|Year : 2011 | Volume
| Issue : 1 | Page : 7-13
Comparative evaluation of shear bond strength of various esthetic restorative materials to dentin: An in vitro study
N Manuja1, IK Pandit2, N Srivastava2, N Gugnani2, R Nagpal3
1 Department of Pediatric Dentistry, Kothiwal Dental College, Moradabad - 244 001, Uttar Pradesh, India
2 Department of Pediatric Dentistry, DAV Dental College, Yamunanagar, Haryana, India
3 Department of Conservative Dentistry, Kothiwal Dental College, Moradabad - 244 001, Uttar Pradesh, India
|Date of Web Publication||23-Apr-2011|
Department of Pediatric Dentistry, Kothiwal Dental College and Hospital, Kanth Road, Moradabad - 244 001, Uttar Pradesh
| Abstract|| |
Aim: To comparatively evaluate the shear bond strength of recent tooth-colored restorative materials to dentin. Materials and Methods: Flat dentinal surface were prepared from 60 caries free, extracted human permanent molars and were mounted in acrylic rings. These were randomly divided into four groups - Group A to Group D, according to the restorative material used i.e. Glass ionomer cement (Fuji IX), Giomer (Beautifil), an Ormocer-based composite (Admira) and Nano Ceramic restorative material (Ceram X). These restorative materials were applied on dentinal surface of all the specimens using nylon cylinders. The mounted samples were stored in distilled water for 24 hours and thermocycled. They were then subjected to shear bond strength test using universal testing machine. Data was analyzed using a one-way analysis of variance (ANOVA) and student's 't'-test. Results: Ceram X (16.63±0.94 MPa) and Admira (17.31±0.95 MPa) were comparable in their bond strength values, but depicted significantly higher bond strength when compared to Beautifil (12.39±1.05 MPa) and Fuji IX (7.76±1.07 MPa). Conclusion: Nano-ceramic and ormocer-based restorative materials showed better bonding potential to dentin as compared to GIC and Giomer.
Keywords: Giomer, nano-ceramic, ormocer, shear bond strength
|How to cite this article:|
Manuja N, Pandit I K, Srivastava N, Gugnani N, Nagpal R. Comparative evaluation of shear bond strength of various esthetic restorative materials to dentin: An in vitro study. J Indian Soc Pedod Prev Dent 2011;29:7-13
|How to cite this URL:|
Manuja N, Pandit I K, Srivastava N, Gugnani N, Nagpal R. Comparative evaluation of shear bond strength of various esthetic restorative materials to dentin: An in vitro study. J Indian Soc Pedod Prev Dent [serial online] 2011 [cited 2014 Apr 16];29:7-13. Available from: http://www.jisppd.com/text.asp?2011/29/1/7/79913
| Introduction|| |
Dental caries still continues to be a major problem in dentistry, and thus, should receive significant attention as far as its management is concerned. Today every focus is diverted to conserve tooth structure using restorative materials, which adhere to tooth structure by minimal intervention and are tooth colored to provide esthetics. Strong durable bond between dental biomaterials and tooth substrate are essential, not only from a mechanical stand point, but also from biological and esthetic perspectives.
Adhesive dentistry has made tremendous advances since the inception of enamel etching by Buonocore in 1955 using phosphoric acid.  Adhesion of restorative materials to enamel has become a routine and reliable aspect of modern restorative dentistry, but dentinal adhesion has proved to be more difficult and less predictable. Much of the difficulties in bonding to dentin are a result of the complex histological structure and variable composition of the dentin itself. While enamel is 92% inorganic hydroxyapatite by volume, dentin is (on an average) only 45% inorganic.  Moreover, unlike enamel, which has a regular arrangement of hydroxyapatite crystals, dentinal hydroxyapatite is randomly arranged in an organic matrix consisting primarily of collagen. Apart from this, dentinal adhesion is also affected by its tubular structure with the presence of odontoblastic processes, permeability of dentin, intrinsic and extrinsic wetness of dentin. ,
The quest for the ideal restorative material is an on-going challenge. In recent years, development and interest in esthetic dentistry has increased. Adhesive techniques have greatly expanded the horizon of esthetic dentistry. The introduction of glass ionomers by Wilson and Kent  and Dr. Bowen's invention of composite resins have served to revolutionize the field of modern day dentistry. The poor wear characteristics, esthetics and long setting reaction of glass ionomer have been modified to a considerable extent with the newer materials showing comparable properties to the composite resin materials. 
