- Email: [email protected]
Edge strength of resin-composite margins
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Edge strength of resin-composite margins D.C. Watts ∗ , M. Issa, A. Ibrahim, J. Wakiaga, K. Al-Samadani, M. Al-Azraqi, N. Silikas Biomaterials Research Group, School of Dentistry and Photon Science Institute, The University of Manchester, Higher Cambridge Street, Manchester M15 6FH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Marginal integrity is a major clinical problem in restorative dentistry. The aim
Received 25 April 2007
of this study was to evaluate the applicability of an edge strength measurement device in
Accepted 30 April 2007
an in vitro test to determine the force required to fracture flakes of material by a Vickers indentation at progressively increasing distances from an interface edge of bulk material. Methods. Five representative resin-composites were investigated. Fourteen disks of speci-
Keywords:
mens (12 mm diameter × 2.5 mm thick) were prepared for each material. These were divided
Edge strength
into seven sub-groups corresponding to different edge-distances (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and
Marginal fracture
1.0 mm). An edge strength measurement device (CK10) (Engineering Systems, Nottingham,
Shrinkage stress
UK) was used. The mode of the failure of each specimen was examined under the integral
Resin-composites
microscope of the CK10. Results. The force (N)-to-fracture at a distance of 0.5 mm from the edge was defined as the edge strength. The highest failure force (edge strength) was observed for Tetric Ceram (174.2 N) and the lowest for Filtek Supreme (enamel) (87.0 N). Correlations between the failure-forces to fracture materials with edge-distance were regression analyzed giving coefficients (r) ranging from 0.94 (p = 0.02) to 0.99 (p = 0.01). Two modes of failure were observed: chipping and – generally at greater distances – cracking. Significance. Edge strength is a definable and potentially useful parameter to characterize this aspect of clinically related behavior. A standardized distance of 0.5 mm from the specimen’s edge, when chipping failure prevails, is suitable and convenient as a reference point. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Resin-composites have been used in dentistry for approximately 40 years. Since their introduction they have been improved dramatically. As a result they are now used routinely for posterior restorations as well as the anterior ones. Since the new millennium, the majority of dental schools have included classes I and II composite restorations in the operative dentistry curriculum [1]. Despite their success, resin-composites still face some clinical problems and their longevity can be compromised, especially in the adverse conditions of the oral environment.
∗
Marginal integrity is one of these challenging problems. Photo-polymerization may be responsible for generating internal stresses through shrinkage that can be critical in marginal regions, leading to “chipping” fractures. Although partial edge fracture of the restoration does not necessarily cause loss of retention of the restorative material, it often leads to other detrimental effects, such as marginal leakage that causes staining at the cavity walls. Thus, one of the biggest advantages of aesthetic restorative materials, namely their visual similarity to sound teeth, is greatly diminished. Apart from this aesthetic problem there is also the risk of secondary caries. This risk is particularly serious, as the gaps formed by
Corresponding author. Tel: +44 161 2756749. E-mail addresses: [email protected] (D.C. Watts), [email protected] (N. Silikas). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.04.006
130
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
edge fracture cannot be reached by tooth brushing or similar preventive care procedures. Therefore, the objective of achieving improved marginal integrity has become very important in the designs of new resin-composite materials. This goal is particularly difficult to achieve as marginal integrity should be maintained over the whole life span of the restoration, resisting temperature changes and frequent mechanical loading that can promote marginal fracture. Failure of resin-composite restorations can occur when they are subjected to heavy masticatory loads. One of the common failure modes of the restoration is fracture. Although the restorations should be properly designed to avoid fracture, they may nevertheless fail in this way catastrophically. Numerous clinical studies have reported marginal degradation for certain composites, especially near occlusal contacts [2–7] causing economic loss and patient discomfort. Thus, mechanical properties are important and should be optimized for the successful longevity of resin-composite restorations. To overcome these clinical problems appropriate laboratory methods are required that can predict key aspects of the clinical performance of resin-composites [1]. The structural integrity of a restoration under the influence of occlusal forces can be predicted by appropriate measurements of its material strength, especially the specific tendency for marginal fracture. The ability of restorative materials to withstand fracture of a thin edge can be described as “edge strength”. A dedicated measurement device (Engineering Systems, Nottingham) can be applied to determine edge strength of dental materials. This determines the force required to fracture flakes of material by a Vickers indentation at progressively increasing distances from an interface edge of bulk material. The aims of this study were (i) to determine the force needed for edge fracture of resin-composites (ii) to investigate the relationship between the force needed to fracture and the increasing distance from the edge. The hypothesis tested was that the force to fracture would increase progressively with distance from the edge.
