Abstract
Objective
To test and compare the two-body wear rate of three CAD/CAM polymer materials and the influence of specimen geometry, antagonist material and test set-up configuration.
Methods
Three CAD/CAM polymeric materials were assessed: a thermoplastic polyetheretherketone (PEEK), an experimental nanohybrid composite (COMP) and a PMMA-based material (PMMA). Crown-shaped and flat specimens were prepared from each material. The specimens underwent thermo-mechanical loading (50 N, 5/55 °C; 600,000 chewing cycles) opposed to human enamel and stainless steel antagonists. Half of the specimens of each group were loaded with a sliding movement of 0.7 mm, the remaining half without. Thereby, 24 different test set-ups were investigated ( n = 12). Wear of the materials and antagonists was evaluated with a match-3D procedure. The topography of all surfaces was examined with scanning electron microscopy (SEM). Data were statistically evaluated with four-/one-way ANOVA followed by Scheffé post hoc test and unpaired t -test ( p < 0.05).
Results
All PEEK specimens showed significantly less material loss than COMP and PMMA specimens when loaded laterally. Within the axial loaded groups this was only true for the flat specimens tested with enamel antagonists. Crown specimens of these groups exhibited lower loss values than flat ones. Lateral force application led mostly to significantly higher material loss than the axial load application. On the antagonist side, no impact of CAD/CAM polymer material, antagonist material, force application and specimen geometry was found.
Significance
Wear of PEEK was lower than that of the resin-based materials when lateral forces were applied, but showed comparable antagonist wear rates at the same time.
1
Introduction
Computer-aided design/computer-aided manufacturing (CAD/CAM) technique allows milling of different materials with high precision. Because the demand for metal-free treatment options in dentistry is still increasing, several CAD/CAM polymers have been introduced for dental restorations alternatively to ceramics . The actually growing interest in tooth-colored high-performance polymers can be also attributed to improvements in CAD/CAM-technology, faster processing and lower costs as well as improved mechanical properties in combination with the advantage of using them in thinner thicknesses as compared to ceramics . However, CAD/CAM polymeric materials such as composite- and PMMA-based materials are still rather used as long-term provisionals and are more and more considered for definitive restorations . Several studies investigated the performance of CAD/CAM resin FDPs regarding color stability and mechanical properties and obtained comparable or even better results as compared to glass-ceramics .
In contrast to ceramics, the major advantage of polymers is the low elastic modulus, which allows for better absorption of functional stresses by deformation . Another advantage is the low abrasiveness of the enamel antagonists . Two studies assessing the wear behavior of CAD/CAM polymers and ceramics have shown that polymers generated the least amount of antagonistic enamel wear . In contrast to ceramics, CAD/CAM resins caused no enamel cracks .
With regard to the restoration wear itself, metal alloys and ceramic materials were proven to be very wear-resistant in general . In contrast, composite resin materials and unfilled polymers cannot withstand a more accentuated material loss . Although the wear of ceramic materials is supposed to be similar to that of enamel, there are inconsistent results indicating even more occlusal contact wear than for composites . Different test set-ups or selection of subjects might be the reason for this . Nonetheless, the wear of natural teeth can significantly increase with an antagonist supplied with a ceramic restoration . In literature, there is a scarcity of studies dealing with the wear of resins as FDPs or veneering materials .
A rather new polymeric material in this field of dental research is polyetheretherketone (PEEK) – a polymer from the main group PAEK (polyaryletherketone). PEEK is either available as industrially pressed blanks for CAD/CAM milling, as industrially pre-pressed pellets or in granular form. However, the latter two forms require thermo-pressing or melting processing. Due to the excellent physical and biological properties, PEEK has gained wide acceptance in medicine and has attracted attention in prosthodontics in recent time and has been suggested being a potential material for fixed dental prostheses (FDPs) . Investigations on performance of three-unit PEEK FDPs have shown that industrial pre-fabrication of blanks (CAD/CAM/pellets) increased the stability and reliability of the restorations. Less plastic deformation and higher mean fracture loads were observed compared to FDPs pressed from granular material. Also, higher Weibull moduli were achieved . Concerning the adhesion of PEEK to dimethylmethacrylates, the initial difficulties were overcome and the results obtained in studies on bond strengths of PEEK to other resins as well as on load-bearing capacity of FDPs are very promising .