The glass ionomer cement is a product of an acid-base reaction between basic fluoroaluminosilicate glass powder and polycarboxylic acid in the presence of water. The glass ionomer cement bonds directly to teeth and has a potential effect of remineralization because of fluoride content. Conventional glass ionomers reveal only moderate wear resistance, poor fracture toughness and rather rough surface conditions.  The clinical use of conventional glass ionomer has been improved by the development of highly viscous glass ionomer cements (Fuji IX) and composite-based restorative materials like Giomer, Ormocer and Nano-ceramics.
Giomer is a new group of direct restorative materials and adhesives that offers aesthetics, handling and physical properties of composite resins with added benefits of high radiopacity, anti-plaque effect, fluoride release and recharge. These hybrid aesthetic restorative materials are based on pre-reacted glass-ionomer (PRG) technology to form a stable phase of glass ionomer in the restorative.  This technology can be further classified into two categories: F-PRG (reaction of entire glass) and S-PRG (reaction of only the glass surface). Chemically, they consist of fluoroboroaluminosilicate glass (81.5% w/w) reacted with polyalkenoic acid in water prior to inclusion into the resin base of bis-GMA (bisphenol A glycidyl methacrylate) and TEGDMA (Triethylene glycol dimethacrylate).  Beautifil is one of the products of Giomer category having S-PRG technology. 
ORMOCERs (ORganically MOdified CERamics) are a new class of materials, developed by the Fraunhofer Research Institute at Wurzburg, Germany. These compounds are also known in the literature as 'Ormosils' (Organically Modified Silicates). These are hybrid polymers synthesized by the sol-gel process. New multifunctional urethane and thioether oligo (meth) acrylate alkoxysilanes as sol-gel precursors have been developed for preparation of inorganic/organic copolymer composites as dental restorative materials. Ormocer consists of three main units, each of which influences their properties. The 'Organic Unit' (organic polymers) is responsible for the cross-linking capacity, polarity, hardness and optical behavior of the Ormocer. The inorganic unit (glass ceramic) is responsible for the thermal expansion as well as the chemical and thermal stability. The 'inorganic-organic unit' (polysiloxanes) is responsible for the elasticity, interfacial properties and processing properties. ,
Since nanotechnology was introduced to dentistry, nanocomposites with filler sizes ranging from 0.01 to 0.04 mm have been developed.  Nanocomposites have many advantages, such as reduced polymerization shrinkage, increased mechanical properties, improved optical characteristics and better gloss retention. , Wear resistance of nanocomposites has been shown to be comparable or superior to that of microfill and microhybrid composite resins.  The organically modified, ceramic-based, nano-ceramic composite also was developed using the same technology. It contains a methacrylate-modified silicon-dioxide-containing nano-filler and resin matrix that is replaced by a matrix full of highly dispersed methacrylate-modified polysiloxane particles.  A nano-ceramic resin composite, Ceram-X comprises organically modified ceramic (Ormocer) nanoparticles and glass fillers (1.1-1.5 mm). The filler concentration of Ceram-X is 76% by weight and 57% by volume. According to manufacturer's data, these nano-ceramic particles are inorganic-organic hybrid particles. Both, nano-ceramic particles and nano-fillers have methacrylate groups available for polymerization. 
