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

Effect of three different preparation designs on the marginal adaptation of indirect overlay restoration fabricated from lithium disilicate ceramic material: An in-vitro comparative study

Objectives

Marginal adaptation is considered one of the key factors influencing the success of indirect restorations. This study aimed to estimate the marginal fit of lithium disilicate overlays with three distinct preparation designs before and after cementation.

Methods

Thirty maxillary first premolars were divided into the hollow chamfer design (HCD) group, butt-joint design (BJD) group, and conventional occlusal box design (COD) group (n = 10 each). The samples were scanned using an intra-oral scanner, and overlays were fabricated using computer-assisted design and milled on a computer-assisted machine. The finished restorations were luted using a self-adhesive resin RelyX Ultimate. The marginal gap was assessed using a digital microscope with 230X magnification power. Statistical analysis was conducted using analysis of variance and post hoc (Bonferroni correction) tests, assuming a significance level of 5%.

Results

The HCD and BJD groups recorded significantly lower marginal gap, (11.39 ± 0.72, 16.29 ± 0.75) and (11.59 ± 0.75, 16.93 ± 0.65) respectively, than the COD group (24.57 ± 1.18, 34.45 ± 1.09) both pre- and post-cementation.

Conclusion

This study demonstrated that modification of tooth preparation plays a significant role in the marginal adaptation of the lithium disilicate overlays. The gap was smaller with the HCD and BJD than with the COD, with a statistically significant difference.

Marginal and internal adaptation of lithium disilicate partial restorations: A systematic review and meta-analysis

Aim:

The aim of this meta analysis was to evaluate the influence of the processing method on the marginal and internal gaps of lithium disilicate inlays/onlays.

Settings and Design:

A systematic literature review was conducted using the PubMed/Medline, Embase, Scopus, and Cochrane Library databases. This review was registered on the PROSPERO platform.

Materials and Methods:

The studies were selected according to the marginal and internal gaps of two different fabrication methods for lithium disilicate (milled and pressed).

Statistical Analysis Used:

The meta analysis was performed based on the Mantel–Haenszel and inverse variance methods, using the random effects model and a 95% confidence interval.

Results:

From all databases, 127 studies were identified. Four in vitro studies were included in the qualitative analysis and three in the meta analysis. Moreover, 197 restorations were evaluated (103 pressed and 94 milled). During the evaluation of only the internal gap, there was a statistically significant difference favoring the pressed technique (P = 0.002). There was no statistically significant difference in the analyses of the marginal gap (P = 0.530) and the total gap (P = 0.450).

Conclusion:

Both the techniques provided acceptable marginal and total gaps, although the pressed technique revealed a more favorable internal adaptation than the milled onlays/inlays.

Characteristics of a new dental stone mixed by shaking

This study evaluated the physical and mechanical properties of a dental stone mixed by shaking. A shake-mix dental stone (Shake! Mix STONE; SM) was characterized in comparison with three conventional dental stones. The fluidity at pouring time, setting time, density, powder particle distributions, linear setting expansion, compressive strength and surface reproduction of detail for dental stones were investigated. The marginal adaptations of cast crowns to dies made with each stone were also determined. SM had higher fluidity and faster setting time than the other stones (p<0.05). The setting expansion of SM at 2 h was lower than those of other two stones (p<0.05). The 15-min compressive strength of SM was higher than the others (p<0.05). There were no significant differences in the marginal adaptations of the cast crowns fabricated using all the stones (p>0.05). In spite of the different mixing method, the shake-mix type dental stone had comparable physical and mechanical properties to the conventional dental stones.

Effect of cement application techniques on the adaptation and retention of provisional crowns

BACKGROUND

A well-made and properly cemented provisional restoration is essential for the health of abutment teeth and periodontium. Few focused studies are available on the effect of cementation techniques on the fit as well as on the retention of the provisional crowns.

