Evaluation of resin composite materials. Part I

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Research Article

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Evaluation of resin composite materials. Part I: In vitro investigations ROLAND FRANKENBERGER, DMD, PHD, FICD, FRANKLIN GARCÍA-GODOY, DDS, MS, ULRICH LOHBAUER, DIPL ENG, ANSELM PETSCHELT, DMD, PHD & NORBERT KRÄMER, DMD, PHD ABSTRACT: Purpose: To evaluate different resin-based composites using a variety of in vitro investigation methods to predict their clinical behavior. Methods: Materials selected for this study were Heliomolar radiopaque (microfilled), Tetric Ceram, Pertac II (minifilled hybrids), Ariston pHc (ion releasing hybrid), and Solitaire I (hybrid with porous fillers). The evaluated in vitro criteria were three-body wear according to the ACTA method, microtensile bond strengths to enamel and dentin, flexural strength (four-point bending strength), flexural fatigue behavior (flexural fatigue limit), and calcium ion release (for Ariston pHc). Results: Concerning wear resistance, Ariston pHc (20.5 µm after 200,000 cycles) was inferior to the other materials (13.7-15.9 µm). Microtensile bond strengths to dentin were similar for Heliomolar (32.0 MPa), Tetric Ceram (30.4 MPa; both bonded with Syntac Classic), and Pertac II (30.8 MPa; bonded with EBS Multi) being above Solitaire I (22.5 MPa; bonded with Solidbond) being above Ariston pHc (13.2 MPa; bonded with Ariston Liner). Enamel bond strengths for Heliomolar (40.0 MPa), Tetric Ceram (36.5 MPa), and Pertac II (38.9 MPa) were significantly higher than for Solitaire I (26.6 MPa) which was above Ariston pHc (7.2 MPa). Heliomolar, Tetric Ceram, and Pertac II revealed higher µ-TBS to enamel than to dentin, Ariston showed the contrary, and Solitare exhibited no difference between enamel and dentin µ-TBS. Solitaire I exhibited a lower initial flexural strength than the other materials, the computed fatigue strength of the material even dropped to the level of glass ionomer cements (17.9 MPa). Long-term calcium release data for Ariston exhibited a continuously high calcium release becoming lower at the end of the observation beyond 21 months (Am J Dent 2005;18:23-27). CLINICAL SIGNIFICANCE: The in vitro investigation of the failed materials Soitaire I and Ariston pHc exhibited distinct weak links for both materials. When compared to well-suited microfilled and hybrid composites, flexural properties and enamel bonding for Solitaire and adhesion as well as wear resistance for Ariston were inferior. �: Dr. Roland Frankenberger, Department of Operative Dentistry and Periodontology, University of ErlangenNuremberg, Glueckstrasse 11, D-91054 Erlangen, Germany. E-�: [email protected]

Introduction All over the world, tooth-colored materials have gained significant popularity for patients. This observation is attributed to the alleged toxicity of amalgam as well as to higher esthetic demands of the patients.1,2 One of the major trends in today's dentistry is the simplification of bonding and application procedures when resincomposites are used which have been repeatedly reported to be technique-sensitive.3 Consequently, the market was flooded with new materials within a few years, and in many cases the mean survival time on the market is rather short. It is logical that when a successor is introduced on the market, the quality of the predecessor may not have been ideal. The main problem with clinical studies is that when clinical studies are once started, there may elapse a certain period of time until really helpful results are demonstrated. When significant observations occur after a few years, the particular material presently under investigation may no longer be on the market. 2,4,5 The present study thoroughly investigated two innovative materials promising to provide interesting properties. However, both materials are not on the market anymore, indicating that apparently something went wrong, or needed improvement(s). Solitaire I,a one of the first packable composites was marketed in 1997 as low-shrinkage composite with condensable characteristics and a new filler technology involving porous particles.7,8 The packability should facilitate the often difficult reconstruction of proximal contact areas. Ariston pHcb followed a non-adhesive concept, capable of "controlling" the marginal gap by the release of sufficiently