2.
Materials and methods
Five different light-cured resin-composites restorative materials were used. Materials and manufacturers’ details are listed in Table 1. Fourteen disks of specimens (12 mm diameter × 2.5 mm thick) were prepared for each material. The specimens were fabricated at room temperature 23 ± 1 ◦ C, according to manufacturers’ instructions. They were placed with a stainless
steel hand instrument in two increments into a Teflon mould (12 mm diameter) opening from both sides. A separating medium (a thin film of petroleum jelly) was used to facilitate removal of the specimens and to prevent premature fracture of the specimen edge. The upper and lower specimen surfaces were formed against transparent matrix strips placed under and over the mould using glass microscope slides (22 mm × 22 mm, BDH Borosilicate glass). Any excess material was expelled by applying pressure on the mould, between the two glass slides. The layering technique for incremental filling of the mould was employed. The specimens were light-cured for 40 s with an Optilux 501 curing unit at an irradiance of 500 mW/cm2 (Optilux, Demetron, USA), at each increment. The specimens were optimally cured and tested after 7 days water-storage at 37 ◦ C. After curing, specimens were abraded with 800 grit silicon carbide paper to remove excess flash and the specimens were then removed from the mould using light finger pressure and then examined at ×10 for air voids. Specimens with large or an excessive number of small air voids were excluded and replaced. After preparation, all specimens for each material were stored in distilled water at 37 ◦ C for a week using glass containers before loading. Specimen disks were divided into seven sub-groups, each corresponding to a different edge-distance. Each sub-group was comprised of two specimens and three measurements were made on each specimen. A fresh specimen was then used for each successive distance, to avoid test errors as a result of crack propagation, since each disk specimen was relatively small. An average value for the force measured was calculated for each distance. Sub-groups 1–7 were for the following ‘edge’ distances: 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm. An edge strength measurement device (CK10) (Engineering Systems, Nottingham, UK) was used. The CK10 is a bench top, vertical loading compression testing machine. It has been designed to facilitate the study of engineering materials and can be used for edge toughness testing (edge flaking) of brittle materials [8]. The incorporated acoustic sensor enables the detection of cracking and has been used in the field of ceramics to study their cracks [9]. The device is made up of three main segments: the microscope, the specimen holder and the control panel (Fig. 1). It has a load range capacity of 0–10,000 N, a built-in acoustic sensing facility, with a data output display on the front panel. It determines the failure-force that is required to fracture flakes of material by a Vickers diamond indenter (polycrystalline diamond Rockwell C indenter 120◦ ), at any selected distance from an interface edge of bulk material. The crosshead speed was set at 1 mm/min.
Table 1 – Type and composition of the materials tested Materials Z 250 QuiXfil Tetric Ceram Venus Filtek Supreme (enamel)
Type
Shade
Batch no.
Manufacturer
Hybrid composite Posterior hybrid composite Micro-hybrid composite Micro-hybrid composite Nano-composite
A2 Universal A2 A2 Translucent
20030329 0306000653 E46159 020028 3910A1E
3 M ESPE USA Dentsply DeTrey GmbH Germany Ivoclar Vivadent, Schaan, Liechtenstein Heraeus kulzer, Wehrheim, Germany 3 M ESPE USA
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
Fig. 1 – Schematic diagram of the CK10 testing machine used to measure the force (N)-to-fracture at different interval distances from the edge.
2.1.
Fig. 2 – Bar chart graph of the mean force (N) to fracture, at a distance of 0.5 mm from the edge: defined as edge strength, for all materials investigated. (1) Tetric Ceram; (2) Z250; (3) Venus; (4) QuiXfil; (5) Filtek Supreme (enamel).
Observations
The mode of the failure of each specimen was examined under the integral microscope of the CK10. Failures were classified into two types: a. Specimen chipped-off: an edge fragment chipped off. b. Specimen cracking: the specimen was not completely chipped, with a missing fragment but was microscopically cracked. One-way analysis of variance ANOVA, followed by the multiple comparison Tukey test at the significance level of 0.05 was used to evaluate the variable of distance from the specimen edge for all materials. These tests were also used to compare the ‘edge strength’ at 0.5 and 1.0 mm. Regression analysis was applied.