OBJECTIVE

To investigate the effect of six cement application techniques on the adaptation and retention of provisional crowns.

METHODS

Seventy-two provisional crowns (Bisacryl resin) were fabricated over identical self-cured resin dies and were divided into six groups (N= 72; 6 ×n= 12) of cement application techniques (Temp Bond NE, Kerr Corporation). Group-A (Bulk Fill); Group-B (Cement on inner walls); Group-C (Cement on axial walls); Group-D (Cement on crown margin); Group-E (Cement on tooth margin); Group-F (Cement with vent-holes). Adaptation was assessed before and after cementation with digital caliper (0.001 mm). Universal testing machine was used for retention measurement in Mega Pascal's (MPa). Data was analyzed by SPSS using Paired T-Test and Tukey's test (p< 0.05).

RESULTS

The highest and lowest difference between the two measurements was observed for the Group A (0.095 mm) and Group C (0.030 mm) respectively. Paired T-Test revealed significance (p< 0.05) between before and after cementation. Group-F (0.6094 MPa) and Group-E (0.1858 MPa) showed the highest and lowest retention respectively. Group-E (Cement on tooth margin) with lowest retention significantly differed (Tukey's Test; p< 0.05) from other groups.

CONCLUSIONS

Bulk filling of the provisional crown with luting cement significantly increases the adaptation discrepancies. Discrepancies are more when luting cement is applied on the occlusal surface of the crowns. However, it didn't influence the retention of the crowns. Cement application on axial walls and cervical third of intaglio surface of crowns was the method of choice with regards to adaptation and retention combined. Addition of vent-holes improved the adaptation with best possible retention.

Micro-CT evaluation of marginal and internal fit of cemented polymer infiltrated ceramic network material crowns manufactured after conventional and digital impressions

PURPOSE

Purpose of this study was to evaluate the accuracy of fit of cemented polymer infiltrated ceramic network (PICN) material crowns manufactured after digital and conventional impression techniques using micro computed tomography (CT). Furthermore to determine the cement space volume and porosities in the cement layer.

METHODS

A molar typodont tooth was prepared for PICN material crowns and replicated thirty times. The dies were randomly divided into three groups of 10 specimens each according to the impression technique: 3M True Definition Scanner (TDS), cara TRIOS (Trios) and Impregum Penta Soft (Impregum). PICN material crowns were milled for each specimen from Vita Enamic blocks and cemented on their respective dies. The absolute marginal discrepancy (AMD), internal fit (IG), total cement space volume (TVC) and marginal porosities (VP) were measured using Micro-CT.

RESULTS

Mean and standard deviations values in μm for the AMD were: TDS 140.1 (28.4); Trios 253.7 (56.8); Impregum 220.2 (101.1). IG values in μm: TDS 173.1 (27.7); Trios 222.2 (22.4); Impregum 211.6 (55.9). TVC in mm³: TDS 19.82 (2.9); Trios 23.67 (2.01); Impregum 23.77 (5.09). VP in mm³: TDS 0.38 (0.09); Trios 0.36 (0.10); Impregum 0.51 (0.31).

CONCLUSIONS

TDS group showed significantly better marginal and internal fit than the Trios group. No difference of the parameters was detected between the Impregum and both digital groups which implies that the digital impression technique is suitable in the manufacturing process of PICN material crowns.

In vitro Evaluation of the Marginal Fit and Internal Adaptation of Zirconia and Lithium Disilicate Single Crowns: Micro-CT Comparison Between Different Manufacturing Procedures

Background:

Prosthetic precision can be affected by several variables, such as restorative materials, manufacturing procedures, framework design, cementation techniques and aging. Marginal adaptation is critical for long-term longevity and clinical success of dental restorations. Marginal misfit may lead to cement exposure to oral fluids, resulting in microleakage and cement dissolution. As a consequence, marginal discrepancies enhance percolation of bacteria, food and oral debris, potentially causing secondary caries, endodontic inflammation and periodontal disease.

