Effect of Mixing and Impression Techniques Using Vinyl Polysiloxane (VPS) on the Accuracy of Fixed Partial Dentures

Two critical factors that influence the accuracy of an impression include the proper manipulation of the impression materials and the technique used to make the impression. The purpose of this study was to clinically evaluate the effect of different mixing techniques on the accuracy of vinyl polysiloxane (VPS) impressions by assessing metal framework fit of fixed partial restorations. The study included 92 consecutive patients diagnosed with partial edentulism and treated with fixed partial denture restorations. The mixing technique was one of the two following mixing methods: hand mixing technique (45 patients), with the putty material mixed according to the manufacturer’s instructions; or mechanical mixing technique (47 patients), with the putty material mixed by a Pentamix device. Under both mixing methods, vinyl polysiloxane was used as the impression material. Two impression techniques were randomly used by the operators (One/Two-stage putty –wash impression techniques). The accuracy of the metal framework restorations was tested clinically and radiologically, resulting in significant statistical difference (p = 0.04) between different mixing techniques. The mechanical mixing produced more accurate restorations (metal framework misfit only in 14.9% of patients vs. hand mixing 31.1%). Regarding the impression techniques, the two-stage impression technique was found to be significantly more accurate (p = 0.04), resulting in 14.6% ill-fitted metal frameworks vs. 31.8%, in the one-stage technique. It can be concluded that mechanical mixing yields more accurate impressions leading to more accurate restorations, especially when combined with two-stage impression technique.

The Effect of Cooling Times on the Bonding Between Porcelain and Metal Alloys Fabricated by Different Techniques: In Vitro Study

Aim: The aim of this study was to evaluate the effect of different cooling times on shear bond strength between cobalt–chromium metal frameworks fabricated by different techniques and veneering porcelain. Materials and Methods: One hundred twenty cobalt–chromium metal ceramic samples were obtained by three different techniques (casting, milling, and laser sintering). During the porcelain firing, fast and slow cooling protocols were applied. Ten specimens from each group were determined for all groups. The shear bond strength of the metal ceramic samples was measured by a universal testing machine with a constant crosshead speed of 0.5 mm/min. One sample from each group was evaluated by the scanning electron microscopy analysis in terms of surface change and fracture morphology. Variance analysis and Tukey test were used to analyze statistically significant differences between groups. Results: It was seen that the difference between the bond strengths of the metal frameworks obtained by different techniques was statistically significant ( P < .05). All groups except for difference between the bond strengths of all the frameworks metal groups with G-Ceram porcelain were statistically significant ( P < .05). The best bond strengths that were statistically significant were fast cooling G-Ceram (19.65 ± 2.65 MPa) with metal frameworks fabricated by laser sintering, fast cooling with metal framework fabricated by milling Noritake (19.17 ± 2.91 MPa), and the metal framework fabricated by casting was found to be slow cooling Noritake (12.99 ± 2.08 MPa) were seen. Conclusion: The porcelain cooling times had significant effect on the shear bond strength of porcelain to casting, milling and laser sintering alloys.

Fabrication of Metal-Reinforced Complete Dentures Using the CAD-CAM Technique

The digital manufacture of complete dentures would greatly simplify the workflow; however, the metal-reinforced complete dentures production method has not been well established. This article describes a technique of fabricating metal-reinforced complete dentures in a milling machine using the geometry guide, a negative impression of the occlusal surface of the metal-reinforced complete denture designed using the computer-aided design software. The geometry guide supports and stabilizes the artificial teeth and metal framework in the correct position in the surrounding resin. Fabrication of metal-reinforced dental prosthesis using the digital technique is possible with this method.

Fractographic analysis of two fractured removable partial denture metal frameworks

This study aimed to analyze two cases of fracture of the metal framework of Co-Cr-based removable partial dentures, using fractography to identify a true cause of these failures. The first case concerned the coupling between the smaller and larger connectors; the patient brought his fractured removable partial dentures to the clinic. The second case referred to a larger connector; structure of the patient fractured during the test. For failure analysis, images of two fractured metal frameworks were taken by scanning electron microscopy to identify the origin of the fracture and to determine the mechanisms involved in the process. Subsequently, energy dispersive X-ray spectroscopy was used for elemental chemical mapping of the fracture interface. The fractography indicated material fatigue as a possible cause in the first case and an error in the casting process in the second. The energy dispersive X-ray spectroscopy test, performed in the specimen of case B, showed predominant and characteristic peaks for Co and Cr, with a higher concentration of chromium. The fractures identified were associated with inclusions and with material fatigue and a number important characteristics were evidenced by the technique.