3.
131
illustrated graphically in Fig. 2. The highest failure force (edge strength) was observed for Tetric Ceram (174.2 N) and the lowest for Filtek Supreme (enamel) (87.0 N). Statistical analysis showed that there were statistical significant differences in the forces needed to fracture at each distance (p < 0.000) between materials and (p < 0.002) overall between the distance of 0.5 and 1.0 mm. The correlations between the failure-forces to fracture materials with edge-distance were regression analyzed giving coefficients (r) ranging from 0.94 (p = 0.02) to 0.99 (p = 0.01). Fig. 3 shows a typical regression plot of force (N)-to-fracture versus edge-distance for Filtek Supreme (enamel). The fracture force at any edge-distance can thus be expressed using the linear formula: F =S×d+Y where F is the force (N) to produce specimen failure at any given distance d (mm) from the edge. S is the slope of the regression line and Y is the intercept.
Results
The mean force (N) and standard deviation, needed to fracture the specimens of each material, at different distances from the specimen edge (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm) were obtained. The force (N)-to-fracture at a distance of 0.5 mm from the edge was defined as the edge strength. The values of edge strength for all materials are presented in Table 2 and
Table 2 – Edge strength date and predominant failure modes Materials Z 250 QuiXfil Tetric Ceram Venus Filtek Supreme (enamel)
Edge strength (N) 167.8 101.8 174.2 117.2 87.0
S.D. (7.9) (30.1) (22.8) (8.7) (27.1)
Predominant failure mode Chipped Chipped Chipped Cracked Cracked
Fig. 3 – Regression of load (N)-to-fracture values at different distance from the edge for Filtek Supreme (enamel) resin-composite material.
132
3.1.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
Observation on the indentation mode failure
Two modes of failure were observed: cracking and chipping. The indentation failure modes are shown in Table 2. Tetric Ceram, QuixFil and Z250 predominantly chipped off except for one or two distances that were cracked. Venus and Filtek Supreme (enamel) specimens, however, cracked at all edge-distances.
posites which is comparable in magnitude to the transverse strength of amalgam [13]. Fukushima et al. have shown that the extent of marginal degradation on posterior composites is less than that occurring on dental amalgam [14]. The incisal edge strength of porcelain laminate veneers, for restoring mandibular incisors has been determined indirectly by means of fracture strength [15].
4.1. Edge strength resistance at 0.5 mm distance away from the edge
4.
Discussion
The specialized instrument used for measurement determined the critical force for failure that occurred when the indenter was loaded at a given distance from the specimen edge. This force was found to be approximately proportional to the distance of the indenter from the edge. Thus, the hypothesis was confirmed. This observation seems to be associated with the cross-sectional area affected by the loading. This means that when the area increased the fracture force increased. This is analogous to the clinical observations of Krejci et al. on wear of posterior restorative materials [10]. At occlusal contact, wear was inversely related to the size of contact area of the opposing enamel cusps. The need for relevant in vitro laboratory measurements continues to be expressed [1]. Clinical studies can be costly and time consuming. Thus, it would be advantageous to be able to predict relative tendencies for clinically-related modes of edge-failure or marginal breakdown for sets of resincomposite formulations through in vitro measurements [4]. Hence the CK10 instrument was used, in this study, to commence the accumulation of a body of data on these materials. Restorations of posterior teeth with resin-based materials (resin-composites) frequently show relatively early evidence of marginal deterioration in several modes including chipping [11]. This presents dentists with difficult decisions as to whether to watch and wait, recontour, repair or replace [3]. Hence, a measurement of edge strength is desirable to characterize resin-based materials designed for stress-bearing situations and offers an insight into the forces and force directions causing marginal chips [11]. Microfill resin-composites have shown more marginal breakdown than hybrid composites. Marginal breakdown also shows an inverse correlation with fracture toughness for resin-composites [4]. Microfill composites and certain small particle hybrids are more susceptible to marginal degradation than posterior composites with coarser filler particles [4]. Of the