Objective:

The aim of the present in vitro study was to evaluate the marginal and internal adaptation of zirconia and lithium disilicate single crowns, produced with different manufacturing procedures.

Methods:

Forty-five intact human maxillary premolars were prepared for single crowns by means of standardized preparations. All-ceramic crowns were fabricated with either CAD-CAM or heat-pressing procedures (CAD-CAM zirconia, CAD-CAM lithium disilicate, heat-pressed lithium disilicate) and cemented onto the teeth with a universal resin cement. Non-destructive micro-CT scanning was used to achieve the marginal and internal gaps in the coronal and sagittal planes; then, precision of fit measurements were calculated in a dedicated software and the results were statistically analyzed.

Results:

The heat-pressed lithium disilicate crowns were significantly less accurate at the prosthetic margins (p<0.05) while they performed better at the occlusal surface (p<0.05). No significant differences were noticed between CAD-CAM zirconia and lithium disilicate crowns (p>0.05); nevertheless CAD-CAM zirconia copings presented the best marginal fit among the experimental groups. As to the thickness of the cement layer, reduced amounts of luting agent were noticed at the finishing line, whereas a thicker layer was reported at the occlusal level.

Conclusion:

Within the limitations of the present in vitro investigation, the following conclusions can be drawn: the recorded marginal gaps were within the clinical acceptability irrespective of both the restorative material and the manufacturing procedures; the CAD-CAM processing techniques for both zirconia and lithium disilicate produced more consistent marginal gaps than the heat-pressing procedures; the tested universal resin cement can be safely used with both restorative materials.

Evaluation of marginal fit and internal adaptation of zirconia copings fabricated by two CAD - CAM systems: An in vitro study

Statement of Problem:

Three main factors which determine the success of an All-ceramic restoration are esthetic value, resistance to fracture and third being the marginal fit. Marginal fit and internal adaptation are crucial factors in increasing the longevity of the restoration. Newer and economical CAD CAM systems have been introduced claiming better marginal fit and adaptation of All ceramic crowns. CAD CAM systems involves scanning of the die or the tooth preparation and milling of the restoration, which may have variations among the systems available.

Aim of the Study:

Our study intended to check the marginal fit and internal adaptation of commonly used CAD CAM systems namely CERAMILL and CEREC -In Lab MC XL.

Materials and Methods:

Two groups of typodont teeth (n = 10) were prepared using a standardized protocol to receive All ceramic copings. 10 samples of Group A were used for fabrication of copings using CERAMILL system and 10 samples of Group B were used for fabrication of copings using CEREC -In Lab MC XL system. They were then luted with glass ionomer cement under mild finger pressure. Samples were embedded in resin and sliced longitudinally. They were then viewed under stereomicroscope and readings were measured along 15 points using ImageScope software. The P value was set at 0.05 at 95% confidence interval with 80% power. The data were checked for normality and unpaired t-test was used to evaluate the results of the two groups.

Results:

The overall internal adaptation was 61.5 ± 5.2 μm for CERAMILL and 56.9 ± 5.7 μm for CEREC -In Lab MC XL (P < 0.05). The marginal fit for CERAMILL was 83 μm and for CEREC -In Lab MC XL was 68 μm (P < 0.05).

Conclusion:

The marginal adaptation of CEREC -In Lab MC XL (68 μm) was found to be superior to CERAMILL (83 μm) (P < 0.05). Both the CEREC -In Lab MC XL and CERAMILL copings demonstrated internal adaptation and marginal fit within acceptable discrepancy range. When corroborating both the internal adaptation and marginal fit, CEREC -In Lab MC XL was found to be better than CERAMILL.

In vitro evaluation of the marginal integrity of CAD/CAM interim crowns

STATEMENT OF PROBLEM

The accuracy of interim crowns made with computer-aided design and computer-aided manufacturing (CAD/CAM) systems has not been well investigated.

