Ni-Cr based dental alloys; Ni release, corrosion and biological evaluation.

In the last years the dental alloy market has undergone dramatic changes for reasons of economy and biocompatibility. Nickel based alloys have become widely used substitute for the much more expensive precious metal alloys. In Europe the prevalence of nickel allergy is 10-15% for female adults and 1-3% for male adults. Despite the restrictions imposed by the EU for the protection of the general population in contact dermatitis, the use of Ni-Cr dental alloys is on the increase. Some questions have to be faced regarding the safety risk of nickel contained in dental alloys. We have collected based on many EU markets, 8 Ni-Cr dental alloys. Microstructure characterization, corrosion resistance (generalized, crevice and pitting) in saliva and the quantities of cations released in particular nickel and CrVI have been evaluated. We have applied non parametric classification tests (Kendall rank correlation) for all chemical results. Also cytotoxicity tests and an evaluation specific to TNF-alpha have been conducted. According to the obtained results, it was found that their behavior to corrosion was weak but that nickel release was high. The quantities of nickel released are higher than the limits imposed in the EU concerning contact with the skin or piercing. Surprisingly the biological tests did not show any cytotoxic effect on Hela and L929 cells or any change in TNF-alpha expression in monocytic cells. The alloys did not show any proinflammatory response in endothelial cells as demonstrated by the absence of ICAM-1 induction. We note therefore that there is really no direct relationship between the in vitro biological evaluation tests and the physico-chemical characterization of these dental alloys. Clinical and epidemiological studies are required to clarify these aspects.

Adhesion of oral streptococci to all-ceramics dental restorative materials in vitro.

In recent years, patients have benefited from the development of better and more esthetic materials, including all-ceramics dental restorative materials. Dental plaque formation on teeth and restorative materials plays an important role in the pathogenesis of oral diseases. This study investigates initial adhesion of stationary phase streptococcal species to different all-ceramics dental restorative materials. The saliva-coated materials were incubated with the bacteria for 1 h in an in vitro flow chamber which mimics environmental conditions in the oral cavity. Number and vitality of adhering bacteria were determined microscopically after staining. Surface roughness and the composition of the materials had no distinctive influence on bacterial adhesion. However, S. mutans and S. sobrinus adhered about tenfold less numerous to all materials than the other streptococcal species. Further, there was a correlation between bacterial vitality and materials' glass content. The results showed that early plaque formation was influenced predominantly by the presence of the salivary pellicle rather than by material dependent parameters whereas the composition of the all-ceramics appeared to have influenced the percentage of viable cells during the adhesion process. This presented in vitro technique may provide a useful model to study the influence of different parameters on adherence of oral streptococcal species.

Anisotropy of nickel release and corrosion in austenitic stainless steels.

The study of 316L-type stainless steel reveals a significant anisotropy of nickel release that is dependent on the orientation of the test surface with respect to the casting and rolling direction. Cross-sectional specimens (transversal cuts with respect to the rolling direction) show a substantially higher sensitivity to corrosion phenomena compared with longitudinal cuts and they release nickel ions at rates 10-100 times higher. These findings indicate that orientation needs to be taken into account when interpreting test results, in particular when comparing different grades of austenitic stainless steel, as well as in product and production design.

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

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

Reliability and strength of all-ceramic dental restorations fabricated by direct ceramic machining (DCM).

All-ceramic dental bridges for the molar region are not yet available at reasonable costs. The novel direct ceramic machining (DCM) process allows an easy, reliable and rapid fabrication for all-ceramic dental restorations with high mechanical strength and good biocompatibility. In DCM, an enlarged framework is easily milled out of a pre-fabricated porous ceramic blank made of zirconia. After sintering to full density, no further time-consuming hard machining with diamond tools is needed. For individual esthetical requirements, the framework is coated with a veneer porcelain. Compared to the commercially available In-Ceram Alumina and IPS Empress2 restorations, the mechanical strength of zirconia frameworks is twice as high, allowing the restorations to bear the high mastication forces in the molar region. In terms of reliability, zirconia bridges fabricated by the DCM process are also superior to In-Ceram Alumina and IPS Empress2. A clinical study of three-unit dental bridges in the molar region found no problems after the first year of observation.

Flexural strength of Cerec 2 machined and jointed InCeram-Alumina and InCeram-Zirconia bars.