materials currently measured, a nano-filled composite exhibited the lowest edge strength. Previously there were approaches to measure the edge strength indirectly. The edge and bulk strengths of powdered gold have been compared with other filling materials to determine the bending strength [12]. The study concluded that bending strength was the best indirect representative of the edge strength of metallic restorative materials. The modulus of elasticity has also been determined as an indirect measure of rigidity and transverse strength and thus as an index of edge strength for composites and amalgam. Bryant and Mahler pointed out that the transverse strength of microfilled composite is approximately half that of macrofilled com-
Fractures at the margins have been cited as a major problem regarding the failure of posterior composites [16]. The marginal fracture or the edge strength resistance may be related to some other physical property of the composites, specifically fracture toughness [4]. Chipping of a material is often easier (i.e. lower force required) close to an edge, in that forces required to chip a material will tend to be greater at increasing distances from the edge. Hence, edge strength is a potentially definable and useful parameter to characterize this aspect of clinical behaviour. Since edge strength could be measured in varying distances from the edge an arbitrary distance had to be chosen as a comparable reference point. This was selected as being 0.5 mm away from the edge of the specimen. The data measured at 0.5 mm distance may be more clinically relevant to marginal breakdown of restorations than for greater distances. Fractures that occurs within restorative resin materials have been seen mainly either through the bulk of material or through material near the interface [17]. The surface chipping, bulk fracture and incisal wear were correlated with various mechanical properties of the materials [18]. Tyas reported that a significant correlation was found between surface chipping/bulk fracture and fracture toughness (p = 0.002), elastic modulus (p = 0.006) and tensile strength (p = 0.045). Furthermore, the microfine composites were more susceptible to chip and bulk fracture than small particle or hybrid composites when used for anterior high stress situations such as class IV cavities. The results of some clinical studies led to the conclusion that certain small particle and microfilled composites showed greater tendencies for marginal fracture and localized wear in contact areas than the large heavily filled particle composites [2,3,6,7,19].
4.2.
Observation on the indentation mode failure
Apart from cracks, chipping of some of the tested specimens have been observed in this study. The process starts with cracks within the materials and more force is capable of chipping-off the material between the crack and the margin or edge of the specimen. In the present study the observations showed that, some of the materials chipped off or cracked, which could be attributable to the volume, shape and the distribution of its inorganic filler. The specimens of the Venus were cracked at all the distances away from the edge. On the other hand, Tetric Ceram, QuiXfil and Z250 had two types of mode of failure, either crack and/or chipped-off which could be attributed to the difference in composition and other mechanical or physical properties between those materials.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
5.
Conclusions
• At each distance from the edge, there was a critical indentation force-to-failure. • The force-to-fracture increased linearly, for all materials, as the distance from the edge increased. • The force-to-fracture at 0.5 mm distance from the edge was defined as the edge strength value. • The modes of failure varied between materials, but chipping was the predominant mode.
references
[1] Sarrett D. Clinical challenges and the relevance of materials testing for posterior composite restorations. Dent Mater 2005;21:9–20. [2] Bryant R, Hodge K. A clinical evaluation of posterior composite resin restoration. Aust Dent J 1994;39: 77–81. [3] Bryant RW, Marzbani N, Hodge K. Occlusal margin defects around different types of composite resin restorations in posterior teeth. Oper Dent 1992;17:215–21. [4] Ferracane J, Condon J. In vitro evaluation of the marginal degradation of dental composites under simulated occlusal loading. Dent Mater 1999;15:262–7. [5] Mazer R, Leinfelder K, Russell C. Degradation of microfilled posterior composite. Dent Mater 1992;8:185–9. [6] Stangel I, Barolet R. Clinical evaluation of two posterior composite resins: two-year results. J Oral Rehab 1990;17:257–68.
