INTRODUCTION

Nowadays, there is a high acceptance that a retention phase plays an important role for stability of orthodontic results1). Unlike removable retainer, the fixed retainers are preferred by orthodontists because of their aesthetic and long-term use. However, because of poor fitness, unwanted torque and high failure rate, today lingual retainers commonly referred to by patients as "permanent" retainers, are often just a tempo-rary solution2).

The technology of fixed retainer has changed little over the last 40 years. In 1977, Zachrisson introduced multistranded lingual retainer with different shapes. Alternatively, aesthetic resin fiberglass bands were introduced with higher failure rates3).

Additionally, fixed retainers can be manufactured using CAD--CAM systems. The CAD/CAM retainer saves much time and more efficient than conventional4). Unfortunately, limited studies are present in this area and the CAD--CAM techniques as well as types of wires vary for each firm. Two Recent studies reported the CAD/CAM fixed retainer of a nickel-titanium and Zirconium5,6).

Recently, a new type of Poly-ether-ether-ketone (PEEK) fixed lin-gual retainer using digital scanning and then CAD design of a milled PEEK. This appliance is giving a high-strength in white shades with passive retention of anterior teeth7).

PEEK is a high-performance semi-crystalline thermoplastic, con-sisting of three aromatic rings connected by two ether groups and one carbonyl group. PEEK has unique mechanical and chemical properties

and proposed as a functional and esthetic metal free in various applica-tions. However, PEEK has an inert hydrophobic surface and low surface free energy resulting in poor adhesion properties between PEEK and resin composite8).

Different surface treatment and adhesives for PEEK have been mainly studied using shear bond strength tests9). Therefore, the objective of this study will be optimization of a new PEEK CAD/CAM fabricated fixed lingual retainer to human enamel using various surface treatment and adhesive system.

MATERIALS AND METHODS

PEEK pads retainer fabricationIn this study, 60 freshly extracted, intact and caries free human

maxillary premolars of orthodontic treatment were involved. After cleaned, 0.1% thymol disinfected and polished with pumice for 10 sec-onds, teeth were randomly divided into 10 groups of 6 premolars each. To mimic the human dentition, 6 teeth were matched with each other to form a contact area. Then, Self-cured acrylic resin (ScandiQuick, ScanDia, Hagen, Germany) injected for each specimen with the teeth long axis perpendicular to the base of the mold.

Next, the teeth model was digitized using Smart optic digital scan-ner to create a file of virtual model. This file was opened using the

International Medical Journal Vol. 28, Supplement No. 1, pp. 69 - 73 , June 2021

DENTISTRY

Optimization of CAD/CAM Fabricated PEEK Orthodontic Fixed Lingual Retainer Adhesion to Enamel

Riyadh Abdulhamza Ruwiaee1), Akram Faisal Alhuwaizi2)

ABSTRACTObjective: Since PEEK fixed retainers are inert with low surface energy, the aim of present study was to optimize the proto-

col of PEEK surface adhesion.Material and Methods: The study involved CAD/CAM PEEK retainer pads. Specimens were assigned randomly to 9 groups

(n = 6) with a combination of three different surface treatments (98% sulfuric acid, sandblasting with 50 μm alumina, and com-bination of both) and three different bonding materials (Single Bond Universal (SBU), Visio.link, and Heliobond). After surface treatment, SEM and FTIR analysis were examined. After bonding with 3M Unitek Transbond system to human premolar teeth, shear bond strength (SBS) and interface were also evaluated. SBS data were analyzed statistically using one-way ANOVA and post-hoc Tukey's test.

Results: Regardless of adhesion, acid etching yielded significantly higher SBS than sandblasting and combination. FTIR spectra showed new sulfonate groups and SEM revealed a porous surface of etched PEEK. The use of MDP and silane contain-ing SBU on etched PEEK showed the highest SBS. Acid etching showed resin-enamel failure, while sandblasting showed res-in-PEEK failure.

