Fit of cast commercially pure titanium and Ti-6Al-4V alloy crowns before and after marginal refinement by electrical discharge machining

Effect of Cu and Ti electrodes on surface and electrochemical properties of Electro Discharge Machined (EDMed) structures made of Co-Cr and Ti dental alloys

OBJECTIVES

Previous studies have shown that the use of Cu electrodes compromises the electrochemical properties of Co-Cr and Ti alloys used for the fabrication of implant retained superstructures by Electro Discharge Machining (EDM). A possible solution is the use of Ti instead of Cu electrodes and thus the aim of this study was to evaluate the effect of Cu and Ti electrodes on surface and electrochemical properties of two types of dental alloys used for fabrication of implant retained superstructures after EDM.

METHODS

Three full arch frameworks were prepared from a Co-Cr and three from Ti6Al7Nb alloy. One framework from each alloy was used as control, one was subjected to EDM with Cu electrodes and the last one with Ti electrodes. Morphological and elemental characterization was studied by SEM/EDX. The electrochemical properties of the alloys were evaluated by Open Circuit Potential (OCP) and Linear Sweep Voltammetry (LSV) in Ringer's solution. Electrochemical data were analyzed statistically by one way ANOVA and SNK multiple comparison tests at a = 0.05 RESULTS: All groups demonstrate the typical surface after EDM treatment with almost circular valleys and an increase in C and O content compared to control groups. Both alloys demonstrated an uptake of C and Cu by Cu electrodes and C and Ti after treatment with Ti electrodes. The use of Cu electrodes had a detrimental effect on corrosion resistance of Ti alloy.

SIGNIFICANCE

The use of Ti electrodes mitigates the degradation of electrochemical properties compared to Cu electrodes and from this standpoint is safer for the EDM of implant retained superstructures made of Co-Cr and Ti alloys.

Comparative in vitro study of the accuracy of impression techniques for dental implants: Direct technique with an elastomeric impression material versus intraoral scanner

Background

The aim of this study was to compare a conventional technique (elastomeric impression material - EIM) and a digital technique (scanner digital model – SDM) on a six-analog master model (MM) to determine which was the most exact.

Material and Methods

Twenty impressions were taken of a master model (EIM) and twenty scanned impressions (SDM) (True Definition). A coordinate measuring machine (CMM) was used to measure the distances between adjacent analogues (1-2, 2-3, 3-4, 4-5, 5-6), intermittently positioned analogues (1-4, 3-6) and the most distal (1-6). Reference values were established from the master model, which were compared with the two impression techniques. The significance level was established as 5% (p<0.05).

Results

The precision of each technique was compared with MM. For adjacent analogues (1-2), no significant differences were found between EIM-MM (p=0,146). For intermittently positioned analogues (1-4), SDM did not show significant differences with MM (p=0.255). For the distance between distal analogues (1-6), significant differences were found between both techniques and MM (p=0.001).

Conclusions

In a clinical situation with < three implants, EIM is more exact than SDM, but in cases of four implants SDM is more exact. For rehabilitations (> four implants), neither technique can be considered accurate although error falls within the tolerance limits established in the literature (30-150µm).

Key words:Digital workflow, full arch scan, intraoral scanner, CAD/CAM, polyether impression, accuracy.

Effect of electrical discharge machining on dental Y-TZP ceramic-resin bonding

PURPOSE

The study determined (i) the effects of electrical discharge machining (EDM) on the shear-bond strength (SBS) of the bond between luting resin and zirconia ceramic and (ii) zirconia ceramic's flexural strength with the three-point bending (TPB) test.

METHODS

Sixty 4.8mm×4.8mm×3.2mm zirconia specimens were fabricated and divided into four groups (n=15): SBG: sandblasted+silane, TSCG: tribochemical silica coated+silane, LTG: Er:YAG laser treated+silane, EDMG: EDM+silane. The specimens were then bonded to a composite block with a dual-cure resin cement and thermal cycled (6000 times) prior to SBS testing. The SBS tests were performed in a universal testing machine. The SBS values were statistically analyzed using ANOVA and Tukey's test. To determine flexural strength, sixty zirconia specimens were prepared and assigned to the same groups (n=15) mentioned earlier. After surface treatment TPB tests were performed in a universal testing machine (ISO 6872). The flexural strength values were statistically analyzed using ANOVA and Tukey's test (α=0.05).

RESULTS

The bond strengths for the four test groups (mean±SD; MPa) were as follows: SBG (Control), 12.73±3.41, TSCG, 14.99±3.14, LTG, 7.93±2.07, EDMG, 17.05±2.71. The bond strength of the EDMG was significantly higher than those of the SBG and LTG (p<0.01). The average flexural strength values for the groups SBG (Control), TSCG, LTG and EDMG were 809.47, 800.47, 679.19 and 695.71MPa, respectively (p>0.05).

CONCLUSIONS

The EDM process improved the SBS. In addition, there was no significant adverse effect of EDM on the flexural strength of zirconia.