In view of the limited data available on wear behavior of CAD/CAM polymer materials, especially PEEK, the purpose of the current study was to evaluate and compare the two-body wear rate of thermoplastic PEEK, experimental CAD/CAM nanohybrid composite and PMMA-based material. Different test set-ups regarding configuration, antagonist material and force application were investigated. The null hypothesis was that the wear rate of CAD/CAM polymers and antagonists of all tested groups would be similar, regardless of the test method used.
2
Materials and methods
Table 1 provides detailed information regarding the materials (lot numbers, manufacturers, composition) used in this study. The test design is presented in Fig. 1 . The following variables and configurations were investigated and combined:
| 1. Materials: | Thermoplastic PEEK (Dentokeep, nt-trading, Karlsruhe, Germany; LOT: 11DK14001) Experimental CAD/CAM nanohybrid composite (Ivoclar Vivadent, Schaan, Lichtenstein; LOT: HT A2/C14 28923) PMMA-based CAD/CAM material (artBloc Temp, Merz Dental, Lütjenburg, Germany; LOT: 44308) |
| 2. Configurations: | Crown and flat specimens |
| 3. Antagonists: | Standardized human enamel antagonists Standardized stainless steel antagonists (SD Mechatronik, Feldkirchen-Wetsterham, Germany) |
| 4. Wear simulation: | With lateral movement Without lateral movement (axial) |
| Test group | Product name | Abbreviation | Lot number | Manufacturer | Composition |
|---|---|---|---|---|---|
| Thermoplastic PEEK | Dentokeep | PEEK | 11DK14001 | nt-trading, Karlsruhe, Germany | PEEK |
| Experimental CAD/CAM nanohybrid composite | – | COMP | HT A2/C14 28923 | Ivoclar Vivadent, Schaan, Lichtenstein | Different nano-adhesives and approx. 80% of filler |
| PMMA-based CAD/CAM material | artBloc Temp | PMMA | 44308 | Merz Dental, Lütjenburg, Germany | PMMA, OMP = organic modified polymer network |
Based on the above mentioned variables and configurations, a total of 24 different test groups were evaluated. Each group consisted of 12 specimens. To avoid any operator bias and ensure constant quality, specimens were made by one qualified technician, who was unaware of the study design and aims.
2.1
Fabrication of flat specimens
Flat specimens of the three different materials were manufactured by cutting standardized geometries of 10 mm length, 0.5 mm width and a thickness of 2 mm under constant water-cooling using a red handpiece (GENTLEpower LUX 25 LP, KaVo Dental, Biberach/Riß, Germany) and a diamond cutting disk (Diamand disk, 924.104.180, Komet Dental, Gebr. Brasseler, Lemgo, Germany). The specimens were then embedded in an acryl resin (Scandiform/ScandiQuick, SCAN-DIA, Hagen, Germany) and polished with silicon carbide (SiC) paper in 5 steps under water-cooling up to P4000 (P240, P500, P1200, P2400, P4000, Struers, Ballerup, Denmark and Tegramin-20, MD-Fuga 200 mm, Struers).
2.2
Fabrication of crown specimens
A standardized anatomically supported base metal alloy model (Remanium 2000, Dentaurum, Ispringen, Germany) with an elastic modulus of 210 GPa was used to fabricate crowns . The model abutment tooth was designed according to a molar crown design utilizing a complete crown preparation with a 1.2 mm deep chamfer margin design, an occlusal reduction of 1.5 mm and a total convergence angle of 6°. The model was scanned and a master STL-file of the molar crown was designed (Ceramill Motion 2 System, AmannGirrbach, Koblach, Austria). The Cerec inLab system (Sirona, Bensheim, Germany) was used for the experimental CAD/CAM nanohybrid composite and PMMA-based CAD/CAM material, while the ZENO Tec System (ZENO 4030 M1, Wieland + Dental, Pforzheim, Germany) was employed for the thermoplastic PEEK.
