|Year : 2013 | Volume
| Issue : 2 | Page : 43-48
Evaluation and comparison of the effect of enamel preparation designs on fracture resistance of micro-filled and nano-filled composite resin: An in vitro study
Jyothi Shashidhar1, C Shashidhar2
1 Department of Pedodontics and Preventive Dentistry, M.G.V'S K.B.H. Dental College and Hospital, Panchavati, Nashik, Maharastra, India
2 Department of Conservative Dentistry and Endodontics, M.G.V'S K.B.H. Dental College and Hospital, Panchavati, Nashik, Maharastra, India
|Date of Web Publication||3-Aug-2013|
1663/56, "Chandragiri", 11th Cross, Siddaveerappa Extension, Shamanur Road, Davangere - 577 004
Source of Support: None, Conflict of Interest: None
Background and Objectives: The main objective of this study was to compare and evaluate the effect of enamel preparation designs; bevel and stair step chamfer, on fracture resistance of composite resins (micro-filled and nano-filled) using incremental technique. Materials and Methods: The sample group consisted of 60 non-carious maxillary permanent central incisors. They were divided into four groups of 15 teeth each. According to the group, the teeth were prepared and restored with corresponding design and material. The teeth were subjected to compression bending test under instron testing machine then were examined under stereomicroscope. The results were subjected to statistical analysis. Results: Mean peak failure load (Newton) of among experimental groups was observed in the order; Group I (Bevel with micro-filled material) was 255.6 ± 65.0 N and Group II (bevel with nano-filled material) l was 282.1 ± 67.7 N. Group III (Stair-step chamfer with micro-filled material) was 249.8 ± 78.0 N. and Group IV (Stair-step chamfer with nano-filled material) was 283.3 ± 52.0 N. Results of the analysis of variance revealed there was no significant difference in the mean peak failure load values of the four different groups. Failure mode evaluation revealed, majority of the failures occurred as cohesive and mixed type for all the experimental groups. Adhesive type failure was observed maximum (47%) in the bevel group. Stair-step chamfer preparation showed the greatest potential for application, but also involved sacrificing less amount of tooth structure adjacent to fractured edge. Conclusion: This study concludes that both the preparations, bevel as well as stair-step chamfer are equally good to resist fracture when restored either with micro-filled or nano-filled composite resins.
Keywords: Bevel, micro-filled composite, nano-filled composite, stair step chamfer
|How to cite this article:|
Shashidhar J, Shashidhar C. Evaluation and comparison of the effect of enamel preparation designs on fracture resistance of micro-filled and nano-filled composite resin: An in vitro study. J Res Dent 2013;1:43-8
|How to cite this URL:|
Shashidhar J, Shashidhar C. Evaluation and comparison of the effect of enamel preparation designs on fracture resistance of micro-filled and nano-filled composite resin: An in vitro study. J Res Dent [serial online] 2013 [cited 2020 Aug 15];1:43-8. Available from: http://www.jresdent.org/text.asp?2013/1/2/43/116029
| Introduction|| |
A fractured or missing anterior tooth has a negative effect on a person's physical attractiveness that can affect the person's self-esteem.  Conventional methods for the treatment of coronal fractures of anterior teeth were used intra-coronal gold inlay restorations, compacted gold or gold foil, fused ceramic porcelain, full coverage crowns, or pin retained composite or acrylic restorations, which were associated with the removal of healthy tooth tissue to obtain the adequate mechanical retention of the restorations. 
The shortcomings of glass ionomer restoration are generally retention and esthetics.  Hence, the choice of treatment procedure mostly narrows down to restoration with the composite resins. Materials evolved from acrylic resins to composite resins that varied in filler particle size and amount.  After the composite resins were used in conjunction with the acid etching, the conservative restoration of fractured incisal edges became an accepted clinical procedure. ,
Fracture resistance of a material is a measure of its ability to retard crack initiation and propagation.  High fracture resistance of restorative material is required in clinical situations where high impact stresses are experienced and incisal angle restoration of a fracture in anterior tooth is one such demanding situation. Nano-filled composite may represent an important milestone in this development which shows easy adaptation, low level stickiness; excellent handling properties, out-standing polishability, long-term color stability, and highly aesthetic properties make them to use both anterior and posterior restorations.  Micro-filled composite resin traditionally has been recommended for anterior restorations because of the high luster when polished.  The low filler particle percentage of micro-filled resins gives a translucent appearance following restoration completion; the outline of restoration can be apparent.  The drawback of micro-filled composite resin is the secondary fracture potential.  It affords a significant advance in esthetics. They are indicated for non-stress bearing restorations only.
