To assess the impact of irradiation on Martens hardness parameters of different PEEK qualities filled with titanium dioxide (TiO 2 ), namely PEEK/0%, PEEK/20%, and PEEK/>30%.
For Martens hardness (HM) measurements, 40 specimens of each PEEK quality were fabricated and air-abraded with 50 μm Al 2 O 3 . HM parameters of PEEK specimens were measured initially and stepwise after irradiating for 5, 10, 20, 30, 40, 60, 80, 100, 180, 360 and 540 s using light units with different wavelength: Elipar S10 (430–480 nm), EyeVolutionMAX (385–390 nm + 465–470 nm), Translux CL (380–500 nm) and bre.Lux Power Unit (370–500 nm). HM parameters of 10 human teeth were measured initially on enamel and dentin. Data was analysed using 3-way ANOVA with partial eta-squared ( η P 2 ) and post-hoc Tuckey-HSD-test (p < 0.05).
PEEK qualities followed by the wavelength showed the highest effect on Martens hardness (p < 0.013). PEEK/>30% (197.35 ± 19.9 N/mm 2 ), followed by PEEK/20% (191.45 ± 15.49 N/mm 2 ) showed significantly higher values for HM than PEEK/0% (189.55 ± 16.89 N/mm 2 ). PEEK/>30% (5.49 ± 0.4 kN/mm) and PEEK/20% (5.38 ± 0.26 kN/mm 2 ) presented higher indentation modulus (E IT ) than PEEK/0% (4.77 ± 0.36 kN/mm 2 ). Irradiated with wavelength of 430–480 nm (PEEK/0%: 193.28 N/mm 2 , PEEK20%: 198.83 N/mm 2 , PEEK/>30%: 200.5 N/mm 2 ) indicated higher HM compared to specimens irradiated with 380–500 nm (PEEK/0%: 186.63 N/mm 2 , PEEK20%: 191.05 N/mm 2 , PEEK/>30%: 196.13 N/mm 2 ). Irradiation using 430–480 nm (PEEK/0%: 4.95 kN/mm 2 , PEEK20%: 5.52 kN/mm 2 , PEEK/>30%: 5.59 kN/mm 2 ) and 370–500 nm (PEEK/0%: 4.92 kN/mm 2 , PEEK20%: 5.43 kN/mm 2 , PEEK/>30%: 5.53 kN/mm 2 ) indicated higher E IT values compared to specimens irradiated with 380–500 nm (PEEK/0%: 4.72 kN/mm 2 , PEEK20%: 5.34 kN/mm 2 , PEEK/>30%: 5.47 kN/mm 2 ). Duration of irradiation presented no impact on results. Enamel (HM: 2263.6 ± 405.16, E IT : 63.16 ± 19.24) and dentin (HM: 468.2 ± 30.77 N/mm 2 , E IT : 14.14 ± 4.59 kN/mm 2 ) presented significantly higher HM and E IT than the tested PEEK qualities (p < 0.001).
Irradiation with different wavelength impacted HM parameter. The increase of TiO 2 percentage in PEEK matrix improved the HM parameter. However, PEEK showed significantly lower HM parameter than human teeth.
The high performance thermoplastic polyetheretherketone (PEEK) is meanwhile known as alternative to conventional metal framework materials because of its favourable and outstanding mechanical properties . In comparison to metals, conventional polymer materials are generally characterized by high elasticity and ductility, which may be limiting factors for the application of polymer materials in dental use. Here, the material must withstand forces during either fabrication or mastication, retain their flexural strength and fracture toughness and be resistant to corrosion, friction and wear .
Actual developments encourage the trend to design materials that are similar to the natural tooth structure concerning mechanical and optical properties. Accordingly, the low elastic modulus for unfilled PEEK materials of 3–4 GPa is a drawback that limits its application for particular indications in dentistry. However, developments have shown that the elastic modulus as well as mechanical properties of PEEK can be improved by adding different fillers . PEEK is either reinforced with carbon or glass fibres of varying lengths or spherical filler particles like barium-phosphate (BaSO 4 ) or titanium dioxide (TiO 2 ) with filler contents up to 30%. In general, fibre reinforcement provides the PEEK structure with better biomechanical performance due to their superior properties in tension and flexure and are proven to reach flexural moduli and strengths seven times those of materials with spherical filler particles . However, the improved mechanical properties seem to depend on the fibre length, the fibre type, the orientation and the filler content . To the present time in dentistry, PEEK is only doped with TiO 2 particles that provide the required biocompatibility and serve as whitening pigments in addition . TiO 2 -filled PEEK examined an increase in flexural strength and elastic modulus with increasing proportion of powder . In comparison to TiO 2 , filler particles of BaSO 4 had no substantial influence on the mechanical properties of PEEK which was assumed to a greater hardness of TiO 2 towards BaSO 4 . Providing PEEK materials with increasing hardness and optimized colour characteristics at the same time might facilitate the range of indication.
Generally, the hardness of materials is defined as resistance to deformation and thus is an indicator for durability . Regarding to polymer-based materials, the hardness may be influenced by several factors like the yield strength, tensile strength and the elastic modulus . Because of the reason, that on the one hand deformation during loading is assumed to be elastic and plastic in nature and that on the other hand the visco-elastic recovery of materials was proven to have a significant effect on the outcome of the hardness parameters, the Martens hardness test method is obviously advantageous for testing dental materials . Moreover, the hardness of polymer-based materials can be affected by the presence, quantity and size of filler particles as well as by external influences like UV-radiation . Even tough PEEK is known to be resistance to radiation , investigations have shown that certain wavelength can affect the mechanical and chemical surface properties . This is an important aspect, as dental materials are frequently encountered by radiation of different wavelength in dentistry either in laboratory during veneering procedures or in practice during the cementation of restorations.