After seating of the crowns by using occlusion spray (Arti-Spray, white, BK 285, Dr. Jean Bausch KG, Cologne, Germany), all crowns were polished under standardized conditions (Abraso-Starglanz, bredent, Senden, Germany). The inner surfaces of the crowns were air-abraded before cementation using alumina powder (10 s, 0.5 bar, distance: 10 mm) with a mean particle size of 50 μm (Fineblaster type FG 3, Sandmaster, Zofingen, Switzerland) and then cleaned in an ultrasonic bath filled with distilled water for 10 min (Sonorex, Bandelon electronic, Berlin, Germany). Subsequently, the crowns were adhesively cemented on the base metal alloy abutments using a self-adhesive resin cement (Clearfil SA Cement, Kuraray, Tokyo, Japan) according to the manufacturer’s instruction. A special cementing device was used to ensure that the crowns were centrally loaded at a standardized force of 150 g for 10 min. Each specimen was light-cured from each aspect (buccal/distal/oral/mesial) for 40 s with a LED light-curing unit using the standard program with a light intensity of 1200 mW/cm 2 (Elipar S10, 3M ESPE, Seefeld, Germany). Subsequently, specimens were stored for 24 h in distilled water at 37 °C. Afterwards, all crown specimens were digitized. For this purpose, scan powder (Met-L-Chek Developer D 70, Helling, Heidgraben, Germany) was applied to the investigated surfaces. Scanning was performed with a triangulation sensor from two different directions (Laserscan 3D, SD Mechatronik) to preserve the surface geometry before wear simulation.
2.3
Fabrication of the antagonists
Mesiobuccal cusps of extracted human maxillary permanent molars were used as enamel antagonists. All teeth originated from an anonymous collection of several dentists in the Munich area. All teeth were disinfected by immersing them in a 0.5% chloramine solution (Chloramine-T; Sigma–Aldrich Laborchemikalien, Seelze, Germany, LOT 53110, CAS No. 7080-50-4) at room temperature for a maximum period of one week after extraction. Afterwards, they were stored in distilled water at 5 °C for a maximum time period of 6 months according to the ISO 11405/TR. Crowns were then separated to obtain mesiobuccal cusps. Subsequently, a standardized spherical cusp shape with a diameter of 1 mm generated using a bench drill with 40 μm and 8 μm grit (BT-BD 1020 D, Einhell Germany, Landau/Isar, Germany). Afterwards, specimens were polished with a goat hair brush (Abraso-Starglanz, Bredent) and embedded in a round stainless steel mold using amalgam (Dispersalloy, Dentsply DeTrey, Konstanz, Deutschland, LOT: 120823).
In addition, hemispherical stainless steel specimens (X5CrNi18-10, Steel no. k.h.s DIN 1.4301, SD Mechatronik) with an elastic modulus of 210 GPa, served as antagonists. The diameter of the cusps was 6 mm .
In order to allow optimal wear quantification and superimposition, each antagonist received three notches using a diamond disk (924.104.180, Komet Dental). Subsequently, all antagonists were digitized as described above.
2.4
Wear simulation
Specimens and antagonists were mounted in a chewing simulator (Chewing Simulator CS-4, SD Mechatronik) ( Fig. 2 ). Each crown and each flat specimen was tested with steel and human antagonists, respectively. Half of the specimens were loaded with a vertical load of 50 N and a sliding movement of 0.7 mm for 600,000 chewing cycles whereas the other half was loaded in the same manner, but without lateral movement. During wear simulation, thermo-mechanical loading was applied in distilled water at temperatures of 5 °C and 55 °C with duration of 60 s for each cycle.
After wear simulation, the surfaces of the crown specimens and all antagonists were digitized again as described above. The datasets before (baseline reference) and after wear simulation were superimposed using a three-notch alignment and match-3D procedure and images were generated displaying the differences. Material loss was computed (Debian, Match 3D, developed by Dr. Wolfram Gloger).
