This study aimed to assess the effect of different storage media on the hardness and monomer elution of CAD/CAM composite blocks.
Five resin-composite blocks (RCB), one polymer-infiltrated ceramic network (PICN) block (Enamic (EN)), one ceramic-filled poly ether ether ketone (PEEK) block (Dentokeep (DK)), and one feldspathic ceramic block. Microhardness was measured using a Vickers indenter tester (FM-700, Future Tech Corp., Japan). In addition 4 conventional resin-composites were investigated for monomer elution using high performance liquid chromatography (HPLC) after storage in different media for 3 months. The data were analysed by three-way ANOVA, two-way ANOVA, one-way ANOVA, Tukey’s post hoc test and the independent t-test ( α = 0.05 for all tests).
The specimens stored in the water had a hardness reduction ranging from 0.9% to 24.4%. In artificial saliva, the specimens had a hardness reduction ranging from 2.8% to 23.2%. The hardness reduction percentage in 75% Ethanol/Water (E/W) ranged between 3.8% and 35.3%. All materials, except GR (resin-composite block) and DK (Polyetheretherketone (PEEK)), showed a variable extent of monomer elution into 75% E/W with significantly higher amounts eluted from conventional composites. GRA and GND (conventional resin-composites) eluted TEGDMA in artificial saliva and GRA eluted TEGDMA in water.
The hardness of CAD/CAM composite blocks was affected by different storage media, and they were not as stable as ceramic, with PICN exhibited superior hardness stability to all of the resin-composite blocks in all the storage media and was comparable to ceramic block. The hardness reduction percentage of the CAD/CAM composite blocks was influenced by the filler loading and resin-matrix composition.Minimal or no monomer elution from CAD/CAM blocks was detected.
Ceramics have many advantages, including biocompatibility, aesthetics and strength [ ]. However, material stiffness and brittleness are drawbacks, which affect the clinical durability and their milling process [ , ]. Ceramics are highly abrasive, and hence might cause wear and roughness to opposing enamel [ , ]. The ceramic brittleness influences the restoration longevity as it usually fails through crack propagation that ends in catastrophic failure [ ]. Also, ceramic might chip or crack during processing/machining [ ]. Resin-composite materials have been undergoing improvements in many aspects related to their mechanical properties by means of innovative composition including monomer resins, initiation systems, and higher filler loading and polymerisation modes (high temperature and/or high pressure polymerisation) [ ]. Consequently, resin-composites exhibit improvements in tensile and compressive strength, hardness, elastic modulus and wear resistance [ , ].
Aesthetic CAD/CAM processed restorative materials can be either ceramics (glass ceramics, polycrystalline alumina and zirconia) or resin-based composites [ , , ]. Compared to ceramics, CAD/CAM composite materials exhibit less hardness and stiffness, therefore causing less enamel wear clinically [ ], and they are easily fabricated and repaired. Further, they are less brittle than ceramics [ ] and consequently catastrophic failure in clinical conditions and chipping during manufacturing are less likely [ ]. Based on their structure, composite blocks can be either polymer-infiltrated ceramic network (PICN), which is a porous ceramic network infiltrated with a polymer network, or a resin-composite block (RCB) that is similar to conventional resin-composite but manufactured under high pressure and high temperature [ , , ]. In addition, PEEK material designed for CAD/CAM systems is gaining more attention in prosthodontics applications [ , ].
Resin-composites might release low molecular weight monomers such as HEMA and TEGDMA, high molecular weight monomers such as BisGMA and UDMA, free radicals and photoinitiator molecules [ ]. Monomer elution can compromise the material biocompatibility, as well as mechanical properties [ ]. Many factors influence monomer elution from resin-composite, including the degree of conversion, the solvent type, the chemical structure of the eluted molecules [ ], the filler composition and the microstructure [ ]. CAD/CAM composite blocks have a higher degree of conversion and less residual monomer than conventional resin-composites, due to high temperature and high pressure polymerisation [ , ]. UDMA, is the main monomer used in CAD/CAM composite blocks, and less monomer release has been reported in UDMA-containing resin-composite [ , ]. Moreover, CAD/CAM composite blocks are more resistant to breakdown and components leaching out due to their increased hardness compared to conventional composites [ , ].
