The damage tolerance of dental restorative materials was analyzed.
Grinding with diamond burs to simulate adjustments was applied.
The initial materials strength of all tested materials decreased significantly.
The novel interpenetrating and polymer containing composites ENAMIC and PICN exhibit a high damage tolerance upon typical clinical bur grinding procedures.
To analyze the damage tolerance of indirect restorative materials after grinding with diamond burs to simulate adjustments by clinicians and technicians.
Seven commercially available restorative materials (Mark II, ENAMIC, In-Ceram Alumina, VM 9, In-Ceram YZ, IPS e.max CAD) and an experimental one (PICN) were analyzed. Forty bending bars per material were fabricated according to manufacturer’s instructions and lapped with 15 μm diamond suspension. The initial flexural strength was determined in three-point-bending on 10 specimens. Additionally the elastic modulus and Poisson’s ratio were determined by the resonant frequency method. The remaining bending bars were divided into six groups ( n = 5) and subjected to standardized grinding with three different diamond grit burs (coarse, 151 μm; medium, 107 μm and extra fine 25 μm) and two grinding directions (transversal and longitudinal). The ground specimens were subsequently loaded to fracture and analyzed by SEM.
Except for the YTZP bending bars, the initial materials strength of all tested materials decreased significantly with all diamond burs upon adjustments in both transversal and longitudinal grinding directions. The resistance of the ground materials to strength reduction follow the order from highest to least damage tolerant material: PICN > ENAMIC > Mark II > VM 9 > In-Ceram Alumina > IPS e.max CAD. The loss in strength of all examined materials after longitudinal grinding is generally less compared to transversal grinding. The lowest loss in strength occurred for VM 9 (7.79%) and ENAMIC (9.18%) upon longitudinal grinding direction with extra fine and medium diamond grit bur, respectively.
The damage tolerance of restorative materials upon adjustments depends on specific mechanical properties and the adjustment procedure. The outcomes of the simulated grinding protocols of this study can be adopted clinically in terms of the selection of appropriate materials, burs and adjustment parameters.
Several potential materials’ damaging processes are involved during fabrication of indirect dental restorations such as crowns and inlays.
First, producing restorations using CAD–CAM (computer-aided design–computer-aided manufacturing) involves milling pre-fabricated blocks with diamond burs (mostly under water coolant). Depending on material properties such as hardness, elastic modulus and the brittleness index the milling process can introduce flaws (such as microcracks and chipping) to the material . In addition, the extent of strength degradation will vary according to the milling parameters, type of burs and materials damage tolerance. CAD–CAM machining related damage of ceramic restorations may be the cause of premature clinical fractures .
Second, before or after restorations are cemented/bonded to the prepared tooth (with cements or adhesives) the intaglio surfaces as well as the occlusal contacts with the antagonist are usually adjusted directly in the mouth. For the adjustment procedures either a high speed air-driven turbine or a dental handpiece with diamond burs are used. The adjustment procedure can be a material damaging process. The degree of damage depends on grinding parameters such as load, cooling, rotations per minute (rpm) and abrasive bur grit size .
The initial or intrinsic strength of prosthetic CAD/CAM materials depends on the specific microstructure and is influenced by the manufacturing process, for instance pressing or casting. The blank should be relatively major defect/flaw free.
Seven materials, including four CAD/CAM ceramics (Mark II, IPS e.max CAD, In-Ceram YZ and In-Ceram Alumina), one veneering ceramic (VM 9) and two interpenetrating phase composites (ENAMIC and PICN as experimental material) were investigated. Contrary to manufacturer’s instructions In-Ceram Alumina was investigated without a veneering layer. Similar to ENAMIC, the PICN material is an interpenetrating phase composite with two continuous networks of ceramic and polymer and described in earlier papers . The PICN test material had a combination of 69 vol.% ceramic and 31 vol.% polymer, whereas ENAMIC exhibits a ratio of 75 vol.% ceramic to 25 vol.% polymer.
The materials used in the present study were tested in previous investigations related to sharp and blunt indentation induced damage . These studies have shown that all the above-mentioned materials under investigation were less susceptible to blunt compared to sharp indentations. The polymer-infiltrated-ceramic-network materials appeared to exhibit the most significant R -curve behavior, that is, high resistance to crack propagation and relatively high contact induced loading displacements with plastic deformation . The so-called R -curve (crack growth resistance) behavior occurs because of the microstructure toughening interactions behind the crack tip. That is, for materials exhibiting R -curve behavior the crack will initially extend in a stable manner followed by unstable crack extension with crack arrest and again a stable crack extension. For brittle materials not exhibiting R -curve behavior the crack will propagate in an unstable manner until it arrests in the compressive region where a declining stress intensity factor exists .
From the results of the previous studies, the interpenetrating phase composites ENAMIC and PICN (with their polymeric reinforcement phase) are expected to exhibit a highly damage tolerant behavior upon adjustment machining procedures investigated in the present study.
