The fatigue behavior of five restorative materials; two yttria-stabilized tetragonal zirconium dioxide (3Y-TZP), (LAVA (LVs), EVEREST (KVs)); a lithium disilicate LD (IPS e.max CAD Ivoclar-Vivadent); two lithium (di)-silicate/phosphate glassceramics (Suprinity PC and Celtra Duo), are estimated. Table 1 describes the materials used, their manufacturers, and post-processes. Table 2 lists the mechanical properties of the restoration materials used. The materials selected in this analysis have been chosen to cover a wide variety of newly developed restorative materials currently available on the market.
Table 1
Description of materials used, their manufacturers, post-processes
Material | Commercial name ( manufacturer | Processes | Ref |
3Y-TZP sandblasted | LAVA (LVs) 3M Espe, Seefeld, D | Roughness (µm) = 0.26 Sample: Bars 40 mm × 3 mm × 5 mm, each bar was subjected to four firing cycles (930°C, 900°C, 890°C, 880°C) | [12] |
3Y-TZP sandblasted | EVEREST ZS (KVs) KaVo Dental GmbH, Biberach, D | Roughness (µm) = 0.71 Sample: Bars 40 mm × 3 mm × 5 mm, each bar was subjected to four firing cycles (930°C, 900°C, 890°C, 880°C) | [12] |
lithium disilicate | IPS e.max CAD Ivoclar-Vivadent | e.max CAD specimens underwent a crystallization firing for 8min at 840 ◦C (heating rate 55 ◦C/min) or 10min at 850 ◦C (heating rate 30 ◦C/min) | [10] |
Pre-sintered Lithium Silicate/phosphate (LSP) glass-ceramic | Suprinity, VITA Zahnfabrik | e.max CAD specimens underwent a crystallization firing for 8min at 840 ◦C (heating rate 55 ◦C/min) or 10min at 850 ◦C (heating rate 30 ◦C/min) | [10] |
Fully-sintered lithium silicate/Phosphate (LSP)glass-ceramic | Celtra Duo, Dentsply DeTrey | Data not available from the ref. [19] | [10] |
Table 2
Material properties for the restored first molar model.
Material | Young’s modulus (GPa) | Poisson ratio (ν) | Yield strength (MPa) | Flexural strength (MPa) | Compressive strength (MPa) | Shear strength (MPa | ref |
Indenter ( mechanical properties similar to enamel) | 84.1 | 0.33 | | 11.5 | 384 | 60 | [13] |
3Y-TZP (LVs) | 205.2 | 0.32 | | 1282 | | | [12] |
3Y-TZP(KVs) | 205.2 | 0.32 | | 836 | | | [12] |
LD (IPS e.max CAD) | 102.7 | 0.215 | | 338.5 | | | [10] |
(LSP) Celtra Duo | 107.9 | 0.222 | | 488.2 | | | [10] |
(LSP) Suprinity | 104.9 | 0.208 | | 328.7 | | | [10] |
Dentin | 18.6 | 0.31 | | 105.5 | 267 | 12–138 | [13] |
Dual cure resin cement thickness 0.05–0.08 mm for full zirconia, | 8 | 0.3 | | | | 34.4 | [13] |
Die (grade 4 Gypsum) | 14 | 0.35 | 29 | | | | [13] |
A 3D model was obtained by a 3D scanner of the lower first molar crown and shown in Fig. 1. The model consists of a crown, layers of cement, and death (dentine). The sample Stereolithography (STL) files are imported into solid modeling software (SolidWorks 2018, DS Solidworks Corp, USA). The model is transferred into the FEA software (ANSYS, Inc., USA). A 6 mm hemispherical indenter is used and is made of material with enamel-like mechanical properties. A 50 µm thick layer of adhesive resin cement is molded.
The key steps to clinically predict the long-term survival of dental restoration are a) obtain fatigue data from laboratory experiments, b) evaluate the actual multi-state stresses under the oral load in the actual crown restoration, and c) use appropriate fatigue failure theories.
Two types of loads are studied, the first type: a compressive force acting in the tooth axial, the second type: axial load followed by a sliding motion. The model of the first molar under axial loading is shown in Fig. 1(right). The model is fixed on the lower surface. The life of two clinical years is believed to equate to one million cycles. The frictional interaction between occlusal surfaces and indenters is simulated. Furthermore, the crown is thought to be perfectly bonded with dentin.
Static test is conducted to simulated the occlusal load using a hemispherical indenter. Nine samples are manufactured. A 3D finite element model is constructed that simulates the real experimental setup. The sample preparation techniques can be summarized as follows; nine silicone impressions (3M ESPE, United States) were made in order to duplicate the prepared plastic tooth for the first mandibular molar (Nissin Dental Model, Japan), all impressions poured into type IV gypsum die (Dentona gypsum material, Germany). Each master dies derived from one of the nine recorded impressions was then scanned (Dentsply Sirona in EosXF, United States), and used to design zirconium full contoured crowns using CAD/CAM system using (Sirona CAD/CAM Mcx5milling machine Software inlab sw16.1). For zirconia, the corresponding occlusal crown thickness was 1,4 mm. 0.8 mm is the proximal wall thickness. A thickness of 50 µm of cement is used. The crown's STL file was produced and sent to the milling unit. Following the instructions of the manufacturer, all specimens were sintered in a sintering furnace after the milling process ended. Crowns is seated and cemented using dual-cure resin cement Panavia cement on the gypsum dies (Kuraray America Inc, USA). The indenter is located and balanced on the occlusal surface so that it has three points of contact with the crown. The crowns were then subjected to axial compressive load centered in the central fossa until fracture. A radius of 3.8 mm was selected for the spherical indenter. The load was applied at a cross-head speed of 0.5 mm/min on a computer-controlled electromechanical universal testing system (Model WDW-20).