Wear, strength, modulus and hardness of CAD/CAM restorative materials

Abstract

Objective

To measure the mechanical properties of several CAD/CAM materials, including lithium disilicate (e.max CAD), lithium silicate/zirconia (Celtra Duo), 3 resin composites (Cerasmart, Lava Ultimate, Paradigm MZ100), and a polymer infiltrated ceramic (Enamic).

Methods

CAD/CAM blocks were sectioned into 2.5 mm × 2.5 mm × 16 mm bars for flexural strength and elastic modulus testing and 4 mm thick blocks for hardness and wear testing. E.max CAD and half the Celtra Duo specimens were treated in a furnace. Flexural strength specimens (n = 10) were tested in a three-point bending fixture. Vickers microhardness (n = 2, 5 readings per specimen) was measured with a 1 kg load and 15 s dwell time. The CAD/CAM materials as well as labial surfaces of human incisors were mounted in the UAB wear device. Cusps of human premolars were mounted as antagonists. Specimens were tested for 400,000 cycles at 20 N force, 2 mm sliding distance, 1 Hz frequency, 24 °C, and 33% glycerin lubrication. Volumetric wear and opposing enamel wear were measured with non-contact profilometry. Data were analyzed with 1-way ANOVA and Tukey post-hoc analysis (alpha = 0.05). Specimens were observed with SEM.

Results

Properties were different for each material (p < 0.01). E.max CAD and Celtra Duo were generally stronger, stiffer, and harder than the other materials. E.max CAD, Celtra Duo, Enamic, and enamel demonstrated signs of abrasive wear, whereas Cerasmart, Lava Ultimate, Paradigm MZ100 demonstrated signs of fatigue.

Significance

Resin composite and resin infiltrated ceramic materials have demonstrated adequate wear resistance for load bearing restorations, however, they will require at least similar material thickness as lithium disilicate restorations due to their strength.

Introduction

A recent study reported that lithium disilicate was chosen as the material of choice for 20% of surveyed dentists for posterior crowns and 55% of dentists for anterior crowns . When used with in-office CAD/CAM systems, lithium disilicate materials, such as e.max CAD (Ivoclar Vivadent, Schaan, Lichtenstein), are provided to the dentist in a softened state to expedite milling. After milling, this material is crystallized in a furnace to improve its mechanical properties. In order to save the clinician time, new CAD/CAM restorative materials have been introduced which do not require heat treatment to achieve acceptable strength.

The first generation of these materials included “in factory” polymerized resin composites. Paradigm MZ100 (3M ESPE, St Paul, MN, USA) is based on the resin composite Z100 (3M ESPE), which is filled with 85 wt% zirconia and silica particles ranging from 0.01 to 3.5 μm. More recently, Lava Ultimate (3M ESPE) was introduced, an 80 wt% nano-filled resin composite based on Filtek Supreme Ultra (3M ESPE). Cerasmart (GC America, Alsip, IL, USA), a 71 wt% filled nano-composite, is a similar material. Both Paradigm MZ100 and Lava Ultimate have higher flexural strength and fracture toughness than resin composites polymerized by a dental curing light. These resin-composites are likely fabricated at an elevated temperature and pressure . A study of commercial composites polymerized at 250 MPa and 180 °C reported increases of 54–89% in flexural strength and 2–86% in fracture toughness over the same composites light polymerized at ambient conditions. The high temperature/high pressure (HT/HP) conditions were theorized to have reduced the size and number of defects in the composite microstructure as evidenced by a significantly higher density of HT/HP polymerized composites . Further investigation showed HT/HP composites demonstrated higher glass transition temperatures, an indication of greater polymer crosslink density . Unfilled resin polymerized with HT/HP showed improvements in mechanical properties, however, less dramatic than with filled resin composites. Therefore, HT/HP conditions may also have an effect on the resin-filler interaction .

A novel technique is employed to manufacture the polymer-infiltrated ceramic network (PICN) material, Enamic (VITA, Bad Säckingen, Germany). In this material, ceramic particles are partially sintered and then infiltrated with a low-viscosity polymer by capillary action. Unlike resin composites, PICNs contain two interconnected networks, one polymer and one ceramic. Scanning electron microscopy (SEM) imaging and energy-dispersive X-ray spectroscopy (EDX) analysis of this material revealed a predominantly leucite and secondary zirconia crystalline structure surrounded by a resin polymer . An initial investigation of a PICN material revealed the infiltrated ceramic had a higher flexural strength than the fully sintered ceramic or the pure polymer. Increasing the porosity of the PICN material increased its flexural strength and decreased its modulus and hardness. Also, crack bridging by the ductile polymer phase of the PICN material is thought to improve its strength and toughness .