Effects of polishing methods on Candida albicans adhesion on cast pure titanium surfaces

OBJECTIVE

To explore the effects of different polishing methods on Candida albicans adhesion to cast pure titanium surfaces.

METHODS

Pure titanium test specimens randomly divided into 4 groups: not specially treated (CT), manual polishing (MPT), electrolytic polishing (EPT), and centrifugal barrel polishing (CPT). Surface roughness was measured with surface profilometer. Contact angles of water, diiodomethane, and ethylene glycol on specimen surface were measured, and the free energy was calculated. Each group were immersed in a C. albicans suspension for 48 hours, and cell count of C. albicans adhesion were taken. Morphology of C. albicans adhesion was observed by SEM.

RESULTS

Surface roughness (Ra) were in the following order: CT > EPT > MPT > CPT. The amount of C. albicans adhesion to the surface of CPT was significantly less than CT, MPT, and EPT by cells count (P < 0.05). In SEM, less C. albicans aggregation formed on the surface of CPT than other 3 groups.

CONCLUSIONS

Specimens treated by the CPT method showed superior effect to the specimens treated with MPT and EPT methods in reducing the adhesion of C. albicans.

Evaluation of internal and marginal fit of two metal ceramic system - in vitro study

INTRODUCTION

The high strength porcelain fused metal crowns have been extensively used in dentistry. However, the fit is the most encountered problems in porcelain fused metal crowns. This mainly depends on the fabrication technique.

AIM

The purpose was to compare the internal and marginal fit of laser sintered and conventionally casted Cobalt-Chromium copings.

MATERIALS AND METHODS

Twenty stainless steel dies of dimension 15 x 10 mm with 1 mm finish margin were fabricated using CAD-CAM technology. Twenty dies were divided into two groups Group 1 and Group 2 containing 10 samples each. All 20 dies were scanned using LAVA 3M scanner and data were used to fabricate metal copings using Laser sintering technique (Group-1) and Conventional casting technique (Group-2). Copings were cemented onto respective dies and finished and standardized sectioning were made. The sectioned models were scanned under stereomicroscope at 50 x magnification for internal and marginal fit evaluation.

RESULTS

Mean and standard deviation of internal and marginal discrepancy of laser sintered copings/conventional cast metal copings was 107.6 ± 10.9μ and 102.1 ± 17.2μ/187.09 ± 11.47 μ and 176.57 ± 25.82 μ respectively. Statistical analysis showed the laser sintered copings have lesser internal and marginal discrepancy than conventional casted copings with p value < 0.001.

CONCLUSION

The laser sintered Co-Cr copings showed better internal and marginal fit when compared to that of conventional Co-Cr casted copings.

Tensile and Flexural Strength of Commercially Pure Titanium Submitted to Laser and Tungsten Inert Gas Welds

This study evaluated the tensile and flexural strength of tungsten inert gas (TIG) welds in specimens made of commercially pure titanium (CP Ti) compared with laser welds. Sixty cylindrical specimens (2 mm diameter x 55 mm thick) were randomly assigned to 3 groups for each test (n=10): no welding (control), TIG welding (10 V, 36 A, 8 s) and Nd:YAG laser welding (380 V, 8 ms). The specimens were radiographed and subjected to tensile and flexural strength tests at a crosshead speed of 1.0 mm/min using a load cell of 500 kgf applied on the welded interface or at the middle point of the non-welded specimens. Tensile strength data were analyzed by ANOVA and Tukey's test, and flexural strength data by the Kruskal-Wallis test (α=0.05). Non-welded specimens presented significantly higher tensile strength (control=605.84±19.83) (p=0.015) and flexural strength (control=1908.75) (p=0.000) than TIG- and laser-welded ones. There were no significant differences (p>0.05) between the welding types for neither the tensile strength test (TIG=514.90±37.76; laser=515.85±62.07) nor the flexural strength test (TIG=1559.66; laser=1621.64). As far as tensile and flexural strengths are concerned, TIG was similar to laser and could be suitable to replace laser welding in implant-supported rehabilitations.

Influence of the casting technique and dynamic loading on screw detorque and misfit of single unit implant-supported prostheses

OBJECTIVE

This study aimed to evaluate the influence of the casting procedure and cyclic loading of prosthetic frameworks on detorque of prosthetic screws and marginal misfit of single unit implant-supported prostheses.

MATERIALS AND METHODS

Twenty specimens were obtained, each one consisting of a set of an implant (external hexagon 3.75 × 13 mm - Branemark type), a prosthetic abutment (entirely calcinable or overcasted UCLA) and a prosthetic screw. After the specimens were obtained, the prosthetic screws were tightened with 30 Ncm torque and released 24 h later in order to evaluate initial detorque. The screws were retightened and marginal gaps were assessed. All specimens were submitted to 10(6) loading cycles, performed with 2 Hz frequency and 130 N load. The specimens were re-evaluated for marginal misfit and detorque after the mechanical loading (final marginal misfit/final detorque). The results were submitted to analysis of variance for repeated measurements, followed by Tukey HSD test (α = 0.05).