high amounts of fluoride and calcium ions, which was promised to be caries inhibiting. From a chemical point of view, Ariston was a resin-based composite charged with special ion releasing filler particles.9-14 This led to the term "smart material", because it was claimed to release high amounts of fluoride and buffer acids by the release of hydroxy ions in the case of lowered pH values inside the gap. Calcium ions indeed have a positive effect on remineralization, fluoride ions accelerate the remineralization and intervene the metabolism of plaque bacteria.10,12,14 Ariston revealed an ion release twice as high than known from GICs, and its cumulative curve tended to increase over time instead of tending towards constant values.10,11 Ariston offered an interesting philosophy proposing the use of a resin composite without bonding, however with a sufficiently high amount of ion release to prevent recurrent caries.15 Several studies reported successful ion releases of Ariston to be promising in vitro,9-11,13 and resulting in antibacterial activity of the material.16,17 This study surveyed a variety of in vitro data to filter clinically relevant parameters. This should help to effectively preclinically evaluate future dental biomaterials before marketing.

Materials and Methods Three-body wear resistance - Wear resistance by means of three-body wear was determined using the ACTA machine.18,19 The samples were made in a specially manufactured mold exactly copying the high-grade-steel sample holding wheel. The samples were fixed on the sample wheel of the three-body testing device using Rocatecc as pre-treatment for adhesively fixed resin composites. The wheels were stored for 2 weeks and

24 Frankenberger et al subsequently ground under profuse water cooling in ascending grits up to #1000. The wear test was carried out as sliding contact wear test with millet seed as abrasive medium. The wear medium consisted of 150 g millet seed shells (ground in a coffee grinder) in 275 mL distilled water with 1 g sodium azide having been added as a sterilizing agent to suppress bacterial growth. The test wheels were pressed against the antagonist wheel by 15N spring force. The sample holding wheel rotated at 1 Hz. To simulate the sliding action of the opposing teeth, the surface speed of the antagonist wheel differed by 15% (slip). After every 105 rotations the abrasive medium was changed. After 200,000 cycles of the test wheel, the worn and unworn areas were 3D-scanned by use of a profilometer.d The sample wheel was mounted on an axis having been rotated by a computercontrolled micro-step motor. For each specimen (n=9), 160 tracks across the worn area were scanned (x/y/z resolution: 50µm, 50µm, 0.5µm). The computer-based analysis of the profilometrically assessed data was computed using the software package Xpert for Windowse 2.0 The principle of this program is based on a reference plane allowing a 3D data survey of the space beneath it. This plateau was graphically marked at the unworn area of the sample. Only areas revealing more than 70% of the measured points below the reference plane computed with Xpert were arbitrarely defined as worn area and consecutively considered for further data analysis. Microtensile bond strengths to enamel and dentin - A total of 25 caries-free human third molars was used in this investigation. The teeth were stored in 0.1% thymol solution at ambient temperature for less than 4 weeks after extraction and were consecutively debrided and examined to ensure that they were free of defects. For dentin bond strength testing, the occlusal enamel was removed and 700-900 µm thick enameldentin disks were cut from the mid-coronal level of the tooth, perpendicular to the tooth axis by slow-speed diamond-sawf sectioning under continuous water cooling. A standardized smear layer was created in both corresponding surfaces by wetsanding with 600-grit sandpaper for 60 seconds. Eighteen teeth were randomly assigned to four dentin adhesives. The dentin surfaces were treated with the adhesive systems Syntac Classic,b EBS Multi,c Solidbonda for Solitaire I, and Ariston Linerb for Ariston pHc according to the manufacturer's instructions except for Syntac Classic which was applied upon etching dentin for 15 seconds like alternatively recommended in the manufacturer's protocol.21 The crowns of the flattened teeth were then reconstructed with four 1 mm-layers of the corresponding resin composites each layer being light-cured for 40 seconds with an Elipar Trilightc curing unit without soft-start mode. The intensity of the light was checked periodically with a radiometerg to ensure that an intensity 650 mW/cm2 were exceeded. The µ-TBS specimens were stored in distilled water for 24 hours at 37°C and then sectioned. The peripheral areas of the tooth were removed resulting in a 5 mm x 5 mm square central part. The remaining specimen was sectioned into five slices, which were sectioned again to receive 25 resin-dentin beams. The saw was adjusted to steps of 1 mm, due to the thickness of the blade (300 µm) resulting in sticks with a crosssectional area of 700 x 700 µm (0.5 mm2). From the resulting 25 central sticks of each tooth, 10 were selected revealing a dis-

