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| Year : 2006 | Volume
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| Issue : 4 | Page : 152-158 |
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| Wear analysis of nano ceramic composites against a ceramic antagonist |
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AC Krithika1, D Kandaswamy2, Emmanuel Solomon Sathish2
1 Department of Conservative Dentistry and Endodontics, Sri Ramachandra Dental College, Porur, Chennai 600116, India
2 Department of Conservative Dentistry and Endodontics, Meenakshi Ammal Dental College 8 Hospital, Maduravoyil, Chennai, India
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 Abstract | | |
Wear is a natural process and is defined as loss of material from the surface caused by mechanical action alone or through a combination of mechanical and chemical action. The purpose of this article is to measure the three body occlusal contact area wear of different restorative materials against a ceramic antagonist since ceramic restorations are common against composite restorations.
How to cite this article:
Krithika A C, Kandaswamy D, Sathish ES. Wear analysis of nano ceramic composites against a ceramic antagonist. J Conserv Dent 2006;9:152-8 |
How to cite this URL:
Krithika A C, Kandaswamy D, Sathish ES. Wear analysis of nano ceramic composites against a ceramic antagonist. J Conserv Dent [serial online] 2006 [cited 2023 Dec 7];9:152-8. Available from: https://www.jcd.org.in/text.asp?2006/9/4/152/42318 |
| Introduction | |  |
One of the important but clinically neglected parameters in detecting the efficiency of a posterior restorative material is wear resistance. Wear is a natural process and can occur in natural teeth or restorative material. An ideal restorative material should neither wear against nor produce wearing of the antagonist natural tooth.
Clinically, wear can occur at the occlusal contact area (OCA) and contact free area (CFA). Much of the studies done earlier, concentrated on the generalized contact free area wear of the material. Although this type of wear pattern is important, localized occlusal contact area wear of a material is of great clinical concern. This occlusal contact area wear can be attributed to the direct tooth contact - two body wear (e.g. Bruxism) or indirect tooth contact wear through trapped food particles - three body wear.
The aim of this study is to measure the three body occlusal contact area (OCA) wear of packable composites, nano ceramic composite, indirect composite material and amalgam against ceramic antagonist using a three dimensional, non-contact surface profilometer.
| Materials and Methods | | |
The materials used in this study [Table 1] are Group I - dental silver amalgam (DPI alloy and mercury), Group II - indirectly processed composite material (SR Adoro), Group III and IV- direct packable composite material (Surefil, Filtek P60), Group V - direct universal nano ceramic composite (Ceram.X Mono). Ceramic antagonist balls of 0.5 cm diameter were made with pressable all ceramic material (Empress Esthetic) [Figure 1].
The samples were prepared in the following way:
In Group I, Silver amalgam was triturated in a mechanical amalgamator and hand condensed in the brass mould according to the manufacturer's instructions. Samples were allowed to set overnight and then the samples were polished with amalgam polishing wheels. In Group III, IV, V composite material was placed into the brass moulds with composite filling instrument. Brass mould was of 1 cm diameter and 2mm depth. Care was taken to avoid voids. Material was covered with a cellophane sheet. A glass slab was placed with pressure over the cellophane sheet to extrude the excess material. Light curing was done with quartz tungsten halogen light-curing unit with 450nm wavelength and 400mW/ cm 2 intensity. The 2 mm depth samples were bulk cured at four points along the circumference for 20 seconds as per manufacturer's instructions with a total curing time of 80 seconds. Similar methodology was followed as in groups III, IV, V to make Group II SR Adoro samples. The indirect composite material was initially cured in the brass moulds using tungsten halogen light and then placed in the Lumamat 100 curing system (Ivoclar Vivadent) according to the manufacturer's instructions. The curing cycle consisted of simultaneous light curing at 350 nm and a thermal curing of 104'C. Samples from all the above groups were finished and then polished with composite polishing kit (Shofu). All the samples were stored in distilled water at 37C.