Conclusion: The strongest adhesive system was acid etching with SBU, while the best adhesive after sandblasting PEEK was visio.link. Lastly, combining sandblasting and acid etching gave no benefits over etching alone.

KEY WORDSPEEK, lingual retainer, adhesion

Received on March 31, 2021 and accepted on April 30, 20211) Student, Orthodontic Department, College of Dentistry, University of Baghdad Baghdad, Iraq2) Professor, Orthodontic Department, College of Dentistry, University of Baghdad Baghdad, IraqCorrespondence to: Riyadh Abdulhamza Ruwiaee(e-mail: riyadhalseebawi@yahoo.com)

69

C 2021 Japan University of Health Sciences & Japan International Cultural Exchange Foundation

Ruwiaee R. A et al.70

Exocad software system in order to design a virtual PEEK retainer. The final design had 6 PEEK pads and 5 connectors. The 12 mm2 pad sur-face areas of 3 mm width and 4 mm height with 0.8 mm thickness were designed. Also, occlusal supports for easier seating and accuracy were made (Fig. 1).

Then after, the virtual design was sent to the milling machine using CORITEC 250i CAD/CAM system milling engine (Imes-Icore GmbH, Leibozgraben, Germany) loaded by blank of dental PEEK (JUVORATM Dental Disc; JUVORA Ltd, Wyre, Lancashire, UK) to fabricate the final PEEK retainer. It is essential that the final PEEK designs are fitted pas-sively with the help of the occlusal support guidance and provided with smooth polished surfaces.

PEEK pad surface treatment and conditioningThe air-dried PEEK pad specimens were randomly divided into

three groups according to surface treatments:

A- Sulfuric acid Group (N = 18): The pads surfaces were etched with 98% sulfuric acid (RCI Labscan Limited, Bangtorad, Samutsakorn, Thailand) for 60 seconds, then rinsed off with distilled water for 60 sec-onds and air-dried.

B- Sandblasting Group (N = 18): The pads were sandblasted with 50 ųm Al2O3 particles (Tec-line®, BJ, Amsterdam, Netherlands) for 15 seconds at a pressure of 0.4 MPa and a distance of 10 mm perpendicular to the pad surface. The specimens rinsed with distilled water for 60 sec-onds and then air-dried.

C- Combination Group (N = 18): Pads were first sandblasted treated followed by acid etching and dried.

Next, the surface conditioning was divided into three subgroups (n = 6) of different adhesive systems (Table 1).

FTIR and SEM evaluation of PEEK padTwo additional control untreated peek pads surface were analyzed

using the Fourier transform infrared spectroscopy (FTIR spectrometer; Spectrum Two; PerkinElmer, Waltham, MA, USA). Then, the FTIR

spectra was compared with the FTIR spectra of the same sample after acid etching to confirm the sulfonating reaction. The measurements were interpreted using NIOS2 main software.

The surface morphology of 2 additional PEEK pad samples from each group of acid etched, sandblasted and combination of both as well as 2 control samples were randomly evaluated using scanning electron microscopy (SEM; Hitachi S-3400 N; Hitachi High Technologies Scientific Instruments, Wokingham UK). Treated pad morphology were observed and analyzed to compare with the control groups.

Teeth etching, conditioning and retainer bondingAfter 20 seconds pumice cleaned, water rinsed and air-dried, teeth

lingual surfaces were etched with 37% phosphoric acid (3M ESPE, Deutschland GmbH, Germany 497909 MDP) for 30 seconds, water rinsed and 15 second dried.

Then, a primer (Transbond XT system; 3M Unitek) was uniformly applied using a microbrush, gently air-blown, and photo polymerized for 20 seconds.