OBJECTIVE: The flexural strength of Cerec 2 InCeram-Alumina and InCeram-Zirconia bars is evaluated. The focus of the in vitro study is to identify a jointing procedure for InCeram which may be used for producing full-ceramic fixed-partial-denture frameworks. METHODS: Six groups (n=15) of machined and jointed InCeram-Alumina (T1-T5) and InCeram-Zirconia (T6) bars (3x4x13mm(3)), respectively, were examined using a 3-point-bending test. InCeram-Alumina joint-free controls were: machined (C1), slip cast (C2, C3) and cut from the block (C4) bars. Machined joint-free InCeram-Zirconia bars were used as controls (C5). InCeram-Alumina slip was used for jointing T1-T5 and InCeram-Zirconia slip for bars T6. Bars were jointed in groups T1 and T2 using butt joint (S1), in T3 and T4 oblique (S2, S3) and in T5 and T6 rounded (S4) joint shapes. RESULTS: Two-way analysis of variance showed significant differences between materials (p<0.001) and jointing shapes (p<0.001). The rounded (S4) shape showed the highest flexural strength of 434 (65) MPa of InCeram-Alumina (T5) and 475 (54) MPa of InCeram-Zirconia (T6) bars, respectively but machined/joint-free InCeram-Alumina (511 (59) MPa, C1) and machined/joint-free InCeram-Zirconia (624 (58) MPa, C5) were significantly (p<0.01/p<0.001) stronger. No significant differences (p>0.05) were found between machined/jointed InCeram-Zirconia (475 (54) MPa, T6), joint-free InCeram-Alumina slip cast (498 (125) MPa, C2) and joint-free InCeram-Alumina machined bars (511 (59) MPa, C1). SIGNIFICANCE: Compared to conventional slip cast InCeram-Alumina the flexural strength of machined/jointed InCeram-Zirconia appears to be adequate for fixed-partial-denture frameworks.

Strength and microstructure of IPS Empress 2 glass-ceramic after different treatments.

PURPOSE: This investigation was designed to determine whether heat pressing and/or simulated heat treatments affect the flexure strength and microstructure of the lithium disilicate glass-ceramic of the IPS Empress 2 system. MATERIALS AND METHODS: Four groups of the lithium disilicate glass-ceramic were prepared as follows: group 1 = as-received material; group 2 = heat-pressed material; group 3 = heat-pressed and stimulated initial heat-treated material; and group 4 = heat-pressed and simulated heat-treated material with full firings for a final restoration. Three-point bending tests and scanning electron microscopy (SEM) analysis were conducted. RESULTS: The flexure strength of group 2 was significantly higher than that of group 1. However, there were no significant differences in strength among groups 2, 3, and 4, or between groups 1 and 4. The SEM micrographs of the lithium disilicate glass-ceramic showed a closely packed, multidirectionally interlocking microstructure of numerous lithium disilicate crystals protruding from the glass matrix. The crystals in the glass matrix of the heat-pressed materials (groups 2, 3, and 4) were a little more homogeneous and about 2 times bigger than those of the as-received material (group 1). These changes of the microstructure were greatest between groups 1 and 2. However, there were no marked differences among groups 2, 3, and 4. CONCLUSION: Although there were significant increases in the strength and some changes of the microstructure after the heat-pressing operation, the combination of heat pressing and simulated subsequent heat treatments did not produce an increase of strength of IPS Empress 2 glass-ceramic.

[Clinical study of zirconium oxide bridges in the posterior segments fabricated with the DCM system]. / Klinische Studie von Zirkonoxidbrücken im Seitenzahngebiet hergestellt mit dem DCM-System.

