A 5-year retrospective study of cobalt-chromium-based fixed dental prostheses

Cobalt chromium alloys in fixed prosthodontics: Investigations of mechanical properties and microstructure

STATEMENT OF PROBLEM

Cobalt chromium (Co-Cr) alloys possess beneficial mechanical properties because alloys, even in thin sections, can resist high mastication forces and exhibit an acceptable bond to the surface porcelain layer. Traditional manufacturing techniques of Co-Cr alloys such as casting have been replaced with newer fabrication techniques, such as milling, laser melting, and presintered milling. Despite scarce documentation, these new manufacturing techniques are being used to fabricate dental and implant constructions.

PURPOSE

This in vitro study investigates the hardness, yield strength, elastic modulus, and microstructure of the most commonly used Co-Cr alloys for fixed prosthodontics based on manufacturing technique. In addition, this study investigates the effect of heat treatment on the mechanical properties and microstructure of these materials.

MATERIAL AND METHODS

Five Co-Cr alloys were included (dumbbell and rectangular shaped) based on four manufacturing techniques: cast, milled, laser melted, and presintered milled. Commercially pure titanium grade 4 and titanium-6 aluminum-4 vanadium ELI (extra low interstitial) were included for comparison, and yield strength and elongation after fracture were evaluated. The specimens were tested for hardness using the Vickers test and for elastic modulus using a nondestructive impulse excitation technique. The microstructure of selected specimens was analyzed using focused ion beam-scanning electron microscopy (FIB-SEM) and energy dispersive X-ray spectroscopy (EDS).

RESULTS

The mechanical properties depend on the manufacturing technique used; the laser-melted and presintered Co-Cr specimens demonstrated the highest mechanical properties, followed by the milled and cast groups. Both the laser-melted and the presintered milled Co-Cr specimens showed smaller grain size compared with the cast and milled Co-Cr specimens. The titanium-6 aluminum-4 vanadium ELI demonstrated higher hardness and yield strength compared to commercially pure titanium grade 4. No major differences were observed for the selected materials regarding the mechanical properties and microstructural appearance after heat treatment.

CONCLUSIONS

The laser melting and presintered milling techniques produced higher mechanical properties compared with the cast and milled Co-Cr. These findings were confirmed through microstructural analysis with respect to the grain size, precipitation, and number of pores.

Up to 10 years clinical performance of zirconia ceramic and metal-ceramic fixed partial dentures: A retrospective study

STATEMENT OF PROBLEM

Completely veneered zirconia ceramic (ZC) fixed partial dentures (FPDs) have been reported to have a higher incidence of ceramic chipping than the standard metal-ceramic FPDs. However, data from comparative long-term studies are sparse.

PURPOSE

The purpose of this retrospective study was to compare the long-term survival, chipping-free survival, and success of ceramic-veneered high noble metal alloy (HN), base metal alloy (cobalt-chromium alloy) (CC), and ZC FPDs and to isolate risk factors for the incidence of failure, veneer chipping, and overall complications.

MATERIAL AND METHODS

Data from 289 study participants (58.7% women; mean age, 57.97 ±11.51 years) provided with a total of 400 FPDs comprising 197 (49.3%) HN FPDs, 121 (30.3%) CC FPDs, and 82 (20.5%) ZC FPDs fabricated from presintered 3 mol% yttria-stabilized zirconia (mean time of service, 6.85 ±3.25 years) were evaluated. Of these, 278 (69.5%) FPDs were provided by dentists and 122 (30.5%) by dental students. Five- and 10-year survival, chipping-free survival, and success rates were calculated using the Kaplan-Meier method and compared with the log-rank test. Risk factors were assessed using Cox regression analysis. The study was exploratory, so all P values were considered exploratory and descriptive.

RESULTS

The 5- and 10-year survival rates were 94.7% and 77.8% for HN, 93.7% and 81.2% for CC, and 92.9% and 53.3% for ZC FPDs, respectively, indicating no clear difference in survival among the framework materials. The 5- and 10-year chipping-free rates were 94.1% and 82.8% for HN, 96.1% and 78.9% for CC, and 82.6% and 62.2% for ZC FPDs, respectively, indicating a difference between HN and ZC, and CC and ZC restorations. The 5- and 10-year success rates were 83.7% and 55.3% for HN, 86.5% and 51.4% for CC, and 68.9% and 30.2% for ZC FPDs, respectively, indicating a difference between HN and ZC, and CC and ZC restorations. The Cox regression model indicated framework material as an influencing risk factor for success, and this influence persisted when chipping was investigated. Compared with zirconia, the use of a metal alloy reduced the risk of chipping or the occurrence of complications by up to one-third.

CONCLUSIONS

All FPDs showed high 5-year survival rates and acceptable 10-year survival rates with no strong differences among the materials. Higher success and chipping-free rates were observed for both HN and CC restorations compared with ZC restorations. Metal frameworks had a lower risk for complications or veneer chipping than zirconia frameworks.

