In vitro Wear of 4 different Universal Composites
Patricia Matias1a, Jean-François Roulet2b*, Nader Abdulhameed2c, Chiayi Shen2d
1Department of Dentistry, University of Brasilia, Brasília DF 70910-900, Brazil
2Department of Restorative Dental Sciences and Center of Dental Biomaterials, College of Dentistry, University of Florida, Gainesville FL 32610-0415, USA
- DDS student, Scientific Initiation Scholar
- Prof. Dr. med. dent., Director of Center for Dental Biomaterials
- BDS, MS, Visiting scientist, Candidate PhD Material Sciences
- PhD, Professor
Professor Jean-François Roulet, DDS, PhD, Prof hc
Director of Center for Dental Biomaterials, Department of Restorative Dental Sciences, College of Dentistry
University of Florida, 1395 Center Drive, Room D9-6 PO Box 100415, Gainesville FL 32610-0415, USA
Tel / Fax: (352) 273-5850
*(all pictures and tables are shown in .pdf document)
Objectives: To test the hypotheses that: (1) there is no difference in the volumetric wear among composites tested, and (2) there is no difference in the wear rates calculated from the linear relationship of wear increase over cycling.
Methods: Two composites comprising pre-polymerized particles (Herculite-Précis [H], Tetric-N-Ceram [T]), one composite with very fine glass fillers (Charisma Opal [C]), and one composite with mixture of agglomerated and nonagglomerated silica, and zirconia fillers (Filtek Z 350 XT [F]) were tested in a chewing simulator (CS 4.8, SD Mechatronik) with spherical Steatite antagonists (Ø 6 mm). Eight specimens of each composite were made by applying two increments into aluminum specimen-holders with a cylindrical cavity (Ø 8 mm, depth 1.5 mm), light cured (Bluephase G2; 1383 mW/cm2) for 20 s, polished to high gloss, and subjected to mastication cycles (59 N, 1.2 Hz, lateral movement 0.7 mm) and thermocycles (5/55°C; 116 s per cycle) simultaneously. After each 100, 500, 1,000, 2,000, 5,000, 10,000, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, 110,000, and 120,000 mastication cycles, 3D images of worn surfaces were captured with Laserscanner LAS-20 (SD Mechatronik), and volumetric wear in mm3 was calculated by Geomagic software.
Results: Five samples were lost due to separation at interface between increments. The means of total volumetric wear (mean±SD) after 120,000 cycles are 0.78±0.26 mm3, 0.91±0.15 mm3, 0.99±0.29 mm3, and 1.15±0.36mm3 for F, H, T and C. Wear rate of each surviving sample between 2,000 and 120,000cycles was calculated by linear regression (R2>0.99 for all specimens). The wear rates (mean±SD ; μm3/cycle) are, 5.97±2.29×103, 6.85±1.06×103, 8.91±2.81×103 and 6.43±0.58×103 for F, H, T and C. GLM shows there is statistically significant difference in wear rate among the four materials (p=0.0488), in total volumetric wear among the four composites (p=0.1183) and wear of antagonist (p=0.3027)
Conclusions: The first hypothesis was accepted and the second hypothesis was rejected. To prevent separation between increments, future specimen preparation should consider bulk fill.
Keywords: composite, in vitro wear, chewing simulator
When composite resins were introduced into the market, depending on the filler content, some of them were recommended for anterior and posterior use. Adaptic (Johnson & Johnson, New Brunswick NJ, USA) was chosen as composite material for a clinical study mainly based on mechanical and physical data (1). Furthermore the authors reported wear results obtained with a tooth brushing machine. Adaptic showed similar wear than amalgam, when abraded with a slurry of heavy CaCO3, but 4x less wear when abraded with pumice. This was not confirmed clinically. In the 3-year report, the same authors (2) using the USPHS criteria for evaluating restorations described a dramatic decrease in the quality of the occlusal anatomy from 44 “Alpha” at baseline to 6 “Alpha” and 36 “Bravo” ratings, which was interpreted as wear. This result was confirmed by Roulet (3). However, in this study the wear was measured using a 3 coordinate measuring machine. Using a 100 µm grit the x, y, and z coordinates wear was determined for approximately 60 points per occlusal surface. The wear was determined as average difference from a plane defined by the cusp tips. It was found for Adaptic, that after 3 years the average vertical wear was 224 ± 151 µm. Using better equipment, it became possible to distinguish between wear in the occlusal contact area (OCA) and the contact free area (CFA). It was found that the OCA:CFA ratio equals to an average of 2.5. (4). Different equipment was used to accomplish this: Profilometer (5,6), 3 coordinated table using a long lens to determine the vertical dimension (7) and a computer controlled 3-coordinated table with a mechanical switch for the vertical dimension (8). Today laser scanner measures fast and efficient occlusal anatomy and wear (9).
