Thermal cycling distortion of porcelain fused to metal fixed partial dentures

Marginal fit of 3-unit implant-supported fixed partial dentures: Influence of pattern fabrication method and repeated porcelain firings

BACKGROUND

Marginal fit significantly impacts the long-term success of dental restorations. Different pattern fabrication methods, including hand-waxing, milling, or 3D printing, may affect restorations accuracy. The effect of porcelain firing cycles on the marginal fit of metal-ceramic restorations remains controversial, with conflicting findings across studies.

PURPOSE

The aim was to evaluate the potential effects of multiple porcelain firings (3, 5, 7 cycles) as well as pattern fabrication method (conventional hand-waxing, milling, and 3D printing) on the marginal adaptation of 3-unit implant-supported metal-ceramic fixed partial dentures. It was hypothesized that neither the wax pattern fabrication method nor repeated ceramic firings would significantly affect the marginal adaptation of metal-ceramic crowns.

METHODS

In this in-vitro study, 30 Cobalt-Chromium alloy frameworks were fabricated based on pattern made through three techniques: conventional hand-waxing, CAD-CAM milling, and CAD-CAM 3D printing (n = 10 per group). Sixteen locations were marked on each abutment to measure the vertical marginal gap at four stages: before porcelain veneering and after 3, 5, and 7 firing cycles. The vertical marginal gap was measured using direct microscopic technique at ×80 magnification. Mean vertical marginal gap values were calculated and two-way ANOVA and Tukey's post hoc tests were used for inter-group comparisons (α = 0.05).

RESULTS

The 3D printing group showed significantly lower (P<0.001) mean vertical marginal gaps (60-76 μm) compared to the milling (77-115 μm) and conventional hand-waxing (102-110 μm) groups. The milling group exhibited a significant vertical gap increase after 3 firing cycles (P<0.001); while the conventional (P = 0.429) and 3D printing groups (P = 0.501) showed no significant changes after 7 firing cycles. Notably, the vertical marginal gap in all groups remained below the clinically acceptable threshold of 120 μm.

CONCLUSION

CAD-CAM 3D printing provided superior marginal fit compared to CAD-CAM milling and conventional hand-wax pattern fabrication methods. The impact of porcelain firing on the mean marginal gap was significant only in the milling group. All three fabrication techniques yielded clinically acceptable vertical marginal adaptation after repeated firings. Additive manufacturing holds promise to produce precise implant-supported prostheses.

Comparative assessment of marginal accuracy of grade II titanium and Ni-Cr alloy before and after ceramic firing: An in vitro study

Objective:

The aims of the study are to assess the marginal accuracy of base metal and titanium alloy casting and to evaluate the effect of repeated ceramic firing on the marginal accuracy of base metal and titanium alloy castings.

Materials and Methods:

Twenty metal copings were fabricated with each casting material. Specimens were divided into 4 groups of 10 each representing base metal alloys castings without (Group A) and with metal shoulder margin (Group B), titanium castings without (Group C) and with metal shoulder margin (Group D). The measurement of fit of the metal copings was carried out before the ceramic firing at four different points and the same was followed after porcelain build-up.

Results:

Significant difference was found when Ni–Cr alloy samples were compared with Grade II titanium samples both before and after ceramic firings. The titanium castings with metal shoulder margin showed highest microgap among all the materials tested.

Conclusions:

Based on the results that were found and within the limitations of the study design, it can be concluded that there is marginal discrepancy in the copings made from Ni–Cr and Grade II titanium. This marginal discrepancy increased after ceramic firing cycles for both Ni–Cr and Grade II titanium. The comparative statistical analysis for copings with metal-collar showed maximum discrepancy for Group D. The comparative statistical analysis for copings without metal-collar showed maximum discrepancy for Group C.

Prosthetic misfit of implant-supported prosthesis obtained by an alternative section method

PURPOSE

Adequate passive-fitting of one-piece cast 3-element implant-supported frameworks is hard to achieve. This short communication aims to present an alternative method for section of one-piece cast frameworks and for casting implant-supported frameworks.

MATERIALS AND METHODS

Three-unit implant-supported nickel-chromium (Ni-Cr) frameworks were tested for vertical misfit (n = 6). The frameworks were cast as one-piece (Group A) and later transversally sectioned through a diagonal axis (Group B) and compared to frameworks that were cast diagonally separated (Group C). All separated frameworks were laser welded. Only one side of the frameworks was screwed.

