Effect of Zinc Oxide-Eugenol Temporary Restorations on Bond Strength of Composite Resin

Introduction

Zinc oxide-eugenol (ZO-E) is a widely used temporary restorative material in both restorative dentistry and endodontics. ZO-E is well known for its sedative, analgesic and antibacterial properties (1, 2), which make it an ideal material to use in the treatment of pulp symptoms such as reversible pulpitis. ZO-E is also routinely used to fill endodontic access cavities during the course of root canal treatment due to its ability to impede bacterial ingress, its wear resistance and its strength against occlusal forces (2). In the modern era of dentistry, composite resins are increasingly chosen over amalgam due to their aesthetic tooth-colored nature, as well as their adhesive ability which allows for conservative cavity preparations and support of tooth structure. However, the effect of ZO-E on the bonding of composite has been equivocal in the related literature. According to previous research, ZO-E decreases the bond strength of composite resin materials to tooth surfaces (3) and increases microleakage (4). On the other hand, there are other studies which have shown that the interaction between free radicals and eugenol is associated with the suppression of composite polymerization (5, 6). The reversible chemical reaction of ZO-E starts with a chelation reaction, which produces solid zinc-eugenolate when zinc oxide is mixed with eugenol in the presence of water (7,8). When the zinc-eugenolate encounters liquid from the dentinal tubules or the oral cavity, eugenol is released. Eugenol, a phenolic compound, reacts with free radicals and inhibits polymerization of resin monomers. A study by Hansen and Asmussen in 1987 (6) concluded that microscopic remnants of ZO-E and eugenol in the dentinal tubules inhibited the polymerization reaction of composite resin materials, and this may decrease the bond strength of permanent composite restorations (6). This was further reinforced in more recent studies conducted by Carvalho et al. (9, 10), who maintained that any form of residue from previous temporary restorative materials and/or cements is detrimental to the bond strength of dentine adhesives and self-etch adhesive systems. Such reduction could potentially affect the bond of the permanent composite resin restoration. A study conducted by Carvalho et al. (10) indicated that it is more likely that the presence of residue causes this decrease rather than the presence of eugenol per se.  On the other hand, other studies have demonstrated that with a strong enough etchant such as orthophosphoric acid, which is used with most (if not all) modern composites, any microscopic remnants of ZO-E are removed before resin bonding takes place, and therefore the effect of ZO-E on resin bonding is negligible (11, 12). Furthermore, other studies have reported that the effects of ZO-E on microleakage (13), adhesive bond strength (14) and composite bond strength (15) depend on the powder to liquid ratio used for temporary restoration. As a result of the variations in experimental methods and observations, there are varying conclusions and theories surrounding the effects of ZO-E when used as a temporary restorative material before the use of definitive composite resin. Previous research has investigated the effects of ZO-E on composite materials, adhesives and luting cements (4, 11, 16-18), however, there are only a few studies (15, 18) conducted on composite resins that are currently used. Hence, the present study aimed to investigate the effect of ZO-E on the shear bond strength of a newer generation of composite resin material.

