Impact of plasma treatment of PMMA-based CAD/CAM blanks on surface properties as well as on adhesion to self-adhesive resin composite cements

Abstract

Objectives

To test surface energy, roughness and tensile bond strength (TBS) to self-adhesive resin composite cements without/with plasma treatment combined with different conditioning methods of PMMA-based CAD/CAM blocks.

Methods

PMMA specimens (10 mm × 10 mm × 2 mm) were fabricated ( N = 260), polished and air-abraded (50 μm Al 2 O 3 , 5 s, 0.05 MPa). Twenty specimens were selected for surface energy and roughness measurements (without/with plasma n = 10 per group). The remaining specimens ( n = 240) were used for TBS testing without/with plasma treatment and following conditioning methods ( n = 20 per test group): (i) without conditioning, (ii) Visio.link, (iii) VP connect and luted with Clearfil SA Cement and RelyX Unicem Automix. Specimens were aged (24 h 37 °C water + 5000 thermal cycles, 5 °C/55 °C), TBS was measured and failure types were assessed. Data were analyzed using descriptive statistics, Mann–Whitney- U and Kruskal–Wallis- H tests, unpaired t -test, Chi 2 and the Spearman correlation test.

Results

Plasma treatment of PMMA increased the surface energy significantly ( p < 0.001) but had no impact on the surface roughness ( p = 0.718). Groups without plasma treatment showed higher TBS than plasma treated groups ( p < 0.001 to p = 0.011), except PMMA conditioned with VP connect and luted using RelyX Unicem Automix ( p = 0.03). Clearfil SA Cement showed higher TBS compared to RelyX Unicem Automix, except for groups conditioned with Visio.link. Both resin composite cements showed the highest TBS for groups conditioned with Visio.link. Also, among Clearfil SA Cement, conditioning with VP connect showed comparable TBS to Visio.link.

Significance

Plasma treatment of PMMA did not increase the adhesion to self-adhesive resin composite cements.

Introduction

Resin fixed dental prostheses (FDPs) can be fabricated either conventionally or using a computer-aided design/computer-aided manufacturing (CAD/CAM) technique. Manufacturing costs for the CAD/CAM process are often lower and the restorations can be produced more rapidly compared to the conventional fabrication process . CAD/CAM resins present higher mechanical properties compared to the conventionally polymerized ones, as CAD/CAM resins are industrially fabricated under high pressure and temperature . A study by Başaran et al. pointed out that conventionally fabricated composites revealed a lower initial fracture load than CAD/CAM manufactured composite FDPs. Alt et al. found that industrially polymerized three-unit FDPs showed significantly higher fracture load after 3 months of water storage at 37 °C and 5000 cycles thermal aging compared to manually polymerized ones. Stawarczyk et al. investigated the fracture load of 3-unit FDPs after water aging for up to 6 months or thermal loading aging for up to 1.2 mio cycles and observed significantly more stable and higher results for CAD/CAM resins compared to conventionally polymerized ones, as well as a lower fracture load for glass-ceramic FDPs. Due to their less abrasive effect on the opposing enamel , as well as their good optical properties, CAD/CAM resins can be an alternative material to glass-ceramics . Additionally, resins offer more suitable CAD/CAM milling properties and can be used at a thinner thickness than glass-ceramic materials .

It is claimed that resin materials show a higher discoloration rate compared to ceramics. However, a study tested the color stability of CAD/CAM and conventional resins and compared the results with glass-ceramics . The authors reported comparable color stability of PMMA-based CAD/CAM materials to glass-ceramics and significantly higher stability to conventional resins. A disadvantage of PMMA-based CAD/CAM restorations in prosthetic dentistry remains the difficulty to achieve adequate bond strength to the resin composite cement . The use of air-borne particle abrasion is one of the most common methods for conditioning polymeric materials, which cleans and at the same time increases the surface area . Furthermore, using different adhesives can improve the bond strength to PMMA-based CAD/CAM materials . Surface treatment by plasma is a potential option, which represents a process of raising the surface energy of different materials and leads to an improvement of the bonding characteristics. Plasma treatment is based on an ionized gas with an essential equal density of positive and negative charges. These charges are important for an alternating electrical field at radio or microwave frequencies to electrodes. Excited molecules will decay and excite other molecules, which leads to an interaction with the surface in a dry chemical way, thereby forming a new surface layer. Typical gases used for treatment of polymers are air, oxygen, nitrogen, helium, argon, and ammonia. Plasma can exist over an extremely wide range of temperatures and pressures . Low-density and high-density plasma can be distinguished. In general, plasma densities of 10 9 –10 10 cm −3 are considered as low and densities of 10 11 –10 12 cm −3 as high.

