What influence does pretreatment with argon plasma have?
PMMA chips on the models (Image: Dr. Reza Saeidi Pour, ZTM Otto Prandtner).
If the vertical occlusion is to be raised as part of a prosthetic treatment, temporary restorations (long-term temporary occlusal veneers) are often initially used to test the desired bite elevation. CAD/CAM blanks made of PMMA are often used as the material of choice. Their advantages include good mechanical and optical properties. There are a few material details that need to be taken into account when it comes to fastening.
The
Industrial polymerization of the PMMA blanks means that there are hardly any free carbon-carbon double bonds on the surface that can bind to other plastics (e.g. fastening composites). In order to activate the PMMA surface, appropriate pretreatment methods are necessary. In the in-vitro study from the LMU Munich presented here, the scientists examined the influence of the Argon plasma pretreatment in combination with
PMMA chips in the upper jaw on the model (Image: Dr. Reza Saeidi Pour, MDT Otto Prandtner).
- various adhesives and
- self-adhesive fastening materials
on the bond strength of PMMA restorations. Furthermore, after the pretreatments tested here, two important parameters that are crucial for a good bond were examined:
- the surface tension and
- the roughness of the CAD/CAM plastic (PMMA).
Process of the investigation
Pretreatment of the test specimens, conditioning, fixing and artificial aging
Plasma pretreatment of the PMMA test specimen surface
A total of 260 PMMA test specimens were produced for the study. These were machine polished and carefully corundum blasted with aluminum oxide powder (50 µm) for 5 seconds at a pressure of 0,5 bar at a distance of 10 mm. Of these test specimens, 20 were selected for measuring surface energy and roughness and 10 of each were pretreated with argon plasma from an easy-to-use handpiece. The remaining 240 test specimens were used to measure the bond strength. While 120 test specimens were pretreated with argon plasma (Fig. 1), the remaining 120 remained untreated.
The preparation for insertion was as follows: (i) without conditioning, (ii) visio.link (bredent), (iii) VP connect (Merz Dental). The test specimens were then attached either with Clearfil SA Cement (Kuraray Noritake) or with RelyX Unicem Automix (3M) and thus bonded and thermally aged 5000 times between 5 °C and 55 °C in thermal load cycles. Each group consisted of 20 test specimens. After the test specimens were artificially aged, the bond strengths were measured and their fracture patterns were assessed (see Fig. 2).
Statistical analysis of the data
Overall, it was observed that the plasma treatment of PMMA significantly increased the measured surface energy, but had no influence on the surface roughness. However, groups without plasma treatment showed higher bond strengths than plasma-treated groups. The exception was the pretreatment with VP connect in combination with RelyX Unicem Automix. Here the bond strengths increased significantly after the plasma pretreatment. In general, Clearfil SA Cement showed higher bond strengths to PMMA plastic than the self-adhesive luting composite RelyX Unicem Automix. After conditioning with visio.link, both self-adhesive luting materials showed the highest bond strengths. Within the group cemented with Clearfil SA Cement, conditioning with VP Connect showed comparable bond strengths to visio.link.
Different fracture patterns after completed tensile test (4.2: cohesive fracture; 1.16P: adhesive fracture of the trigger sleeve)
Conclusion
Plasma treatment of PMMA restorations increases the surface tension, but not the bond strength to self-adhesive luting materials. It can therefore be noted that usability is improved after plasma pretreatment, although the bond strength is not affected. In this article we describe what exactly plasma pretreatment is and how it works Article on the EADT news blog!
Published in: Liebermann A, Keul C, Bähr N, Eichberger M, Edelhoff D, Roos M, Stawarczyk B. Impact of plasma treatment of PMMA-based CAD/CAM blanks on surface properties as well as on adhesion to self-adhesive resin cements . Dent Mater 2013;29(9):935-944.
Authors: Anja Liebermann, Annett Kieschnick, Bogna Stawarczyk
- surface tension describes the surface property “wettability”, ie how well the surface can be wetted with the fastening material. This value influences, among other things, the network.
- Surface energy (interfacial tension) is the work that must be done to increase the surface area of a phase. This value is determined, for example, by the contact angle. The smaller this angle is, the better the wetting of this surface is because the liquid can be distributed more easily over the surface.
- Bond strength describes the adhesion of two materials. It results from the force that is used until the two materials break apart and the adhesive area between the materials, ie force (N) / adhesive area (mm2) = MPa.