Therefore, literature shows that a surface pretreatment of PEEK by air-abrasion and additional condition using an adhesive system is necessary in order to overcome the inert surface character and to enable adequate bonding to other dental materials. In particular, the adhesive system visio link (bredent) resulted in highest bond strengths . As visio.link is polymerised by light curing, the air-abraded PEEK surface is exposed to radiation of certain wavelengths.
Due to the fact, that TiO 2 is also known to be influenced by UV-radiation , the present study investigates the impact of radiation with light of different wavelength on the Martens hardness parameter of three different PEEK qualities. Moreover, the initial hardness parameter of PEEK are compared to those of enamel and dentin, as the potential of PEEK increases for several indications in dental use and versatile information about the high-performance thermoplastic are needed. The tested null hypothesis was that an increased percentage of TiO 2 in the PEEK matrix as well as the light unit with different wavelength show no impact on Martens hardness parameter, regardless on the duration of radiation.
Materials and methods
Forty PEEK specimens for each PEEK quality (Tizian PEEK: PEEK/0%; Dentokeep: PEEK/20%; breCAM BioHPP: PEEK/>30%; N = 120, Table 1 ) with a surface area of approximately 16 mm 2 and a thickness of 3 mm were cut under dry conditions using a handpiece (KaVo EWL K9, KaVo Dental, Biberach/Riß, Germany) and embedded in acrylic resin (ScandiQuick A and B, ScanDia, Hagen, Germany, Lot No.09201 and 09202). The specimens were polished up to P1200 (SiC paper, Struers, Willich, Germany) for 20 s with an automatic polishing device (Tegramin 20, Struers) under permanent water cooling. After ultrasonically cleaning (L&R Transistor/Ultrasonic T-14, L&R, Kearny, NJ, USA) for 60 s in distilled water, the specimens were air-abraded (basis Quattro IS, Renfert, Hilzingen, Germany) with alumina particles (Al 2 O 3 , Orbis Dental, Münster, Germany) using following air-abrading parameters: particle size 50 μm, duration 10 s, distance 1 mm at 45°, a pressure of 0.4 MPa and ultrasonically cleaned and air-dried again. Air-abrasion was used in order to adjust the surface modification of PEEK that is recommended to achieve adequate bonding to other dental resin composite materials like luting or veneering materials. Specimens of each PEEK quality were divided into 4 groups according to light units with different wavelength Elipar S10: 430–480 nm, EyeVolution MAX: 385–390 nm + 465–470 nm, Translux CL: 380–500 nm and bre.Lux Power Unit: 370–500 nm. For Elipar S10 and Translux CL the light intensities were determined using a precise dental radiometer (Bluephase Meter II, Ivoclar Vivadent, Schaan, Liechtenstein) ( Table 2 ).
|Product name||Abbreviation||Composition||Manufacturer||Lot. No.|
|Poly-etheretherketone||Tizian PEEK Blank||PEEK/0%||Polyetheretherketone||Schütz Dental Group||2014004126|
|Dentokeep PEEK Disc||PEEK/20%||Polyetheretherketone, filled with 20% TiO 2||NT-Trading||11DK14001|
|breCAM.BioHPP milling blank, dentin shade 2||PEEK/>30%||Polyetheretherketone, filled with >30% TiO 2||Bredent||438251|
|Category||Application||Product name||Manufacturer||Technical details|
|Light intensity in mW/cm 2||Wavelength in nm|
|LED||Chairside||Elipar S10||3M, Seefeld, Germany||1200||430–480|
|Labside||EyeVolution MAX||Dreve, Unna, Germany||–||1 × 385–390|
|6 × 465–470|
|Halogen||Chairside||Translux CL||Heraeus Kulzer, Hanau, Germany||450||380–500|
|Labside||Bre.Lux Power Unit||Bredent, Senden, Germany||220||370–500|
To analyse the Martens hardness parameters (HM in N/mm 2 and E IT in kN/mm 2 ), a universal hardness testing machine (ZHU 0.2/Z2.5, Zwick Roell, Ulm, Germany) was used. Therefore, 10 PEEK specimens for each group were irradiated using the corresponding light unit which was either directly hold onto the specimen surface with a distance of 5 mm or the specimen was put into the polymerisation furnace. Generally, the specimens were stored in dry atmosphere. The duration of irradiation was 540 s in total, while hardness was measured 12 times on each specimen including initially and longitudinally after 5, 10, 20, 30, 40, 60, 80, 100, 180, 360 and 540 s on each specimen, resulting in 480 measurements for each PEEK quality. Therefore, the diamond intender pyramid ( α = 136°) of the testing machine was pressed vertically into the air-abraded and irradiated specimen surface with a load of 9.807 N for 10 s ( Fig. 1 a). The maximum depth of the indenter in PEEK was 0.04 mm. The intender displacement represented the total amount of elastic deformation of the surface along with the plastic depth of the impression . Martens hardness (HM) and indentation modulus (E IT ) values were calculated (testXpert V12.3 Master, Zwick) according to following equations (DIN EN ISO 14577) :
HM = F A s ( h ) = F 26.43 * h 2
E IT = ( 1 − ν S 2 ) * ( 1 E r − ( 1 − v i 2 ) E i ) − 1 w i t h E r = π 2 C A p