RESULTS

No statistically significant differences were found on detorque values of the prosthetics screws for all groups and intervals evaluated (p = 0.8922). The entirely calcinable abutments showed higher initial marginal misfit compared to the overcasted ones (p = 0.0438). There was no statistically significant difference on marginal misfit before and after mechanical loading for both groups (p > 0.05).

CONCLUSIONS

It can be concluded that the overcasted abutments showed lower misfit values when compared to the entirely casted abutments. No difference was observed on detorque values of prosthetic screws. After mechanical loading there was no difference on marginal misfit and detorque between the groups.

Marginal discrepancies of Ni/Cr crowns for a soft tissue-level, trans-mucosal implant system

OBJECTIVE

To examine the marginal fit of frameworks for implant-supported single crowns built with or without the use of prefabricated plastic copings.

METHODS

Sixty premolar sized metal crown frameworks for cemented implant restorations were prepared by two separate dental laboratories. Twenty were prepared with the use of original plastic copings for burn-out (OPC), twenty without any prefabricated plastic coping (WPC), and finally twenty with modified plastic copings (MPC). Specimens were inspected at 100× magnification. Marginal discrepancies can emerge at the vertical level as marginal gaps (MG) or horizontal differences in the circumferential precision of the restorations (HEX). HEX and MG were recorded at 10 different points in each metal crown. The ANOVA test was performed to estimate if there were statistically significant differences between the two dental laboratories. ANOVA Test was also used between Groups OPC, WPC, and MPC to evaluate differences in the mean MG and HEX values. The Tukey's post hoc test was also performed to determine whether couples of data sets were different or not.

RESULTS

No statistically significant differences regarding HEX and MG were found among the two dental laboratories. Significant differences were found among OPC, WPC, and MPC groups both for MG and HEX (P < 0.05). Tukey post hoc tests revealed that there were statistically significant differences (P-value < 0.05) in all pairs of groups and for both dimensions measured. The OPC group showed horizontal and vertical over-extension. The WPC group margins appeared vertically under-extended but horizontally over-extended. The WPC groups showed unpredictability of results in the vertical dimension. The MPC group margins demonstrated horizontal over-extension and appeared vertically slightly under-extended. The frameworks belonging to this group appeared the most precise in the vertical dimension.

CONCLUSIONS

Casting Ni/Cr alloys without the use of prefabricated plastic components leads to unpredictable precision in the vertical marginal discrepancies. The use of plastic copings results in more predictable but horizontally and vertical over-extended margins when the snap-on mechanism is not removed properly after the casting due to the hardness of the Ni/Cr alloys. The removal of the snap-on mechanism after the wax-up and before casting (MPC group) results in more uniform and smaller vertical marginal discrepancies.

Misfit of pure titanium frameworks: effect of veneer coverage and spark erosion process

OBJECTIVE. The purpose of this study was to evaluate the influence of veneer application on the misfit level of implant-supported frameworks. MATERIALS AND METHODS. Thirty commercially pure titanium (Tritan, Dentaurum, Pforzheim, Germany) frameworks were fabricated from a metallic index containing five Branemark-type multi-unit abutments. Analogs of the abutments were positioned into the framework to manufacture an index for each framework, which permitted the evaluation of the marginal gap caused only by the veneer coverage. The frameworks were grouped (n = 10) in the following manner: (G1) heat-cured acrylic resin (Clássico, Clássico, São Paulo, Brazil); (G2) light-cured resin (Versyo.com, Heraeus Kulzer, Brazil); and (G3) porcelain (Triceram, Dentaurum, Pforzheim, Germany). Marginal refinement with spark erosion was then conducted. The marginal gap was verified before and after the veneer coverage and the spark erosion procedure, following the single screw test protocol (tightening force of 10 Ncm). Data were submitted to an analysis of variance (ANOVA) in a split-plot design for repeated measurements followed by a Tukey test (p = 0.05). RESULTS. The veneer application was associated with a significant increase in the mean misfit values of all groups. The lowest values were presented by G2. After the spark erosion process, the mean misfit value decreased only on G3. CONCLUSIONS. Heat-cured acrylic resin and porcelain produced the highest values of marginal gaps, whereas light-cured acrylic resin produced the lowest. In addition, the spark erosion process was effective only in the marginal gap of the porcelain application group.

A comparison of the marginal adaptation of cathode-arc vapor-deposited titanium and cast base metal copings

STATEMENT OF PROBLEM

A new fabrication process where a titanium coping, with a gold colored titanium nitride outer layer, can be reliably fused to porcelain; however, the marginal adaptation characteristics are undetermined.

PURPOSE

The purpose of the study was to compare the clinically acceptable marginal adaptation (CAMA - defined as a marginal gap mean of ≤ 60 μm) rates of cathode-arc vapor-deposited titanium and cast base metal copings to determine whether the titanium copings would produce a higher CAMA rate than the cast base metal copings.