Materials and Methods

One hundred extracted human molar teeth were used for this study. The inclusion criteria consisted of  sufficient horizontal cross-sectional surface area to which composite could be bonded, absence of visible caries or cracks, and sufficient root length to ensure adequate retention during mounting for the test procedure. The teeth were obtained from the collection of extracted teeth held by the University of Western Australia Dental School. Prior to use, all teeth had been disinfected with formalin and sodium hypochlorite and they were then stored in a saline and 0.2% thymol solution. The samples were also stored in the same solution throughout the experiment except when being prepared or tested. A flat surface of dentine was prepared perpendicular to the long axis of each tooth using a high-speed handpiece and diamond flat fissure or wheel burs. A minimum surface area of 30mm² ensured adequate space to which a composite rod of 7mm diameter and 10mm length could be bonded to the tooth. The teeth were divided into two groups. Group 1 was the control group in which the prepared surfaces of the teeth were not exposed to any substances before bonding the composite rod to the dentine. Group 2 was the treatment group which had ZO-E applied to the prepared dentine surface prior to bonding the composite rods. The ZO-E was mixed at a powder to liquid ratio of 10g:1g, according to the manufacturer’s instructions. The ZO-E was left on the teeth for ten days and the specimens were stored in the saline/thymol solution during this period. The ZO-E was then removed with hand instruments and saline irrigation. Complete removal was verified microscopically. Details of all materials used in this experiment are shown in Table I. The composite resin used was 3M ESPE Z100 (3M ESPE Dental Products, St. Paul, Minn. USA) which was bonded to the dentine of all the teeth according to the manufacturer’s instructions.  First, 37.5% orthophosphoric acid etch (Scotchbond Multipurpose Etchant - 3M ESPE Dental Products, St. Paul, Minn. USA) was placed on the dentine for 20 sec. This was followed by a 10-sec rinse and then Scotchbond Universal Bond (3M ESPE Dental Products, St. Paul, Minn. USA) was placed on the surface and agitated with a microbrush for 20 sec and then light-cured for 20 sec. A 7 mm diameter and 10 mm long clear matrix (Odus Universal Strips, Odus Dental, Vevey, Switzerland) was used to place the composite rods. The composite resin rod was placed in five 2 mm increments and light-cured for 20 sec each with a Bluephase Style light-curing unit (Ivoclar Vivadent, Schaan, Liechtenstein) which has a light intensity of 1200 mW/cm². Once the composite rods were placed, the specimens were left for 24 h inside a specimen jar with a saline-saturated piece of gauze to provide an environment with 100% humidity in order to ensure complete polymerization of the material. All of the teeth were then mounted in acrylic blocks within stainless steel tubes. A universal testing machine (Instron, Norwood, MA, USA) was used for shear testing of the composite rod bond strengths. The stainless steel tubes containing the teeth in acrylic were secured in the vice of the machine and a perpendicular force was applied at a crosshead speed of 1mm/min to the composite resin rod with a blunt, rounded tip.  A small countersunk depression was made at the midpoint of each composite rod to prevent slipping of the Instron attachment tip. The tip of the universal test machine was applied to this countersunk depression. The force required to shear the composite from the tooth was recorded at the point where complete fracture occurred. In addition, the location of the fracture was assessed by examining the specimens under a stereomicroscope at 2.5× magnification and the fractures were recorded as either a fracture between the composite-tooth interface (adhesive failure), a fracture of the tooth or a fracture within the composite (cohesive failure). The nature of the fracture was also assessed as either a ‘clean break’ when there was no dentine attached to the broken composite rod, or a ‘half-clean break’ when a small area of dentine remained on the composite rod after fracture.

Data were analyzed statistically through Stata Software (StatCorp, Texas, USA) using an unpaired two-sample t-test with unequal variance and compared with a significance level of 0.05

Table I. The materials used and their compositions

Results   

Data were gathered from 31 control samples and 40 ZO-E samples for the analysis. The other 29 teeth had to be excluded from the final analysis since they dislodged from the stainless steel tubes in which they were mounted, or the fractures were not caused by composite-dentine bond failure. Results of the shear bond strength tests are shown in Table II. The mean forces required to break the bond between composite and dentine were 137.15±46.82 N and 140.08 ± 40.39 N for the control and treatment groups, respectively. The raw data shows the average force required to break the bond between tooth and composite was very similar for the two groups. Since the P-value of 0.783 was greater than 0.05, there was no statistically significant difference between the forces required for both groups. Therefore, application of ZO-E prior to placing composite rods did not influence the bond strength of the composite resin to dentine when 37% phosphoric acid and Scotchbond Universal Adhesive were used to prepare the dentine surface before placing the composite. The type of fractures in proportion to the number of teeth are shown in Figure 1. None of the composite rods were completely covered in dentine after fracturing off the tooth. The majority of the samples in Group 1 had a “clean break” compared to those of the Group 2. The teeth with “clean breaks” all underwent adhesive failures of the bond between the teeth and composite resin. There were no cases of cohesive failure of the composite resin specimens with half-clean breaks had a mixture of adhesive failure of the bond plus fracture of the dentine, with the latter being a cohesive failure of the dentine.

Table II.   Mean (Newtons), median and standard deviations for the two groups

Figure 1.  The number of “clean breaks” and “half -clean breaks” in each group. A “clean break” designates that no dentine was attached to the broken composite rod, while a ‘half -clean break’ designates that there was a small area of dentine remaining attached to the composite rod after fracture