CAD/CAM composite mechanical properties, such as flexural strength, modulus of resilience and hardness, are essential in terms of predicting material clinical success and performance [ , ]. Some studies have investigated the hardness of CAD/CAM composite materials [ , ] at dry condition, and others with long-term water storage [ ] or short-term storage in food simulating agents [ ] and some drinks like coffee [ ] and acidic drinks [ ]. There is a paucity of research regarding the monomer elution of the newly introduced CAD/CAM composite materials, particularly in simulated oral conditions (ageing). One study evaluated the monomer elution of some CAD/CAM composite blocks for up to 2 months in ethanol and distilled water [ ]. However, no study has evaluated the effect of different storage media, such as 75% ethanol, water and artificial saliva, on microhardness and monomer elution over relatively long-term storage times (1 and 3 months). Therefore, this study aimed to assess the long-term effect of different storage media on the microhardness (CAD/CAM composite blocks) and monomer elution (CAD/CAM composite blocks and conventional resin-composites) of the investigated materials. The null hypotheses were 1) there is no significant difference in the hardness reduction percentage between the investigated materials provoked by various storage media and storage times 2) there will be no difference in the monomer elution quantity between the CAD/CAM composite blocks and conventional resin-composites provoked by various storage media and storage times.
For simplicity, the term CAD/CAM composite block is used to describe PICN and resin-composite blocks (resin-composite designed for CAD/CAM systems); otherwise, each will be referred to as PICN or RCB.
Materials and methods
Eight CAD/CAM blocks were investigated for hardness reduction: Five resin-composite blocks (RCB) (Grandio blocs (GR), Lava™ Ultimate (LU), Cerasmart (CS), BRILLIANT Crios (BC), Block HC (HC)), one polymer-infiltrated ceramic network (PICN) block (Enamic (EN)), one ceramic-filled poly ether ether ketone (PEEK) block (Dentokeep (DK)), and one feldspathic ceramic block (Vitabloc Mark II (VM)), as shown in Table 1 .
|Composition by weight||Manufacturer||Filler
|Feldspathic ceramic block||Vitabloc Mark II (VM)||Fine-particle feldspar ceramic||0||Vita Zahnfabrik, Germany||0.00(0)|
|Polymer-infiltrated ceramic network (PICN)||VitaEnamic
|86% fine structure feldspar ceramic||14%
UDMA + TEGDMA
|Vita Zahnfabrik, Germany||85.1(0.1)|
|Resin-composite blocks (RCB)||Grandio Blocs
|86% nanohybrid fillers||14%
UDMA + DMA
|VOCO GmbH Germany||84.6(0.01)|
|80% silica and zirconia nano particles||20%
Bis-GMA, UDMA, Bis-EMA, TEGDMA
|3 M™ESPE™ USA||74.8(0.1)|
|71% silica and barium glass nanoparticles||Bis-MEPP, UDMA, DMA||GC dental products, Europe||66.1(0.2)|
|70% glass and amorphous silica||Cross-linked methacrylates (Bis-GMA, Bis-EMA, TEGDMA)||COLTENE, Switzerland||70.1(0.05)|
|61% silica powder, microfumed silica, and zirconium silicate||UDMA + TEGDMA||Shofu
|80% PEEK||NT-Trading, Germany||27.5(0.06)|
|73% zirconium silicate micro fine ceramic particles||UDMA, UDA||Shofu Japan||–|
|nanohybrid Silica-based||UDMA and other DMA||GC dental products, Europe||–|
|89% glass ceramic and silica-nanoparticles||Bis-GMA, Bis-EMA, TEGDMA||VOCO GmbH Germany||–|
|79–81% nanohybrid fillers||20−21% UDMA + TEGDMA
Each CAD/CAM block was sectioned by a diamond blade (MK 303, MK Diamond, CA, USA) mounted on a saw (Isomet 1000 Precision Saw, Buehler Co, IL, USA) under constant water irrigation into rectangular bars of 16 × 4 × 2 mm dimensions. Each specimen was wet ground and polished using a lapping machine (MetaServ 250, Buehler Co, IL, USA) with a series of silicon carbide papers of P320, P500, P1200, P2400, and P4000 grit (Buehler Co, IL, USA) under water cooling and then polished with 0.25 μm diamond suspension (Meta Di Supreme, Buehler Co, IL, USA). All specimens were then cleaned in an ultrasonic bath (Ultrasonic Cleaning System, L&R Co, NJ, USA) with distilled water for 5 min. Forty-eight specimens were prepared, comprising 6 of each of the 8 materials. The microhardness of each material (n = 6, 5 readings per specimen) was measured under dry conditions at day 0, and then 2 samples (n = 2) of each material were immersed in 10 ml of either water, artificial saliva or 75% E/W, and stored at 37 °C in individual glass vials. Microhardness was measured after 30 and 90 days. The Macknight-Hane and Whitford formula was used to prepare artificial saliva (AS) [ ]. All the materials used and their concentrations are shown in Table 2 .