The aim of the present study was to analyze the impact of simulated clinical and technical adjustments on the flexural strength of dental ceramics and interpenetrating phase composites. The specific objectives were to evaluate and compare the damage tolerance of dental restorative materials following diverse standardized grinding protocols and varying abrasive diamond burs.
The seven investigated materials (listed in Table 1 , with abbreviations used in the following text) included Mark II, ENAMIC, PICN test material, In-Ceram Alumina, VM 9, In-Ceram YZ (all Vita Zahnfabrik, Bad Saeckingen, Germany) and IPS e.max CAD (Ivoclar Vivadent, Schaan, Liechtenstein).
|Material (abbreviation)||Category||Initial strength, σ f (MPa)||Elastic modulus, E (GPa)||Poisson’s ratio, ν||IS Fracture toughness, K 1C a (MPa√m)||Hardness, H a (GPa)|
|Mark II (MarkII)||Feldspar glass ceramic||122.18
|ENAMIC||Interpenetrating phase composite (polymer infiltrated ceramic network)||152.27
|PICN test material (PICN)||Interpenetrating phase composite (polymer infiltrated ceramic network)||158.53
|In-Ceram Alumina (ICAlumina)||Glass infiltrated aluminum oxide ceramic||548.48
|Feldspar veneering ceramic||121.60
|Yttria stabilized tetragonal zirconia polycrystals||1222.13
|IPS e.max CAD
|Lithium disilicate crystals embedded in a glassy matrix||397.23
The test specimens in this study were bending bars of the test materials ( Table 1 ). MarkII, PICN, ICAlumina, YTZP and emaxCAD bars were cut from blocks with a diamond saw. YTZP bending bars were prepared oversized to compensate the sintering shrinkage and sintered according to manufacturer’s instructions. For the VM9 bending bar preparation, a mold to shape and a furnace (Vita Vacumat 4000, Vita Zahnfabrik) to sinter the specimens were used. The sintering was conducted according to manufacturer’s instructions. ICAlumina bars were glass-infiltrated after cutting followed by firing and excess glass removal by sandblasting according to manufacturer’s instructions. The pre-crystallized emaxCAD bending bars were placed on the recommended crystallization tray and crystallized according to manufacturer’s instruction for partially reduced IPS e.max CAD restorations. Subsequently the bending bars were lapped (MDF 400 PR, Bierther sub-micron, Bad Kreuznach, Germany) then diamond grit polished (15 μm diamond suspension) to a size of 18 mm × 4 mm × 1.2 mm and edges were chamfered according to ISO 6872 in order to minimize stress concentration due to machining flaws. Due to machining (lapping, chamfering) of YTZP bending bars, a regeneration firing according to manufacturer’s instructions was conducted.
Characterization of mechanical properties
The initial flexural strength of the materials was determined by three-point bending until fracture in a universal testing machine (Z010, Zwick/Roell, Ulm, Germany) with a crosshead speed of 0.5 mm/min. The initial fracture stress σ f ( n = 10), was calculated according to ISO 6872 :
σ f = 3 F l 2 w h 2
where F is the fracture load, l the roller span (here 15 mm), w the width and h the height of the bar.
The dynamic elastic modulus ( E ) and Poisson’s ratio ( ν ) were determined five times on one specimen per material with dimensions of 40 mm × 15 mm × 2 mm by the impulse excitation of vibration analysis according to ASTM E 1876 and the equations for rectangular bars:
E = 0.9465 m f f 2 b L 3 t 3 T 1
ν = E 2 G − 1
G = 4 L m f t 2 b t R
where m , b , t and L correspond to the mass, width, thickness and length of the bar, respectively; f f and f t represent the fundamental resonant frequency of the bar in flexure and torsion, respectively; T 1 and R are correction factors and G corresponds to the dynamic shear modulus.
Standardized adjustment apparatus
To simulate adjustments of dental restorations, an apparatus was employed to ensure a standardized grinding procedure ( Fig. 1 ). The hand piece (4911; KaVo Dental GmbH, Biberach/Riß, Germany) was fixed and the specimens (bending bars prepared according to Section 2.2.1 ) were placed longitudinal or transversal with respect to the bur orientation into slots. A pneumatic control releases and retracts (with adjustable time intervals) the specimen holder which was attached to a weight (100 g).
In vitro adjustment protocols and measurement of damage tolerance
The specimens (prepared according to Section 2.2.1 ) were divided into six groups of five bending bars per material to apply the following adjustment procedures. Three different diamond grit burs (Hager & Meisinger GmbH, Neuss, Germany), C (107–181 μm diamond grain size), M (64–126 μm) and EF (10–36 μm) according to ISO 7711-3 were employed and displayed in Fig. 2 to grind (with 20,000 rpm) the specimens for 2 s and 100 g load in both transversal and longitudinal directions with respect to the bending bar length (cf. bending bar slots in Fig. 1 ). The head of the burs were spherical (2.3 mm diameter). Each bur was replaced after grinding 10 bending bars of one material type to ensure the cutting efficiency of the bur.