MATERIAL AND METHODS

Thirty-seven cathode-arc vapor-deposited titanium copings and 40 cast base metal copings were evaluated using an optical microscope. Fifty vertical marginal gap measurements were made of each coping, and the mean of these measurements was used to form the gap score. A 1-tailed t test was used to compare the CAMA rates, and the Satterthwaite t-score was used to analyze the consistency of the coping adaptation (α =.05).

RESULTS

CAMA was achieved by 24 of the 37 (64.86%) titanium copings compared to 19 of the 40 (47.50%) base metal copings. A 1-tailed t test produced a Z-score of 1.533 (1-tailed P=.063), which allowed acceptance of the study hypothesis with only a modest risk of a Type I error.

CONCLUSIONS

Cathode-arc vapor-deposited titanium copings exhibited a higher rate of CAMA compared to base metal copings.

Internal adaptation of cast titanium crowns

As the adaptation of titanium crowns obtained by Rematitan Plus investment, specific for titanium, is not recognized to be suitable, this study evaluated the effect of the concentration of the specific liquid and the temperature of the mold of investments on the internal misfit of crowns cast on commercially pure titanium. Individual dies of epoxy resin were obtained, representing teeth prepared for full-crown restoration with a 6-degree axial surface convergence angle and shoulder (1.0 mm). For the waxing of each crown, a ring-shaped stainless steel matrix (8.0mm internal diameter; 7.5 mm height) was adapted above the individual dies of epoxy resin. The Rematian Plus investment was mixed according to the manufacturer's instructions using two different concentrations of the specific liquid: 100%, 75%. Casting was performed in a Discovery Plasma Ar-arc vacuum-pressure casting machine with molds at temperatures of 430 degrees C, 515 degrees C and 600 degrees C. The crowns were cleaned individually in a solution (1% HF + 13% HNO3) for 10 min using a ultrasonic cleaner, with no internal adaptations, and luted with zinc phosphate cement under a 5 kg static load. The crown and die assemblies were embedded in resin and sectioned longitudinally. The area occupied by cement was observed using stereoscopic lens (10X) and measured by the Leica Qwin image analysis system (mm(2)). The data for each experimental condition (n=8) were analyzed by Kruskal-Wallis non-parametric test (á=0.05). The results showed that liquid dilution and the increase in mold temperature did not significantly influence the levels of internal fit of the cast titanium crowns. The lowest means (+/-SD) of internal misfit were obtained for the 430 degrees C/100%: (7.25 mm(2) +/-1.59) and 600 degrees C/100% (8.8 mm(2) +/-2.25) groups, which presented statistically similar levels of internal misfit.

An initial study of diffusion bonds between superplastic Ti-6Al-4V for implant dentistry applications

STATEMENT OF PROBLEM

Production of precisely fitting fixed partial denture implant superstructures with titanium alloys is limited by casting techniques that introduce distortion. After alignment of the framework with existing implants, the remaining misfit may generate stresses that cause screw loosening and adversely affect the implant/bone interface.

PURPOSE

The purpose of this study was to prepare diffusion-bonded joints between superplastic (SP) Ti-6Al-4V plates and indenters (representing analogs to implant-supported fixed frameworks and abutments) and determine if this process has potential for producing strong, dimensionally precise prostheses.

MATERIAL AND METHODS

Seven sets of trial indenter and plate specimens with dimensions of 6.4 mm x 6.4 mm x 5 cm and 8.5 mm x 1.5 cm, respectively, were prepared. Several indenter designs were used (35- or 45-degree half-angle, presence or absence of a notch, and SP versus no SP condition for Ti-6Al-4V); all plates were prepared from SP Ti-6Al-4V. For the results-guided experimental design, there was 1 trial indenter/plate combination for each design/processing condition. Diffusion bonding was performed at 10(-6) Torr, while the temperature was increased 10 degrees C/min to 900 degrees or 920 degrees C. Following 10 minutes of equilibration, the indenter was pressed 2.5 mm into the plate at 0.13 mm/min. Joint strength was evaluated in tension, and the ductile or brittle character of fracture surfaces was assessed by the presence or absence of a dimpled rupture surface from secondary electron SEM observations. Fractured specimens were cross-sectioned and examined with an optical microscope to evaluate overall joint integrity and quality, and used for Vickers hardness measurements to gain insight into the variation in mechanical properties of the indenter and plate with distance from the joint. One-way ANOVA (alpha=.05) was used to compare hardness at the joint for the trial specimen with highest joint strength with hardness values for adjacent regions at 125-mum intervals in the indenter and plate. The Ryan-Einot-Gabriel-Welsch (REGW) multiple range test was used to identify any specific location having significantly (alpha=.05) different hardness. Backscattered electron SEM observations were also performed on the cross-sectioned specimens to investigate whether a layer of alpha-stabilized titanium, which would decrease joint strength, was present. Fits of Ti-6Al-4V implant analogs prepared by this diffusion-bonding process were assessed qualitatively from visual observation.