Next, the adhesive, TransbondTM LR (3M Unitek AG, Monrovia, CA, USA), was applied to the PEEK pad base and pressed onto the enamel surface using an equal force with easier guidance of occlusal supports. The excessive adhesive eliminated and light-cured for 40 sec-onds at the mesial and distal sides for 20 second each. After of occlusal rests removing and connectors separating, the specimens stored in dis-tilled water for 24 h at 37□ before shear bonding test.

Shear bond strength of PEEK padSBS of each group was tested by a computer?controlled universal

testing machine (Laryee WDW-50, Beijing, China). Load was applied using a knife-edge shaped with 0.5 mm/min crosshead speed and the bonding surface parallel to the loading piston (Fig. 2). SBS was expressed in Mega-Pascals (MPa), calculating with dividing the maxi-mum value of load at failure in newtons by the bonding area in square millimeters (equal to 3 x 4 = 12 mm2). The higher SBS PEEK pad will be the most appropriate adhesive system. Thus, the treatment and adhe-

Figure 1: The Exocad virtual PEEK retainer (Pads and connectors).

Table 1: Surface conditioning proceduresProduct

Manufacturer Composition Applicationname

Single Bond 3M ESPE, Deutschland GmbH, Germany MDP phosphate monomer, dimethacrylate 1) Apply a thin layer by rubbing for 20 sUniversal resins, HEMA, VitrebondTM copolymer, filler, 2) Gentle air stream for 5 s ethanol, water, initiators, silane 3) Light cure 10 s

Visio.link Bredent GmbH & CoKG, Senden, Germany MMA, PETIA, dimethacrylates, photoinitiators 1) Apply a thin layer 2) Light cure 90 s

Heliobond Ivovlar Vivadent AG, Schaan, Liechtenstein Bis-GMA, TEG-DMA 1) Apply a thin layer 2) Light cure 10 s

MMA: methylmethacrylate; PETIA: pentaerythritol triacrylate; MDP: 10-methacryloyloxydecyl dihydrogen phosphate; HEMA: 2-hydroxyethyl methacrylate; Bis-GMA: bisphe-

nol-A-diglycidylmethacrylate; TEGDMA: triethylene glycol dimethacrylate.

Optimization of CAD/CAM Fabricated PEEK Orthodontic Fixed Lingual Retainer Adhesion to Enamel 71

sion system were optimized for PEEK retainer design.

Adhesive failure analysisAfter failure occurred, the specimens were visually examined under

a stereomicroscope (Stereomicroscope, Olympus, Japan) at x 1.6 magni-fication. Failure modes were classified into four types: (1) PEEK-resin failure (2) enamel-resin failure, (3) PEEK or enamel cohesive failure, and (4) mixed failure mode. Next, specimens were randomly chosen from each mode of failure for SEM observation at x 10 magnification.

Statistical analysisFor statistical analysis, SPSS version 22, IBM, Armonk, NY, USA

was used. The Shapiro-Wilk test was performed to verify the normality of SBS data distribution. While, one-way analysis of variance (ANOVA) and post-hoc Tukey's test examined the effect of different surface treatments, bonding materials and interactions of the two factors on SBS (a significant level of 5%).

RESULTS

FTIR and SEM evaluation of PEEK pad surfaceThe presence of sulfonate groups on the 98% sulfuric acid treated

PEEK samples were confirmed using Fourier transform infrared spec-troscopy (FTIR). In SPEEK samples, a new peak appeared at 1416 cm-1 owing to new sulfonate at the aromatic C---C band at 1486 cm-1 (Figs. 3

blue color). Moreover, two new absorption peaks appeared at 1009 cm-1 and 1097 cm-1, and were, respectively, assigned to the O = S = O sym-metric and asymmetric vibrations (Figs.3 red color).

While, SEM analysis at 1,000 x magnification showed that the con-trol group demonstrated a smooth, polished surface. The acid etched samples characterized by a sponge-like porous fiber network. while, sandblasting exhibited irregular, fissured surfaces with embedded alumi-na oxide. the mountain region with shallow valley were distributed throughout the surfaces of the combination groups (Figs. 4) respectively.

