between enamel and dentine surfaces prepared with the same type of dental bur. The smoothest surfaces were those completed with tungsten carbide finishing burs. The diamond bur preparations were intermediate in the roughness assessment and the tungsten carbide crown burs gave the roughest surfaces. There were no significant differences in the contact angle measurements for the various groups. It was concluded that the surface roughness of enamel and dentine prepared by different rotary instruments had no significant influence on the wettability of distilled water on these surfaces.     

Effect of seating force,margin design,and cement on marginal seal and retention of complete metal crowns

PURPOSE: The purpose of this study was to determine the marginal discrepancy and retention of silver-palladium crowns cemented with zinc phosphate (Phosphacap) and glass-ionomer cement (Fuji Cap 1) using different seating forces on preparations with various margin designs. MATERIALS AND METHODS: Crown preparations with three finish lines--chamfer, shoulder, and shoulder with a 45-degree bevel--were sequentially prepared on a dentoform premolar. A metal die for each of the three finish lines was constructed. Complete metal crowns were fabricated for each metal die using a silver-palladium alloy. Three different seating forces--25, 100, and 300 N--were used to load the crowns until initial set of the cement. The marginal discrepancy was calculated by measuring the change in crown height before and after cementation using a digimatic indicator. Retention was determined by measuring the tensile strength using the Lloyd universal testing machine. RESULTS: The higher seating forces produced better crown seating but had no significant effect on crown retention. The shoulder and shoulder with bevel finish lines provided better crown retention than the chamfer. Glass-ionomer cement provided greater crown retention than zinc phosphate cement. No significant correlation between marginal seating and crown retention was revealed using a Pearson analysis. CONCLUSION: Marginal seal was not influenced by either margin design or type of luting cement, but was improved with higher seating force. Crown retention was affected by the margin finish line and the luting agent.     

Microcomputed tomography evaluation of cement shrinkage under zirconia versus lithium disilicate veneers

Statement of problemComputer-aided design and computer-aided manufacturing (CAD-CAM) technology and the improved translucency of recently developed high-strength monolithic zirconia could make them clinically acceptable for veneers if bonding to zirconia was as predictable as to glass-ceramics. Few studies have compared how resin cements behave between glass-ceramic and zirconia veneers before and after polymerization.PurposeThe purpose of this in vitro study was to evaluate the volumetric polymerization shrinkage of resin cement, marginal discrepancy, and cement thickness before and after polymerization for glass-ceramic and zirconia veneers with light-polymerizing resin cement.Material and methodsTen lithium disilicate veneers and 10 zirconia veneers were fabricated with a CAD-CAM workflow on extracted human maxillary anterior teeth with intact enamel surfaces. Zirconia veneers were treated with airborne-particle abrasion, and lithium disilicate veneers were etched with 5% hydrofluoric acid. All specimens were treated with ceramic primer and cemented with a light-polymerized resin cement. All specimens were scanned before and after resin cement polymerization by microcomputed tomography. The data were processed by the Amira software program to compare polymerization volumetric shrinkage, cement thickness, and marginal discrepancy. The data were compared by using a t test and analysis of variance (α=.05). Two bonded veneers were loaded in a mastication simulator for 400 000 cycles to investigate the effect of cyclic fatigue loading.ResultsMean volumetric polymerization shrinkage was 4.2 ±0.8% for the lithium disilicate group and 6.4 ±3.5% for the zirconia group. No significant difference was found for volumetric shrinkage between materials (P=.132). The mean ±standard deviations of the marginal discrepancies before and after polymerization were 178 ±41 μm and 158 ±37 μm for lithium disilicate and 115 ±33 μm and 107 ±32 μm for zirconia. A smaller marginal discrepancy was found for both materials after polymerization (P=.011) and for zirconia compared with lithium disilicate (P=.004). The mean ±standard deviation cement thickness values before and after polymerization were 157 ±27 μm and 147 ±27 μm for lithium disilicate and 162 ±53 μm and 147 ±52 μm for zirconia. Smaller cement thickness was found after polymerization (P<.001), whereas no significant difference was found in cement thickness between materials (P=.144). No changes were noted in marginal discrepancy and cement thickness as a result of the fatigue loading.ConclusionsThe difference in the volumetric polymerization shrinkage of cement between lithium disilicate and zirconia veneers was not statistically significant. Polymerization shrinkage resulted in smaller marginal discrepancy and cement thickness for both veneer materials.     

Marginal Discrepancy of All-Ceramic Crowns Cemented on Implant Abutments

Purpose The purpose of this investigation was to determine the mean marginal discrepancy of all-ceramic crowns cemented on implant abutments. Materials and Methods Five Brånemark CeraOne abutments were connected to implant fixtures embedded in acrylic resin blocks. The marginal discrepancy was measured using a video camera connected to a digitizing board at four locations on each abutment for five samples in each of the following groups: all-ceramic caps (caps), all-ceramic crowns (crowns), and all-ceramic crowns cemented with zinc phosphate cement (cemented crowns). Results The mean marginal discrepancies were as follows: caps, 99.0 μm (SD, 16); crowns, 117.8 μm (SD, 20); and cemented crowns, 168.8 μm (SD, 23). Statistically significant differences were found between all three groups at the 99% level of confidence. In addition there were two significant differences between abutments. Conclusions Subgingival marginal discrepancies of the magnitude measured in this study have been shown to cause periodontal problems. It is probable that the marginal discrepancy of CeraOne restorations, as tested, can be improved. Efforts should be made to enhance the marginal accuracy of these restorations.     