RESULTS

The results on the tightened side were significantly lower in Group C (6.43 ± 3.24 µm) when compared to Groups A (16.50 ± 7.55 µm) and B (16.27 ± 1.71 µm) (P<.05). On the opposite side, the diagonal section of the one-piece castings for laser welding showed significant improvement in the levels of misfit of the frameworks (Group A, 58.66±14.30 µm; Group B, 39.48±12.03 µm; Group C, 23.13±8.24 µm) (P<.05).

CONCLUSION

Casting diagonally sectioned frameworks lowers the misfit levels. Lower misfit levels for the frameworks can be achieved by diagonally sectioning one-piece frameworks.

Effect of firing cycle and surface finishing on the sag resistance of long-span metal ceramic framework using base metal alloys-an in vitro study

BACKGROUND

The search for the ideal restorative material or combination of materials in dentistry is still the subject of modern dental research. In clinical practice, there are increased numbers of ceramo-metal restorations being fabricated which replace multiple missing teeth in a single framework. The literature is scanty with regard to investigations of the 'sag' resistance of base metal alloys commonly used for ceramo-metal restorations and specifically for long-span restorations. An in vitro study was carried out to investigate the effects of surface finishing and simulated porcelain-firing on the sag resistance of long-span ceramo-metal frameworks using base metal alloys.

METHOD

Four types of alloys were used. A total of 80 samples were selected for the study and they were divided into four groups of 20 samples each. 'As cast' metal specimens in group I, group II and with surface finishing in group III and group IV. Each test sample was mounted on the sample holding accessory of the custom made thermo mechanical analyser and subjected to three firing cycles. Each sample was evaluated for the sag-related deflection recorded by the dial gauge with an accuracy of 1 µm. The data recorded was subjected to a statistical analysis using unpaired t-test to compare the relative difference in the sag-related deflection values.

RESULTS

It was observed that the values for sag-related deflection were significantly less for the 'as cast' samples of all the alloy groups. Nickelchromium (NiCr) and cobaltchromium (CoCr) exhibited value of 19.4 μm and 14.8 μm, respectively.

CONCLUSION

Among the four groups of alloys tested, CoCr based alloy exhibited the least amount of sag-related deflection. The results show that the sag-related deflection was less in Co-Cr alloys as compared with the NiCr alloys, still Ni-Cr alloys in fixed dental prostheses frameworks are used due to their desirable physical and chemical properties.

Influence of cold working and thermal treatment on the fit of implant-supported metal-ceramic fixed partial dentures

STATEMENT OF PROBLEM

The application of porcelain may alter the fit of metal-ceramic frameworks. In fixed partial denture frameworks, this altered fit may cause marginal opening and require sectioning and soldering of the prosthesis.

PURPOSE

The purpose of this study was to compare the fit of implant-supported metal-ceramic fixed partial dentures when different thermal treatments were performed.

MATERIAL AND METHODS

Fifteen implant-supported metal-ceramic fixed partial denture frameworks were fabricated and divided into 3 treatment groups: Group 1 = cold working and thermal oxidation; Group 2 = unrestrained thermal treatment, cold working, and oxidation; and Group 3 = investment-restrained thermal treatment, cold working, and oxidation. After the appropriate treatment procedures, porcelain was applied. Measurements were made with a digital caliper, with the help of fiduciary marks, after each step in 3 dimensions and at 2 locations. These measurements were compared with those of the as-cast framework (baseline) to determine framework distortion. Distortion data among groups were compared with repeated-measures analysis of variance and Tukey's Studentized range test (P<.05).

RESULTS

Absolute values of distortion (regardless of direction) after thermal treatment and after porcelain application were not significantly different among groups. However, total raw values of distortion, which describe the specific direction of distortion, were significantly different for 4 of the 6 recorded measurements between Groups 1 and 3 (P<.05). Although the absolute distortion values did not differ among groups, heat treatment and finishing of the alloy affected the timing and direction of distortion.

CONCLUSION

Within the limitations of this study, heat treatment of implant-supported metal-ceramic fixed partial denture frameworks before cold working did not decrease the total absolute distortion. Definite patterns of distortion were, however, found in Groups 1 and 3, suggesting that specific manipulations and heat treatments of the alloy affected the direction of distortion.