Shear bond strength and failure analysisRegarding surface treatment, the sulfuric acid treated groups

showed significantly higher SBS, followed by the combination groups and lastly the sandblasted groups regardless of adhesive type. While, regarding adhesive system, Single Bond Universal adhesive with acid surface treatment gave significantly higher bonding strength than all the other groups (Table 2).

Predominantly, the acid treated and combination groups showed res-in-enamel failure with a minority of mixed failure pattern. While, the sandblasted groups showed 100% resin-PEEK failure. No cohesive fail-ures were observed in both PEEK or enamel (Table 2).

DISCUSSION

Surface treatmentThe SEM images showed changes from a plain homogenous pattern

to a complex fiber network with porosities and blister-like sub-surface in the sulfuric acid treated group compared to the control group. This surface roughness of the sulfuric acid treated group was similar to that found by previous studies10,11). It seems that micro-topographical changes of PEEK surface after sulfuric acid etching enhanced the penetration of the resin adhesive, resulting in the increased SBS by a mechanical inter-lock between the etched PEEK and adhesives.

The presence of sulfonate groups on the treated samples was con-firmed using FTIR in agreement with Wang et al.12). A new peak appeared at 1416 cm-1, owing to new sulfonate substitution at the aro-matic C---C band at 1486 cm-1. Moreover, two new absorption peaks appeared at 1009 cm-1 and 1097 cm-1, and were, respectively, assigned to the O = S = O symmetric and asymmetric vibrations.

Sandblasting can make the PEEK surface morphology change, help-ing the dental resin penetrate into the framework material to enhance micromechanical interlocks and ultimately increasing bond strength13,14). Previous studies reported a significant increase in SBS values after sandblasting of PEEK9,15,16).

In the sandblasting groups, Heliobond gave a SBS of 8.6 MPa,

Figure 2: A universal shear bond testing machine of the speci-mens.

Figure 3. FTIR spectra of Non-sulfonated and sulfonated PEEK (SPEEK) samples of 98% sulfuric acid treated PEEK at the same reaction time of 60 s.

Figure 4: Scanning electron microscopy (SEM) images of the dif-ferent surface treatment at a magnification of 10,000x. (A) non treated PEEK (B) 98% sulfuric acid etching; (C) Air abrasion with impeded alumina particle (red arrow) (D) Combination of air abrasion and 98% sulfuric acid treatment).

Ruwiaee R. A et al.72

which is less than the 13.5MPa found by Schmidlin et al.13) using the same protocol. Moreover, the highest SBS values (12.292 MPa) were shown in specimens conditioned with Visio.link, which is less than the 19.86 MPa of Caglar et al.16). This may be because the latter used filled PEEK.

SEM images of the sandblasted specimens showed irregular fissure without any porous pattern, which increases the contact area and is suit-able for the flow of adhesive as shown by Ates et al.15). Moreover, alu-mina particles embedded in the PEEK surface were observed in agree-ment with Silthampitagl et al10).

Surface conditioningPretreatment alone is not sufficient to guarantee long-term stable

bonding of resin to PEEK surfaces17). The application of adhesive results in PEEK coating with a high wettability to resin-matrix materials16).

This study used the 10-dimethacrylates (10-MDP) of Single Bond Universal, pentaerythritol triacrylate (PETIA) of Visio.link, and methyl-metacrylate (MMA) of Heliobond to examine SBS of 3MTM TransbondTM LR adhesive to PEEK. The bonding success between PEEK and resin is related to the contents and solvents of the adhesive system.

The sulfuric acid Single Bond Universal group showed significantly higher SBS values than visio.link and Heliobond. This is in agreement with Lee et al.18) and may be attributed to the fact that MDP has higher effect than MMA-containing bond materials on porous PEEK surface. As it has a hydrophobic methacrylate terminal end and a hydrophilic phosphate terminal end, copolymerizing resin monomers and chemical-ly binds to oxides, respectively19). Furthermore, Lee et al.18) found that Single Bond Universal showed significantly higher SBS than All-bond universal adhesive on etched PEEK, although both adhesives contain phosphate monomer (MDP). This may be because of silane component of Single Bond Universal.