American Journal of Dentistry, Vol. 18, No. 1, February, 2005

tance to the pulp of 2.0 ± 0.5 mm resulting in 20 microtensile specimens for each group (n=20).22 For evaluating enamel bond strength, the teeth were cut longitudinally and the buccal and lingual aspects of the teeth were flattened on an area of 4 mm x 4 mm without dentin exposure using 600 grit sandpaper under continuous water cooling. Then the same specimen preparation procedure was carried out like in the dentin specimens, without rubbing of adhesive components. From the resulting sticks of the teeth, 20 with an enamel thickness of 0.5 ± 0.1 mm were selected resulting in 20 microtensile beams in each group (n=20). For the simulation of mixed cavities, also during enamel bonding procedures the primers were applied according to the manufacturer's recommendations for use. The sticks were mounted in a Zwick fixing deviceh with glue waxi and debonded using an universal testing machineh with a 50N load cell traveling at a crosshead speed of 1mm/ minute. µ-TBS was determined by computing the quotient of maximum load (N) and adhesion area. Specimens having failed during the specimen preparation process were recorded as pretest failures and were included in the results as 0 MPa. Flexural strength and flexural fatigue limit - Each resin composite under investigation was placed in a special mold (2 x 2 x 25mm) and light cured on five overlapping points on each upper and lower side of the specimen. Every point has been cured for 40 seconds as per manufacturer's recommendation and ISO 4049 standard. Thirty two beams for each group were manufactured and stored for 1 day in distilled water at 37°C. The flexural strengths were measured using the four-pointbending test (n=12). The specimens were loaded until fracture with a crosshead speed of 0.75 mm/minute in a universal testing machine.h The flexural fatigue limits (FFL) of the composite materials were determined under equivalent test conditions and for 105 cycles at a frequency of 0.5 Hz (n=20). The "staircase" approach methodology23,24 was used to evaluate the FFL values: the cyclic stress tests were conducted sequentially, with the maximum applied stress in each succeeding test being increased or decreased by a fixed increment of stress, according to whether the previous test resulted in failure or not. The first specimen was tested at approximately 50% of the initial flexural strength value. As the data are concentrated around the mean stress, the number of specimens required is less than with other methods.23,24 The tests were carried out at 37°C under distilled water. Fractographic examination was performed under a light microscopej on all specimens and under a scanning electron microscope (SEM) ISI SR 50k on representative specimens (five for each set; Fig. 1). Long-term calcium release of Ariston pHc - Ten discs with 11 mm diameter and 2 mm thickness were light cured according to the manufacturer's recommendations. The specimens were stored in 0.9% saline solution. To prevent the leaching process from an acidic influence of CO2, both discs and glass vials were fully rinsed and covered with nitrogen gas. After 1 day and subsequently after 1, 2, 3, 4, 5, 6, 9, 12, 15, 18, 21, 24, 27 and 30 months, calcium ion release measurements were carried out. The solution has been changed after every single measurement. The remaining solutions were subjected to complexometric titration using a 0.001M ethylenediamine-tetraacetate disodium

American Journal of Dentistry, Vol. 18, No. 1, February, 2005

Table 1. Mean ACTA-wear rates. Same superscript letters indicate no statistically significant difference (ANOVA, modified LSD; P< 0.05). ____________________________________________________________________________________________________ Material (n=9)

Mean wear rate [µm](SD)

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Ariston pHc 20.5 (4.4)B Solitaire I 13.7 (3.3)A Pertac II 14.6 (3.5)A Tetric Ceram 15.1 (2.8)A Heliomolar 15.9 (3.4)A ____________________________________________________________________________________________________ Table 3. Results for flexural strength (4-PB). Same superscript letters indicate no statistically significant difference (ANOVA, modified LSD; P< 0.05). The flexural fatigue limits are displayed as percentage of the initial strength in the third column. ____________________________________________________________________________________________________ Material