Today's dental reconstructive therapeutic concepts require restoration of high esthetic quality and excellent biocompatibility. Full ceramic reconstructions accomplish these requirements but only for anterior teeth and premolars. For all-ceramic bridges the mechanical strength was insufficient to withstand the posterior chewing forces. Frequently the interdental connectors cracked, and the only way to prevent these fractures was to overconture the connectors to a size of approx. 16 mm2. The high-tech ceramic zirconia is a potential alternative for three-to five-unit full ceramic bridges in the functionally loaded posterior segments. Experimental zirconia bridges which were fabricated using the DCM system (Direct Ceramic Machining System at the ETH Zurich, were tested in vitro. The frameworks were digitally enlarged by 20% and were easily milled from a presintered yet porous zirconia blank. After the milling process, the framework was densely sintered and shrank to its original size. Due to these positive in-vitro results a clinical investigation was started. 22 veneered zirconia bridges were luted; 19 molars and 25 premolars were prepared. The connectors, max. 7 mm2, of all these bridges, have been functionally loaded by antagonists. After a mean observation time of 385 days (307 days to 488 days), all 22 bridges did not show any cracks in the framework or in the veneering porcelain. The patients commented particularly on the low heat conduction rate of zirconia. The only endodontic problem which occurred could not be directly connected to the type of bridge framework. The reliability of zirconia bridges in this investigation was connected to the DCM-Process. No statement about other zirconia-systems can be made on the results of this study.

Perfusing dentine with horse serum or physiologic saline: its effect on adhesion of dentine bonding agents.

Freshly prepared dentine specimens of human teeth were perfused with either horse serum or physiologic saline. After application of AllBond2, ART Bond, Syntac or an experimental dentine bonding agent called P-Bond, a composite cylinder was added and cured at the same time. After 1500 thermal cycles with constant imitation of intrapulpal pressure, the shear bond strengths were measured. Resulting shear bond strength values were analysed with Students t-test or Mann-Whitney Test. The values for AllBond2 were not significantly different. The values for ART Bond (P < 0.05) and for Syntac (P < 0.05) were significantly higher if the dentine was perfused with horse serum. For P-Bond (P < 0.001) the values were significantly higher if the dentine was perfused with physiologic saline. According to these results it does not seem to be appropriate to take a clear decision as to which of the two perfusing media investigated might be more suitable to imitate in vivo conditions.

Effects of preparation and luting system on all-ceramic computer-generated crowns.

PURPOSE: Computer-aided design/computer-integrated machining (CAD/CIM) allows defect-oriented custom-shaping of the inside surfaces of all-ceramic crowns. The purpose of this study was to examine the effect of inside crown form on fracture strength of cemented and bonded crowns. MATERIALS AND METHODS: Four preparation types were used: (1) "classic" with a butt shoulder of 1.2 mm, abutment height of 4 mm, and 6-degree convergence, (2) like type 1 with mesio-occlusodistal cavity, (3) like type 1 with height reduced by 50%, and (4) like type 1 with abutment reduced by 100% plus a pulp chamber cavity. Crowns were CAD designed on preparations 1 to 4 using identical outside morphology. Machined crowns were placed on abutments (a) without any media as controls (n = 15), (b) cemented (n = 15), and (c) bonded (n = 15), and were loaded until fracture. RESULTS: Zinc phosphate-cemented crowns (1b, 2b, 3b, and 4b) showed significant (P < 0.001) increase of fracture load values compared to uncemented control crowns (1a, 2a, 3a, 4a). Fracture load values of bonded crowns (1c, 2c, 3c) were significantly (P < 0.001) higher than those for cemented crowns. Bonded crowns with thick occlusal dimensions (3c and 4c) showed the highest fracture load values. CONCLUSION: Bonded all-ceramic CAD/CIM crowns with defect-oriented inside morphology and increased occlusal dimensions showed high fracture load values.

A new all-ceramic post and core system: clinical, technical, and in vitro results.

Root-filled teeth with fractured or discolored coronal aspects invariably need to be restored by crowns. The prepared abutment tooth is usually reinforced by a metallic post and core system. The grayish discoloration of the root, and consequently of the gingiva, caused by the metal color may be an enormous esthetic disadvantage in the anterior teeth. In 1993 ceramic posts made of zirconia were introduced by the authors, allowing a new all-ceramic concept for nonvital abutment teeth. A new ceramic post and core system has now been developed with the idea of further improving esthetic appearance. In this system the core material is heat pressed directly onto the zirconia post. This article describes the material and the fabrication procedures (chairside and in the laboratory) of the system. Clinical results are presented. The retention of the core material is evaluated by in vitro tests.

Film thickness of various dentine bonding agents.