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[7] Tyas MJ, Truong VT, Goldman M, Beech DR. Clinical evaluation of six composite resins in posterior teeth. Austr Dent J 1989;34:147–53. [8] http://www.engsys.co.uk. [9] Wu H, Roberts S, Derby B. Residual stress and subsurface damage in machined alumina and alumina/silicon carbide nanocomposite ceramics. Acta Materalia 2001;49:507–17. [10] Krejci I, Lutz F, Zedler C. Effect of contact area size on enamel and composite wear. J Dent Res 1992;71:1413–6. [11] Quinn JB, Vaderhobli R. Edge chip geometry based on force direction in dental materials. In: IADR. 2006. [12] Iwaku M, Nagata N, Hosoda H, Fusayama T. Edge strength of powdered gold fillings. J Dent Res 1966;45:1468–72. [13] Bryant R, Mahler D. Modulus of elasticity in bending of composites and amalgams. J Prosthet Dent 1986;56:243–8. [14] Fukushima J, Setcos JC, Phillips R. Marginal fracture of posterior composite resins. J Am Dent Assoc 1988;117:577–83. [15] Wall J, Reisbick M, Johnston W. Incisal-edge strength of porcelain laminate veneers restoring mandibular incisors. In J Prostho 1992;5:441–6. [16] Roulet J. The problems associated with substituting composite resins for amalgam: a status report on posterior composites. J Dent 1988;16:101–13. ¨ [17] Zidan O, Asmussen E, Jorgensen K. Microscopical analysis of fracture restorative resin/etched enamel bonds. J Dent Res 1982;90:286–91. [18] Tyas M. Correlation between fracture properties and clinical performance of composite resins in posterior teeth. Aust Dent J 1990;35:46–9. [19] Lambrechts P, Vanherle G, Vuylsteke M, Davidson CL. Quantitative evaluation of the wear resistance of posterior dental restorations: a new three dimensional measuring technique. J Dent 1984;12:252–67.
available at www.sciencedirect.com
journal homepage: www.intl.elsevierhealth.com/journals/dema
Edge strength of resin-composite margins D.C. Watts ∗ , M. Issa, A. Ibrahim, J. Wakiaga, K. Al-Samadani, M. Al-Azraqi, N. Silikas Biomaterials Research Group, School of Dentistry and Photon Science Institute, The University of Manchester, Higher Cambridge Street, Manchester M15 6FH, UK
a r t i c l e
i n f o
a b s t r a c t
Article history:
Objectives. Marginal integrity is a major clinical problem in restorative dentistry. The aim
Received 25 April 2007
of this study was to evaluate the applicability of an edge strength measurement device in
Accepted 30 April 2007
an in vitro test to determine the force required to fracture flakes of material by a Vickers indentation at progressively increasing distances from an interface edge of bulk material. Methods. Five representative resin-composites were investigated. Fourteen disks of speci-
Keywords:
mens (12 mm diameter × 2.5 mm thick) were prepared for each material. These were divided
Edge strength
into seven sub-groups corresponding to different edge-distances (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and
Marginal fracture
1.0 mm). An edge strength measurement device (CK10) (Engineering Systems, Nottingham,
Shrinkage stress
UK) was used. The mode of the failure of each specimen was examined under the integral
Resin-composites
microscope of the CK10. Results. The force (N)-to-fracture at a distance of 0.5 mm from the edge was defined as the edge strength. The highest failure force (edge strength) was observed for Tetric Ceram (174.2 N) and the lowest for Filtek Supreme (enamel) (87.0 N). Correlations between the failure-forces to fracture materials with edge-distance were regression analyzed giving coefficients (r) ranging from 0.94 (p = 0.02) to 0.99 (p = 0.01). Two modes of failure were observed: chipping and – generally at greater distances – cracking. Significance. Edge strength is a definable and potentially useful parameter to characterize this aspect of clinically related behavior. A standardized distance of 0.5 mm from the specimen’s edge, when chipping failure prevails, is suitable and convenient as a reference point. © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Resin-composites have been used in dentistry for approximately 40 years. Since their introduction they have been improved dramatically. As a result they are now used routinely for posterior restorations as well as the anterior ones. Since the new millennium, the majority of dental schools have included classes I and II composite restorations in the operative dentistry curriculum [1]. Despite their success, resin-composites still face some clinical problems and their longevity can be compromised, especially in the adverse conditions of the oral environment.