RESULTS

The maximum joint strength of 820 MPa was achieved for a diffusion bonding temperature of 900 degrees C for an SP Ti-6Al-4V indenter with a 35-degree half-angle and no notch. This joint strength is nearly 90% of the maximum tensile strength of the parent Ti-6Al-4V, which can range from approximately 930 to 1015 MPa. The hardness at the joint was significantly higher (P<.05) than the hardness of the indenter and plate at 125-mum distances from the joint. The mean hardness of the indenter at 125 mum from the joint was significantly greater (P<.05) than the mean hardness of the plate at a distance of 500 mum from the joint. All other mean hardness values at the different measurement distances from the joint were not significantly different. Ductile fracture occurred for all superplastic processing conditions, and no alpha-titanium layer was present. Minimal asperities were observed with the optical microscope, and fits of implant prosthesis analogs were considered acceptable.

CONCLUSIONS

A 900 degrees C processing condition for diffusion-bonding an SP Ti-6Al-4V indenter with a 35-degree half-angle and no notch to a Ti-6Al-4V SP plate yielded a joint with nearly the same strength as the parent alloy. Use of this processing temperature with a 0.13 mm/min rate of pressing the indenter into the plate yielded minimal distortions for implant prosthesis analogs when observed visually.

Effect of Different Finishing and Polishing Agents on the Surface Roughness of Cast Pure Titanium

PURPOSE

The aim of this study was to evaluate the effect of finishing and polishing agents on surface roughness of cast commercially pure titanium using scanning electron microscope (SEM) analysis.

MATERIALS AND METHODS

A standardized square steel die measuring 10 x 10 mm with a thickness of 2 mm was machine cut. An impression of this die was used to create wax patterns for casting. Sixty specimens were cast in commercially pure titanium. These were divided into three groups (A, B, and C) of 20 specimens each. Group A specimens were polished with black, brown, and green rubber discs followed by green polishing compound with buff. Group B specimens were polished with black, brown, and green rubber cones, buffed with yellow polishing cake designed for gold alloy. Group C specimens were polished with silicium carbide cones and buffed with orange polishing cake. Surface roughness of the test specimens was measured in microns with a perthometer. Data were analyzed with ANOVA and Tukey's honest significant difference (HSD) multiple comparison tests among the different groups. Qualitative analysis was done by SEM photomicrographs.

RESULTS

Surface roughness values R(a) for Groups A, B, and C were 0.68 microm, 0.78 microm, and 0.27 microm, respectively. SEM photomicrographs and the statistical analysis revealed that the finishing and polishing were better with Group C test specimens with lower surface roughness values compared with groups A and B. Tests showed that Group C was statistically smoother (p< or = 0.01).

CONCLUSION

Within the limitations of this study, surface roughness was less on cast CpTi specimens that were finished and polished from the cutters designed specifically for titanium.

Surface and elemental alterations of dental alloys induced by electro discharge machining (EDM)

OBJECTIVES

To evaluate the surface and elemental alterations induced by electro discharge machining (EDM) on the surface of dental cast alloys used for the fabrication of implant retained meso- and super-structures.

METHODS

A completed cast model of an arch that received dental implants was used for the preparation of six wax patterns which were divided into three groups (Au, Co and Ti). The wax patterns of the Au and Co groups were invested with conventional phosphate-bonded silica-based investment material and the Ti group with magnesia-based investment material. The investment rings of the Au and Co groups were cast with an Au-Ag alloy (Stabilor G) and a Co-Cr base alloy (Okta C), respectively, while the investment rings of group Ti were cast with cp Ti (Biotan). One casting of each group was subjected to electro discharge machining (EDM); the other was conventionally ground and polished. The surface morphology and the elemental compositions of conventionally and EDM-finished surfaces were studied by SEM/X-ray EDS analysis. Six spectra were collected from each surface employing the area scan mode and the mean value of each element between conventionally and EDM-finished surfaces was statistically analyzed by t-test (a=0.05). Then the specimens of each group were cut perpendicular to their longitudinal axis and after metallographic grinding and polishing the cross-sections studied under the SEM.

RESULTS

The EDM surfaces showed a significant increase in C due to the decomposition of the dielectric fluid during spark erosion. Moreover, a significant Cu uptake was noted on these surfaces from the decomposition of the Cu electrodes used for EDM. Cross-sectional analysis showed that all alloys developed a superficial zone (recast layer) varying from 2 microm for Au-Ag to 10 microm for Co-Cr alloy.

SIGNIFICANCE

The elemental composition of dental alloy surfaces is significantly altered after EDM treatment.

Cementable implant crowns composed of cast superstructure frameworks luted to electroformed primary copings: an in vitro retention study

OBJECTIVE

The aim of this in vitro study was to investigate, on ITI solid abutments, the retention values of single crowns fabricated using an alternative prosthetic solution: secondary cast superstructure luted to an electroformed primary coping.

