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

The Effect of Framework Fabrication Technique on the Fit Accuracy of Full Arch Screw Retained Implant Supported Prostheses

Purpose

The purpose of this study was to compare the accuracy of fit of Co-Cr full arch screw-retained implant-supported fixed dental prosthesis fabricated among three different methods: conventional casting, milling, and additive manufacturing technology.

Materials and methods

A master model of a completely edentulous mandible with five internal connection implants was utilized. Thirty full arch Co-Cr screw-retained implant-supported frameworks were fabricated by three different methods: conventional casting, milling, and additive manufacturing (AM) technology. The marginal fit was measured using a coordinate measuring machine in x-, y-, and z-axes, as well as the three-dimensional discrepancy. The casting group were measured twice: before the adaptation procedure and again after the adaptation procedure (sectioning and laser welding). For comparisons of marginal fit of frameworks between different groups one-way analysis of variance and Games Howell test was used. Paired t-test was used to compare cast frameworks before and after adaption.

Results

There were statistically significant differences in marginal fit and width distortion between groups (P <.05). The mean of total distortion of each group was 94.6 µm (SD 50.5 µm) for casting group before adaptation, 92.44 µm (SD 49.6 µm) for casting group after adaptation, 71.4 µm (SD 37.2 µm) for additive manufacturing group, while for the milling group the total distortion was 50.1 µm (SD 27.5 µm).

Conclusion

Full arch screw-retained implant-supported frameworks fabricated with any of the three fabrication techniques using cobalt-chromium material exhibited acceptable marginal fit. Milling fabrication technique showed the most accurate marginal fit. Adaptation procedure for the cast group has significantly improved the marginal fit.

Comparison of porcelain veneer fracture in implant-supported fixed full-arch prostheses with a framework of either titanium, cobalt-chromium, or zirconia: An in vitro study

Objectives

The aim of this study was to compare porcelain veneer strength on screw‐retained implant‐supported fixed full‐arch prostheses with a framework of either milled titanium (Ti), cobalt–chromium (CoCr), and yttria‐stabilized zirconia (Y‐TZP) in an in vitro loading model.

Materials and Methods

Fifteen screw‐retained maxillary implant‐supported full‐arch prostheses (FDP), five each of Ti, CoCr, and Y‐TZP frameworks with porcelain veneers were included. All FDPs were subjected to thermocycling before loading until fracture of the veneer. The load was applied at the distal fossa of the occlusal area of the pontic replacing 24. Fracture loads were analyzed, and the fracture quality was assessed. Statistical analysis on the fracture load was performed using Kruskal–Wallis test. The statistical significance was set at p < .05.

Results

There was no statistical significance found between the groups regarding fracture load. The highest and lowest load was seen within the CoCr FDP, varying between 340 and 1484 N. Different types of fracture appearances were seen. The Y‐TZP FDPs had a higher number of fractures locally in the loaded area while CoCr and Ti more often showed cracks in the anterior region, at a distance from the loaded area.

Conclusions

Within the limitations of this study, the conclusion was that framework material may affect the fracture behavior of maxillary full‐arch bridges; however, there were no differences in veneer fracture strength when frameworks of Ti, CoCr, or Y‐TZP were compared.

Implant framework misfit: A systematic review on assessment methods and clinical complications

BACKGROUND

The fit of implant-supported prostheses is of prime importance for the long-term success of implant therapy.

PURPOSE

This systematic review aimed to evaluate recent evidence on current techniques for assessing implant-framework misfit, its associated strain/stress, and whether these misfits are related to mechanical, biological, and clinical consequences.

MATERIALS AND METHODS

An electronic search for publications from January 2010 to October 2020 was performed using the Pubmed, Embase, Web of Science, and Cochrane Library databases with combined keywords on implant-framework misfit assessments and related clinical complications. Inclusion and exclusion criteria were applied. After full-text analyses, data extraction was implemented on current techniques of misfit assessment and the relationship between the misfit and the induced strain/stress.

RESULTS

A total of 3 in vivo and 92 in vitro studies were selected, including 47 studies on quantifying the degree of implant-framework misfit with dimensional techniques, 24 studies measuring misfit-induced strain/stress with modeling techniques, and 24 studies using both methods. The technical details, advantages, and limitations of each technique were illustrated. The correlation between the implant-framework misfit and the induced strain/stress has been revealed in vitro, while that with the biological complications and implant/prostheses failure was weak in clinical studies.