∗
Marginal integrity is one of these challenging problems. Photo-polymerization may be responsible for generating internal stresses through shrinkage that can be critical in marginal regions, leading to “chipping” fractures. Although partial edge fracture of the restoration does not necessarily cause loss of retention of the restorative material, it often leads to other detrimental effects, such as marginal leakage that causes staining at the cavity walls. Thus, one of the biggest advantages of aesthetic restorative materials, namely their visual similarity to sound teeth, is greatly diminished. Apart from this aesthetic problem there is also the risk of secondary caries. This risk is particularly serious, as the gaps formed by
Corresponding author. Tel: +44 161 2756749. E-mail addresses: [email protected] (D.C. Watts), [email protected] (N. Silikas). 0109-5641/$ – see front matter © 2007 Academy of Dental Materials. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.dental.2007.04.006
130
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
edge fracture cannot be reached by tooth brushing or similar preventive care procedures. Therefore, the objective of achieving improved marginal integrity has become very important in the designs of new resin-composite materials. This goal is particularly difficult to achieve as marginal integrity should be maintained over the whole life span of the restoration, resisting temperature changes and frequent mechanical loading that can promote marginal fracture. Failure of resin-composite restorations can occur when they are subjected to heavy masticatory loads. One of the common failure modes of the restoration is fracture. Although the restorations should be properly designed to avoid fracture, they may nevertheless fail in this way catastrophically. Numerous clinical studies have reported marginal degradation for certain composites, especially near occlusal contacts [2–7] causing economic loss and patient discomfort. Thus, mechanical properties are important and should be optimized for the successful longevity of resin-composite restorations. To overcome these clinical problems appropriate laboratory methods are required that can predict key aspects of the clinical performance of resin-composites [1]. The structural integrity of a restoration under the influence of occlusal forces can be predicted by appropriate measurements of its material strength, especially the specific tendency for marginal fracture. The ability of restorative materials to withstand fracture of a thin edge can be described as “edge strength”. A dedicated measurement device (Engineering Systems, Nottingham) can be applied to determine edge strength of dental materials. This determines the force required to fracture flakes of material by a Vickers indentation at progressively increasing distances from an interface edge of bulk material. The aims of this study were (i) to determine the force needed for edge fracture of resin-composites (ii) to investigate the relationship between the force needed to fracture and the increasing distance from the edge. The hypothesis tested was that the force to fracture would increase progressively with distance from the edge.
2.
Materials and methods
Five different light-cured resin-composites restorative materials were used. Materials and manufacturers’ details are listed in Table 1. Fourteen disks of specimens (12 mm diameter × 2.5 mm thick) were prepared for each material. The specimens were fabricated at room temperature 23 ± 1 ◦ C, according to manufacturers’ instructions. They were placed with a stainless
steel hand instrument in two increments into a Teflon mould (12 mm diameter) opening from both sides. A separating medium (a thin film of petroleum jelly) was used to facilitate removal of the specimens and to prevent premature fracture of the specimen edge. The upper and lower specimen surfaces were formed against transparent matrix strips placed under and over the mould using glass microscope slides (22 mm × 22 mm, BDH Borosilicate glass). Any excess material was expelled by applying pressure on the mould, between the two glass slides. The layering technique for incremental filling of the mould was employed. The specimens were light-cured for 40 s with an Optilux 501 curing unit at an irradiance of 500 mW/cm2 (Optilux, Demetron, USA), at each increment. The specimens were optimally cured and tested after 7 days water-storage at 37 ◦ C. After curing, specimens were abraded with 800 grit silicon carbide paper to remove excess flash and the specimens were then removed from the mould using light finger pressure and then examined at ×10 for air voids. Specimens with large or an excessive number of small air voids were excluded and replaced. After preparation, all specimens for each material were stored in distilled water at 37 ◦ C for a week using glass containers before loading. Specimen disks were divided into seven sub-groups, each corresponding to a different edge-distance. Each sub-group was comprised of two specimens and three measurements were made on each specimen. A fresh specimen was then used for each successive distance, to avoid test errors as a result of crack propagation, since each disk specimen was relatively small. An average value for the force measured was calculated for each distance. Sub-groups 1–7 were for the following ‘edge’ distances: 0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm. An edge strength measurement device (CK10) (Engineering Systems, Nottingham, UK) was used. The CK10 is a bench top, vertical loading compression testing machine. It has been designed to facilitate the study of engineering materials and can be used for edge toughness testing (edge flaking) of brittle materials [8]. The incorporated acoustic sensor enables the detection of cracking and has been used in the field of ceramics to study their cracks [9]. The device is made up of three main segments: the microscope, the specimen holder and the control panel (Fig. 1). It has a load range capacity of 0–10,000 N, a built-in acoustic sensing facility, with a data output display on the front panel. It determines the failure-force that is required to fracture flakes of material by a Vickers diamond indenter (polycrystalline diamond Rockwell C indenter 120◦ ), at any selected distance from an interface edge of bulk material. The crosshead speed was set at 1 mm/min.