The clinical success of the newer restorative materials depends upon a good adhesion with the dentinal surface to resist various dislodging forces acting within the oral cavity. So, keeping in mind the various mechanical and esthetic properties of recently available tooth-colored restorative materials and due to lack of sufficient literature about these materials, the present study has been done with an objective to comparatively evaluate the shear bond strength of Glass ionomer cement (Fuji IX), Giomer (Beautifil), an Ormocer-based composite (Admira) and Nano-ceramic restorative material (Ceram X).
| Materials and Methods|| |
A total number of 60 caries-free, extracted human permanent molars, were selected for the study. The teeth were thoroughly cleaned and stored in normal saline at room temperature till the study was conducted. The stored teeth were retrieved from normal saline and depth holes measuring 1.5 mm were drilled in the deepest part of central fossa of each tooth with the help of a round diamond bur. This was done to standardize the depth of dentin as it also affects dentin bond strength. All the sixty samples were ground on an orthodontic trimmer to expose a flat dentinal surface. This was followed by manual polishing of the dentinal surface with wet 600 grit silicone carbide paper. The specimens were embedded in auto-polymerizing acrylic resin in the aluminum moulds. The specimens were placed perpendicular to the acrylic resin surface. Just before the placement of restorative material, each sample was thoroughly washed with water, dried with air from an oil-free air source. An adhesive tape with a punch hole of diameter 3 mm was applied on the flat dentinal surface of each mounted tooth to delineate the area for bonding.
The samples were randomly divided into four groups - Group A to Group D according to the restorative material used [Table 1], consisting of 15 samples in each group.
Glass ionomer cement, Fuji IX (GC Corporation, Japan). The dried dentinal surface was conditioned with 10% polyacrylic acid for 10 s. Powder and liquid were mixed in a ratio of 3 : 1. Nylon cylinder (3 mm of internal diameter and 2.5 mm height) was placed perpendicular to the dentinal surface. Mixed glass ionomer cement was inserted into these cylinders. The nylon cylinders were removed after 30 s. After that GC Fuji varnish was applied on the entire surface.
Giomer, Beautifil (Shofu Inc., Kyota, Japan). The dried dentinal surface was treated with a mixture of FL Primer A and B for 10 s. Excess primer was removed with gentle air drying, then the low-viscosity FL Bond was placed and light cured for 10 s. Nylon cylinder (3 mm of internal diameter and 2.5 mm height) was placed perpendicular to the dentinal surface. Beautifil restorative was filled into these cylinders and light cured for 40 s. After that the nylon cylinders were removed.
Ormocer, Admira (VOCO Dental, Germany). Dentinal surface was conditioned with 37% phosphoric acid (Vococid, VOCO) for 15 s, rinsed with water for 20 s and blot dried. Admira Bond was applied according to manufacturer's instructions. Admira restorative material was placed inside the nylon cylinder (3 mm of internal diameter and 2.5 mm height) perpendicular to the dentinal surface and light cured for 40-60 s.
Nano-ceramic restorative, Ceram X (Dentsply, USA). Dentinal surface was conditioned with 37% phosphoric acid (Etchant, 3M ESPE, St. Paul, MN, USA) for 15 s, rinsed with water for 20 s and blot dried. Prime & Bond NT was applied and left undisturbed for 20 s, air-thinned for 5 s and light cured for 10 s. Nylon cylinder (3 mm of internal diameter and 2.5 mm height) was placed perpendicular to the dentinal surface. Ceram X restorative was filled into these cylinders and light cured for 40 s. After that the nylon cylinders were removed.
The mounted samples were stored in distilled water at room temperature for 24 h. All samples were subjected to thermocycling for 2000 cycles between 5 o and 55 o C with a dwell time of 30 s. The samples were then subjected to shear bond strength test using universal testing machine (Instron, Lloyd, LR 100, UK). The specimens were mounted in a jig, while a straight knife-edge rod was applied at the tooth-restoration interface at a crosshead speed of 0.5 mm/min. Load was applied until restoration failure occurred.
