NareshKumar*, Adrian Shortall**
To investigate the influence of mixing methods on bi-axial flexure strength (BFS) of experimental resin-based composites (RBCs).
BFS of two experimental RBCs either hand-spatulated or mechanically-mixed and one commercial RBC (30 specimens per group) was determined following 1 week immersion in distilled water at 37±1 ºC. Scanning electronmicroscopy (SEM) of fractured surfaces of eachRBC specimen was performed.Results were evaluated by one-wayANOVA, post-hocTukeymultiple comparison tests (P=0.05) and Weibull statistics.
Asignificant difference was identified between theme an BFS of them echanically-mixed (95±13MPa) and hand-spatulated (83±15 MPa) experimental RBCs (P<0.05). The commercial RBC exhibited a significantly higher mean BFS (135±20 MPa) compared with both hand-spatulated and mechanically-mixed RBCS (P<0.05). The Weibull modulus of BFS data of hand-spatulated RBC was considered to be significantly decreased compared with theWeibull modulus of both mechanically-mixed and commercial RBCs as the 95% confidence intervals did not overlap. Greater porosity was observed in the hand-spatulated RBC in contrast to the mechanically-mixed and commercial RBCs under SEM examination.
Mixing method has a significant effect on the BFS of RBCs and mechanical-mixing of experimentalRBCsmay be considered as amore reliablemethod.
Resin-based composite;Mixing;Porosity,Bi-axial flexure strength
Resin-based composites (RBCs) have been developed and marketed over the last 4 decades and are being increasingly used in restorative dentistry as an alternative to dental amalgam as a result of improvements in materials and clinical techniques. Comparable or even improved survival of large composite restorations relative to amalgam counterparts has been reported in a 12 years retrospective study. Research drives composite development and informs best clinical practice. Research into experimental and commerciallyavailableRBCs is being continuously undertaken in terms of refining material constituents and as a result many material properties have been refined.
Most research investigations employ commercial RBCs for comparative evaluation. Interpretation of significant findings fromsuch studies is handicapped both bymultiple differences in the formulations of commercial RBCs, and the fact that manufacturers are understandably reluctant to reveal precise details of formulation differences in proprietary products. It is difficult to identify themost specific component causing variations in properties. The use of experimental RBCswith controlled formulations is important as it allows systematic investigation of variables controlling RBC behaviour thus allowing hypothesis testing of fundamental concepts. Mixing methods for various composites include hand-mixing, vacuum-mixing and centrifugation. During hand-spatulation, the chances of introducing porosities into the material are high and the person mixing the material may be at an increased risk of exposure to a high level of volatile constituents. Vacuum-mixing and centrifugation methods are used to reduce the porosities. Since 1970, dual asymmetric centrifugation (DAC) has been considered to be a suitable technology for the speedy mixing of viscous composites. In DAC, a supplementary counter-clockwise rotation of mixing cup takes place around its own vertical axis in addition to clockwise rotation of normal centrifugation. During mixing of a material in theDACmachine, thematerial is continuously pushed into the corner between bottom and cup wall by conventional centrifugation, whereas the additional rotation forces move the material towards the centre of mixing cup. Consequently, two opposite rotating movements lead to shear forces and result in a homogenousmix
The mechanical and physical properties of materials under investigation are likely to be affected due to resultant porosities during hand-spatulation which may consequently decrease the reliability of data. Thus, the purpose of this investigationwas to highlight the influence of hand-spatulated and mechanically-mixed model resin composite formulations on bi-axial flexure strength.
Model resins were prepared with BisGMA/ TEGDMA at 60/40 mass ratio containing the photoinit iator camphoroquinone co-init iator dimethylaminoethyl methacrylate and inhibitor butylated hydroxytoluene in the concentrations of 0.2%, 0.3%, and 0.1% by mass respectively. Resins were filled to 75 mass% filler particles by either hand-spatulation or mechanical-mixing.Hand-spatulationwas carried out in a glass beaker using a glass rod (7 mm diameter). A centrifugal mixing device (Speed-Mixer, DAC 150 FVZK, Hauschild Engineering, Germany) was used to mechanically incorporate the filler. A commercially available microhybrid RBC (Z100 MP RestorativeTM batch 8YR; shade A3) (3M ESPE Dental Products, St. Paul,MN,USA)was evaluated as a control group (table 1)
Nominally identical disc-shaped specimens (12 mm diameter, 1 mm thickness) of each RBC (n=30) were produced and bi-axial flexure strength (BFS) of all RBC specimens was determined as reported in the previous study by Curtis et al. (2008). In the current study each specimenwas cured for 40 s in contrast to 20 s curing time employed in the previous study and specimens were stored wet in distilled water at 37±1 ºC for 1 week prior to testing. Specimens were selected by the non-probability purposive sampling technique. Each specimen was inspected for obvious surface defects and discarded ifnecessary. The selection of the number of specimens for each RBC group was justified, as a sample size of at least thirty specimens for Weibull statistics with due consideration of material and testing cost has been suggested in the literature.