Table 1 – Type and composition of the materials tested Materials Z 250 QuiXfil Tetric Ceram Venus Filtek Supreme (enamel)
Type
Shade
Batch no.
Manufacturer
Hybrid composite Posterior hybrid composite Micro-hybrid composite Micro-hybrid composite Nano-composite
A2 Universal A2 A2 Translucent
20030329 0306000653 E46159 020028 3910A1E
3 M ESPE USA Dentsply DeTrey GmbH Germany Ivoclar Vivadent, Schaan, Liechtenstein Heraeus kulzer, Wehrheim, Germany 3 M ESPE USA
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
Fig. 1 – Schematic diagram of the CK10 testing machine used to measure the force (N)-to-fracture at different interval distances from the edge.
2.1.
Fig. 2 – Bar chart graph of the mean force (N) to fracture, at a distance of 0.5 mm from the edge: defined as edge strength, for all materials investigated. (1) Tetric Ceram; (2) Z250; (3) Venus; (4) QuiXfil; (5) Filtek Supreme (enamel).
Observations
The mode of the failure of each specimen was examined under the integral microscope of the CK10. Failures were classified into two types: a. Specimen chipped-off: an edge fragment chipped off. b. Specimen cracking: the specimen was not completely chipped, with a missing fragment but was microscopically cracked. One-way analysis of variance ANOVA, followed by the multiple comparison Tukey test at the significance level of 0.05 was used to evaluate the variable of distance from the specimen edge for all materials. These tests were also used to compare the ‘edge strength’ at 0.5 and 1.0 mm. Regression analysis was applied.
3.
131
illustrated graphically in Fig. 2. The highest failure force (edge strength) was observed for Tetric Ceram (174.2 N) and the lowest for Filtek Supreme (enamel) (87.0 N). Statistical analysis showed that there were statistical significant differences in the forces needed to fracture at each distance (p < 0.000) between materials and (p < 0.002) overall between the distance of 0.5 and 1.0 mm. The correlations between the failure-forces to fracture materials with edge-distance were regression analyzed giving coefficients (r) ranging from 0.94 (p = 0.02) to 0.99 (p = 0.01). Fig. 3 shows a typical regression plot of force (N)-to-fracture versus edge-distance for Filtek Supreme (enamel). The fracture force at any edge-distance can thus be expressed using the linear formula: F =S×d+Y where F is the force (N) to produce specimen failure at any given distance d (mm) from the edge. S is the slope of the regression line and Y is the intercept.
Results
The mean force (N) and standard deviation, needed to fracture the specimens of each material, at different distances from the specimen edge (0.4, 0.5, 0.6, 0.7, 0.8, 0.9 and 1.0 mm) were obtained. The force (N)-to-fracture at a distance of 0.5 mm from the edge was defined as the edge strength. The values of edge strength for all materials are presented in Table 2 and
Table 2 – Edge strength date and predominant failure modes Materials Z 250 QuiXfil Tetric Ceram Venus Filtek Supreme (enamel)
Edge strength (N) 167.8 101.8 174.2 117.2 87.0
S.D. (7.9) (30.1) (22.8) (8.7) (27.1)
Predominant failure mode Chipped Chipped Chipped Cracked Cracked
Fig. 3 – Regression of load (N)-to-fracture values at different distance from the edge for Filtek Supreme (enamel) resin-composite material.
132
3.1.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
Observation on the indentation mode failure
Two modes of failure were observed: cracking and chipping. The indentation failure modes are shown in Table 2. Tetric Ceram, QuixFil and Z250 predominantly chipped off except for one or two distances that were cracked. Venus and Filtek Supreme (enamel) specimens, however, cracked at all edge-distances.
posites which is comparable in magnitude to the transverse strength of amalgam [13]. Fukushima et al. have shown that the extent of marginal degradation on posterior composites is less than that occurring on dental amalgam [14]. The incisal edge strength of porcelain laminate veneers, for restoring mandibular incisors has been determined indirectly by means of fracture strength [15].
4.1. Edge strength resistance at 0.5 mm distance away from the edge
4.