Marginal fit of cemented and screw-retained crowns incorporated on the Straumann (ITI)® Dental Implant System: an in vitro study

Aim: The aim of this study was to assess the marginal fit of crowns on the Straumann (ITI)^® Dental Implant System with special consideration of different casting dental materials. Material and methods: Sixty porcelain‐fused‐to‐metal crowns were fabricated: 18 crowns on standard cone abutments with an impression cylinder, partially prefabricated analogs, no coping and screw‐retained (A); 18 crowns on solid abutments without an impression device, no analogs, no coping and cemented (B); and 18 crowns on solid abutments using an impression transfer cap, an analog with a shoulder, no coping and cemented (C). In each group, six crowns were made on epoxy mastercasts (Bluestar^®), six on synthetic plaster (Moldasynt^®) and six on super hard stone (Fujirock^®). Six additional crowns were fabricated with the transversal screw retention system onto the Octa^® system with impression transfer caps, metal analogs, gold copings and screw‐retained (D). Impregum^® was used as impression material. Crowns of B and C were cemented with KetacCem^®. Crowns of A and D were fixed with an occlusal screw torqued at 15 N cm. Crowns were embedded, cut and polished. Under a light microscope using a magnification of × 100, the distance between the crown margin (CM) and the shoulder (marginal gap, MG) and the distance between the CM and the end of the shoulder (crown length, CL) was measured. Results: MGs were 15.4±13.2 μm (A), 21.2±23.1 μm (B), 11±12.1 μm (C) and 10.4±9.3 μm (D). No statistically significantly differences using either of the casting materials were observed. CLs were ?21.3±24.8 μm (A), 3±28.9 μm (B), 0.5±22 μm (C) and 0.1±15.8 μm (D). Crowns were shorter on synthetic casting materials compared with stone casts (P<0.005). Conclusions: CMs fit precisely with both cemented and screw‐retained versions as well as when using no, partial or full analogs.     

The effect of thermal stress on the marginal seal of cast gold full crowns

An experiment was conducted to observe the effect of fluctuations in temperature on the marginal seal of full veneer cast gold crowns. Forty crowns were fabricated and allotted into two groups of 20 crowns each. One group was cemented with zinc phosphate and the other group, with zinc polycarboxylate cement. Ten crowns from each group were subjected to thermal stress of fluctuating temperatures (4 and 60 C) and the remaining ten crowns from each group were held at the constant temperature of 37 +/- 1 C. The experiment demonstrated the following results: --Microleakage was evident in all groups. --The degree of microleakage with zinc phosphate was less than with zinc polycarboxylate cement. --Contrary to the claim that zinc polycarboxylate cement adheres to enamel through chelation, we were unable to observe better results. The degree of penetration of dye in the tooth-cement interfaces of crowns cemented with zinc polycarboxylate was not less despite the remaining layers of enamel for bonding. The cause of microleakage with cast gold restorations is apparently multifactorial. Research should formulate a luting agent which forms a strong bond to gold alloy and tooth substrate and has a high tensile and shearing strength.     

The Effect of Die Spacer on Retention and Fitting of Complete Cast Crowns

PURPOSE: This study evaluated the effect of die spacer on the fit and retention of complete cast crowns by using three different cements. MATERIALS AND METHODS: Standardized full crown restoration preparations were completed on 99 extracted molar teeth, impressions were made with poly(vinyl siloxane), and stone dies were made. Dies were covered with four layers of die spacer using three techniques: (1) covering the occlusal and 1/3 of the axial surfaces, (2) covering the occlusal and 2/3 of the axial surfaces, and (3) covering the entire preparation except the apical 0.5 mm of the preparation. Complete metal crowns were cast using Pors-on 4 alloy. Crowns were then assigned to one of three luting agent groups: resin modified glass ionomer cement, resin cement, or zinc phosphate cement. The castings were placed on their respective teeth and the marginal opening was recorded by two methods: 72 specimens were examined before and after cementation using optical microscopy with 0.001 mm resolution, and 27 specimens were examined after cementation with scanning electron microscopy. After cementation, the teeth were thermocycled for 700 cycles between 5 degrees C and 55 degrees C. The tensile retentive strength was measured on a universal testing machine with a crosshead speed of 0.5 mm/min. The data obtained for the fitting were recorded in millimeters and the data for the tensile retentive strength were recorded in KgF. The statistical analysis was performed by analysis of variance and post hoc Tukey's test (p< 0.05). RESULTS: Before cementation, better marginal fit was obtained when the die spacer covered all but the area 0.5 mm short of the margin of the preparation; however, after cementation, the resin modified glass ionomer cement group had the best fit with the same application of die spacer. Castings luted with resin cement required the greatest tensile force to produce cement failure. CONCLUSIONS: Increasing the area of the die surface covered with spacer improved the fit of the cast restoration. After cementation, the resin modified glass ionomer showed better adaptation; however, the optical microscopy and scanning microscopy correlate well. Resin cement had the highest resistance to tensile forces.