The aim of this study was to investigate the effect of low-density cold active inert argon plasma gas treatment on PMMA properties, as well as the combination of plasma, different conditioning methods and self-adhesive resin composite cements on the tensile bond strength. The hypothesis tested whether plasma treatment increases the surface energy and roughness of PMMA as well as tensile bond strength to self-adhesive resin composite cements.

Materials and methods

This study tested the impact of cold active inert argon gas plasma treatment on PMMA-based CAD/CAM artBloc Temp material on surface and bonding properties. The bonding properties were tested without or with conditioning, using Visio.link or VP connect. Luting was performed using two self-adhesive resin composite cements: RelyX Unicem Automix and Clearfil SA Cement ( Table 1 ).

Table 1
Summary of the products, manufacturers, composition, Lot. numbers and the application steps used in this study.
Products Manufacturer Composition Lot. No. Application steps
PMMA based polymeric CAD/CAM block
artBloc Temp Merz Dental (Lütjenburg, Germany) PMMA * unfilled 53008
Treatment
Plasma Reinhausen Plasma (Regensburg, Germany) 20 s at a pressure of 0.2 MPa
Conditioning
Visio.link Bredent (Senden, Germany) MMA * , PETIA * , photoinitiators 114784 Applied and light-polymerized for 90 s (bre.Lux Power Unit, Bredent)
VP connect Merz Dental (Lütjenburg, Germany) MMA * VP 22912 Applied and air-dried for 180 s
Self-adhesive resin composite cements
RelyX Unicem Automix 3M ESPE (Seefeld, Germany) Methacrylated phosphoric esters, dimethacrylate organic fillers 475760 40 s light-polymerization (3M ESPE Elipar S10)
Clearfil SA Cement Kuraray Medical Inc., Sakazu (Kurashiki, Okayama, Japan) PASTE A: MDP * , Bis-GMA * , TEGDMA * , dimethylacrylate, Ba–Al fluorosilicate glass, SiO 2 , benzoylperoxide, initiators
PASTE B: Bis-GMA, dimethacrylate, Ba–Al fluorosilicate glass, SiO 2 , pigments 058 AAA

* PMMA, polymethylmethacrylate; MMA, methyl-methacrylate; PETIA, pentaerythritoltriacrylate, MDP, 10-methacryloyloxydecyl-dihydrogenphosphat; TEGDMA, triethylenglycol-dimethacrylate; Bis-GMA, bisphenol-A-diglycidylmethacrylate.

Specimen preparation

Nine CAD/CAM blanks were cut in 260 discs (thickness: 3 mm) with a low-speed diamond saw (Secotom-50, Struers, Ballerup, Denmark) under water-cooling, and were subsequently embedded in chemical polymerized acrylic resin (ScandiQuick, ScanDia, Hagen, Germany). The resin specimens were polished (Tegramin-20; Struers, Ballerup, Denmark) with a series of silicon carbide papers up to P4000 (Struers) under water application. Thereafter, specimens were air-abraded for 5 s using alumina powder with a mean size of 50 μm under pressure of 0.05 MPa (nozzle distance: 1 cm, angle: 45°). The specimens were cleaned for 10 min in an ultrasonic bath containing 80% ethanol (Bransonic Ultrasonic Cleaner 3510 E-DTH, Branson, Danbury, USA) and air-dried.