PURPOSE

The purpose of this in vitro study was to evaluate the marginal integrity of interim crowns made by CAD/CAM compared with that of conventional polymethylmethacrylate (PMMA) crowns.

MATERIAL AND METHODS

A dentoform mandibular left second premolar was prepared for a ceramic crown and scanned for the fabrication of 60 stereolithical resin dies, half of which were scanned to fabricate 15 Telio CAD-CEREC and 15 Paradigm MZ100-E4D-E4D crowns. Fifteen Caulk and 15 Jet interim crowns were made on the remaining resin dies. All crowns were cemented with Tempgrip under a 17.8-N load, thermocycled for 1000 cycles, placed in 0.5% acid fuschin for 24 hours, and embedded in epoxy resin before sectioning from the mid-buccal to mid-lingual surface. The marginal discrepancy was measured using a traveling microscope, and dye penetration was measured as a percentage of the overall length under the crown.

RESULTS

The mean vertical marginal discrepancy of the conventionally made interim crowns was greater than for the CAD/CAM crowns (P=.006), while no difference was found for the horizontal component (P=.276). The mean vertical marginal discrepancy at the facial surface of the Caulk crowns was significantly greater than that of the other 3 types of interim crowns (P<.001). At the facial margin, the mean horizontal component of the Telio crowns was significantly larger than that of the other 3 types, with no difference at the lingual margins (P=.150). The mean percentage dye penetration for the Paradigm MZ100-E4D crowns was significantly greater and for Jet crowns significantly smaller than for the other 3 crowns (P<.001). However, the mean percentage dye penetration was significantly correlated with the vertical and horizontal marginal discrepancies of the Jet interim crowns at the facial surface and with the horizontal marginal discrepancies of the Caulk interim crowns at the lingual surface (P<.01 in each instance).

CONCLUSIONS

A significantly smaller vertical marginal discrepancy was found with the interim crowns fabricated by CAD/CAM as compared with PMMA crowns; however, this difference was not observed for the horizontal component. The percentage dye penetration was correlated with vertical and horizontal discrepancies at the facial surface for the Jet interim crowns and with horizontal discrepancies at the lingual surface for the Caulk interim crowns.

Die spacer thickness reproduction for central incisor crown fabrication with combined computer-aided design and 3D printing technology: an in vitro study

STATEMENT OF PROBLEM

The inability to control die spacer thickness has been reported. However, little information is available on the congruency between the computer-aided design parameters for die spacer thickness and the actual printout.

PURPOSE

The purpose of this study was to evaluate the accuracy and precision of the die spacer thickness achieved by combining computer-aided design and 3-dimensional printing technology.

MATERIAL AND METHODS

An ivorine maxillary central incisor was prepared for a ceramic crown. The prepared tooth was duplicated by using polyvinyl siloxane duplicating silicone, and 80 die-stone models were produced from Type IV dental stone. The dies were randomly divided into 5 groups with assigned die spacer thicknesses of 25 μm, 45 μm, 65 μm, 85 μm, and 105 μm (n=16). The printed resin copings, obtained from a printer (ProJet DP 3000; 3D Systems), were cemented onto their respective die-stone models with self-adhesive resin cement and stored at room temperature until sectioning into halves in a buccolingual direction. The internal gap was measured at 5 defined locations per side of the sectioned die. Images of the printed resin coping/die-stone model internal gap dimensions were obtained with an inverted bright field metallurgical microscope at ×100 magnification. The acquired digital image was calibrated, and measurements were made using image analysis software. Mixed models (α=.05) were used to evaluate accuracy. A false discovery rate at 5% was used to adjust for multiple testing. Coefficient of variation was used to determine the precision for each group and was evaluated statistically with the Wald test (α=.05).