Today composite’s wear resistance is much improved mainly due to refinement of filler technology (10). Clinical studies document the excellent longevity of posterior composite restorations if applied correctly (11,12,13,14,15); therefore, it seems that wear is not the main issue anymore. Wear behavior of restorative materials is still important and in focus, since today more and more really occlusal bearing restorations are placed clinically due to the lately observed expansion of indication for composites. Palaniappan et al. (16) reported for hybrid composites a vertical substance loss in the same magnitude as for enamel. However, if looked at the wear in volume, enamel was worn significantly less than the 3 tested composites. Frankenberger et al. (17) observed significant wear of nanohybrid and fine hybrid composite restorations in extended class II cavities after 8 years of service.
With more nanoparticle-based composite materials being introduced, there is a need of investigating wear resistance of those materials. Therefore, the objective of this investigation was to measure in vitro wear of 4 nano particle based commercial universal composites. The null hypotheses are (1) there is no difference in the volumetric wear among composites tested, and (2) there is no difference in the wear rates calculated from the linear relationship of wear increase over cycling.
2. Materials and Methods
The four universal composites were received and the samples prepared according to standard procedures being equal for each brand. The manufacturer, filler composition and Lot numbers are displayed in Table 1. Eight samples were prepared for each brand (n=8), which resulted in total 32 samples.
Thirty-two aluminum sample holders (inner Ø 7.9 mm depth 1.5 mm) were grit blasted with 27 µm aluminum oxide (EtchMaster Tips Small, Groman, USA) then one coat of universal bond (Monobond Plus, Ivoclar Vivadent, Liechtenstein) was added and left for 60 s, followed by air blasting to evaporate the solvent. Then one coat of adhesive (Optibond FL 2, Kerr, USA) was applied and light cured for 10 s using the Bluephase G2 (Ivoclar Vivadent, Liechtenstein) at “HIGH Power” mode delivering 1383 mW/cm2 at a distance of 1.5 mm (verified with MARC Resin calibrator, BlueLight Analytics Inc., Halifax, NS, USA). The composites were filled into the sample holders in two increments, and each was light cured for a total of 40 s that delivered 55 J/cm2 (1383 mW/cm2x40).
The composite surfaces where finished and polished by using (Sof-Lex Disks, 3M, USA), light orange disc for finishing and yellow disc for polishing for 10-15 s. All samples were stored in distilled water for 3 weeks at 37 ⁰C. Steatite balls (Ø 6 mm) mounted into aluminum holders with composite were used as antagonists. One antagonist per sample (n=32) was used and discarded after finishing all cycles. The samples were randomly distributed to the chewing simulator chambers (CS-4, Mechatronik, Germany) using random numbers (Table 2; Remington and Schork, 1970). The chewing simulator was run according to the parameters listed in Table 3. The composite samples were scanned after 100, 500, 1,000, 2,000, 5,000, 10,000, 20,000, 30,000, 40,000, 50,000, 60,000, 70,000, 80,000, 90,000, 100,000, 110,000, and 120,000 mastication cycles.
By using geometric software (Geomagic Control 2014, Geomagic, Cary, NC, USA), the scanned data were used to measure the wear of the samples after each round. The flat surface of the sample was used as a reference plain. All wear facets at 120,000 cycles were examined with a digital microscope and digital images recorded (Keyence VHX 1000, Keyence Corporation of America, Elmwood Park, NJ, USA).