In general, for surface energy, surface roughness and TBS tests, plasma treatment was performed with a low-density cold active inert argon gas plasma beam for 20 s (0.2 MPa) over a distance of 10 mm ( Fig. 1 ) directly before performing the tests or following luting steps.

Fig. 1
Plasma pre-treatment of PMMA surfaces.

Surface energy measurement

Twenty specimens were selected for surface energy measurement. Ten of the specimens were plasma treated. Surface energy was tested by the sessile drop technique using a contact angle meter (EasyDrop, Krüss, Hamburg, Germany) with two different liquids of different polarity: distilled water and diiodomethane 99% (Cat: 15.842-9, Sigma–Aldrich, Steinheim, Germany, Lot. No: S65447-448) at room temperature, separately. The water- or diiodomethane-drop was registered with a CCD-camera making a digitalized photo standardized after exactly 5 s. A special program (Easy Drop DSA4; Krüss) measured the static contact angle with the help of the diameter and the height of the sessile drops. Surface energy was calculated on basis of the contact angle measurements with water and diiodomethane according to the Ström database .

<SPAN role=presentation tabIndex=0 id=MathJax-Element-1-Frame class=MathJax style="POSITION: relative" data-mathml='cosθ=σS−σLSσL’>cosθ=σSσLSσLcosθ=σS−σLSσL
cos θ = σ S − σ LS σ L

σ L is the surface energy of the liquid; σ S is the surface energy of the solid; and σ LS is the interface surface energy.

Surface roughness measurement

For the subsequent determination of surface roughness, the same specimens as for surface energy measurement ( n = 20) were immediately used. Roughness was investigated using a surface measurement unit (MarSurf M400+SD26, Mahr, Göttingen, Germany). Each specimens was measured six times with a track of 6 mm, three times horizontal and three times perpendicular. The distance between the tracks was 0.25 mm. The mean roughness value R a of each specimen was calculated.

Tensile bond strength (TBS) measurement

The remaining specimens ( n = 240) were divided according to treatment, conditioning and self-adhesive resin composite cement used ( Fig. 2 ). Each tested group contained 20 specimens. Products, manufacturers, compositions, Lot. numbers and application steps are summarized in Table 1 .

Fig. 2
Specimens divided according to treatment, conditioning and resin composite cement level for testing TBS.

Acrylic cylinders with an inner diameter of 2.9 mm were placed on the plasma non-treated/treated and conditioned PMMA, filled with resin composite cement and loaded (750 g) by means of a luting apparatus. Excess resin composite cement was removed from the bonding margin using microbrushes (Ivocalr Vivadent, Schaan, Liechtenstein) and specimens were polymerized ( Table 1 ). Subsequently, specimens were stored in distilled water at 37 °C for 24 h and then aged for 5000 thermal cycles between 5 °C and 55 °C with a dwell time of 20 s in each bath (Thermocycler THE 1100, SD Mechatronik, Feldkirchen-Westerham, Germany) .

Zwick 1445 machine (Zwick, Ulm, Germany) was used for measuring TBS. Specimens were positioned axially to the loading direction in the jig of the Zwick machine, which provided a moment-free axial force application. A collet held the PMMA tube while an alignment jig allowed self-centering of the specimen. The jig was attached to the load cell and tensioned at a crosshead speed of 5 mm/min apart by upper chain, allowing the whole system to be self-aligning ( Fig. 3 ). The load at failure was recorded and TBS were calculated according to the following equation: TBS = F / A = N/mm 2 = MPa, F : load at fracture (N), A : bond area (mm 2 ).

Fig. 3
Design of tensile strength measurements.

The failure types were analyzed under a stereomicroscope (Axioskop 2 MAT, Karl Zeiss Mikroskopie, Göttingen, Germany) after debonding and classified as follows: (i) adhesive – between the substrate and the resin composite cement; (ii) cohesive in the resin composite cement; and (iii) cohesive in artBloc Temp.