RESULTS

The accuracy, expressed in terms of the mean differences between the prescribed die spacer thickness and the measured internal gap (standard deviation), was 50 μm (11) for the 25 μm group simulated die spacer thickness, 30 μm (10) for the 45 μm group, 15 μm (14) for the 65 μm group, 3 μm (23) for the 85 μm group, and -10 μm (32) for the 105 μm group. The precision mean of the measurements, expressed as a coefficient of variation, ranged between 14% and 33% for the 5 groups.

CONCLUSIONS

For the accuracy evaluation, statistically significant differences were found for all the groups, except the group of 85 μm. For the precision assessment, the coefficient of variation was above 10% for all groups, showing the printer's inability to reproduce the uniform internal gap within the same group.

Internal fit of pressed and computer-aided design/computer-aided manufacturing ceramic crowns made from digital and conventional impressions

STATEMENT OF PROBLEM

No studies have evaluated the internal adaptation of pressed and milled ceramic crowns made from digital impressions.

PURPOSE

The purpose of this in vitro study was to evaluate the internal fit of pressed and milled ceramic crowns made from digital and conventional impressions.

MATERIAL AND METHODS

Thirty polyvinyl siloxane (PVS) impressions and 30 Lava COS impressions made of a prepared dentoform tooth (master die) were fabricated. Thirty crowns were pressed in lithium disilicate (IPS e.max Press), and 30 crowns were milled from lithium disilicate blocks (IPS e.max CAD) (15/impression technique) with the E4D scanner and milling engine. The master die and the intaglio of the crowns were digitized with a 3-dimensional laser coordinate measurement machine. The digital master die and intaglio of each crown were merged. The distance between the die and the intaglio surface of the crown was measured at 3 standardized points. One-way ANOVA was used for statistical analysis (α=.05).

RESULTS

One-way ANOVA revealed that the internal gap obtained from the Lava/press group (0.211 mm, ±SD 0.041) was significantly greater than that obtained from the other groups (P<.001), while no significant differences were found among PVS/press (0.111 mm ±SD 0.047), PVS/CAD/CAM (0.116 mm ±SD 0.02), and Lava/CAD/CAM (0.145 mm ±SD 0.024).

CONCLUSIONS

The combination of the digital impression and pressed crown produced the least accurate internal fit.

Evaluation of the Luting Cement Space for Provisional Restoration by using Various Coats of Die Spacer Materials-An Invitro Study

AIM

The present study was to evaluate the space provided for the temporary luting cement, after the application of various coats of die spacers, during the fabrication of provisional crowns and bridges.

MATERIALS AND METHODS

A total of 50 specimens of dental stone with provisional crowns on all these samples were prepared and were divided into five groups based on the application of various coats of different die spacers. Later these specimens were sectioned buccolingually and were observed using a stereomicroscope under 100X magnification. The images thus obtained were evaluated and noted for the amount of space between the inner surface of the provisional crown and the specimens at five different locations using Image Pro 6.0 Express software and the values were subjected to one-way ANOVA test, and unpaired t-test.

RESULTS

There was a significant increase of luting space thickness with various die spacer applications than the specimens of control group.

CONCLUSION

Specimens of double coat applications of silver and gold die spacers showed higher luting cement space than the separating media application specimens.

Cement selection for implant-supported crowns fabricated with different luting space settings

PURPOSE

To measure and compare the retentive strength of cements specifically formulated for luting restorations onto implant abutments and to investigate the effect of varying cement gap on retention strength of implant-supported crowns.

MATERIALS AND METHODS

Standard titanium abutments were scanned by means of a 3D digital laser scanner. One hundred and sixty standard metal copings were designed by a Computer Aided Design/Computer Aided Manufacturing (CAD/CAM) system with two cement gap values (20 and 40 μm). The copings were cemented to the abutments using the following eight cements with one being the control, zinc oxide temporary cement, while the other seven were specifically formulated implant cements (n = 10): Premier Implant Cement, ImProv, Multilink Implant, EsTemp Implant, Cem-Implant, ImplaTemp, MIS Crown Set, and TempBond NE. The specimens were placed in 100% humidity for 24 hours, and subjected to a pull-out test using a universal testing machine at a 0.5 mm/min crosshead speed. The test results were analyzed with two-way ANOVA, one-way ANOVA, post hoc Tamhane' s T2, and student's t-tests at a significance level of 0.05.

