The current in vitro study was conducted to assess the microhardness and compressive strength of bulk-fill flowable nano-hybrid resin composite contaminated during packing with hemostatic agent, alcohol, artificial saliva and powdered gloves. One hundred disc-shaped resin composite specimens were prepared using split mold. The contamination of resin composite was performed after packing the first increment (3 mm). After packing the second increment (3mm), the resin composite specimens were photocured for 40 sec. The contaminated specimens were allocated into four groups (n = 20) according to the contaminant used either hemostatic agent (Group 1), alcohol (Group 2), artificial saliva (Group 3) and powdered gloves (Group 4). The specimens of control group (n = 20) were not contaminated during packing. Each group (n = 20) was assessed to test the microhardness (n = 10) and the compressive strength (n = 10). The microhardness and compressive strength were tested one-day post-photocuring (n = 5) and one-month post-photocuring (n = 5).
Bulk filling of cavity would be advantageous if compared with incremental layering of resin composite in reducing treatment time for cavity restoration, polymerization stress, contraction stress and improving esthetic quality . Flowable composite can be used as a stress-breaker intermediate layer between restoration and substrate to relieve the stress associated with polymerization shrinkage . Our study used bulk-fill flowable resin composite as it become common to be used by the clinicians especially in class II subgingival caries, root caries, deep cavities. These areas are difficult to be properly isolated and are accidentally liable to contamination. The mechanical properties of resin composites are significantly influenced by the filler particle shape, size range, and volume content . The introduction of nanometer sized particles is thought to offer superior esthetics and polishability in addition to excellent wear resistance and strength . Nanohybrid resin composites include a mixture of nanosized and conventional filler particles .
The current study used artificial saliva among the used contaminants; because salivary contamination to resin composite during packing is common in some clinics where rubber dam is not used; or when leakage occurs due to improper use of rubber dam. The study also used alcohol among contaminants because many dentists may digitally manipulate the resin composite with their fingers that could be contaminated with alcohol. Aluminium chloride (AlCl3) is one of the hemostatic agents used at a concentration between 0–25% to promote hemostasis before placement of a restoration by protein precipitation and constriction of blood vessels. Aluminium chloride (25%) was used in the current study because of its minimum tissue irritation, ease of use and effective results . Many clinicians manipulate the resin composite material during packing with gloves containing powder, causing its contamination. Therefore, powdered gloves were used in this study as contaminant during packing resin composite.
Microhardness is one of the most useful properties to assess because it is closely correlated with resistance to abrasion when used for restoration in load bearing areas . Compressive strength is a vital test for the selection of core material since most of the masticatory forces are of compressive nature .
It was observed that the tested contaminants did not affect the resin composite surface microhardness at one-day and one-month post-photocuring. However, the compressive strength of resin composite was significantly decreased after being contaminated with the tested contaminants at one-day and one-month post-photocuring. This might be due to the application of the contaminants between the two increments, not on the surface of the specimen. Vickers hardness test could only assess the surface microhardness of resin composite specimen . Moreover, Celluloid strip and glass slab were used in specimen preparation, so oxygen inhibited layer was not formed on the resin composite specimen surface . Absence of oxygen inhibited layer on the specimen surface might be the cause of proper resin polymerization with subsequent increased surface microhardness.
Our results were inconsistent with Widiandini et al. who demonstrated that the salivary contamination did not affect the compressive strength of nanohybrid composite . This inconsistence of results could be due to the application of the contaminant in that study onto the surface of the specimen, while the contaminants in the current study were applied in-between increments. However, Cobanoglu et al. stated that when saliva encounters the dentin surface, a saliva layer is deposited on dentin surface; water is evaporated and leaves a glycoprotein layer . Likewise, Shimazu et al. found that the bond strength among composite resin layers is reduced when it is contaminated with saliva . Similarly, significant decrease of the compressive strength of resin composite contaminated with artificial saliva could be due to presence of organic adherent layer and other elements of saliva in between increments interfering with proper polymerization.
The US Food and Drug Administration (FDA) defined ethanol as a liquid that simulates fatty foods and alcoholic beverages. In several studies, ethanol led to degradation or “softening” of composites and reduction of its microhardness . It was previously revealed that ethanol has a more aggressive potential and causes higher water sorption and solubility than water or artificial saliva . Ethanol contamination was found in a previous study to inhibit the resin composite polymerization causing significant decrease of surface microhardness. However, the resin composite surface microhardness was only affected which can be removed by routine polishing resulting in hardness that was similar to the uncontaminated resin composite . The conflict of results between previous studies and our current study could be due to the use of nanohybrid resin composite with high mechanical properties. Nanofillers can accomplish more close contact with the matrix resin than microfillers, therefore nanofilled composites can achieve high hardness as well as good polishability [16, 19].
It was previously postulated that manual manipulation of resin composite with powdered latex gloves should be avoided . Martins et al. found that the flexural strength of resin composite manipulated with powdered gloves was reduced . It was previously revealed that sulfides released from latex gloves inhibited the polymerization of the silicone in impression materials based on polyvinyl siloxanes, when it reacts with chloroplatinic acid from silicones . The results of the current study confirmed previous studies. Our findings could be the result of the physical barrier action of powder particles deposited on the resin composite interfering with complete polymerization and subsequently affecting the mechanical properties namely the compressive strength of resin composite.
It was previously demonstrated that the acidic pH of Al Cl3 used as hemostatic agent resulted in smear layer removal, and dentin etching effects [13, 14]. Similarly, the chloride ions could penetrate the uncured resin matrix or in-between the fillers and might slightly etch the resin matrix or the fillers reducing the compressive strength of the resin composite specimen.
At one-month post-photocuring, the control group recorded a significantly higher compressive strength mean value when compared to one-day post-photocuring value. Our findings confirmed the results of Gornig et al. . However, the contaminated groups showed no significant difference between compressive strength mean value at one-day and one-month post-photocuring. The stability of compressive strength of contaminated resin composite at one-month post-photocuring indicates the negative effect of hemostatic agent, alcohol, artificial saliva and powdered gloves on the compressive strength.