Various enamel preparation techniques have been recommended such as a butt joint margins, feather edge margins,  bevels,  chamfer preparation,  short bevel,  and long bevel have been utilized to help esthetics.  Later other investigators compared chamfer and bevel preparation for retention and found that chamfered preparations provided greater restoration fracture resistance and higher retention of tooth structures. ,
Albers introduced the stair step chamfer preparation for class IV restorations he describes this preparation design as a chamfer that follows the vertical and horizontal anatomical contours, making the preparation look like stair-steps.  This preparation shows good esthetic results because the chamfer margins and vertical contours between the lobes overlap making difficult to locate the chamfer margins. Finite element analysis showed that stress in chamfer and stair-step chamfer models was more homogenously distributed while stress in bevel models was relatively concentrated at lingual regions. A study indicates that the stair-step chamfer and 2-mm bevel should be recommended for clinical restoration.  The use of bevels resulted in improved fracture resistance of composite restorations. 
The aim of the study was to evaluate and compare the effect of enamel preparation designs, bevel, and stair step chamfer on fracture resistance of composite resins (micro-filled and nano-filled) using the incremental technique.
| Materials and Methods|| |
Sixty human, non-carious permanent maxillary central incisors were collected and stored in distilled water; the adherent blood and soft-tissue were removed immediately after procurement. Then they were stored in 0.5% chloramine-T bacteriostatic/bactericidal solution for 1 week. Following which, they were stored in distilled water at temperature of 4°C. In order to reduce deterioration the storage medium was replaced each fortnightly.  All teeth selected for testing were used within 1 month of procurement.
Using a PVC pipe tube of length 2.5 cm, all specimens were mounted in the same degree of angulation and vertical projection with roots embedded in self-curing acrylic resin. The cervical line of each tooth was made to coincide with the level of acrylic resin or upper edge of the tube so as to achieve parallelism and standard inclination of the incisal edges.
A standardized mesio-incisal fracture was created which coincides with 5 mm gingivally and 5 mm distally along the incisal edge. Then they were joined to form a base of an imaginary triangle with apex corresponding to mesio-incisal line angle. Experimental fractures were made using slow-speed diamond disc at a constant speed of 150 rpm. The mesiodistal widths were measured at the cut surface and were found almost identical in all groups. 
The specimens were divided in four groups of 15 samples each and color-coded with colored adhesive tapes and numbered.
Experimental enamel preparation designs were used on labial aspect of the teeth. The designs evaluated in the study were the bevel and the stair-step chamfer, and lingually all preparations were restricted to an inclined bevel extending 2 mm cervically from the fractured edge.
Enamel preparation design: Bevel-color code: Black (Group I) and white groups (Group II)
Using a standard diamond rotary bur (Mani Inc. Diamond bur TC-2 1, ISO NO. 160/014), a 45° bevel extending 2 mm beyond the fractured incisal edge and through the entire enamel thickness, was given on the cavosurface margin of the tooth which were to be restored with micro-filled and nano-filled composite respectively.
Enamel preparation design: Stair-step chamfer-color code: Red (Group III) and yellow (Group IV)
Using a standard diamond rotary bur (Mani Inc. Diamond bur SO-20, ISO NO. 288/0 12), a stair-step chamfer preparation was placed on the labial surface as described by Harry F. Albers. The stair-step chamfer included approximately half the thickness of the enamel in depth. The preparation of stair-step chamfer was also limited in the restricted area marked 2 mm cervically from the fractured edge. Two gradually merging vertical and horizontal chamfers each were placed alternatively to achieve enamel preparation design resembling stair steps which were to be restored with micro-filled and nano-filled composite respectively.
The enamel margins were acid etched with 35% phosphoric acid gel for 15 s, then thoroughly rinsed with water for 15 s and air dried. 
Adper Single bond 2 Adhesive (3M ESPE, USA) was applied on the prepared surface using the painting brush according to manufacturer's instruction. The adhesive was thereafter light cured for 20 s on each surface before restoring with composite resin.
Using micro-filled composite, Heliomolar (ivoclar vivadent Inc, USA) and nano-filled composite (Filtek TM Supreme XT, 3M ESPE), these preparations were restored with the use of "incremental" technique and cured with LED curing unit (Translux® Blue, Hareus Kulzer), [Figure 1] wavelength 440-480 nm and finishing of the incisal edges was done with superfine aluminum oxide flexible abrasive disks (Soflex-3M ESPE, USA).
|Figure 1: Light Emitting Diode (LED) unit to cure the composite material|
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Specimens were aged for 24 h in distilled water at 37° in an incubator and then subjected to cantilever bending test using universal testing machine (Instron T-Series TINIUS OLSEN. Instron Corp. England, UK) to determine the resistance to fracture of composite restorations. The readings were noted to determine the peak force at failure in Kilograms (Kg) then converted to Newtons (N).
Fracture mode evaluation
After the testing of preparations, the specimens were examined under stereomicroscope at ×40 magnification [Figure 2] to evaluate the site where failure occurred. The labial surfaces of samples were examined to determine the mode of failure of the composite resin.
|Figure 2: Microscopic view of A) adhesive, C) cohesive and M) mixed failure|
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The actual mode of failure was recorded according to the following criteria;
Adhesive (A): Failure at the tooth resin interface,
Cohesive (C): Complete failure within the resin restoration,
Mixed (M): Partial fractures of resin restoration and partial adhesive failure at the interface.
| Results|| |
In analyzing the results of experiment under methods considered in this research work, the following statistical tests were employed.