Long-term retrospective clinical study of tooth-supported fixed partial dentures: A multifactorial analysis

PURPOSE

To assess the influence of several factors on the survival of tooth-supported fixed partial dentures (FPDs) with vital and non-vital abutments.

MATERIALS AND METHODS

The present retrospective cohort study was based on the records of patients treated with 3-to-7-unit tooth-supported FPDs with a minimum follow-up time of 6 months after prosthesis delivery. Cumulative survival rate (CSR) was calculated over the maximum follow-up period. Cox regression models were used to evaluate the association between the clinical covariates and prosthesis failure.

RESULTS

A total of 331 FPDs in 229 patients were included. The CSRs were 90.1% and 77.6% after 5 and 10 years and 67.9% and 52.1% after 15 and 20 years, respectively. Tooth-supported FPD failure was more prevalent within the first years of prosthesis delivery. Loss of one or more abutment teeth and loss of prosthesis retention were the main reasons for failure. Smoking and type of prosthesis material significantly influenced the survival of FPDs. Abutment vitality, position of the non-vital abutment, or prosthesis length did not show any significant influence on the occurrence of prosthesis failure.

CONCLUSION

Smoking and the type of prosthesis material are suggested to contribute to an increased rate of FPD failure irrespective of abutment vitality.

A retrospective clinical study of fixed tooth- and implant-supported prostheses in titanium and cobalt-chromium-ceramic: 5-9-year follow-up

OBJECTIVES

The aim of this retrospective study was to evaluate the clinical outcome of fixed tooth- and implant-supported protheses manufactured in porcelain veneered cobalt-chromium (CoCr) or titanium with a follow-up period of 5-9 years.

MATERIALS AND METHODS

This study included 63 patients with a total of 86 fixed dental protheses (FDPs) (53 implant-supported and 33 tooth-supported). In total, 67 were short-span FDPs (3-5 units) and 19 were long-span FDPs (6-12 units). The FDPs were evaluated using a modified version of the California Dental Association (CDA).

RESULTS

The binary regression analysis indicated that neither CoCr nor titanium had a statistically significant effect on the odds of success or survival of either tooth- or implant-supported FDPs. However, the success of FDPs was negatively affected by greater FDP length and general tooth wear. The survival of FDPs was negatively affected by increased FDP longevity.

CONCLUSIONS

This study found no statistically significant effect on the odds of success and survival outcomes for any combination of tooth-supported, implant-supported, porcelain-veneered CoCr, or porcelain-veneered titanium FDPs. As the number of FDPs was limited, the results should be interpreted with caution.

CLINICAL RELEVANCE

This study shows that the choice between CoCr porcelain and titanium porcelain in fixed protheses did not have a statistically significant effect on the outcome.

Cobalt-chromium alloys fabricated with four different techniques: Ion release, toxicity of released elements and surface roughness

OBJECTIVE

To investigate the metal ion release, surface roughness and cytoxicity for Co-Cr alloys produced by different manufacturing techniques before and after heat treatment. In addition, to evaluate if the combination of materials affects the ion release.

METHODS

Five Co-Cr alloys were included, based on four manufacturing techniques. Commercially pure titanium, CpTi grade 4 and a titanium alloy were included for comparison. The ion release tests involved both Inductive Coupled Plasma Optical Emission Spectrometry and Inductive Coupled Plasma Mass Spectrometry analyses. The surface analysis was conducted with optical interferometry. Cells were indirectly exposed to the materials and cell viability was evaluated with the MTT (3-(4.5-dimethylthiazol-2-yl)-2.5-diphenyltetrazolium bromide) method.

RESULTS

All alloys showed a decrease of the total ion release when CpTi grade 4 was present. The total ion release decreased over time for all specimens and the highest ion release was observed from the cast and milled Co-Cr alloy in acidic conditions. The cast and laser-melted Co-Cr alloy and the titanium alloy became rougher after heat treatment. All materials were within the limits of cell viability according to standards.

SIGNIFICANCE

The ion release from Co-Cr alloys is influenced by the combination of materials, pH and time. Surface roughness is influenced by heat treatment. Furthermore, both ion release and surface roughness are influenced by the manufacturing technique and the alloy type. The clinical implication needs to be further investigated.

Cobalt-chromium alloys in fixed prosthodontics in Sweden

Aim: The aim of this study was to compile the usage of Co-Cr alloys in fixed prosthodontics (FP) among dental laboratories in Sweden.

Methods: From March to October 2015, questionnaires were sent to 542 registered dental laboratories in Sweden. The questionnaires were divided in two parts, one for fixed dental-supported prosthodontics (FDP) and one for fixed implant-supported prosthodontics (FIP). Reminders were sent three times.