CONCLUSIONS

Dimensional and modeling techniques are available to measure the implant-framework misfit. The passivity of implant-supported fixed prostheses appeared related to the induced strain/stress, but not the clinical complications. Further studies combining three-dimensional (3D) assessments using dimensional and modeling techniques was needed.

Stress distribution on different bar materials in implant-retained palatal obturator

Implant-retained custom-milled framework enhances the stability of palatal obturator prostheses. Therefore, to evaluate the mechanical response of implant-retained obturator prostheses with bar-clip attachment and milled bars, in three different materials under two load incidences were simulated. A maxilla model which Type IIb maxillary defect received five external hexagon implants (4.1 x 10 mm). An implant-supported palatal obturator prosthesis was simulated in three different materials: polyetheretherketone (PEEK), titanium (Ti:90%, Al:6%, V:4%) and Co-Cr (Co:60.6%, Cr:31.5%, Mo:6%) alloys. The model was imported into the analysis software and divided into a mesh composed of nodes and tetrahedral elements. Each material was assumed isotropic, elastic and homogeneous and all contacts were considered ideal. The bone was fixed and the load was applied in two different regions for each material: at the palatal face (cingulum area) of the central incisors (100 N magnitude at 45°); and at the occlusal surface of the first left molar (150 N magnitude normal to the surface). The microstrain and von-Mises stress were selected as criteria for analysis. The posterior load showed a higher strain concentration in the posterior peri-implant tissue, near the load application side for cortical and cancellous bone, regardless the simulated material. The anterior load showed a lower strain concentration with reduced magnitude and more implants involving in the load dissipation. The stress peak was calculated during posterior loading, which 77.7 MPa in the prosthetic screws and 2,686 με microstrain in the cortical bone. For bone tissue and bar, the material stiffness was inversely proportional to the calculated microstrain and stress. However, for the prosthetic screws and implants the PEEK showed higher stress concentration than the other materials. PEEK showed a promising behavior for the bone tissue and for the integrity of the bar and bar-clip attachments. However, the stress concentration in the prosthetic screws may represent an increase in failure risk. The use of Co-Cr alloy can reduce the stress in the prosthetic screw; however, it increases the bone strain; while the Titanium showed an intermediate behavior.

Can ceramic veneer spark erosion and mechanical cycling affect the accuracy of milled complete-arch frameworks supported by 6 implants?

STATEMENT OF PROBLEM

Milling is a well-established method for manufacturing prosthetic frameworks. However, information about the influence of ceramic veneer and spark erosion on the accuracy of the all-on-six complete-arch fixed frameworks manufactured from different materials is lacking.

PURPOSE

The purpose of this in vitro study was to compare the accuracy of milled complete-arch fixed frameworks with zirconia, cobalt-chromium, and titanium at different steps of their manufacturing process and the influence of mechanical cycling.

MATERIAL AND METHODS

Fifteen milled complete-arch fixed frameworks, supported by 6 implants, were made in zirconia, cobalt-chromium, and titanium (n=5). The fit was measured by the single-screw test protocol. Stress was measured by photoelastic analysis. The loosening torque was evaluated by tightening the screws, retightening them after 10 minutes, and then evaluating the loosening torque 24 hours later. Thereafter, all frameworks received ceramic veneer, and the previous tests were repeated. Cobalt-chromium and titanium frameworks received spark erosion after ceramic veneer, and all analyses were repeated. Before and after mechanical cycling, loosening torque was evaluated. The results were subjected to 2-way repeated-measures ANOVA and the Bonferroni test (α=.05).

RESULTS

Titanium presented higher fit values than zirconia (P=.037) and similar to cobalt-chromium frameworks (P>.05) at baseline. After ceramic veneer, higher fit levels were observed for zirconia (P=.001) and cobalt-chromium (P=.008). Titanium showed higher stress values (P<.05) regardless of time. Baseline for all materials presented lower stress values (P<.05). Higher loosening torque values were found for the titanium group at baseline (P<.001) and after ceramic veneer (P<.001). Spark erosion improved fit and loosening torque values only for cobalt-chromium (P<.05). Mechanical cycling did not influence the loosening torque (P>.05).

CONCLUSIONS

Titanium milled complete-arch fixed frameworks presented poorer fit values than zirconia, although the loosening torque at baseline was higher. Ceramic veneer increased the fit levels for zirconia and cobalt-chromium, decreased the loosening torque values for cobalt-chromium, and enhanced stress levels. Spark erosion can be a reliable technique to improve fit and loosening torque for cobalt-chromium frameworks. Mechanical cycling did not decrease loosening torque.