Celtra Duo (Sirona Dentsply, Milford, DE, USA) is a material classified as a zirconia re-inforced lithium silicate CAD/CAM material that may be optionally heat treated. It contains 10% dissolved zirconia in a silica-based glass matrix. Although heat treatment is not necessary for crystallization of the material, flexural strength of fired Celtra Duo has been reported to be considerably greater than the as-milled material .

Previous studies have compared several mechanical properties of these new CAD/CAM materials. Cerasmart and Lava Ultimate were reported to have a higher flexural strength and lower flexural modulus than Enamic . The hardness of Lava Ultimate and Enamic was shown to be lower than enamel whereas e.max CAD was harder than enamel . Less opposing enamel wear was reported against Lava Ultimate than e.max CAD . Regarding material wear, some previous studies reported equivalent wear for Lava Ultimate and Enamic , whereas other studies reported higher wear of Enamic than Lava Ultimate or Cerasmart . Equivalent material and antagonist wear were reported for e.max CAD, Celtra Duo and fired Celtra Duo . To date, there has not been a comparison of the mechanical properties of these new materials available for CAD/CAM fabrication. The purpose of this study was to measure the flexural strength, elastic modulus, hardness, and wear of lithium dislicate (e.max CAD), lithium silicate/zirconia (Celtra Duo), 3 resin composites (Cerasmart, Lava Ultimate, Paradigm MZ100), and a polymer infiltrated ceramic (Enamic). The null hypotheses are that there are no differences in any property for any of the materials tested.

Materials and methods

Microstructural analysis

All materials ( Table 1 ) were sectioned into bars using an Isomet Saw (Buehler, Lake Bluff, IL, USA). Resin-based materials (Cerasmart, Lava Ultimate, Paradigm MZ100)were fractured in half using blunt force with a hammer. Celtra Duo (fired group) and e.max CAD were then fired in a furnace (Programat CS Oven, IvoclarVivadent) following the parameters of the manufacturer (Celtra Duo: 500 °C start, 55 °C/min heating rate, 820 °C final, 1:30 holding time; e.max CAD: 403 °C start, 90 °C/min heating rate, 820 °C final #1, 10:00 holding time #1, 840 °C final #2, 7:00 holding time #2)and etched with a 9.5% hydrofluoric acid (Porcelain etchant, Bisco, Schaumburg, IL, USA) for 1 min. Enamic was prepared by two methods to demonstrate the polymer and ceramic-based components. First, the polymer component of Enamic was burned out in a porcelain oven at 400 °C for 3 h, then the glass component was removed by etching with 9.5% hydrofluoric acid for 1 min. The specimens were rinsed with water for 30 s and stored in ethanol for 24 h. The specimens were then secured to tabs with gold conducting tape, and gold-coated in a vacuum sputter coater (Desk-I; Denton Vacuum Inc., Moorestown, NJ, USA), and examined in an SEM (Quanta FEG 650; FEI, Hillsboro, OR, USA) with the secondary electron imaging mode. Unknown phases were examined for elemental composition using energy dispersive X-ray spectroscopy (EDS) (Team EDS System for SEM with APOLLO XL Silicon Drift Detector; EDAX, Mahwah, NJ, USA). Representative images from each substrate were chosen for presentation in this article.

Table 1
CAD/CAM restorative materials used in this study.
Material Manufacturer Shade
Paradigm MZ100 3M ESPE A2
Cerasmart GC America A2 LT
LAVA Ultimate 3M ESPE A2 LT
Enamic VITA 1M2 Translucent
Celtra Duo Sirona Dentsply A2 LT
e.max CAD Ivoclar Vivadent A2 LT

Flexural strength andmodulus testing

Materials were sectioned into bars (n = 10) of dimensions 2.5 mm × 2.5 mm × 16 mm using an Isomet Saw. After sectioning, all specimens were polished with 320, and 600 SiC abrasive paper (3M ESPE). Celtra Duo (fired group) and e.max CAD were then fired in a furnace following the manufacturer’s parameters (as described above). Bars were stored in distilled water at 37 °C for 24 h. After storage, the specimens were tested in a 3-point bending fixture (12 mm span) mounted in a universal testing machine (Model 4411; Instron, Canton, MA, USA) with a crosshead speed of 1 mm/min.

Hardness testing

Materials were sectioned into 4 mm thick blocks (n = 2) and set into a clear chemically cured embedding medium (95% methyl methacrylate/5% n -butyl embedding epoxy, Fischer Scientific, Pittsburgh, PA, USA). All specimens were wet polished sequentially with 320, 600, and 1200 SiC paper and stored in distilled water at 37 °C for 24 h. After storage, the specimens were placed in a microhardnessindentor (Micromet 5101, Buehler). Five indents were made in each specimen near the center of the specimen at least 0.5 mm away from each other. A Vicker’s indenter was used with a load of 1 kg and a dwell time of 15 s based on recommendations of ASTM C1327 . The major diameters of the Vicker’s indent (d1 and d2) were measured with an optical micrometer, and hardness was calculated with the formula: Hardness = 1850 × Load/(d1 × d2).