|Compound||Compound amount g/l (distilled water)||Manufacturer||CAS|
|Sodium carboxymethyl cellulose (CMC)||10.00||Sigma-Aldrich||9004−32-4|
|Potassium chloride (KCl)||0.625||BHD Chemicals Ltd., Poole, England||7447−40-7|
|Calcium chloride (CaCl 2 )||0.166||Sigma-Aldrich||10043−52-4|
|Potassium dihydrogen phosphate (KH 2 PO 4 )||0.326||May & Baker LTD, Dagenham, England||7778−77-0|
|Magnesium chloride (MgCl 2 )||0.059||Sigma-Aldrich||7786−30-3|
|Potassium hydroxide (KOH)||To adjust pH 6.75||May & Baker Ltd., Dagenham, England||1310−58-3|
Microhardness was measured using a Vickers indenter tester (FM-700, Future Tech Corp., Japan) under a load of 300 g, and a dwell time of 20 s.
The hardness reduction (as a percentage) after 90 days storage was calculated as follows:
Where VHN( d0 ) and VHN( d90 ) are the Vickers hardness numbers at day 0 and day 90, respectively.
The ceramic block (Vitabloc Mark II (VM) was excluded for the monomer elution experiment; seven CAD/CAM blocks and four conventional composites (2 indirect and 2 direct) were investigated in the monomer elution experiment ( Table 1 ). All solvents were HPLC grade. Water, methanol, ethanol, acetonitrile, caffeine, and ethoxylated Bis-phenol A dimethacrylate (Bis-EMA) were from Sigma-Aldrich (UK). Bis-phenol A glycidyl methacrylate (Bis-GMA), triethylene glycol dimethacrylate (TEGDMA), and urethane dimethacrylate (UDMA) were supplied by Röhm GmbH (Germany).
Each CAD/CAM block was sectioned into 10 × 10 × 3 mm dimensions, as in section 2.1 . Conventional resin-composite materials were prepared of 10 × 10 × 3 mm dimensions using a polytetrafluoroethylene (PTFE) mould, and were cured according to their manufacturer recommendations using a LED light-curing unit with an output irradiance of 1200 mW/cm 2 (Elipar™, 3 M ESPE, USA). All specimens were wet ground, polished and cleaned as in section 2.1 . Each specimen had a surface area of 320 mm 2 and a volume of 300 mm 3 . One hundred and sixty-five specimens were prepared, comprising 15 of each material, and divided into three groups (n = 5) immersed in 3 ml of either water, artificial saliva or 75% E/W solution and stored at 37 °C for 1 and 3 months.
Caffeine (CF) was used as an internal standard, and 0.1 mg/mL was added to all storage media. Artificial saliva was prepared using the Macknight-Hane and Whitford formula [ ]. The monomers of interest (Bis-GMA, UDMA, TEGDMA, Bis-EMA) were dissolved in methanol with CF as an internal standard at different concentrations and were assessed by HPLC to identify each eluted monomer retention time and to obtain a calibration curve to quantify each monomer. In addition, control samples of only storage media were assessed by HPLC. The specimens were stored in the dark at 37 °C, and after 1 month, all storage solutions were collected for analysis and replaced with fresh solutions. After 3 months, the storage solutions were collected again for analysis and the total eluted monomer quantity in 3 months was calculated by adding the values obtained at the 2 time points.
Analysis of eluted monomers
The collected solutions at each time point were placed in HPLC vials and assessed by HPLC (Agilent 1100 series, Agilent Technology, Germany) to identify and quantify the eluted monomers using the calibration curves and retention times of the monomers of interest. Chromatography was performed using a reversed-phase column with isocratic separation. HPLC samples of 1 μL were injected into Phenomenex SphereClone 5 μm ODS (2) column of dimensions 4.6 × 250 mm (Phenomenex, USA). Chromatographic separation was achieved using a mixture of acetonitrile and water (65%:35%) at a flow rate of 0.5 ml/min. The column temperature was set at 22 °C with a run time of 70 min for each sample, and the UV detector was set at 205 and 210 nm. Fig. 1 illustrates an HPLC chromatogram of the detected monomers (TEGDMA, UDMA, Bis-GMA) and their retention times.
The calibration curve was obtained by plotting the HPLC peak area of monomer to peak area of CF against the concentration of monomer to the concentration of CF, of the calibration solutions as follows:
Then, a linear regression analysis of the plotted ratio was carried out. The linearity, slope (b), intercept (a), and calibration range are shown in Table 3 . The following equation was used to calculate the eluted monomer concentration (μg/mL):
( R 2 )
|Slope (b)||Intercept (a)||Calibration range (μg/mL)||Calibration range (nmol/mm 2 )|