The wear of the Steatite antagonists was not measured with the laser scanner due to difficulty of establishing reference plane. They were determined indirectly by the geometric relationship (Fig. 1a). The radius (b) of the wear facet was measured using the Keyence digital microscope. Knowing the radius (r) of the sphere, we calculated the height of the abraded dome (h) using the following formula (Fig 1 b),
Sample C1, C3, C4, F2 and F7, which experienced separation of the specimen at the interface between increments before conclusion of the experiment, were excluded from the analyses. Due to imbalance of number of specimen among groups of composites, GLM (SAS, 9.4; SAS Institute Inc., Cary, NC, USA) is used to analyze the variance of wear volume of the composites and antagonists. After the initial wear-in period, linear relationship between the wear volume and number of cycles from 2,000 to 120,000 cycles is apparent for all samples investigated. Linear regression was performed using SAS to determine the slope of the curve. The values represent the wear in μm3/cycle of the sample and is called wear rate in this paper. GLM is used to determine statistical differences of wear rates among the four composite groups. The correlation coefficients (r2) between wear of antagonists and volumetric wear of composites, and between wear of antagonist and wear rate of composites are calculated by linear regressions.
GLM analyses show that after 120,000 chewing cycles there are no statistical differences in total volumetric wear among the four composites (p=0.1183) and wear of antagonist (p=0.3027) with respective composite. Linear regressions of the composite wear volume vs. number of cycles showed the degree of fit (r2) was >0.99 for each of the specimen investigated. GLM analysis of the values of wear rate determined for each specimen shows there is statistical significant difference among composites groups at p=0.0488. It is important to note that the p-value is almost at the point of no significant difference of p=0.05. The mean values along with standard deviation of the total wear volume at 120,000 chewing cycles, wear of respective antagonist and the wear rates are shown in Table 4. The mean accumulative wear volumes with respect to number of cycles for each group of composite results of the composites along with the best fit straight line of the mean values are displayed in Fig. 3. Analysis of correlation shows both wear volume and wear rate increases slightly as the wear of antagonist increases but with low correlation coefficient at r2= 0.0027 and r2= 0.2081, respectively.
Some exemplary pictures of wear facets of the composites are shown in Fig. 4.
The wear of all composites investigated are linear with respect to number of chewing cycles after wear-in period and statistical analyses show that there is significant difference among the composites in wear rate at p=0.0448 and no significant difference (p=0.1183) in final volumetric wear. Therefore, the first hypothesis was accepted and the second hypothesis was rejected.
Various wear testers have been used to investigate the wear behavior of composites since their introduction. Wear simulation is a very complex process and over the last 40 years scientists have tried to build devices capable of simulating the wear of dental restorative materials. The outcome is heavily influenced by a multitude of factors, such as wear type reflected by the wear testing equipment, the load used, the antagonist material and shape, the use of thermocycling and finally of the material that is worn. In the following these factors will be discussed briefly in the context of this study. One family of wear devices uses 3-body wear. This means that a third body, mimicking food, is forced between the two bodies which stress the material with wear. Such devices are the ACTA wear machine (18), the Oregon Health Science University (OHSU) machine (19), the Alabama wear simulator (20), and the CW3 of Peking University (21) and multiple toothbrushing machines (22,23). Common of these devices is that by introducing a third body in suspension affects the results heavily and it is not known which quality of the third body would be clinically relevant for wear of the occlusal surface. The ACTA machine can be run as two body wear tester as well, having the two wheels run in contact. Osiewicz et al. (24) have reported differences in wear between 1 and 62.5 fold more wear for moving from 2-body wear to 3-body wear using the same abrasive, but different material combinations (4 composites for antagonist wheel and 6 composites for other wheel).
Another approach is to use a two body wear. A simple and widely used device is the Taber abraser (25), which comprises two abrasive wheels engaging on a rotating disk under constant pressure. Two body wear can be induced as a pin on block principle where a pin (antagonist) is pressed under constant force onto a rotating disk (26) or oscillated against a flat surface (27). Using a pin on block approach, scientists have tried to simulate chewing movements by having an antagonist lowered on a surface, then slid sideways under load, disengaged from the load and being moved to starting point to begin the next cycle (28). Such devices are the Willitec Chewing simulator (29), the Minnesota artificial mouth (30), the CoCom Chewing simulator (31), the TE88 (32), the Tokyo Medical Dental University Chewing simulator (33) or the Mechatronik Chewing simulator used in the present study. We decided to use a Pin on block chewing simulator, because the load and movements are well controlled and there is no third body to deal with, which makes interpretation of the results less problematic.