4-PB [MPa](SD) FFL[MPa](SD) FFL (n=9) (n=20) [% of 4-PB] ____________________________________________________________________________________________________ Ariston pHc 89.3 (7.9)A 44.3 (6.4) 50 Solitaire I 50.8 (10.9)B 17.9 (5.1) 35 Pertac II 83.7 (14.4)A 47.6 (2.5) 57 Tetric Ceram 75.2 (15.5)A 45.3 (11.8) 60 Heliomolar 85.9 (12.8)A 39.4 (9.1) 46 ____________________________________________________________________________________________________

salt (EDTA) solution (Triplex IIIl) and the calcium specific color indicator Eriochrome Black T.l Calcium ion release was calculated according to the consumed amount of Triplex III solution. An 0.001 M aqueous CaCl2 solution served as a control. The final specimens surface condition after 24 months was examined in an SEM (Fig. 2). Statistical evaluation - The statistical analysis was computed with SPSS for Windows,m version 10.0. The in vitro investigations were analyzed depending on their distribution behavior (Kolmogorov-Smirnov test). Wear, flexural strength, and ion release data were normally distributed and subjected to ANOVA and a modified LSD post hoc routine. Bond strength values were not normally distributed and therefore analyzed to compute differences between dependent groups (Wilcoxon matched-pairs signed-ranks test) and among independent groups (Mann-Whitney U test, correction method according to Bonferroni-Holm) for pairwise comparisons.

Results Three-body wear resistance - The results or ACTA three-body wear are shown in Table 1. Ariston exhibited a significantly lower wear resistance than the other materials under investigation (P< 0.05; Table 1). The resin composites Heliomolar, Tetric Ceram, Pertac II and Solitaire showed no significant difference regarding three-body wear (P> 0.05). Microtensile bond strengths to enamel and dentin - The results of the dentin and enamel µ-TBS investigations are displayed in Table 2. Regarding bond strength to dentin, the reference materials Pertac II (30.8 MPa), Tetric Ceram (30.4 MPa) and Heliomolar (32.0 MPa) resulted in significantly higher bond strengths compared to Solitaire I (22.5 MPa) and Ariston (13.2 MPa) (P< 0.05; Mann-Whitney U-test). Among the latter two materials, Solitaire exhibited significantly higher dentin bond strengths than Ariston (P< 0.05). Facing enamel bond strength, Pertac II (38.9 MPa), Tetric Ceram (36.5 MPa), and Heliomolar (40.0 MPa) showed similar bond strengths (P> 0.05) being both higher than Solitaire I (26.6 MPa) (P< 0.05). Solitaire I revealed significantly higher enamel bond strengths than Ariston (7.2 MPa)(P< 0.05). Only in the enamel µ-TBS specimens of Ariston pHc 40% pre-test failures occured (n=8).

Evaluation of resin composites 25 Table 2. Mean µ-TBS values. Same superscript letters indicate no statistically significant difference among different materials (Mann-Whitney U-test; P< 0.05). Asterisk means significant differences between the different adhesion substrates enamel and dentin (Wilcoxon test; P< 0.05). ____________________________________________________________________________________________________ Material

µ-TBS to dentin [MPa](SD)

Significance

µ-TBS to enamel [MPa](SD)

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Ariston pHc 13.2 (4.2)C * 7.2 (6.4)c Solitaire I 22.5 (9.2)B n.s. 26.6 (7.4)b Pertac II 30.8 (8.8)A * 38.9 (9.2)a Tetric Ceram 30.4 (9.4)A * 36.5 (9.3)a Heliomolar 32.0 (8.1)A * 40.0 (10.0)a ____________________________________________________________________________________________________ Table 4. Results for calcium ion release and mean weight of the specimens over the observation period of 24 months. ____________________________________________________________________________________________________ Time Calcium release [µg/ml](SD) Weight [mg](SD) ____________________________________________________________________________________________________ 1 day 138.1 (7.9) 301.3 (6.3) 1 month 171.3 (16.0) 303.2 (6.6) 2 months 133.9 (8.5) 302.9 (6.5) 3 months 159.4 (11.0) 302.8 (6.5) 4 months 120.6 (10.7) 302.6 (6.6) 5 months 142.3 (11.1) 302.1 (6.8) 6 months 137.1 (7.9) 301.6 (6.9) 9 months 187.8 (15.1) 301.2 (7.1) 12 months 205.4 (23.0) 301.0 (7.2) 15 months 188.7 (19.1) 299.5 (7.3) 18 months 186.5 (1.4) 298.2 (7.4) 21 months 126.8 (35.6) 295.6 (8.3) 24 months 94.0 (27.3) 298.1 (7.5) ____________________________________________________________________________________________________