The data collected was tabulated accordingly and was subjected to statistical analysis. Mean and standard deviations were calculated for each group and analyzed using a one-way analysis of variance (ANOVA). Student's 't'-test was used to identify differences between the two groups.
| Results|| |
Mean shear bond strength values are presented in [Table 2] [Graph 1]-[Additional file 1]. One-way analysis of variance showed highly significant difference among the groups [Table 3]. Ormocer Admira (17.31±0.95 MPa) showed maximum mean shear bond strength and glass ionomer cement Fuji IX (7.76±1.07 MPa) showed minimum mean shear bond strength value amongst all the groups There was highly significant difference in the shear bond strength of Glass ionomer cement with all other groups. Ceram X (16.63±0.94 MPa) and Admira were comparable in their bond strength values, but depicted significantly higher bond strength when compared to Giomer (12.39±1.05 MPa).
|Table 2: Mean shear bond strength values and standard deviation in different study groups |
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|Table 3: Analysis of variance (ANOVA) for shear bond strength of different restorative materials used in the study |
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| Discussion|| |
Various restorative materials have been used since years to preserve the lost tooth structure and maintain form, function and esthetics. Dental amalgam has served as an excellent and versatile restorative material for many years. However, amalgam restorations have many drawbacks like lack of adhesion to the mineralized tissues, lack of esthetics and the unavoidable use of mercury, which may be regarded as harmful component to the patient's health. Therefore there is always a need for restorative materials having superior properties than amalgam like biocompatibility, fracture resistance, good esthetics and a good adhesion of the restorative material to various tooth surfaces.
One of the primary objectives of adhesion is to achieve a strong, durable and predictable union between restorative material and tooth structure. Bonding improves retention and stabilization of a restoration without excessive removal of sound tooth structure. The need for providing large undercuts and auxiliary retentive aids is eliminated. Adhesive restorations are better able to transmit and distribute functional stresses across the bonded interface thereby, reinforcing weakened tooth tissue. Strong adhesion between the tooth and the restorative material prevents formation of marginal gaps occurring due to polymerization shrinkage stress.
Conventional glass ionomer cements are able to adhere physico-chemically to the surface of the cavity because they contain polyalkenoic acids and have a coefficient of thermal expansion, which is almost equal to that of tooth substance.  In addition, the glass particles of these types of cement release fluoride after the restoration hardens, thereby offering an anticariogenic effect along the cavity margins.
Hybrid restorative materials combining the technologies of glass ionomers and resin composites have been developed to help overcome problems of conventional glass ionomers such as moisture sensitivity, low initial mechanical properties and inferior translucency; and at the same time maintain their clinical advantages such as fluoride release and adhesiveness.  A new category of hybrid aesthetic restorative materials which differ from both resin-modified glass ionomers and compomers have been introduced which are known as Giomers.  It should be considered as light-cure composite as it does not have a significant acid-base reaction as part of its curing process and cannot set in the dark. This technology also differs from compomers, in which a variable amount of dehydrated polyalkenoic acid is incorporated into the resin matrix and the acid does not react with the glass until water uptake occurs into the restoration. 
The mean particle size of giomer (non inclusive of PRG particles) is found to be below 1 μm. Yap and Mok  compared the surface finish of a giomer to a composite, a compomer, conventional- and resin-modified glass ionomer cements to conclude that giomers have better surface finish than conventional/resin-modified glass ionomer cements and comparable to composites and compomers.
Dental composites based on ormocer technology and nano-fillers have been recently introduced. Ormocer is a three dimensionally cross-linked copolymer. They are advanced filling materials for use in dentistry which, due to their innovative matrix technology and filler particles represent state-of-the-art science. The technology used in the ormocer materials is different from that of conventional composites. The ormocer consists of an inorganic-organic (inorganic backbone based on SiO 2 functionalized with polymerizable organic units) network matrix formed through poly-condensation. The filler particles are embedded in this cross-linked inorganic and organic network matrix as with proven composite technology. The average particle size is 0.7 μm. 
Ceram-X is a resin composite which contains organically modified ceramic nanoparticles (2 to 3 nm) and nano-fillers (10 nm) that are combined with conventional glass fillers (mean particle size: 1.1 to 1.5 μm). Nanoparticles and nano-fillers comprise a polysiloxane backbone and have methacrylate groups available for polymerization. They are highly dispersed and not present as agglomerates. 