MATERIALS AND METHODS

Fifty standard 4.1 mm ITI implants and 5.5 mm high ITI machined abutments were assembled and mounted in acrylic resin. Implant/abutment assemblies were randomly divided into two groups. In the test group, primary galvanic caps were directly fabricated on implant abutments (A.G.C. Micro machine), and a secondary cast noble alloy superstructure was luted on each primary galvanic cap with a resin cement (Nimetic Cem). In the control group, prefabricated burn-out caps were used for casting the metal frameworks. Test and control crowns were cemented using a resin cement (Panavia 21). After storage at 37 degrees for 24 h, the specimens were subjected to a pull-out test using an Instron universal testing machine. The load required to dislodge each sample and the respective mode of failure were recorded. Means and standard deviations of loads at failure were analyzed using ANOVA. Statistical significance was set at P< or =0.05.

RESULTS

The retention values (+/-SD) of loads at failure were 67.26 (+/-16.61) for the test group and 44.03 (+/-9.45) for the control group. In the test group no separation occurred between the electroformed (galvanic) primary cap and the secondary superstructure.

CONCLUSIONS

The results showed that this prosthetic solution is superior on retentive performance than the conventional cast framework. An added clinical advantage of this novel method is its potential to provide a totally passive fit. Further in vitro and in vivo studies involving multiple-unit restorations are needed in order to more generally validate this prosthetic concept.

Effects of sandblasting and electrical discharge machining on porcelain adherence to cast and machined commercially pure titanium

The aim of this study was to determine the effect of sandblasting and electrical discharge machining (EDM) on cast and machined titanium surfaces and titanium-porcelain adhesion. Twenty machined titanium specimens were prepared by manufacturer (groups 1 and 2). Thirty specimens were prepared with autopolymerizing acrylic resin. Twenty of these specimens (groups 3 and 4) were cast with commercially pure titanium and the alpha-case layer was removed. For control group (group 5), 10 specimens were cast by using NiCr alloy. Groups 2 and 4 were subjected to EDM while groups 1, 3, and 5 were subjected to sandblasting. Surface examinations were made by using a scanning electron microscope (SEM). A low-fusing porcelain was fused on the titanium surfaces, whereas NiCr specimens were covered using a conventional porcelain. Titanium-porcelain adhesion was characterized by a 3-point bending test. Results were analyzed by Kruskal-Wallis and Mann-Whitney U tests. Metal-porcelain interfaces were characterized by SEM. The bond strength of control group was higher than that of the titanium-porcelain system. There was no significant difference between cast and machined titanium groups (p > 0.05). There was no significant difference between EDM and sandblasting processes (p > 0.05). The use of EDM as surface treatment did not improve titanium-porcelain adhesion compared with sandblasting.

Marginal accuracy of titanium copings fabricated by casting and CAD/CAM techniques

STATEMENT OF PROBLEM

Advances in computer-aided design/computer-assisted manufacturing (CAD/CAM) technology purportedly enhance the marginal fit of dental restorations. However, little information is available on the marginal accuracy of restorations manufactured with various CAD/CAM systems.

PURPOSE

The purpose of this study was to evaluate and compare the marginal accuracy and refinement time of titanium copings fabricated by 3 different CAD/CAM systems relative to standard casting techniques.

MATERIAL AND METHODS

Sixty-four stone die duplicates of a human maxillary central incisor, prepared for a metal-ceramic crown, with a uniform chamfer design, were divided into 4 groups (n=16). The specimens were restored with titanium copings using CAD/CAM systems Pro 50 (PRO), DCS (DCS), and Everest (EVE). A conventional titanium casting technique, Biotan (BIO), served as a control. Vertical and horizontal discrepancies between restoration margins and the preparations were each measured before and after manual refinement. This refinement was completed using a disclosing agent and by removing the internal positive defects of the copings. The marginal discrepancies of the copings were evaluated at 4 standard areas using 10 measurements, for a total of 160 measurements of each margin. Repeated-measures ANOVA was used for analyzing marginal accuracy. The coping refinement time was analyzed with the Kruskal-Wallis and post hoc Wilcoxon rank sum tests (alpha=.05).

RESULTS

The marginal discrepancies (microm) ranged from 32.9 to 127.8 before and from 3.4 to 58.4 after the manual refinement of copings. Manual refinement significantly improved the marginal accuracy (P<.0001) when compared with the initial fabrication. The relative (%) gain of marginal accuracy was PRO, 74.1%; DCS, 69.7%; EVE, 68.7%; and the control, BIO, 69.2%. The median duration of manual refinement time in minutes was 6.0 for PRO, 9.5 for DCS, 4.0 for EVE, and 4.0 for BIO (Kruskal-Wallis-test: P<.0001).

CONCLUSION

Manual adjustment significantly improves the marginal accuracy of CAD/CAM system-fabricated titanium copings. The highest marginal accuracy was achieved with the DCS system, using a longer refinement time.

