Abstract
Objectives
To test the tensile bond strength (TBS) between two self-adhesive resin composite cements and PEEK after helium plasma treatment and used glycine as a potential coupling agent incorporated in different adhesives.
Methods
In summary, 896 air-abraded PEEK specimens were fabricated. Half of the specimens were treated with cold active inert helium plasma and the other half were left non-treated. Both groups were then split in two groups: In group 1 ( n = 256), 64 specimens were pre-treated with: (a) soft-liner liquid, (b) visio.link, (c) Ambarino P60 and (d) no pre-treatment (control), respectively. In group 2 ( n = 192), specimens were conditioned accordingly, but the adhesive materials were modified by including a commercially available glycine (Air-Flow PERIO). PEEK specimens were then luted using either RelyX Unicem or Clearfil SA Cement and TBS was measured initially and after 14 days water storage combined with 10’000 thermal cycles (16 specimens/subgroup). Fracture type analysis was performed. For statistical analyses Kolmogorov–Smirnov, Shapiro–Wilk tests, 1-, 4-way ANOVA (post hoc: Scheffé), and t -test were used ( p < 0.001).
Results
Helium plasma pre-treatment without glycine showed no impact on initial TBS ( p > 0.348). In contrast, a combination between glycine application and Softline/Ambarino P60 allowed for significantly higher initial TBS was measured after helium plasma treatment ( p = 0.001). However, this effect was no evident after thermo-cycling. All groups conditioned with visio.link showed the highest TBS values.
Significance
The introduction of amine groups by simple provision of amino acids in the form of glycine can improve the bond strength after helium plasma treatment using different adhesive materials. However, using this simple approach, the method cannot withstand thermal challenge yet.
1
Introduction
Polyetheretherketone (PEEK), a polymer from the group of polyaryletherketone (PAEK), which comes more and more in the focus of prosthetic dentistry as implant, provisional abutment, implant supported bar or clamp material (RDPs) . It is biocompatible and exhibits good mechanical properties . It displays a rather low surface energy and high resistance to surface modification by different chemical treatments and is therefore stable to nearly all organic and inorganic chemicals. However, it still remains a problem to achieve adequate bond strengths to composite resin materials.
A first study on this topic assessed the bonding potential of a self-adhesive resin composite cement (RelyX Unicem) and an adhesive/composite system (Heliobond/Tetric) to differently pre-treated PEEK surfaces and showed that bonding to PEEK was only possible when using a bonding system on an etched surface using sulfuric acid . However, materials were applied on surfaces etched with sulfuric acid, which – from a clinical perspective – may be hazardous in a clinical setting. Additional methods without etching are therefore still warranted by means of simple and safe surface (pre-)treatment modalities. Among the latter, plasma surface treatment was thought to have a potential to raise the surface energy in order to improve the overall bonding characteristics. The physics definition of “plasma” is an ionized gas with an essentially equal density of positive and negative charges. It can exist over an extremely wide range of temperature and pressure . A previous in vitro study showed that the use of methyl methacrylate (MMA)-based adhesives allowed for bonding between PEEK and self-adhesive resin cements, but helium plasma treatment had no impact on bond to resin composite cements . This was explained – in part – by a lack of sufficient functional groups being able to react with methacrylate, because PEEK represents an organic thermoplastic polymer material with highly cross-linked structures. Therefore, no chemical bonding between substrates could be achieved.
Grace and Gerenser were able to demonstrate the induction of amine and imine carbon species as functional groups on the surface of nitrogen plasma-treated polystyrene . The majority of these functional groups usually contained a terminal nitrogen (primary amine or imine). A previous study hypothesized that the thereby induced functional groups on the surfaces of fibre-reinforced composite posts might contain terminal nitrogen, which then reacted with the functional groups in the composite core build-up material and moreover, that these amine and imine functional groups might remain stable over time as compared to the oxygen functional groups. Indeed, nitrogen plasma treatment appeared to increase the tensile-shear bond strength between post and composite and nitrogen functional groups were apparently induced on the surface, which became more stable.
Based on these findings, we aimed to assess the influence of the application of amine groups by simple addition of amino acids (glycine) on the bond strength after helium plasma treatment and different adhesive material applications to two resin cements. The hypothesis was that the induction of functional groups after helium plasma pre-treatment improves the bond strength properties to resin composite cements – even after a thermal challenge.
2
Material and methods
Eight-hundred and ninety-six PEEK specimens (Dentokeep, nt-trading, Karlsruhe, Germany) were cut under water-cooling to a thickness of 2 mm (Secutom50, Struers, Ballerup, Denmark), were embedded in chemically curing acrylic resin (ScandiQuick, ScanDia, Hagen, Germany) and then polished up to silicium carbide paper (SIC) P2400 (PlanoPol-2, Struers) under constant water-cooling. All PEEK specimens were air-abraded with alumina powder with mean size of 50 μm (basic Quattro IS, Renfert, Hilzingen, Germany) at 0.2 MPa for 10 s at 45° angle and were ultrasonically cleaned in distilled water for 5 min. Half of the PEEK specimens ( n = 448) were treated additionally using low-density cold helium plasma for 20 s with pressure of 0.2 MPa at distance of 10 mm. The power of plasma generator was 20 Watts. The other half remained untreated. Afterwards, 64 specimens of each group were adhesively treated with either Soft-Liner Liquid (GC Europe, Leuven, Belgium; group a), visio.link (bredent, Senden, Germany; group b), Ambarino P60 (Creamed, Marburg, Germany; group c) or acted as untreated control group (group d). The treatment was also performed using the same adhesive solutions, which included additionally glycine. For this purpose, 5 g of a commercially available glycine powder (Air-Flow PERIO, EMS, Nyon, Switzerland, Lot. No.: 1207053) with 10 ml of the respective adhesive solutions was mixed until homogeneous solution. Table 1 provides an overview regarding the manufacturers, compositions and the single application steps of each adhesive material used in this study according to the manufacturer’s instructions. The allocation of the different experimental groups and subgroups is depicted in Fig. 1 .
