1

Introduction

For many years resin composites are considered a viable treatment option for all types of restorations . Nowadays direct and indirect resin composites are also frequently applied to build up the occlusion when extensive tooth wear took place . The success of such a treatment will depend on the reason for the wear e.g. erosion, bruxism or a combination of both. Reason for failure of direct resin composite restorations appears to be fracture, wear, and secondary caries . The best correlation of clinical wear, according to a denture model study of 13 experimental hybrid composites, was between wear, fracture toughness, and flexural strength . Subjecting resin composites to dynamical loading prior to fracture testing significantly reduces the fracture strength compared with values obtained after static loading . When selecting a resin composite to increase the occlusal vertical dimension information about the filler is important as the filler largely determines the mechanical properties of the material. The content, size and hardness of the filler, as well as the silanization of the filler will influence the effect on the composite itself as well as the antagonist. Resin composites with quartz and zirconium as the filler will abrade the opposing enamel .

To achieve long-lasting results resin composites should possess a high resistance against fracture and wear. In vivo and in vitro wear have been studied extensively . However, information about the interaction of the wear rates between resin composites with various compositions and in different combinations is still scarce. The aim of this in vitro study was to evaluate the wear between direct and indirect resin composite with different compositions in a two-body and three-body wear test.

2

Materials and methods

The materials tested in this study, their manufacturers and batch numbers are summarized in Table 1 . Three direct resin composites and three indirect resin composites were selected. Within each group of materials differences between the type, size and amount of filler existed. For the direct resin composite the filler-load decreased from APX > Z250 > HM (see Table 1 ). Looking at the hardness of the filler it decreased from Z250 > APX = HM . For the indirect resin composite the filler-load decreased from EST > SFY = ADO (see Table 1 ). The hardness of the filler decreased from EST = SFY > ADO.

The two-body and three-body wear were evaluated with the ACTA wear machine . The wear machine is equipped with two wheels of different diameters which rotate in the same directions with about 15% difference in the circumferential speed while having near contact. Two-body wear can be determined by full contact of the specimen wheel and antagonist wheel, while three-body wear can be determined by using an abrasive medium as third body between both wheels.

The specimen wheel accommodated six different composites. The composite specimens are placed on the circumference of the wheel while the other wheel serves as antagonist. The antagonist wheel was made of stainless steel (SS) with extra hardening of the outer surface or of composite APX or the nano-composite, FS, or Z250. Both FS and Z250 contain zirconium as the filler but in different size. All restorative materials were handled and cured according to the manufacturers’ instructions. The specimen wheels were kept wet at RT at all times throughout a period of 3 years. The antagonist wheel was duplicated by taking an impression of stainless steel wheel by silicon material (Heavy Tray, Correct Flow, Flextine, USA) in a metal ring. Duplicated composite antagonist wheels were made in layers and cured in light oven for 180 s (Denta Color XS, 1992, Kulzer).

For evaluating the two-body wear the wheels were placed in distilled water. For the three-body wear the wheels are placed in a slurry of white millet seeds in a buffer solution (pH 7) as reported previously . For both the two-body wear and the three-body wear the wheels were pressed against each other with a spring force of 15 N. A test run consisted of 200,000 cycles (55½ h) of the specimen wheel at a rotational speed of 1 Hz.

After the experiment, 10 tracings ( n = 10) were taken at fixed positions on the worn surface of specimens (PRK profilometer no. 20702, Perthen GmbH, Hannover, Germany) to determine the loss of material in μm. The average wear rate and their standard deviations was calculated from these profiles. The experimental details of this procedure have been reported previously .

The roughness (Ra) of the specimens was determined by a profilometer (PRK profilometer no. 20702, Perthen GmbH, Hannover, Germany). The surface of the specimens was evaluated by scanning electron microscopy (SEM) (XL 20, Philips, Eindhoven, NL).

A three-way ANOVA test, with wear two- or three-body, the resin composite, and the type of material for the antagonist as variables, was used to test possible significant differences in wear rate. The two- and three-body wear rates were independent statistically analyzed by two-way ANOVA ( P < 0.01) and Tukey post hoc test ( P < 0.01) with the resin composite and the type of material for the antagonist as variables. The correlation coefficient ( r ² ) between the wear rate and the filler content was also calculated. The software used was Sigma Stat 3.1 (SPSS Inc., Chicago, USA).

2

Materials and methods

The materials tested in this study, their manufacturers and batch numbers are summarized in Table 1 . Three direct resin composites and three indirect resin composites were selected. Within each group of materials differences between the type, size and amount of filler existed. For the direct resin composite the filler-load decreased from APX > Z250 > HM (see Table 1 ). Looking at the hardness of the filler it decreased from Z250 > APX = HM . For the indirect resin composite the filler-load decreased from EST > SFY = ADO (see Table 1 ). The hardness of the filler decreased from EST = SFY > ADO.

Table 1

Materials properties of materials used according to the manufacturers data.

Code	Material	Matrix/fillers	Filler d,e	Batch/exp/color
Z250	Filtek Z250 a	Bis-GMA, UDMA, bis-EMA, zirconia, silica	78 0.19–3.3 μm	20050727 2008-06 A3
SFY	Sinfony a	UDMA, Bis-EMA, borosilicate glass, pyrogenic silica	45 0.5–0.7 μm	216232 2008-12/A2
HM	Heliomolar b	Bis-GMA, urethane dimethacrylate, decandiol dimethacrylate, silicon dioxide, ytterbium trifluoride, copolymer	67 0.04–0.2 μm	H24852 2009-08/A3
ADO	Adoro b	Cycloaliphatic dimethacrylate, urethane dimethacrylates, decamethylenedimethacrylate copolymer, highly dispersed silicon dioxide	65 10–100 nm	H22320 2008-06/A3
EAD	Estenia C&B c	Bis-GMA, UDMA, decandiol dimethacrylate, surface treated alumina, silanated glass ceramics	92 2 μm	219AA 2008-05/A2
APX	Clearfil APX c	Bis-GMA, TEGDMA, silanated barium glass filler, silanated silica filler, silanated colloidal silica, dl -camphorquinone	86 3 μm	1098AA 2008-04/A3
APX Antagonist	Clearfil APX c	Bis-GMA, TEGDMA, silanated barium glass filler, silanated silica filler, silanated colloidal silica, camphorquinone	86 3 μm	00480A 2014-08/A2
Z250 Antagonist	Filtek Z250 a	Bis-GMA, UDMA, bis-EMA, zirconia, silica	78 0.19–33 μm	N182171 2013-05/A2
FS Antagonist	Filtek Supreme XT a	Bis-GMA, UDMA, TEGMA, bis-EMA, zirconia filler, silica filler	73 5–75 nm	N105945 2012-06/A3B

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