Discussion
The specialized instrument used for measurement determined the critical force for failure that occurred when the indenter was loaded at a given distance from the specimen edge. This force was found to be approximately proportional to the distance of the indenter from the edge. Thus, the hypothesis was confirmed. This observation seems to be associated with the cross-sectional area affected by the loading. This means that when the area increased the fracture force increased. This is analogous to the clinical observations of Krejci et al. on wear of posterior restorative materials [10]. At occlusal contact, wear was inversely related to the size of contact area of the opposing enamel cusps. The need for relevant in vitro laboratory measurements continues to be expressed [1]. Clinical studies can be costly and time consuming. Thus, it would be advantageous to be able to predict relative tendencies for clinically-related modes of edge-failure or marginal breakdown for sets of resincomposite formulations through in vitro measurements [4]. Hence the CK10 instrument was used, in this study, to commence the accumulation of a body of data on these materials. Restorations of posterior teeth with resin-based materials (resin-composites) frequently show relatively early evidence of marginal deterioration in several modes including chipping [11]. This presents dentists with difficult decisions as to whether to watch and wait, recontour, repair or replace [3]. Hence, a measurement of edge strength is desirable to characterize resin-based materials designed for stress-bearing situations and offers an insight into the forces and force directions causing marginal chips [11]. Microfill resin-composites have shown more marginal breakdown than hybrid composites. Marginal breakdown also shows an inverse correlation with fracture toughness for resin-composites [4]. Microfill composites and certain small particle hybrids are more susceptible to marginal degradation than posterior composites with coarser filler particles [4]. Of the materials currently measured, a nano-filled composite exhibited the lowest edge strength. Previously there were approaches to measure the edge strength indirectly. The edge and bulk strengths of powdered gold have been compared with other filling materials to determine the bending strength [12]. The study concluded that bending strength was the best indirect representative of the edge strength of metallic restorative materials. The modulus of elasticity has also been determined as an indirect measure of rigidity and transverse strength and thus as an index of edge strength for composites and amalgam. Bryant and Mahler pointed out that the transverse strength of microfilled composite is approximately half that of macrofilled com-
Fractures at the margins have been cited as a major problem regarding the failure of posterior composites [16]. The marginal fracture or the edge strength resistance may be related to some other physical property of the composites, specifically fracture toughness [4]. Chipping of a material is often easier (i.e. lower force required) close to an edge, in that forces required to chip a material will tend to be greater at increasing distances from the edge. Hence, edge strength is a potentially definable and useful parameter to characterize this aspect of clinical behaviour. Since edge strength could be measured in varying distances from the edge an arbitrary distance had to be chosen as a comparable reference point. This was selected as being 0.5 mm away from the edge of the specimen. The data measured at 0.5 mm distance may be more clinically relevant to marginal breakdown of restorations than for greater distances. Fractures that occurs within restorative resin materials have been seen mainly either through the bulk of material or through material near the interface [17]. The surface chipping, bulk fracture and incisal wear were correlated with various mechanical properties of the materials [18]. Tyas reported that a significant correlation was found between surface chipping/bulk fracture and fracture toughness (p = 0.002), elastic modulus (p = 0.006) and tensile strength (p = 0.045). Furthermore, the microfine composites were more susceptible to chip and bulk fracture than small particle or hybrid composites when used for anterior high stress situations such as class IV cavities. The results of some clinical studies led to the conclusion that certain small particle and microfilled composites showed greater tendencies for marginal fracture and localized wear in contact areas than the large heavily filled particle composites [2,3,6,7,19].
4.2.
Observation on the indentation mode failure
Apart from cracks, chipping of some of the tested specimens have been observed in this study. The process starts with cracks within the materials and more force is capable of chipping-off the material between the crack and the margin or edge of the specimen. In the present study the observations showed that, some of the materials chipped off or cracked, which could be attributable to the volume, shape and the distribution of its inorganic filler. The specimens of the Venus were cracked at all the distances away from the edge. On the other hand, Tetric Ceram, QuiXfil and Z250 had two types of mode of failure, either crack and/or chipped-off which could be attributed to the difference in composition and other mechanical or physical properties between those materials.
d e n t a l m a t e r i a l s 2 4 ( 2 0 0 8 ) 129–133
5.
Conclusions
• At each distance from the edge, there was a critical indentation force-to-failure. • The force-to-fracture increased linearly, for all materials, as the distance from the edge increased. • The force-to-fracture at 0.5 mm distance from the edge was defined as the edge strength value. • The modes of failure varied between materials, but chipping was the predominant mode.
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