Scanning electron microscopy (SEM) of fractured surface of three disc specimens for each RBC was conducted using a Jeol JSM-840A (Jeol Ltd, Akishima, Tokyo, Japan)with an acceleration voltage of 5 kV. A one-way ANOVA and post-hoc Tukey multiple comparison tests were performed on the BFS data (P=0.05) to highlight any difference between handspatulated, mechanically-mixed and commercial RBCs. Subsequently, BFS data were submitted to Weibull statistics in order to assess the reliability of BFS between material groups and r2-values were obtained using regression analysis of theWeibull data.
Astatistically significant difference between BFS of mechanically-mixed (95±13 MPa) and hand-spatulated (83±15 MPa) RBCs was identified (P<0.05). The commercial RBC exhibited a significantly higher BFS (135±20 MPa) compared with both hand-spatulated and mechanically-mixedRBCs (P<0.05) (Table 2)
TheWeibullmodulus of BFS data of hand-spatulated RBC was considered to be significantly decreased compared with the Weibull modulus of mechanicallymixed and commercial RBCs as the 95% confidence intervals did not overlap. The differences between Weibull modulus ofmechanically-mixed and commercial RBCs were considered non-significant since the 95% confidence intervals overlapped. The r2-values of 0.91, 0.96 and 0.93 were identified for the BFS of handspatualed, mechanically-mixed and commercial RBCs respectively (Table 2). SEM highlighted consistently larger and more numerous microscopic defects in handspatulated RBC compared with mechanically-mixed and commercialRBCs (Figure-1).
The significant decrease in BFS, associated Weibull modulus and r2 values of hand-spatulated RBC compared with mechanically-mixed and commercial RBCs(Table 2) suggests a wider flaw distribution in the former, which was subsequently confirmed by SEMexamination (Figure-2).
flaw or defect may act as a weak inclusion and lead to stress concentrations thus speeding up failure and declining BFS of material. Moreover, it has been suggested in the previous study that nanoparticles are likely to agglomerate and may lead to a decrease in the strength of RBCs. Thus, itmay be assumed in the current study that during hand-spatulation, nanoparticles may have agglomerated greatly compared with mechanicallymixedRBCs.( Figure-3)
The agglomerated particles possess high internal porosity compared with a discrete solid filler and are likely to create regions of stress concentration. Consequently, such regions require less energy to initiate or propagate a crack and lead to failure at low stresses. In addition, agglomerated particles may create a weak resin/filler interface and lead to insufficient load transfer between matrix and particles , thereby resulting in decreased BFS. Furthermore, the substantial reduction in BFS and reliability of the hand-spatulated RBC suggests the possibility of a deleterious effect of porosity on other mechanical and physical characteristics of RBCs. De Gee (1979) reported that vacuummixing of a composite resin led to a90% reduction in porosity and an associated 11.5% increase in diametral tensile strength of the material. McCabe and Ogden (1987) found that 20 seconds spatulation in air of the single paste light-cured composite resin Prisma-Fil led to a mean porosity increase from 0.23% for minimally handled material to 1.53% for hand spatulated material. Diametral tensile strength was reduced by 21% and compressive fatigue strength by 14.6% following air introduction by spatulation. In addition to adversely affecting mechanical properties,increased porosity levels in experimental samples will impact on physical and optical properties. The majority of investigators prepare model RBCs using handspatulation and report significant findings in their research.However, those conclusionsmay not necessarily be valid due to non-homogenous mixing of RBCs. Thus researchwork based on the hand-spatulation ofRBCsmay affect the development of improved materials. Finally, another drawback of hand-spatulation is that mixing speed, pressure and time are not controlled which may also cause variations in resultantRBCbatches.
No significant difference between the Weibull modulus of the mechanically-mixed and the commercial RBCs (Table 2) signifies a narrow distribution of defects and an increased reliability of strength data of both materials. However, the significant difference in themean BFS of these two materials may be attributed to compositional variations. It appears that model RBC formulation based on mechanical-mixing is more homogenous and reproducible compared with handspatulation. As a result, associated research would yield more consistent data patterns which should assist in the understanding and development of improved resin composite materials. Therefore, in order to accurately examine the data of experimental RBCs among researchers and different test centres, the standardisation and reproducibility ofmixingmethod should be optimised to obtain consistently reliable results.
This study indicates that mixing method has a considerable effect on BFS of RBCs. The mechanicallymixed RBCs showed reliability in BFS data, thus, the use of speed mixer may be considered in RBCs research in order to achieve more accuracy in results and consequently meaningful comparisons of data among investigators.
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