The main constituents of Visio.link are MMA and PETIA. PETIA has a high capacity to modify the PEEK surface20). MMA may cause the PEEK surface to swell and the dimethacrylate monomers provide the connection to the composite resins with 2 carboxyl groups as binding sites14). Only multifunctional methacrylate containing adhesive systems give durable bonding of sandblasted PEEK21). In the current study, for the sandblasted groups, the highest SBS values were shown with Visio.link adhesive, in agreement with Stawarczyk et al.9)

Heliobond is a low viscosity unfilled resin bonding agent due to the high TEGDMA content22). This increases the capillary penetration and wetting ability of the resin, resulting in the facilitation of micromechani-cal bonding23). The penetration of resin materials into the porosities con-firmed the mechanical bonding between PEEK and resin materials. This bonding agent had no additional functional monomer to cause any effect of chemical bonding to pretreated PEEK surface24).

Silthampitag et al.10) showed that the SBS of resin composite on PEEK was highest (27.36 MPa) for 98% sulfuric acid and Heliobond and the lowest for sandblasting and Heliobond. These readings were higher than those found in this study and may be because they used Titanium dioxide filled PEEK which has higher stiffness.

The water solvent and ethanol with the co-solvent HEMA of Single Bond Universal adhesive may play an important role in the penetration of resin adhesive deeper in the high porous acid treated PEEK surface, while no solvent is reported in Visio.link and Heliobond adhesives.

The failure mode analysis:In the sulfuric acid and combination groups, the predominant failure

mode showed adhesive failure at composite-enamel interface indicating a strong adhesion between the PEEK pad and resin. While mixed failure mode was seen in 5.6% and 11.1% respectively as found in previous studies18,25). While, in sandblasting groups, the adhesive retained on the enamel of all the 18 samples indicating a weak adhesion between the PEEK pad and resin, which was in line with Stawarczyk et al26).

SUMMARY

Sulfuric acid pretreatment with Single Bond Universal group showed the highest SBS (30.41MPa) which exceeded those found by previous study such as the piranha solution10,20) and sandblasting10,13,26).

Since SBS higher than 10 MPa are considered acceptable, acid etch-ing treatment of PEEK surfaces can be considered a viable surface treat-ment modality for PEEK. While, sandblasting with visio.link adhesive is on the borderline line.

The preferred adhesive for 98% sulfuric acid etched PEEK is Single Bond Universal, while the best adhesive after sandblasting PEEK was visio.link, with the former adhesive system being more superior. Lastly, combination of both sandblasting and acid etching gave no benefits over etching alone regarding SBS and failure mode.

ACKNOWLEDGEMENTS

The authors thank the team in the mechanical engineering depart-ment of the University of Technology, for their support in performing the mechanical testing.

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Table 2: SBS (MPa) and failure type percentage of PEEK pad test. SBS (MPa) Failure type (%)

Surface Bonding Mean S.D. Resin- Resin- Mixed Treatment system enamel PEEK

Sulfuric acid Visio.link 19.861 0.680 6 (100%) - -

Single Bond Universal 30.417 0.734 6 (100%) - -

Heliobond 23.125 1.043 5 (83%) - 1 (17%)

Sandblasting Visio.link 12.292 1.208 - 6 (100%) -

Single Bond Universal 9.931 0.718 - 6 (100%) -

Heliobond 8.611 1.506 - 6 (100%) -

Combination Visio.link 16.528 0.510 5 (83%) - 1 (17%)

Single Bond Universal 14.305 1.009 6 (100%) - -

Heliobond 16.945 1.043 5 (83%) - 1 (17%)

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