Results: In total of 542 dental laboratories, 55% answered the questionnaires. Most dental laboratories use Co-Cr in FP, 134 (74%) in FDP and 89(66%) in FIP. The laboratories used Co-Cr alloys of various compositions in the prostheses, 35 for FDP and 30 for FIP. The most commonly used Co-Cr alloys for tooth-supported FDPs were (a) Wirobond^® 280, (b) Cara SLM and (c) Wirobond^® C. For implant-supported frameworks the frequently used alloys were: (a) Cara SLM, (b) Cara Milled and (c) Wirobond^® 280. Except for the difference in composition of these alloys, they were also manufactured with various techniques. In tooth-supported prostheses the dominating technique was the cast technique while newer techniques as laser-sintering and milling were more commonly reported for implant-supported constructions. A fourth technique; the ‘pre-state’ milling was reported in FDP.

Conclusion: More than 30 different Co-Cr alloys were reported as being used in FP. Thus, there is a need for studies exploring the mechanical and physical behavior and the biological response to the most commonly used Co-Cr alloys.

Fit of cobalt-chromium implant frameworks before and after ceramic veneering in comparison with CNC-milled titanium frameworks

Computer-aided design/computer-aided manufacturing fabrication of implant-supported frameworks is a standard procedure, and the use of ceramic-veneered cobalt-chromium alloys is increasing. However, no data are available concerning the precision of fit of these frameworks and the impact on the fit of the veneering procedure. The aim of this study was to evaluate the fit of computer numeric-controlled-milled cobalt-chromium and titanium implant frameworks for edentulous maxillas, provided with six implants. An additional aim was to evaluate the effect of ceramic veneering on the fit of the cobalt-chromium frameworks. Ten stone casts simulating an edentulous maxilla provided with six dental implants and abutments were produced. One computer numeric-controlled-milled cobalt-chromium framework and one titanium framework were fabricated for each stone cast. Each stone cast and corresponding titanium and cobalt-chromium framework was measured with a coordinate measuring machine in the three-dimensional (X axis, Y axis, and Z axis) directions. Both milled titanium and cobalt-chromium frameworks presented a good fit in the vertical plane (Z axis), 5.3 µm for titanium frameworks and 4.6 µm for the cobalt-chromium frameworks. The titanium frameworks showed a statistically significant smaller mean degree of misfit in the horizontal plane, X (5.0 µm) and Y (2.8 µm) axes as compared with the cobalt-chromium frameworks presenting a mean deviation of 13.5 µm in X axis and 6.3 µm in Y axis (P < 0.001). After ceramic veneering of the cobalt-chromium frameworks, the horizontal distortion significantly decreased from 13.5 to 9.7 µm in X axis (P = 0.007) and from 6.3 to 4.4 µm in Y axis (P = 0.017). The fit of both titanium and cobalt-chromium frameworks was very good. There were small but significant differences in fit between the titanium and cobalt-chromium frameworks, but the difference is of no clinical significance. The ceramic veneering resulted in a minor but significant improvement of the fit for the cobalt-chromium frameworks.

In vitro element release and biological aspects of base-metal alloys for metal-ceramic applications

Objective: The aims of this study were to investigate the release of element from, and the biological response in vitro to, cobalt–chromium alloys and other base–metal alloys used for the fabrication of metal-ceramic restorations.

Material and methods: Eighteen different alloys were investigated. Nine cobalt–chromium alloys, three nickel–chromium alloys, two cobalt–chromium–iron alloys, one palladium–silver alloy, one high-noble gold alloy, titanium grade II and one type III copper–aluminium alloy. Pure copper served as positive control. The specimens were prepared according to the ISO standards for biological and corrosion testing. Passive leaching of elements was measured by using Inductively Coupled Plasma – Mass Spectrometry (ICP-MS) after incubation in cell culture media, MEM, for 3 days. Corrosion testing was carried out in 0.9% sodium chloride (NaCl) and 1% lactic acid for 7 days, and the element release was measured by Inductively Coupled Plasma – Optical Emission Spectroscopy (ICP-OES). The biological response from the extract solutions was measured though MTT cytotoxicity testing and the Hen's egg test-chorio-allantoic membrane (HET-CAM) technique for irritationt.

Results: The corrosion test showed similar element release from base-metal alloys compared to noble alloys such as gold. Apart from the high-copper alloy, all alloys expressed low element release in the immersion test, no cytotoxic effect in the MTT test, and were rated non-irritant in the HET-CAM test.

Conclusions: Minimal biological response was observed for all the alloys tested, with the exception of the high-copper alloy.

Surface characteristics and castability of Zr-14Nb alloy dental castings

The purpose of this study was to evaluate mechanical properties, surface characteristics and castability of Zr-14Nb dental castings. The mean 0.2% proof and ultimate tensile strengths of Zr-14Nb were approximately 68% and 76% those of Ti-6Al-7Nb, respectively, while they were comparable to Co-29Cr-6Mo. Elongation of Zr-14Nb was the highest of all alloys tested. The hardened reaction layer was formed on the surfaces of the Zr-14Nb castings. The layer consisted of oxygen and aluminum originating form investment. The castability of Zr-14Nb was comparable to that of Ti-6Al-7Nb. Dental castings of Zr-14Nb reveal mechanical properties that were within the range of the other dental alloys. Further improvements in castability and minimization of the surface reaction layer are needed for applications in dental prostheses.