Chewing forces are reported in the literature to vary from 20 – 120N (34). Most researchers use 5 Kg (49N), which has been reported by Gibbs et al. (35) to be the average chewing force under normal function. For the present study, 6 kg (58.9N) was chosen, with the idea to be able to discriminate better between the materials having a slightly higher load. However, the higher load apparently was incapable of discriminating the wear rate among the four material groups. Therefore, 5 kg load should be adopted as a standard for future study for ease of comparison. All wear facets exhibited typical grooves resulting from abrasive wear by harder antagonists with unique feature for each material. For Charisma (Fig. 3d), the white lines common for that group of material are not cracks on the surface but wear debris being folded perpendicular to the direction of horizontal movement. Some worn surfaces of Tetric-N-Ceram samples appear to have a round tab to the oval wear spot (Fig. 3b). The likely cause is that samples had shifted in the initial stage of testing.
There is no agreement in the literature about the material and the shape of the antagonists to be used in in vitro wear studies. The following materials have been reported (34): stainless steel, natural teeth, tooth cusps shaped to a specific shape and highly polished, leucite reinforced ceramic (Empress), Steatite (magnesium silicate ceramic), Degusit (aluminum oxide) and Zirconium oxide (36). Average radius of natural cusps is 1.04 and 1.79 mm (34). Artificial materials are used with diameters of 3mm (36), 4mm (20,33), 6mm (37,38) or 12mm (27,32). In the present study 6mm Steatite antagonists were chosen. For horizontal movement, both 0.3 and 0.7 mm have been used, we used 0.7 mm as it is more commonly used. As a measurement tool a laser scanner was used. Heintze et al. (39) have shown that laser scanners give the same results as the ones obtained with optical or mechanical profilometers.
Due to the notable differences in in vitro wear testing methods described above it is almost impossible to directly compare the present results to other studies. Therefore, comparisons only to studies done with Willitec/Mechatronik wear testing machines are reported. As in the studies of Heintze et al. (34,39) and Wang et al. (27) the wear development over the number of cycles was linear. We can confirm Heintze’s data (34,39) that the Antagonist wear is about half the wear of the composite materials. In contrast, the wear reported for a multitude of materials by Ivoclar Vivadent R&D is slightly lower than the wear found in this study. This may be due to the higher load used in the present study and the use of different antagonists (Empress vs Steatite). Lazaridou (38) found for Tetric Evoceram 0.3297 mm3, while Tetric N Ceram in the present study showed 1.15 mm3, which is substantially higher. Again, there are differences in the method, may explain these difference. The load used in the present study was 20% higher, we used thermoclycling, Lazaridou et al. (38) did not.
Most in vitro wear test methods demonstrate a steep increase in wear initially, also called wear-in or run-in phase, and then a flattening of the curve that appears increasing in a linear fashion, thereafter. The wear profile of individual sample tested and the profile of mean of the material groups all exhibited the wear-in pattern (Fig. 2). The duration of wear-in varies among material groups. In the literature, this linear relationship is often recognized (27,34,39,40) but not used for calculating wear rates. Often for comparison, the final volumes of wear were used for comparison. Since the total volume of wear also depends on the number of cycles and the extent of vertical movement, it becomes necessary have both information available for comparison. Wear-in phase does not truly reflect the wear of the material but include the wear generated during the initial stage when the composite and the antagonist are adjusting to accommodate each other in forming a sliding interface. The linear portion of the curve can be used to calculate the wear rate without the influence of the wear-in phase. When the load and the horizontal movement remains the same, the effect of testing duration by cycles disappears when wear rates are used. A straightforward comparison of in vitro wear will become possible. Lastly, the unit for the wear rate should also be standardized. The unit of mm3 is commonly used in discussing of volumetric wear and µm is used in presenting wear in depth. We used the unit µm3/cycle for wear rate in this study. However, the quantity of µm3/cycle is very small, a factor of 103 is needed (Table 4). The unit of mm3/cycle, on the other hand is so big that a factor of 10-6 is needed. As a compromise, we suggest that mm3/megacycle be used in expressing volumetric wear for comparison. As such the values of wear rates shown in Table 4 would be presented without the factor of 103.
The four tested composites showed a linear development of wear over the number of cycles and showed a wear which was comparable to wear in other studies. It can be expected that in the clinical reality they will behave similar than other composites of their class.
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