RESULTS

Statistical analysis revealed significant differences in retention strength across the cement groups (p < 0.01). Resin-based cements showed significantly higher decementation loads than a noneugenol zinc oxide provisional cement (TempBond NE) (p < 0.01), with the highest tensile resistance seen with Multilink Implant, followed by Cem-Implant, MIS Crown Set, ImProv, Premier Implant Cement, EsTemp Implant, and ImplaTemp. Increasing the cement gap from 20 to 40 μm improved retention significantly for the higher strength cements: Multilink Implant, Premier Implant Cement, ImProv, Cem-Implant, and MIS Crown Set (p < 0.01), while it had no significant effect on retention for the lower strength cements: EsTemp Implant, ImplaTemp, and TempBond NE (p > 0.05).

CONCLUSIONS

Resin cements specifically formulated for implant-supported restorations demonstrated significant differences in retention strength. The ranking of cements presented in the study is meant to be an arbitrary guide for the clinician in deciding the appropriate cement selection for CAD/CAM-fabricated metal copings onto implant abutments with different luting space settings.

The effect of occlusal surface relief of dies on marginal adaptation of metal-ceramic casting copings

PURPOSE

The purpose of this study was to evaluate the impact of occlusal relief of dies on internal adaptation of metal-ceramic casting copings.

MATERIALS AND METHODS

Standardized preparations were made on 80 extracted third molar teeth. Impressions were made with poly(vinyl siloxane), and stone dies were prepared. Dies were covered with four layers of die spacer, covering the entire preparation together with the occlusal surface excluding the apical 0.5 mm of the preparation in group 1 (40 specimens), and covering the same area excluding the occlusal surface in group 2 (40 specimens). Copings were cast using nickel-chromium-based metal ceramic alloy and cemented using zinc phosphate cement. The specimens were sectioned along the long axis. Internal discrepancies were recorded with a 0.001-mm resolution stereoscope at 6 points: the middle of the occlusal surface (MO), middle of the lingual wall (ML), middle of the buccal wall (MB), middle of the buccal shoulder finish line (MSH), middle of the lingual chamfer finish line (MCH), and middle of the buccal bevel finish line (MBL). Student's t-test was used for statistical analysis. Significance level was set at p < 0.05.

RESULTS

The marginal discrepancies of group 1 were higher than those of group 2. Significant differences in discrepancies were found on MO (p < 0.0001), MSH (p = 0.012), and MBL (p = 0.035). The bevel margin showed the least marginal discrepancy following occlusal surface of the die with no relief.

CONCLUSION

Leaving the occlusal part of the die uncovered with the die spacer improved the crown seating considerably in the occlusal surface as well as shoulder and bevel margins.

Effect of Abutment Height on Retention of Single Cement-retained, Wide- and Narrow-platform Implant-supported Restorations

BACKGROUND AND AIMS

In contrast to prepared natural dentin abutments,little is known concerning factors influencing the retention of fixed prostheses cemented to implant abutments. The aim of this study was to investigate the effect of im-plant abutment height on the retention of single castings cemented to wide and narrow platform implant abutments.

MATERIALS AND METHODS

Thirty-six parallel-sided abutments (Biohorizon Straight Abutment) of narrow platform (NP) and wide platform (WP) sizes with their analogs were used. In each group of platform size, abutments were prepared with axial wall heights of 5, 4, 3, 2 mm (n=9). On the whole 72 castings were constructed, which incorporated an attachment to allow removal. Castings were cemented to abutments with TempBond®. A uniaxial tensile force was applied to the crown using an Instron machine until cement failure occurred. Analysis of variance of the models were fit to determine the effect of height of abutment of the restorations on the mean tensile strength (α=0.05).