Methodology for wear testing
Wear testing was done with reciprocating compression sliding machine at IIT, Chennai. Test samples were glued with cyanoacrylate to a jig and placed on a platform of the machine. Ceramic antagonist ball was attached to another jig and tightened by screws. This jig was maintained stationary and placed opposite to the test samples. Poppy seeds were added as a third body to simulate food during mastication. [1]
The machine had an induction motor. The rotary action of the induction motor was translated into linear action of the sliding platform. One complete circle drawn by the induction motor translated to one complete horizontal motion of the platform comprising of a forward and a backward stroke of 2mm each. All the samples were subjected to 20,000 similar wear cycles. [1] A constant stress of 20 MPa was maintained between the test samples and ceramic antagonist to simulate the average stress during mastication (3.9 to 17.3 MPa) [2]
Methodology for wear analysis
Material wear (depth in microns) was measured using a three dimensional profilometer (WykoNT1100 SURFACE PROFILER Vecco, Woodbury, NY USA) [Figure 2] with adjacent unworn areas as reference points at Electrical department, Indian Institute of Technology, Chennai. Wyko-NT1100 SURFACE PROFILER is a non-contact optical profiling system that provides high resolution, 3D surface measurement, from sub-nanometer surface roughness to millimeter step-height. This optical profiling system consists of an interferometer. Interferometers employ a system where one beam is reflected from the object under test and the other beam is reflected from a reference mirror. The beams are recombined to create bright and dark bands called fringes that make up the interferogram. Fringes, like lines on a topographic map, represent the topography of the object. Completing the interferometric setup is a CCD detector that registers the interferogram and forwards the frame to the computer for processing using interferometric programs. The wear depth of each sample was measured in the step measurement mode using Vision 32 software. One measurement was made in each sample.
Mean and standard deviation were calculated from the obtained measurements for each group. [Table 2] and [Table 3] Mean values were compared by one-way ANOVA. Multiple range tests by Tukey HSD procedure was employed to identify the significant groups at 5 % level. In the present study, P0.05 was considered as the level of significance.
| Results | | |
Mean value in-group IV (17.970.43) was significantly higher than the mean values in Group I (5.31 0.35), Group Il (5.370.60), Group III (13.760.39) and Group V (8.20 0.49). Further, the mean value in group III was significantly higher than the mean value in Group I, II and V. Also the mean in group V was significantly higher than the mean values in group 1 and II (P<0.05). However, there was no significant difference in mean values between groups I and II (P>0.05).
| Discussion | | |
Wear is the net result of a number of processes such as abrasion, adhesive effects, fatigue and corrosive effects. [3] Abrasive wear occurs when hard asperities plough into softer surfaces. These processes act in different combinations in various classes of dental materials. [3]
Wear resistance of tooth or restorative material is influenced by a number of factors like characteristics of the antagonist material, composition and properties of the wearing material. Shape, hardness, brittleness and roughness of the antagonist can influence the wear of test specimens. Compositional factors such as the type of the filler, filler size and shape, filler volume and inter particle spacing [4] between the fillers, the composition and degree of polymerization of the resin matrix [5] and resin filler interface play a key role in influencing the wear property. In addition, hardness and fracture toughness property might have an influence in the wear resistance behavior of composite materials. [6],[7]
Ideally, in-vitro wear tests should use enamel as antagonist to simulate clinical conditions. Due to the biologic variation in the enamel, enamel alternatives like steatite, degussit and stainless steel balls have been used [8] Hyun-Suk Cha [9] suggested that ceramic could be used as antagonist since ceramic restorations are common these days. Further, ceramic is known to abrade opposing material more than natural enamel [10] and if a material is more resistant to wear against ceramic, it will behave well against enamel also. Thus, ceramic antagonists were used in this study.
This study uses a non -contact three dimensional surface profilometer that determines the wear depth by optical scanning using interferometer. [10] Earlier studies were done using a contact stylus two dimensional profilometer. The disadvantage of this method is that the diamond stylus touches the worn areas for measuring the depth. This may lead to inaccurate results. 3D profilometer used in this study uses an optical scanner for analyzing the surface wear. The vision 32 software associated with the profiler accurately determines the mean wear depth between the worn and unworn areas. Another advantage of this system is that the samples are not damaged after the analysis.