Curing dentine bonding agents create a film thickness on the surface of teeth which are prepared for all-ceramic crowns. The aim of this study was to investigate if the film thickness of dentine bonding agents (DBAs) is acceptable with the fit of definitive restorations of 50-100 microm. AllBond 2, Syntac, ART Bond, P-Bond (an experimental DBA), and the Primer of AllBond 2 were applied onto teeth which were prepared with standardized all-ceramic crown preparations. The DBAs were applied onto the prepared tooth surfaces according to manufacturers' instructions and under a standardized simulation of intrapulpal pressure. After curing the DBAs, the teeth were cut mesiodistally and orolingually and the film thickness was measured under a light microscope. Only the film thicknesses of AllBond 2 and P-Bond were low enough and, therefore, would enable curing to take place immediately after their application.

Effect of cleaning dentine with soap and pumice on shear bond strength of dentine-bonding agents.

This in vitro study reports on the cleaning effect of different soaps on the shear bond strength of various dentine-bonding agents. Human teeth were coated with provisional cements for 24 h or for 14 days. After removing the provisional cements with a scaler, the dentinal surface was cleaned with a cotton pellet and non-fluoridated flour of pumice and soap for 10 sec. Different dentine-bonding agents and a luting resin were bonded to the dentinal surface according to manufacturers' instructions with the bonding agent and the composite material being light-cured at the same time. The bonding agents were tested under intrapulpal pressure and with thermal cycling to imitate physiological conditions. Compared with cleaning the dentine with water and pumice, all soaps investigated in this study decreased the shear bond strength values of the tested dentine-bonding agents considerably.

Bond strength of one-bottle dentin bonding agents on human dentin.

PURPOSE: To evaluate the shear bond strength of one-bottle dentin bonding agents (DBA's) (Prime & Bond 2.1, ART Experimental, Syntac Single Component) on pressurized human dentin. MATERIALS AND METHODS: Freshly prepared dentin specimens of human teeth were perfused with horse serum which was diluted 1:5 in physiologic saline. Three different types of surface treatment were evaluated on the freshly prepared dentin. Group 1: One of the three one-bottle DBA's was applied onto freshly prepared dentin together with a cylinder of composite luting resin and cured. Group 2: A provisional cement (Freegenol, without eugenol, or Temp Bond, containing eugenol) was applied first on the dentin surface for 24 h. Only then was one of the three the one-bottle DBA's added onto the dentin together with a cylinder of composite luting resin after cleaning the dentin surface with pumice. Group 3: One of the three one-bottle DBA's was applied first on the dentin surface and light-cured. Then a provisional cement (Freegenol, without eugenol, or Temp Bond, containing eugenol) was added for 24 h. After cleaning with pumice, the respective one-bottle DBA was applied for a second time onto the dentin together with a cylinder of composite luting resin and light-cured. As control for Group 1 (freshly prepared dentin), the two- or three-step DBA's ART Bond and Syntac were used in a similar way. As control for Group 2 (single application of the DBA's after contamination of the dentin with a provisional cement) and Group 3 (dual application of the DBA's with intermediate contamination of the dentin with a provisional cement) the two-step DBA ART Bond was used. After 1500 thermal cycles with constant imitation of intrapulpal pressure, shear bond strengths were measured. Resulting shear bond strength values were displayed by means of a box plot and they were analyzed statistically by Student's t-Test or one-way ANOVA. RESULTS: Lowest and highest mean shear bond strength values were 0.26 +/- 0.47 MPa (single use of ART Bond with prior application of Temp Bond) and 16.34 +/- 5.02 MPa (dual use of ART Bond with intermediate application of Temp Bond). With respect to the surface treatment significant differences between the DBA's could be found in all groups.

Dentin bond strength of Dyract Cem.