RESULTS

The mean peak removal force for corresponding abutments was significantly different (P < 0.05): (1) with plat-form sizes: WP > NP; (2) with alteration of axial wall height for NP: 5 mm > 4 mm > 3 mm = 2 mm and for WP: 5 mm > 4 mm = 3 mm = 2 mm.

CONCLUSION

The retention of NP cement-retained restorations is influenced by the wall height but not in same manner as WP. Restorations of narrow-platform size with longer abutment exhibited higher tensile resistance to dislodgement.

Marginal discrepancy as affected by selective placement of die-spacer: an in vitro study

An increase in the marginal discrepancy is seen after cementation with a luting agent and provision of cement space with a die-spacer is the most preferred method to avoid it. Recommended thickness of die-spacer is 25-40 μm. Smaller die-spacer thickness was consistently found at the axio-occlusal line angles as compared to the other surfaces which has been postulated to that the spacer paint tends to flow away from the sharp line angles and cusp tips as a result of increased surface tension. The absence of adequate relief spaces in these areas impedes the flow of cement beyond the occlusal portion of the casting, which would result in incomplete seating because of hydraulic pressure. Fifty stone dies were duplicated from a steel die and were divided into five groups of sample size 10, where the die-spacer was selectively placed. Measurements were taken at four points, 90° apart from each other with the help of optical microscope. Later all the castings were cemented using Glass Inomer cement as a luting agent, under a 10 kg static load and measurements were recorded. Statistical analysis showed samples with no spacer had the maximum pre and post cementation gap while the least discrepancy was seen in group with additional layer of die-spacer painted over the axio-occlusal line angle. The results were highly significant which clearly indicated the superiority of this group over others. Within limitations of the study, it can be said that application of additional layer of die-spacer at the axio-occlusal line angle will help in decreasing the post cementation marginal discrepancy in full cast metal crowns.

Effects of clinical experience and explorer type on judged crown margin acceptability

PURPOSE

This study was undertaken to simultaneously compare instrumentation type and operator characteristics in judgments of clinical acceptability of crowns exhibiting a controlled range of marginal gaps. The research was conducted in a laboratory setting and generalizability analysis was used as a statistical technique to identify the sources contributing to variation in the judgment outcome.

MATERIALS AND METHODS

A crown was seated on an ivorine tooth in a device that permitted continuous adjustment in intervals of 25 mum to produce known marginal gaps ranging from zero to 250 microm. Forty-nine students and six faculty members used five types of explorers each to determine, by tactile examination, the point on the controlled increasing marginal gap where they would no longer regard the gap as clinically acceptable.

RESULTS

There were no differences across type of explorer. Operators with clinical experience had a threshold that rejected crowns at a smaller gap than did those operators without clinical experience (p= 0.007). Faculty members maintained a higher individual degree of consistency in their personal judgments than did students (p= 0.02); however, the inter-operator consistency was significantly lower for faculty members than for students (p < 0.05).

CONCLUSIONS

Differences among operators in a simulation of the decision regarding gaps in crowns accounted for 63% of the variance; type of explorer used in assisting this decision accounted for about half as much variance. Faculty members making such judgments exhibited high intra-operator consistency but significantly lower inter-operator consistency than did students. The study suggests that the internal standards dentists use for clinical decision making deserves further study as they may be as significant as the equipment used.

Marginal Accuracy of Castings Produced with Different Investment Systems

BACKGROUND

The use of casting ring to produce accurate castings has been challenged with the introduction of a ringless casting technique. This study compared the marginal accuracy of all - metal complete coverage crowns fabricated with ringless, split plastic ring and metal ring investment systems.