2.5
Statistical analysis
Data were analyzed using SPSS Version 20 (SPSS Inc, Chicago, IL, USA). Firstly, descriptive statistics were calculated for the data. Normality of data distribution was tested using the Kolmogorov–Smirnov test and analysis of variance (four-way ANOVA) was performed with respect to CAD/CAM polymer materials, antagonist materials, force applications and specimen geometry. One-way ANOVA followed by Scheffé post hoc test was used for analyze the effect of CAD/CAM polymer material. Unpaired t -test was used for calculation of impact of antagonist materials, force applications and specimens’ geometry. p -Values smaller than 5% were considered to be statistically significant in all tests. Selected surfaces of the specimens and antagonists were visually analyzed by scanning electron microscopy (SEM) (Carl Zeiss Supra 55 VP Gemini, Carl Zeiss, Oberkochen, Germany) operating at 10 kV with a working distance of 40–50 mm.
2
Materials and methods
Table 1 provides detailed information regarding the materials (lot numbers, manufacturers, composition) used in this study. The test design is presented in Fig. 1 . The following variables and configurations were investigated and combined:
| 1. Materials: | Thermoplastic PEEK (Dentokeep, nt-trading, Karlsruhe, Germany; LOT: 11DK14001) Experimental CAD/CAM nanohybrid composite (Ivoclar Vivadent, Schaan, Lichtenstein; LOT: HT A2/C14 28923) PMMA-based CAD/CAM material (artBloc Temp, Merz Dental, Lütjenburg, Germany; LOT: 44308) |
| 2. Configurations: | Crown and flat specimens |
| 3. Antagonists: | Standardized human enamel antagonists Standardized stainless steel antagonists (SD Mechatronik, Feldkirchen-Wetsterham, Germany) |
| 4. Wear simulation: | With lateral movement Without lateral movement (axial) |
| Test group | Product name | Abbreviation | Lot number | Manufacturer | Composition |
|---|---|---|---|---|---|
| Thermoplastic PEEK | Dentokeep | PEEK | 11DK14001 | nt-trading, Karlsruhe, Germany | PEEK |
| Experimental CAD/CAM nanohybrid composite | – | COMP | HT A2/C14 28923 | Ivoclar Vivadent, Schaan, Lichtenstein | Different nano-adhesives and approx. 80% of filler |
| PMMA-based CAD/CAM material | artBloc Temp | PMMA | 44308 | Merz Dental, Lütjenburg, Germany | PMMA, OMP = organic modified polymer network |
Based on the above mentioned variables and configurations, a total of 24 different test groups were evaluated. Each group consisted of 12 specimens. To avoid any operator bias and ensure constant quality, specimens were made by one qualified technician, who was unaware of the study design and aims.
2.1
Fabrication of flat specimens
Flat specimens of the three different materials were manufactured by cutting standardized geometries of 10 mm length, 0.5 mm width and a thickness of 2 mm under constant water-cooling using a red handpiece (GENTLEpower LUX 25 LP, KaVo Dental, Biberach/Riß, Germany) and a diamond cutting disk (Diamand disk, 924.104.180, Komet Dental, Gebr. Brasseler, Lemgo, Germany). The specimens were then embedded in an acryl resin (Scandiform/ScandiQuick, SCAN-DIA, Hagen, Germany) and polished with silicon carbide (SiC) paper in 5 steps under water-cooling up to P4000 (P240, P500, P1200, P2400, P4000, Struers, Ballerup, Denmark and Tegramin-20, MD-Fuga 200 mm, Struers).
2.2
Fabrication of crown specimens
A standardized anatomically supported base metal alloy model (Remanium 2000, Dentaurum, Ispringen, Germany) with an elastic modulus of 210 GPa was used to fabricate crowns . The model abutment tooth was designed according to a molar crown design utilizing a complete crown preparation with a 1.2 mm deep chamfer margin design, an occlusal reduction of 1.5 mm and a total convergence angle of 6°. The model was scanned and a master STL-file of the molar crown was designed (Ceramill Motion 2 System, AmannGirrbach, Koblach, Austria). The Cerec inLab system (Sirona, Bensheim, Germany) was used for the experimental CAD/CAM nanohybrid composite and PMMA-based CAD/CAM material, while the ZENO Tec System (ZENO 4030 M1, Wieland + Dental, Pforzheim, Germany) was employed for the thermoplastic PEEK.