Most restorative materials are considered biphasic (composite) with one phase embedded in the other. On wearing, the abrading particle tends to preferentially abrade the softer phase leaving the harder material protruding from the surface (plucking effect). In composites, the hard filler particles remain intact and transmit the force to the surrounding resin matrix leading to micro-cracking. [1]
In the present study, the lowest mean wear depth (5.310.35m) was found with amalgam. This can be attributed to the near elimination of 2 phase (AgHg), maintenance of minimal 1 (Sn-Hg) phase with unreacted alloy particles and their ability to withstand some amount of strain under load, by its metallic nature. [10],[11]
According to the results of this study, indirect composite resin (Group II) had mean wear depth (5.370.06m) statistically similar to dental silver amalgam and superior to all the other direct composite materials used here. The higher wear resistance of indirect composite resin material compared to other composites can be attributed to the better polymerization of indirect composite resin as supported by studies of Condon et al [5] . The curing cycle of these resins consists of a more gradual polymerization with additional heat polymerization. This results in increased polymerization [12] and thus increased cross linking [13] protecting the resin from wearing. Better polymerization of the resin matrix increases the mechanical strength of the resin phase. This increases the abrasion resistance of the resin and prevents the filler particle exfoliation on wearing. Further, heat allows for polymerization stress relaxation. Additionally, the addition of silanated prepolymers of nanofillers and urethane dimethacrylate in the material might have a better adhesive bond between the filler and the resin matrix as suggested by the manufacturer. This might help in good stress transfer ability at the resin filler interface slowing down the process of filler particle loss in wearing. [14]
Comparing among the direct composite materials analyzed in this study, nano ceramic composite (group V) with the nano-sized fillers had significantly lower mean wear depth (8.200.49m) than the other two packable composites. Similar findings have been observed in other studies . [4],[5]
The presence of smaller filler particle in improves wear resistance by three ways.
Firstly, smaller the size of the filler, lesser is the interparticle space (IPS) between the individual fillers. [15] This results in a more homogenous material. Minimal IPS means that the fillers are closely packed and smaller amounts of resin are exposed to the surface. Fillers protect thin expanses of resin from abrasive forces as suggested by Yap et al [16],[17] and Mortier et al [18] . Jorgensen observed that an IPS less than 0.01 microns could protect the resin from abrading forces.
Secondly, smaller size of the nano filler might have allowed for the maintenance of smoother surface during clinical use. This reduces the coefficient of friction and thus the wear of the material. [19],[20] Dr. Stolarski noted that when increased friction is present the critical load for wear could fall to 10% of the friction free value. When the surfaces are rough, contact between the two surfaces occurs at certain points, leading to stress concentration and initiation of micro cracks.
The nano ceramic composite (Group V) had 2 nm sized organically modified ceramic particle and 1 µm sized glass filler in its filler composition. This hybrid nature may further decrease the micro crack propagation. This result is in support of the findings by H.H.K.Xu et al. [21] Condon [6] observed a linear relationship between the filler volume and wear resistance with a statistically significant reduction in the wear when the filler volume was less than 48%.
In our study, among all the groups, group III and group IV had the poor wear resistance with mean wear depths of 13.760.39 in and 17.970.43 m respectively. This can be related to the comparatively larger size of minifillers in these composites. Larger filler particle size leads to increase in the inter particle spacing and the material acts as a heterogeneous solid. Thus the resin wears faster leaving the filler exposed. This leads to separation of the phases and subsequent sub surface damage.
Group III had better wear resistance than group IV. This could be because group III direct packable composite had irregular shaped fumed silica filler. Fumed silica is known to have an increased surface area which allows good stress transfer between the resin and filler phases slowing down the process of filler particle loss [14] . According to the manufacturer, the irregular shaped fillers might provide good inter particle locking. This can result in a decrease in the filler loss during wearing.
Zirconia was added as filler in group IV direct packable composite to improve the hardness of the material according to the manufacturer and supported by Say et al .[22] But the results of this study did not show better wear resistance with improved hardness of the filler. The harder zirconia filler tends to transmit rather than absorb the stress, generated through wear abrasion [15] . This leads to accelerated crack propagation and delamination. According to the results of this study, hardness does not influence the wear properties of the composite materials. Won Suck Oh and Xiaoqiang predicted that the relationship of wear to hardness is not valid for materials that are brittle in nature such as composites, since brittle materials wear by micro fracture and delamination. [23]
Degradation of resin and filler over a period of time by water sorption [18] and tribochemical reaction [24] can occur in composites intra-orally similar to corrosive process in amalgams. Thus a long-term clinical study on the wear property of these materials might be useful to confirm the findings of our in-vitro study.
| Conclusion | | |
The results of this study conclude that nano ceramic composites marketed as universal composites can be a good alternative to packable posterior composite resin as a direct tooth colored posterior restorative material.
| References | | |
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Correspondence Address:
A C Krithika
Department of Conservative Dentistry and Endodontics, Sri Ramachandra Dental College, Porur, Chennai 600116
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/0972-0707.42318
[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3] |
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