Wear testing

Materials were sectioned into 7 mm × 11 mm × 4 mm blocks (n = 8). The specimens were mounted in brass holders with self-curing acrylic material (Flash Acrylic, Yates and Bird, Chicago, IL, USA) and wet polished sequentially with 320, 600, and 1200 SiC paper. A final finish with 0.05 μm alumina slurry and a polishing cloth was applied. After polishing, the specimens were sonicated for 5 min to remove the polishing debris. In order to determine enamel-enamel wear, eight extracted maxillary central incisors were mounted flat in brass holders. All specimens were stored for 24 h in 37 °C water prior to testing.

Freshly extracted caries-free maxillary premolars were selected for wear antagonists from collection containers within the dental school following IRB approval (UAB #N150602006). The dimensions of their buccal cusps were standardized with a cone shaped diamond bur (S5030.11.050 1P, Brasseler, Savanna, GA, USA) in a straight handpiece at 25,000 rpm with water. The standardized cusps were then mounted on metal styli and stabilized with self-cure acrylic (Flash Acrylic, Yates and Bird). Prior to testing, the enamel cusps were scanned using a non-contact surface profilometer (Proscan 2000, Scantron Industrial Products Ltd., Taunton, England) with 20 μm resolution.

The specimens were placed in the modified UAB wear testing machine ( Fig. 1 ). The machine operates by applying a vertical load from the antagonist onto the specimen, sliding horizontally, and then repeating the cycle . The specific parameters for this test were a 20 N load, 1 Hz frequency, 2 mm sliding distance, 33% glycerin lubricant, 24 °C, and 400,000 testing cycles. Following wear testing, materials and enamel cusps were re-scanned with the profilometer. The volumetric wear of each material and opposing enamel cusp was determined with ProForm superimposition software (Scantron Industrial Products Ltd.). A representative specimen from each group was imaged with SEM. The enamel specimen was examined in an environmental chamber at 5 Torr pressure.

Fig. 1
Modified UAB wear testing device.

Statistical analysis

Descriptive statistics, such as means and standard deviations, were computed for each property (flexural strength, elastic modulus, hardness, material wear and enamel wear) within each material. The distributions of the measurements for each property were examined using normal probability plots and the Kolmogorv–Smirnov test, and it was determined that the measurements were approximately normally distributed. Analysis of variance (ANOVA) was used to compare the means of the properties for each material. The Tukey–Kramer multiple comparisons test was then used to determine which specific pairs of means were significantly different. Statistical tests were two-sided and were performed using a significance level of 5%. Statistical analyses were performed using SAS, version 9.4 (SAS Institute, Cary, NC, USA).

Materials and methods

Microstructural analysis

All materials ( Table 1 ) were sectioned into bars using an Isomet Saw (Buehler, Lake Bluff, IL, USA). Resin-based materials (Cerasmart, Lava Ultimate, Paradigm MZ100)were fractured in half using blunt force with a hammer. Celtra Duo (fired group) and e.max CAD were then fired in a furnace (Programat CS Oven, IvoclarVivadent) following the parameters of the manufacturer (Celtra Duo: 500 °C start, 55 °C/min heating rate, 820 °C final, 1:30 holding time; e.max CAD: 403 °C start, 90 °C/min heating rate, 820 °C final #1, 10:00 holding time #1, 840 °C final #2, 7:00 holding time #2)and etched with a 9.5% hydrofluoric acid (Porcelain etchant, Bisco, Schaumburg, IL, USA) for 1 min. Enamic was prepared by two methods to demonstrate the polymer and ceramic-based components. First, the polymer component of Enamic was burned out in a porcelain oven at 400 °C for 3 h, then the glass component was removed by etching with 9.5% hydrofluoric acid for 1 min. The specimens were rinsed with water for 30 s and stored in ethanol for 24 h. The specimens were then secured to tabs with gold conducting tape, and gold-coated in a vacuum sputter coater (Desk-I; Denton Vacuum Inc., Moorestown, NJ, USA), and examined in an SEM (Quanta FEG 650; FEI, Hillsboro, OR, USA) with the secondary electron imaging mode. Unknown phases were examined for elemental composition using energy dispersive X-ray spectroscopy (EDS) (Team EDS System for SEM with APOLLO XL Silicon Drift Detector; EDAX, Mahwah, NJ, USA). Representative images from each substrate were chosen for presentation in this article.

Nov 23, 2017 | Posted by in Dental Materials | Comments Off on Wear, strength, modulus and hardness of CAD/CAM restorative materials
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