After seating of the crowns by using occlusion spray (Arti-Spray, white, BK 285, Dr. Jean Bausch KG, Cologne, Germany), all crowns were polished under standardized conditions (Abraso-Starglanz, bredent, Senden, Germany). The inner surfaces of the crowns were air-abraded before cementation using alumina powder (10 s, 0.5 bar, distance: 10 mm) with a mean particle size of 50 μm (Fineblaster type FG 3, Sandmaster, Zofingen, Switzerland) and then cleaned in an ultrasonic bath filled with distilled water for 10 min (Sonorex, Bandelon electronic, Berlin, Germany). Subsequently, the crowns were adhesively cemented on the base metal alloy abutments using a self-adhesive resin cement (Clearfil SA Cement, Kuraray, Tokyo, Japan) according to the manufacturer’s instruction. A special cementing device was used to ensure that the crowns were centrally loaded at a standardized force of 150 g for 10 min. Each specimen was light-cured from each aspect (buccal/distal/oral/mesial) for 40 s with a LED light-curing unit using the standard program with a light intensity of 1200 mW/cm 2 (Elipar S10, 3M ESPE, Seefeld, Germany). Subsequently, specimens were stored for 24 h in distilled water at 37 °C. Afterwards, all crown specimens were digitized. For this purpose, scan powder (Met-L-Chek Developer D 70, Helling, Heidgraben, Germany) was applied to the investigated surfaces. Scanning was performed with a triangulation sensor from two different directions (Laserscan 3D, SD Mechatronik) to preserve the surface geometry before wear simulation.
2.3
Fabrication of the antagonists
Mesiobuccal cusps of extracted human maxillary permanent molars were used as enamel antagonists. All teeth originated from an anonymous collection of several dentists in the Munich area. All teeth were disinfected by immersing them in a 0.5% chloramine solution (Chloramine-T; Sigma–Aldrich Laborchemikalien, Seelze, Germany, LOT 53110, CAS No. 7080-50-4) at room temperature for a maximum period of one week after extraction. Afterwards, they were stored in distilled water at 5 °C for a maximum time period of 6 months according to the ISO 11405/TR. Crowns were then separated to obtain mesiobuccal cusps. Subsequently, a standardized spherical cusp shape with a diameter of 1 mm generated using a bench drill with 40 μm and 8 μm grit (BT-BD 1020 D, Einhell Germany, Landau/Isar, Germany). Afterwards, specimens were polished with a goat hair brush (Abraso-Starglanz, Bredent) and embedded in a round stainless steel mold using amalgam (Dispersalloy, Dentsply DeTrey, Konstanz, Deutschland, LOT: 120823).
In addition, hemispherical stainless steel specimens (X5CrNi18-10, Steel no. k.h.s DIN 1.4301, SD Mechatronik) with an elastic modulus of 210 GPa, served as antagonists. The diameter of the cusps was 6 mm .
In order to allow optimal wear quantification and superimposition, each antagonist received three notches using a diamond disk (924.104.180, Komet Dental). Subsequently, all antagonists were digitized as described above.
2.4
Wear simulation
Specimens and antagonists were mounted in a chewing simulator (Chewing Simulator CS-4, SD Mechatronik) ( Fig. 2 ). Each crown and each flat specimen was tested with steel and human antagonists, respectively. Half of the specimens were loaded with a vertical load of 50 N and a sliding movement of 0.7 mm for 600,000 chewing cycles whereas the other half was loaded in the same manner, but without lateral movement. During wear simulation, thermo-mechanical loading was applied in distilled water at temperatures of 5 °C and 55 °C with duration of 60 s for each cycle.