Over-refractory casting technique as an alternative to one-piece multi-unit fixed partial denture frameworks

STATEMENT OF PROBLEM

Soldering has been suggested to achieve acceptable fit of multi-unit fixed partial denture (FPD) frameworks of 3 or more elements. However, distortion of the entire framework may occur.

PURPOSE

The purpose of this study was to evaluate marginal fit in 1-piece-casting FPD frameworks comparing 2 casting techniques: the conventional technique (CT) and the over-refractory technique (ORT). The techniques were evaluated on castings made from commercially pure titanium (CP Ti; Tritan), titanium-aluminum-vanadium (Ti-6Al-4V), and nickel-chromium (Ni-Cr; VeraBond II) alloys.

MATERIAL AND METHODS

A metal master cast including 3 preparations was created to simulate a 5-unit FPD. The metal cast was duplicated in vinyl polysiloxane for fabrication of 60 working casts. For the CT, 0.7-mm-thick patterns were waxed on the casts using an electrical wax-dipping unit. Patterns were removed from the preparations before investing. For the ORT, the silicone impressions were poured with an investment material (Rematitan for CP Ti and Ti-6Al-4V, and Talladium for Ni-Cr). The patterns were waxed on the refractory casts, and the assembly was invested. Both CT and ORT (n = 10), CP Ti, and Ti-6Al-4V frameworks were cast using an arc-melting titanium vacuum-casting machine. The Ni-Cr frameworks were cast using an automated centrifugal-casting machine. The mean marginal discrepancies of the frameworks were measured at the mesial, distal, buccal, and lingual interfaces. Marginal gap measurements were recorded 3 times for each surface to the nearest 0.5 microm using a traveling microscope (x50). The data were analyzed with 2-way analysis of variance, followed by the Tukey HSD test (alpha=.05).

RESULTS

Marginal discrepancies for the ORT specimens (82.8 +/- 1.7 microm for CP Ti; 64.4 +/- 1.3 microm for Ti-6Al-4V; and 77.2 +/- 1.5 microm for Ni-Cr) were significantly lower than those obtained for the CT specimens (106.3 +/- 2.6 microm for CP Ti; 75.7 +/- 0.8 microm for Ti-6Al-4V; 101.9 +/- 1.7 microm for Ni-Cr) (P < .05). For both techniques, marginal discrepancies were significantly different among the alloys evaluated, presenting the following order from greatest to least marginal discrepancy: CP Ti, Ni-Cr alloy, and Ti-6Al-4V (P < .05).

CONCLUSION

The ORT for multi-unit FPD frameworks demonstrated significantly lower marginal discrepancies than the CT.

The influence of mold temperature on the fit of cast crowns with commercially pure titanium

Commercially pure titanium (CP Ti) has been widely applied to fabricate cast devices because of its favorable properties. However, the mold temperature recommended for the manufacture of casts has been considered relatively low, causing inadequate castability and poor marginal fit of cast crowns. This study evaluated and compared the influence of mold temperature (430°C - as control, 550°C, 670°C) on the marginal discrepancies of cast CP Ti crowns. Eight bovine teeth were prepared on a mechanical grinding device and impressions were used to duplicate each tooth and produce eight master dies. Twenty-four crowns were fabricated using CP Ti in three different groups of mold temperature (n = 8): 430°C (as control), 550°C and 670°C. The gap between the crown and the bovine tooth was measured at 50 X magnification with a traveling microscope. The marginal fit values of the cast CP Ti crowns were submitted to the Kruskal-Wallis test (p = 0.03). The 550°C group (95.0 µm) showed significantly better marginal fit than the crowns of the 430°C group (203.4 µm) and 670°C group (213.8 µm). Better marginal fit for cast CP Ti crowns was observed with the mold temperature of 550°C, differing from the 430°C recommended by the manufacturer.

Surface roughness and fatigue performance of commercially pure titanium and Ti-6Al-4V alloy after different polishing protocols

STATEMENT OF PROBLEM

Surface quality of cast metal is directly related to service performance under fatigue stress. Surface heterogeneities resulting from either finishing or polishing processes or by corrosive agents such as fluoridated solutions, can negatively affect fatigue life. Cast titanium frameworks are difficult to polish, and an accepted polishing protocol has not been established.

PURPOSE

This study evaluated and compared surface roughness of cast commercially pure titanium (CP Ti) and Ti-6Al-4V alloy submitted to conventional or electrolytic polishing, correlating the results with corrosion-fatigue strength testing performed in artificial fluoridated saliva. Specimens were also tested in air at room temperature to evaluate the effectiveness of the corrosion-fatigue test model.

MATERIAL AND METHODS

For each metal, 40 dumb-bell-shaped rods, 2.3 mm in diameter at the central segment, were cast. Conventional polishing was performed on 20 specimens of each metal following the manufacturer's instructions. A source of continuous electrical current was used for electrolytic polishing of the other 20 specimens of each metal, which were immersed in an electrolytic solution containing 5% fluoridric acid, 35% nitric acid, and 60% distilled water. Surface roughness, Ra (microm), was measured with a profilometer, and fatigue tests were carried out with a universal testing machine using a load 30% lower than the 0.2% offset yield strength. After failure, the fractured surfaces were examined using scanning electron microscopy. Surface roughness means were analyzed with a 2-way analysis of variance and the Tukey multiple comparisons test (alpha=.05).