| Materials | Product Name | Manufacturer | Composition | Application steps as recommended by the manufacturer | Lot. no. | Curing light used * |
|---|---|---|---|---|---|---|
| Adhesive systems | Soft-Liner Liquid | GC Europe, Leuven, Belgium | MMA, BPBG, DBP, EtOH | Apply on PEEK surface and leave for 120 s | 1204042 | – |
| visio.link | Bredent, Senden, Germany | MMA, PETIA, dimethacrylates, Photoinitiators | 1. Apply adhesive on the PEEK surface with a brush 2. Light cure for 90 s |
114784 | Brelux Power Unit, Bredent | |
| Ambarino P60 | Creamed, Marburg, Germany | Dimethacrylate based on phosphor acidesters and phosphon acidesters | Apply on PEEK surface and leave for 120 s | 2011004057 | – | |
| Self-adhesive resin cement | RelyX Unicem Automix 2 | 3M ESPE, Seefeld, Germany | Methacrylated phosphoric esters, dimethacrylate organic fillers | 1. light cure for 40 s | 475760 | |
| Clearfil SA Cement | Kuraray Medical Inc. Sakazu, Kurashiki, Okayama, Japan | Bis-GMA, TEGDMA, MDP, organic fillers | 1. light cure for 40 s | 033BBA | ||
* All polymerization lights were chosen according to manufacturer’s specific instructions.
Subsequently, acrylic cylinders (inner diameter of 2.9 mm) were filled with either a self-adhesive resin composite cement RelyX Unicem (3M ESPE, Seefeld, Germany) or Clearfil SA Cement (Kuraray Medical Inc. Sakazu, Kurashiki, Okayama, Japan) for each subgroup ( n = 32). The filled cylinders were positioned on the PEEK surface and luted applying a standardized load of 100 g on the PEEK surface. Excess resin composite cement was carefully removed from the bonding margin using microbrushes (3M ESPE).
Specimens were then stored in distilled water at 37 °C in an incubator (HeraCell 150, Heraeus, Hanau, Germany). Half of the specimens was removed after 24 h ( n = 16) and the tensile bond strength was tested, whereas the other half ( n = 16) was additionally aged for another 14 days and additionally thermo-cycled for 10,000 cycles between 5 and 55 °C with a dwell time of 20 s each distilled water bath (Thermocycler THE 1100, SD Mechatronik, Feldkirchen-Westerham, Germany).
For the tensile bond strength measurements, specimens were tested in a Universal Testing Machine (Zwick 1445, Zwick, Ulm, Germany) and pulled with a crosshead speed of 5 mm/min as depicted in Fig. 2 . Tensile bond strength (TBS) was calculated as follows: fracture load/bonding area = N/mm 2 = MPa.
For fracture type analysis, the debonded area was examined directly after TBS tests under an optical microscope at a magnification of 25× (Wild M3B, Heerbrugg, Switzerland). Failure types were determined as follows: (a) adhesive (no cement remnants left on the PEEK surface), (b) mixed (cement remnants partially left on PEEK with PEEK surface exposed) and (c) cohesive failures.
For the data analysis descriptive statistics were calculated. Normality of data distribution was tested using Kolmogorov–Smirnov test. Analysis of variance was performed with respect to helium plasma treatment, glycine agent, adhesive system, resin composite cement and aging level. Unpaired t -test was used for calculation of impact of aging type. p -Values smaller than 5% were considered to be statistically significant in all tests. The data were analyzed using SPSS Version 20 (SPSS INC, Chicago, IL, USA).
2
Material and methods
Eight-hundred and ninety-six PEEK specimens (Dentokeep, nt-trading, Karlsruhe, Germany) were cut under water-cooling to a thickness of 2 mm (Secutom50, Struers, Ballerup, Denmark), were embedded in chemically curing acrylic resin (ScandiQuick, ScanDia, Hagen, Germany) and then polished up to silicium carbide paper (SIC) P2400 (PlanoPol-2, Struers) under constant water-cooling. All PEEK specimens were air-abraded with alumina powder with mean size of 50 μm (basic Quattro IS, Renfert, Hilzingen, Germany) at 0.2 MPa for 10 s at 45° angle and were ultrasonically cleaned in distilled water for 5 min. Half of the PEEK specimens ( n = 448) were treated additionally using low-density cold helium plasma for 20 s with pressure of 0.2 MPa at distance of 10 mm. The power of plasma generator was 20 Watts. The other half remained untreated. Afterwards, 64 specimens of each group were adhesively treated with either Soft-Liner Liquid (GC Europe, Leuven, Belgium; group a), visio.link (bredent, Senden, Germany; group b), Ambarino P60 (Creamed, Marburg, Germany; group c) or acted as untreated control group (group d). The treatment was also performed using the same adhesive solutions, which included additionally glycine. For this purpose, 5 g of a commercially available glycine powder (Air-Flow PERIO, EMS, Nyon, Switzerland, Lot. No.: 1207053) with 10 ml of the respective adhesive solutions was mixed until homogeneous solution. Table 1 provides an overview regarding the manufacturers, compositions and the single application steps of each adhesive material used in this study according to the manufacturer’s instructions. The allocation of the different experimental groups and subgroups is depicted in Fig. 1 .