PURPOSE: To evaluate the influence of provisional cements and of a dentin bonding agent on the adhesion of a self-curing polyacid-modified resin composite (Dyract Cem) on pressurized human dentin. MATERIALS AND METHODS: Freshly prepared dentin specimens of human teeth were perfused with physiologic saline. Three different types of surface treatment were evaluated. (1) Dyract Cem was applied to freshly prepared dentin without (Group 1.1) or with (Group 1.2) the respective dentin bonding agent (Prime & Bond 2.0) and cured. (2) Freegenol (Groups 2.1 and 2.2) or Temp Bond (Groups 2.3 and 2.4) were applied first on the dentin surface for 24 hours. Only then was Dyract Cem (with or without Prime & Bond 2.0) added onto the dentin after cleaning the dentin surface with pumice. 3) Prime & Bond 2.0 was applied first on the dentin surface and cured. Then Freegenol (Group 3.1) or Temp Bond (Group 3.2) were added for 24 hours. After cleaning with pumice, Prime & Bond 2.0 was applied for a second time on the dentin (= dual application) and finally Dyract Cem was added. As control, a conventional glass ionomer cement (Ketac-Cem Maxicap; Groups 4.1-4.3) was used in a similar way. After 1,500 thermal cycles with constant imitation of intrapulpal pressure, shear bond strengths were measured. Resulting shear bond strength values were displayed by means of a box plot and they were analyzed statistically by Mann-Whitney, Kruskal-Wallis or one way ANOVA tests. RESULTS: Lowest and highest mean shear bond strength values were 0.27 +/- 0.42 MPa (Group 2.4; single use of Prime & Bond 2.0 with prior application of TempBond) and 5.84 +/- 3.36 MPa (Group 3.1; dual use of Prime & Bond 2.0 with intermediate application of Freegenol). A clearly significant difference between groups could only be found when a single or dual use of the dentin bonding agent Prime & Bond 2.0 were combined with either Freegenol (Groups 2.1, 2.2 and 3.1; Kruskal-Wallis: P < 0.01) or Temp Bond (Groups 2.3, 2.4 and 3.2; Kruskal-Wallis: P < 0.001) as additional dentin surface treatment.

Dual application of dentin bonding agents: effect on bond strength.

PURPOSE: To evaluate the effect of a dual application of dentin bonding agents (DBA) on their shear bond strength on dentin with intermediate application of a provisional cement. MATERIALS AND METHODS: Freshly prepared flat dentin surfaces of human teeth were coated with a first layer of one of various dentin bonding agents (All-Bond 2, ART Bond, Syntac or an experimental DBA called P-Bond). After curing the DBA, a provisional cement (Temp Bond, Freegenol or Fermit, a soft provisional diacrylate) was applied to the bonded dentin surface. After 24 hours, the provisional cement was removed and the dentin surface scrubbed with pumice. A second application of the same DBA together with a composite cylinder followed. After curing and 1,500 thermal cycles with constant imitation of intrapulpal pressure, shear bond strengths were measured. RESULTS: Compared to a single application of dentin bonding agents following Temp Bond treatment (SBS in MPa: Syntac: 0.86 +/- 1.75, ART Bond: 0.26 +/- 0.47, P-Bond: 14.90 +/- 4.51) application of DBAs prior to use of Temp Bond as well as after its removal seems to be very beneficial to shear bond strength values (SBS in MPa: Syntac:13.36 +/- 4.70, ART Bond: 16.34 +/- 5.02, P-Bond: 19.04 +/- 2.01). Independent from the provisional cement, the values after serial application of P-Bond provided consistently high bond values which were not statistically different (Kruskal-Wallis, P > 0.05) from values of P-Bond on fresh dentin (18.19 +/- 2.29 MPa). The only exception from these findings in the present study was All-Bond 2. Regardless of the provisional materials used, the bond strength values of All-Bond 2 remained low (maximum SBS with Fermit as intermediate provisional cement: 4.63 +/- 2.91 MPa).

Corrosion considerations in the brazing repair of cobalt-based partial dentures.

Cobalt-based alloys (Co-Cr-Mo) are usually used in dentistry as frameworks for removable partial dentures. In their basic form these structures function successfully. However, modifications or repairs of the frameworks may reduce their resistance to corrosion and, as a consequence, may provoke biologic reactions in the soft tissues. These reactions may be the result of different types of alloys that contact each other and, in the presence of saliva (based on potential differences), produce a galvanic cell. In this study, a clinical situation after repair of a removable partial denture was examined. The metallographic study of the prosthesis revealed a brazed zone where a gold braze was joining the Co-Cr-Mo framework with a Co-Cr-Ni type alloy (without Mo). The latter revealed signs of corrosion. Various electrochemical parameters (Ecorr, Ecouple, icorr, icouple) of these alloys were analyzed in the laboratory. The Co-Cr-Ni alloy had the lowest nobility and underwent galvanic corrosion in a galvanic couple with gold braze.