Comparing the bond strengths to enamel and dentin, for Pertac II, Tetric Ceram, and Heliomolar enamel bond strength was higher than dentin bond strength (P< 0.05), for Ariston it was recorded vice versa (P< 0.05), and for Solitaire no statistical difference between enamel and dentin µ-TBS was computed (P> 0.05; Wilcoxon test). Flexural strength and flexural fatigue limit - The results of the flexural strength analysis are listed in Table 3. Solitaire I revealed a significantly lower initial flexural strength (50.8 MPa) than the other resin composite materials (75.2-89.3 MPa; P< 0.05). The flexural fatigue limits of the materials Ariston pHc, Pertac II, Tetric Ceram, and Heliomolar were computed to be in the range of 39.4-47.6 MPa which is 46-60% of the inital values. Only Solitaire I showed a dramatical loss in long-term flexural strength, expressed by a flexural fatigue limit of 17.9 MPa being 35% of the initially recorded flexural strength. Long-term calcium release - Ariston pHc showed a similarly high release of calcium ions over the whole observation period of 24 months (Table 4, Fig. 1). Between the 21 and 24 months of the investigation, the amount of released ions significantly decreased (P< 0.05, ANOVA). The weight of the specimens remained stable over 24 months (P> 0.05). However, the appearance under the SEM revealed a completely cracked surface (Fig. 2).

Discussion There is no doubt that adhesive dentistry has gained efficacy during the last 15 years. However, the clinical application of conventional resin composites still involves a certain amount of technique-sensitivity during bonding procedures.3 Therefore, innovative concepts have been always in the focus of the practitioner's point of view. One of the first approaches to go another way was represented by the resin-based composite material Solitaire when

26 Frankenberger et al

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Fig. 1. Accumulated and absolute calcium ion release of Ariston pHc over 24 months.

it was introduced in 1997. First of all, the promise that this particular material was supposed to be really packable or even condensable made it very attractive. The packable character was thought to facilitate the reconstruction of anatomically correct proximal contacts, and polymerization shrinkage was announced to be 50% lower than conventional composites on the market. However, scientific reports exhibited inferior mechanical properties for Solitaire I regarding flexural strength and fracture toughness.7,8 Finally, the packable consistency made it necessary to apply a flowable composite as adaptation promotor for better internal adaptation.25 Analyzing the present in vitro data step by step, it is clearly evident that the main problem for Solitaire I definitely was an insufficient flexural strength behavior. The present in vitro results for Solitaire I regarding its flexural fatigue limit result in 17.9 MPa which would be a typical value for a glass ionomer cement but not for a resin-based composite material.7,8 Clinically, this potentially leads to chipping or even bulk fractures. Interestingly, in vitro µ-TBS to enamel was lower than for the reference materials Heliomolar, Tetric Ceram and Pertac II. An insufficient adhesive stabilization caused by a weaker enamel bonding performance could have supported the poor flexural strength in terms of being even more prone to fractures over a certain period of clinical service. Due to the apparently poor adaptation to cavities25 it would have been helpful to work with an intermediary layer of flowable resin composite such as Flowline,a however, this was not the goal of the present study and the manufacturer did not explicitly recommend that additional step in the instructions for use of Solitaire. Ariston pHc was a completely new approach with promising features as well. However, several questions arose from the modified use being different from actually adhesive concepts. Is wear behavior compromised by the release of ions? Does the so-called liner produce any adhesion to enamel or dentin? Are flexural characteristics affected by the ion release? Three-body wear of Ariston was ~30% higher compared to the other materials. This could negatively influence clinical performance, however, probably not during the early years of clinical service. It is known from a previous study comparing the compomers Dyract AP and Hytac, that clinically unacceptable wear normally occurs between 2- and 4-year recalls in