RESULTS

Electrolytic polishing (0.24 +/- 0.05 microm) provided significantly (P <.05) lower surface roughness values than conventional polishing (0.32 +/- 0.06 microm). Regardless of the polishing protocol, surface roughness of Ti-6Al-4V alloy (0.25 +/- 0.06 microm) was significantly lower (P <.05) than that of CP Ti (0.31 +/- 0.05 microm), and the fluoridated environment did not influence fatigue performance. There was no correlation between fatigue performance and surface roughness.

CONCLUSION

Surface roughness of Ti-6Al-4V was significantly lower than that of CP Ti. For cast titanium frameworks, the electrolytic polishing regimen was found to be more effective than the manufacturer's polishing instructions with abrasives and rotary instruments. After polishing, differences in surface roughness values did not affect corrosion-fatigue performance.

In vitro comparative analysis of the fit of gold alloy or commercially pure titanium implant-supported prostheses before and after electroerosion

STATEMENT OF PROBLEM

For implant-supported prostheses, passive fit is critical for the success of rehabilitation, especially when alternative materials are used.

PURPOSE

The purpose of this study was to compare interfacial fit of implant-supported prostheses cast in titanium to those cast in gold alloy.

MATERIAL AND METHODS

Five 3-unit fixed partial dentures were fabricated in gold alloy (Degudent U) as 1-piece castings, and 5 others were similarly cast in commercially pure titanium (Grade 1). The interfacial gaps between the prostheses and the abutments were evaluated with an optical microscope, before and after electroerosion. Readings were made with both screws tightened (10 N.cm torque), and with only 1 side tightened, so as to also evaluate the passive fit of the prostheses. Data were compared statistically by 2-way analysis of variance and the post hoc Tukey multiple range test (alpha=.05).

RESULTS

Before electroerosion, the interfacial gaps for the 1-piece prostheses were significantly smaller (P<.001) in the gold alloy group when the screws were tightened (Au=12.6 +/- 3.0 microm, compared to Ti=30.1 +/- 6.4 microm). When the side opposite the tightened side was analyzed, there was no significant difference between the gold alloy and titanium groups (Au=69.2 +/- 24.9 microm and Ti=94.2 +/- 39.6 microm). The electroerosion procedure significantly (P<.001) reduced the gaps at the interfaces for both groups under all conditions. Comparison between groups after electroerosion did not present significant differences when the side opposite the tightened side was analyzed, but the gold alloy group showed better fit when the tightened side was analyzed (12.8 +/- 1.4 microm for gold alloy; 29.6 +/- 4.4 microm for titanium) and when both screws were tightened (5.4 +/- 2.3 microm for gold alloy; 16.1 +/- 5.5 microm for titanium).

CONCLUSIONS

Cast titanium prostheses, despite showing larger interfacial gaps between the prosthesis and abutment than those obtained with gold alloy, had improved fit after being subjected to electroerosion.

Evaluation of low-fusing ceramic systems combined with titanium grades II and V by bending test and scanning electron microscopy

The bond strength by three point bending strength of two metal substrates (commercially pure titanium or grade II, and Ti-6Al-4V alloy or grade V) combined to three distinct low-fusing ceramic systems (LFC) and the nature of porcelain-metal fracture by scanning electron microscopy (SEM) were evaluated. The results were compared to a combination of palladium-silver (Pd-Ag) alloy and conventional porcelain (Duceram VMK68). Sixty metal strips measuring 25x3x0.5mm were made - 30 of titanium grade II and 30 of titanium grade V, with application of the following types of porcelain: Vita Titankeramik, Triceram or Duceratin (10 specimens for each porcelain). The porcelains were bonded to the strips with dimensions limited to 8x3x1mm. The control group consisted of ten specimens Pd-Ag alloy/Duceram VMK68 porcelain. Statistical analyses were made by one-way analysis of variance (ANOVA) and Tukey test at 5% significance level. Results showed that the bond strength in control group (48.0MPa ± 4.0) was significantly higher than the Ti grade II (26.7MPa ± 4.1) and Ti grade V (25.2MPa ± 2.2) combinations. When Duceratin porcelain was applied in both substrates, Ti grade II and Ti grade V, the results were significantly lower than in Ti grade II/Vitatitankeramik. SEM analysis indicated a predominance of adhesive fractures for the groups Ti grade II and Ti grade V, and cohesive fracture for control group Pd-Ag/Duceram. Control group showed the best bond strength compared to the groups that employed LFC. Among LFC, the worst results were obtained when Duceratin porcelain was used in both substrates. SEM confirmed the results of three point bending strength.