METHODS

A total of 40 all- metal complete coverage crowns were fabricated on a metal die. The crowns were divided in 4 groups (Group A, B, C and D) of 10 patterns each. A ringless system of investing and casting was used for group A whereas a split plastic ring system was used for group B. Groups C and D utilized metal ring with single and double layers of asbestos free cellulose acetate liner respectively for investing and casting procedures. The restorations were seated on the metal die and the vertical marginal discrepancy was evaluated by measuring the gap between the finish line on the die and the margins of the crown on four specific sites with an optical microscope. Statistical analysis was carried out using ANOVA and multiple comparison "t" test.

RESULT

The mean vertical marginal discrepancy for groups A, B, C and D was 95μm, 136μm, 128μm and 104μm respectively. Vertical marginal discrepancy on each surface was compared among the four groups. Difference of vertical marginal discrepancy on buccal surface (p<0.0001) as well as distal surface (p<0.005) was highly significant whereas it was non-significant on lingual and mesial surfaces (p>0.05).

CONCLUSION

Accurate castings with better marginal fit can be produced with ringless casting technique.

Computer aided analysis of digitized dental stone replicas by dental CAD/CAM technology

OBJECTIVES

To determine the reproducibility of digitized dental stone replicas compared to the master model and the reliability of the computer aided analysis.

METHODS

Four master dies, prepared for complete crowns were fabricated in presintered yttria-stabilized tetragonal zirconia (Y-TZP). Eight vinyl polysiloxane impressions (PROVIL novo; Heraeus Kulzer) were taken of each die and stone replicas were poured in type IV stone (Vel-Mix Stone; Kerr). The master dies and the stone replicas were digitized in a touch-probe scanner (Procera Forte; Nobel Biocare AB), to create triangulated surface-models. The point-cloud from the first of the repeated digitizations of each master die was used as CAD-reference-models (CRM). Discrepancies between the points in the triangulated surface-models and the corresponding CRM were measured by a matching-software (CopyCAD 6.504 SP2; Delcam Plc). The distribution of the discrepancies was analyzed and presented in color-difference-maps.

RESULTS

The precision of the measuring method, presented as the repeatability coefficient, ranged between 7 and 16 microm (entire surface), whereas the analysis of the stone replicas revealed a precision (repeatability coefficient) ranging from 19 to 26 microm. The accuracy of the replica to master (the mean discrepancy) ranged from 0.5 to 2.0 microm (95% confidence interval 1.5-2.9 microm).

SIGNIFICANCE

The greatest precision of the measurement was seen in the jacket surface of the die. The size of the stone replicas varied and the repeatability coefficient was on average 15 microm (2-25 microm) greater for the replica-to-master alignment than the repeated digitizations of the master.

Effect of the use of die spacer on the marginal fit of copings cast in NiCr, NiCrBe and commercially pure titanium

The goal of this study was to evaluate the effect of using die spacers on the marginal fit of NiCr (M1) and NiCrBe (M2) alloys and commercially pure titanium (cpTi) (M3) copings cast by the lost wax technique. Using a metal matrix, 45 resin added extra hard type IV stone models were obtained for the fabrication of wax patterns under the following conditions: no die spacer (A), with one die spacer layer (B) and with two die spacer layers (C), with five repetitions for each condition (alloy x die). Each die was waxed and the wax patterns were invested as per manufacturer's instructions. Three wax patterns were embedded in each casting ring, each corresponding to one of the conditions. Each coping, seated to the metal matrix by a seating pressure standardizing device (SPSD), was taken to an optical microscope for measurement of marginal discrepancy. The obtained data (µm) were analyzed statistically by ANOVA and Tukey' test (a=5%). There was statistically significant difference (p<0.05) among the materials (M1=110.67; M2=130.33 and M3=148.33). Regarding the use of the die spacer, there was a statistically significant difference (p<0.05) among the three conditions (A=162.00; B=131.06 and C=96.67). It was concluded that there is less marginal discrepancy with two die spacer layers.