Fig. 2. SEM image of an 24 months aged Ariston pHc specimen for determination of calcium release. The specimen is completely covered with cracks.

clinical investigations. The true effect of the inferior threebodywear results can only be verified in a clinical investigation by means of a randomized prospective trial. Bonding performance was poor to both tooth tissues enamel and dentin (Table 2). Facing this background, another problem may arise from missing cuspal stabilization when the restorations are not bonded at all. This is probable because the in vitro µ-TBS at least to enamel, have been very low, being evident in the µ-TBS study with 7.2 MPa enamel bond strength and a 40% pre-test failure rate. This means that approximately half of the involved enamel is not effectively used for bonding and the other half is bonded poorly. In addition, the polymerization shrinkage of Ariston pHc is comparable to other hybrid composites on the market. This results in gaps all around these restorations.26,27 Flexural strength and fatigue behavior does not seem to be critical from the preclinical point of view for Ariston.28 The same was observed for the calcium ion release which was considerably high over the whole observation period of 21 months in vitro. However, the SEM analysis of aged Ariston specimens resulted in images of completely cracked surfaces indicating that the material seems to disintegrate over time (Fig. 2). According to the present in vitro results, the properties of both materials under investigation may not be sufficient for the proposed indication as amalgam substitutes. For Solitaire I, flexural strength combined with suboptimal enamel bonding

American Journal of Dentistry, Vol. 18, No. 1, February, 2005

performance is critical and may cause fractures over time. For Ariston, the problem is more complex. Even in the case of volumetric stability it is questionable whether cuspal stabilization is achievable without sufficient bonding. This may cause enamel cracks or even tooth fractures. Facing the results obtained by Yap et al29 and Martin et al,30 Ariston should exhibit a considerably high hygroscopic expansion because existing gaps become narrower after water storage. The trends arising from the present in vitro data are that both materials are at risk; Solitaire may be prone to fractures and gaps in enamel, and Ariston may be subject to enamel cracks, postoperative hypersensitivities and marginal discoloration. To definitively evaluate both materials Ariston and Solitaire, a randomized prospective clinical study had to elucidate their suitability. This was the goal of a further study acting as Part II of the present paper. Focusing both parts in vitro and in vivo would allow to judge whether in vitro data are able to predict clinical behavior over a certain period of time. Altogether, the present results indicate that a variety of preclinical in vitro investigations such as bond strength, fatigue and wear behavior is mandatory to screen the properties of dental biomaterials prior to any clinical trial. a. b. c. d. e. f. g. h. i. j. k. l. m.

Heraeus Kulzer, Dormagen, Germany. Ivoclar-Vivadent, Schaan, Principality of Liechtenstein. 3M ESPE, Seefeld, Germany. Perthen, Göttingen, Germany. ATI Technologies, Inc., Thornhill, Ontario, Canada. Isomet, Buehler, Lake Bluff, IL, USA. Demetron/Kerr, Danbury, CT, USA. Zwick Corp., Ulm, Germany. Yeti Dental products Corp., Engen, Germany. Zeiss, Jena, Germany. Leitz, Akashi, Japan Merck Corp., Darmstadt, Germany. SPSS Inc., Chicago, IL, USA

Acknowledgements: This study was supported by materials and a grant from Ivoclar-Vivadent (Schaan, Liechtenstein). The authors are grateful to Mr. Herbert Brönner for his assistance with the calcium release project. Dr. Frankenberger is Associate Professor, Mr. Lohbauer is Materials Science Laboratory Supervisor, Prof. Petschelt is Professor and Chair, and Prof. Krämer is Associate Professor, Department of Operative Dentistry and Periodontology, University of Erlangen-Nuremberg, Erlangen, Germany. Dr. García-Godoy is Professor and Associate Dean, Nova Southeastern University, Health Professions Division, College of Dental Medicine, Fort Lauderdale, FL, USA.

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