Analysis of variance
Analysis of variance (ANOVA) test was used to detect mean fracture resistance of all the four groups namely Group 1 (bevel restored with micro-filled composite), Group II (bevel restored with nano-filled composite) and Group III (stair-step chamfer restored with micro-filled composite) Group IV (stair-step chamfer restored with nano-filled composite), but the results were not statistically significant [Table 1] [Graph 1] [Additional file 1].
Percentages of modes of failure (adhesive, cohesive, and mixed) were compared using Chi-square test and found statistically not significant. Chi-square = 3.49, P = 0.74, NS [Graph 2] [Additional file 2].
| Discussion|| |
Due to recent advances in materials, conservative concepts in restorative dentistry and clinical successes have made composite resins, the material of choice for class IV restorations.  Micro-filled resin has silica filler with an average particle size of 0.04 microns. The particles in the micro-filled are very small so that wear occurs smoothly across the entire restorative surface. These resins are capable of being brought to a high polish due to their extremely small fillers and highly esthetic.
Nano-filled composite has even smaller (nanomers) 20-75 nm in size as well as a combination of nanoclusters (0.6-1.4 μm in size). Filler proportion of approximately 72-78% by weight, which has ultra-fine radiopaque zirconium oxide fillers. This material shows excellent level of stability, satisfactory adaptability, low stickiness, polishability and good stability. Nano-filled material yields a higher resistance to fracture compared to micro-filled.
This result is in agreement with the previous which showed that the compressive, diametral strengths and the fracture resistance of the nano-composite were equivalent to or higher than those of other composites which may be hybrid, compomer and micro-filled. 
It is established that the strength of the bond and degree of retention varies directly with the enamel surface available for acid etching.  In this study, two variants of preparation of enamel (bevel and stair-step chamfer) were evaluated for their effect on the fracture resistance of bonded composite resin.
Currently most clinicians prefer to use bevel because of a conservative approach especially, toward traumatized teeth and to gradient the color change from tooth matter to restoring material. Bagheri J and Denehy.  Showed that increasing the thickness of the bevel, not the length had a significant difference on shear strength. In another study by Xu et al. showed that fracture resistance of a 1-mm bevel preparation was lower than for the 2-mm bevel, plain chamfer, and stair-step chamfer preparations, but was higher than for butt joints. 
Investigators have established that the beveling the enamel surface would increase composite bonding strength to acid-etched enamel and decreased the chance of restoration failure. Enamel bevels are therefore, popular in preparations to allow gradual transition of color from the tooth shade to the matching composite shade, and in an attempt to enhance the strength of the bond.
The peak failure load observed in bevel group restored with micro-filled material was 255.6 ± 65.0 N. The bevel group restored with nano-filled material was 282.1 ± 67.7 N [Table 1].
Bevel preparation yields a higher resistance to fracture in nano-filled material compared to micro-filled material. These differences were also found not statistically significant.
Stair-step chamfer technique of enamel preparation was introduced by Albers.  with the objective of addressing esthetical concerns associated with the chamfer preparation. The chamfer line follows the anatomical contours of the natural human incisors on the buccal surface while having a plane chamfer on the lingual surface.
The peak failure load observed in Stair-step chamfer group restored with micro-filled material was 249.8 ± 78.0 N. The peak failure load in Stair-step chamfer group restored with nano-filled material was 283.3 ± 52.0 N [Table 1].
With micro-filled material, bevel preparation yields a higher resistance to fracture compared to stair-step, i.e. peak failure load of Group I > Group III. Furthermore, with nano-filled material stair-step chamfer yields a higher resistance to fracture when compared to bevel i.e. peak failure load of Group IV > Group II. Furthermore, these differences are not statistically significant. Again the probable reason for the discrepancy of the results could be due to different types of composites.
Discussion of failure mode
In bevel groups, (Group I and Group II) most of the failures were adhesive whereas in stair-step chamfer group (Group III and Group IV) the failures recorded were mixed followed by cohesive thereby indicating high resistance to interfacial failure (adhesive) offered by stair-step chamfer preparation [Graph 2].
The results of this study were in agreement to the findings of previous study where both micro-filled and macro-filled resins showed better adhesion values in chamfer compared to bevel preparation. 
In general it is established that, as the retentive values for the restorative material increased, progressively larger proportions of materials remained bonded to the enamel surface. This is an indication that a large proportion of the break occurred within the restorative material. As the retentive values decreased, less restorative material remained on the enamel surface.
| Conclusion|| |
This study concludes that both the preparations viz. bevel as well as stair step chamfer are equally good to resist fracture when restored either with micro-filled or nano-filled composite resins.
In the present study, every effort was taken to duplicate the oral situations however; the in vivo responses to the enamel preparation design might differ from the results of this study. Nonetheless, clinical implications of this research project are significant. Further in vivo trials with these materials and surface treatments are indicated to confirm the validity of these recommendations.
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[Figure 1], [Figure 2]