Statistical analyses

For power analyses, a pilot study with 15 specimens was performed with Clearfil SA Cement combined with Visio.link to artBloc Temp without plasma treatment (24 ± 6.5 MPa). Two different power analyses with respect to plasma treatment or conditioning method were calculated. A sample size of 20 in each group will have 97% power to detect a difference of 35% in means (8.4 MPa) caused by the application of plasma, assuming that the common standard deviation is 6.5 MPa using a two-sided two group t -test with 0.05 significance level. A sample size of 20 in each group will have 93% power to detect a difference of 35% in means (8.4 MPa) caused by use of a conditioning method, assuming that the common standard deviation is 6.5 MPa using a two-sided two group t -test with Bonferroni corrected significance level of 0.017 (3 between groups comparisons within 3 with adhesive treated groups). Power analyses were calculated using nQuery Advisor (Version 6.04.10, Statistical Solutions, Saugaus Mass) prior to performing this study.

Descriptive statistics and 95% confidence intervals (95% CI) for the surface energy, surface roughness and mean TBS were computed. For TBS means, standard deviations (SD), medians and interquartile ranges (IQR) were provided. Normality of data distribution was tested using Kolmogorov–Smirnov and Shapiro–Wilk tests. The influence of plasma treatment on surface energy was compared by Mann–Whitney- U test and on surface roughness by a two sample Student’s t -test. Association between surface energy and surface roughness was evaluated by the non-parametric Spearman correlation. TBS groups were non-parametric compared with respect to plasma treatment, conditioning method and resin composite cement using the Mann–Whitney- U and Kruskal–Wallis- H tests. The post hoc test for Kruskal–Wallis- H procedure was conducted manually by means of Mann–Whitney- U test for between-group comparisons together with the Bonferroni corrected significance level (0.05/3 = 0.016). Chi 2 test was used to analyze the failure types. The 95% CI for the true relative frequencies of the failure types were gathered from Ciba Geigy Table . All results of statistical analyses with p -values smaller than 0.05 were considered to be statistically significant. To perform statistical analyses, the Statistical Package of Social Science Version 20 (SPSS INC, Chicago, IL, USA) was used.

Materials and methods

This study tested the impact of cold active inert argon gas plasma treatment on PMMA-based CAD/CAM artBloc Temp material on surface and bonding properties. The bonding properties were tested without or with conditioning, using Visio.link or VP connect. Luting was performed using two self-adhesive resin composite cements: RelyX Unicem Automix and Clearfil SA Cement ( Table 1 ).

Table 1
Summary of the products, manufacturers, composition, Lot. numbers and the application steps used in this study.
Products Manufacturer Composition Lot. No. Application steps
PMMA based polymeric CAD/CAM block
artBloc Temp Merz Dental (Lütjenburg, Germany) PMMA * unfilled 53008
Treatment
Plasma Reinhausen Plasma (Regensburg, Germany) 20 s at a pressure of 0.2 MPa
Conditioning
Visio.link Bredent (Senden, Germany) MMA * , PETIA * , photoinitiators 114784 Applied and light-polymerized for 90 s (bre.Lux Power Unit, Bredent)
VP connect Merz Dental (Lütjenburg, Germany) MMA * VP 22912 Applied and air-dried for 180 s
Self-adhesive resin composite cements
RelyX Unicem Automix 3M ESPE (Seefeld, Germany) Methacrylated phosphoric esters, dimethacrylate organic fillers 475760 40 s light-polymerization (3M ESPE Elipar S10)
Clearfil SA Cement Kuraray Medical Inc., Sakazu (Kurashiki, Okayama, Japan) PASTE A: MDP * , Bis-GMA * , TEGDMA * , dimethylacrylate, Ba–Al fluorosilicate glass, SiO 2 , benzoylperoxide, initiators
PASTE B: Bis-GMA, dimethacrylate, Ba–Al fluorosilicate glass, SiO 2 , pigments 058 AAA
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Nov 25, 2017 | Posted by in Dental Materials | Comments Off on Impact of plasma treatment of PMMA-based CAD/CAM blanks on surface properties as well as on adhesion to self-adhesive resin composite cements
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