In the current study, chemically cured Glass ionomer cement, Fuji IX, depicted the least mean shear bond strength value amongst all the groups tested. Various studies have also consistently shown the inferior bonding performance of glass ionomer cement when compared to composited-based restorative materials. ,, Glass ionomers remain as the only material that is self-adhesive to tooth tissue, without any surface pre-treatment.  This material relies on the chemical bond to the tooth rather than micro-mechanical bond, which is a weak bond. Moreover, polyacrylic acid used in chemically cured glass ionomer cement for dentin conditioning acts as a mild etching agent that removes the smear layer, but does not remove smear plugs from the dentinal tubules.
Although the exact mechanism of bonding of glass-ionomer cement to enamel and dentin is still unknown, it seems that the mechanism involves wetting of the tooth surface by the cement and subsequent formation of ionic bonds with the conditioned tooth substrate. It is generally believed that the adhesion of conventional glass ionomer cements might be the result of the development of an ion-exchange mechanism, polyacrylate ions replacing phosphate ions in the surface of hydroxyapatite. 
Giomer Bond is a glass-ionomer base, tricurable, all-in-one, filled adhesive based on PRG technology and consists of 4-AET, 4-AETA, UDMA, HEMA, PRG filler, fluoroaluminosilicate glass, acetone, water and initiator.  Due to inclusion of these fillers, Giomer possibly showed higher shear bond strength as compared to glass-ionomer cement.
Both Ceram X and Admira showed significantly higher bond strengths as compared to Fuji IX and Beautifil. Schirrmeister JF et al .evaluated the clinical performance of Ceram X in combination with an experimental one-bottle etch-and-rinse adhesive (K-0127) and concluded that after four years of clinical service, 92.6% of Ceram X restorations performed clinically well. 
As stated by the manufacturer, the Ormocer-based adhesive, Admira Bond chemically consists of ormocers, Bis-GMA, HEMA, BHT, acetone and organic acids. The ormocer moieties present help to strengthen the resin and composite assembly bonded to dentin via hybrid layer formation. These ormocer moieties are not present in the bonding agent of Giomer. This could have possibly resulted in highest bond strength values for Admira bond / Admira. 
The possible reason for highest bond strength of Ormocer as compared to other tooth-colored restorative materials may also be attributed to less polymerization shrinkage stress due to the presence of BHT (n-butyl hydroxyl toluene) in Admira Bond. Yap and Soh concluded that the polymerization shrinkage of Admira was significantly lower than conventional composite resin.  The process of polymerization can be characterized by pre-post-gel phases. Polymerization stress initially is relieved by the composite flow until it reaches the so-called 'gel-point'. Before this point, the resin-based composite is flexible and accommodates to relieve stress. After this gel point is reached, the composite changes to an unyielding state in which the shrinkage stress is transmitted to the tooth structure. Versluis et al. observed that the longer the pre-gel point time, the less the stress in the post-gel phase.  BHT can cause reduction in reaction speed by chemical inhibition, which occurs as the free radicals are terminated by reacting with the phenolic hydrogen of the BHT molecule (C 15 H 24 O). The phenoxy radicals may then inactivate another free radical C-C or C-O coupling or by loss of another hydrogen atom to form a quinone, which may react further. Therefore, each inhibitor molecule can terminate two or more polymer chains. The conversion proceeds at a reduced rate until the inhibitor is completely consumed. Up to the 'pre-gel' phase, the shrinkage forces can be dissipated but when cross-linking reaches a certain point, molecular displacement becomes impossible. After that, the shrinkage is likely to generate stress.  However, Al-Harbi and Farsi reported no statistically significant difference in the microleakage of class V cavities restored with Admira and Z-100 composite. 
| Conclusion|| |
The current study concluded that new nano-ceramic- and ormocer-based restorative materials have better bonding potential to dentin as compared to GIC and Giomer but they should be subjected to further clinical evaluation.
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[Table 1], [Table 2], [Table 3]
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