Dental Hypotheses

: 2022  |  Volume : 13  |  Issue : 3  |  Page : 94--98

Assessment of the Shear Bond Strength of Composite Resin to Fresh Amalgam Using Different Adhesion Protocols: An In Vitro Study

Haider Hasan Jasim, Mohammed K Gholam, Biland MS Shukri 
 Department of Conservative Dentistry, College of Dentistry, Mustansiriyah University, Baghdad, Iraq

Correspondence Address:
BDS, MSc Haider Hasan Jasim
College of Dentistry, Mustansiriyah University, Baghdad


Introduction: This study was conducted to evaluate and compare the effect of using three adhesive systems on the shear bond strength between composite resin and fresh amalgam. Materials and Methods: Amalgam samples were assigned into three groups according to the adhesive systems being used prior to the composite resin application. Group A (n = 10): universal self-etch bonding system (Single Bond Universal Adhesive, 3M, Neuss, Germany) was applied and light cured. Group B (n = 10): universal self-etch bonding was applied and light cured, followed by a thin layer of nanohybrid flowable composite (Tetric N-Flow, Ivoclar Vivadent, Liechtenstein), then light cured. Group C (n = 10): self-adhesive dual-cure luting resin cement (Calibra Universal, Dentsply Sirona, Charlotte, NC) was applied in equal quantity for each sample over amalgam surface. Composite resin was then applied by plastic instrument in a single increment to the height of the hole (2 mm), then celluloid strip was placed and light cured. After thermocycling, a shear bond test was performed. Results: Resin luting cement interface showed the higher significant bond strength, followed by universal bonding agent − flowable composite interface. The universal bonding interface group yielded the lowest results. Conclusion: The application of self-adhesive dual-cure luting resin cement resulted in significant increase in bond strength between composite resin and fresh amalgam.

How to cite this article:
Jasim HH, Gholam MK, Shukri BM. Assessment of the Shear Bond Strength of Composite Resin to Fresh Amalgam Using Different Adhesion Protocols: An In Vitro Study.Dent Hypotheses 2022;13:94-98

How to cite this URL:
Jasim HH, Gholam MK, Shukri BM. Assessment of the Shear Bond Strength of Composite Resin to Fresh Amalgam Using Different Adhesion Protocols: An In Vitro Study. Dent Hypotheses [serial online] 2022 [cited 2022 Sep 26 ];13:94-98
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Restoration of extensively carious teeth to an optimum state of health, function, and aesthetics, continues to be a challenge for all operating dental surgeons. Although numerous restorative materials have been tried in order to achieve adequate strength, restore aesthetics, and conserve the remaining tooth structure, an ideal restorative material has still eluded researchers.[1]

Amalgam has been used for more than a century and a half to restore extensively carious teeth; its mechanical properties and serviceability in oral environment were acceptable and clinically proven.[2]

However, amalgam does not strengthen the weakened tooth structure because of its high elasticity modulus, and it does not bond to tooth structure; Dental amalgam need more preparation since it didn’t bond to tooth structure, it need undercut design cavity to retain the filling.[3]

Composite materials have been regarded for several years as esthetic substitutes for amalgam.[4] Moreover, composite resins need conservative cavity preparation to act as dentin replacement to reinforce weakened enamel cusp. The adhesion of composite to tooth structure minimizes the removal of sound tissue structure and improves fracture resistance of the restored tooth.[5],[6]

Esthetics is important in restorative dentistry; however, longevity of restorations should be the most important criterion in material selection.[7] Nevertheless, the ability to obtain adequate proximal contacts, potential microleakage when the gingival margin located apical to the cement enamel junction, and wear at composite resin contact area are some problems related to posterior composite restoration. Moreover, placement of the composite in larger cavities will reduce its survival, when compared to smaller cavities.[8] Alvanforoush et al., in their systematic review study, showed that the reasons of failures in composite restoration were secondary caries (25.68%), composite fracture (39.07%), and tooth fracture (23.76%); this could be due to the increased use of composites in larger restorations, and possibly, changes of material characteristics.

In large restorations, concerns of bonding failure due to polymerization shrinkage and low-strength composite resin, as specimen thickness linearly increased the decree of conversion, microhardness, polymerization rate, and refractive index linearly decreased.[9]

An alternative method of treatment incorporating both the desired mechanical properties of amalgam and the esthetic qualities of composite resin in order to solve problems related to both materials is the combined composite–amalgam restoration. [10],[11],[12],[13],[14],[15] It is a one-session procedure, in which composite resin retention is obtained from fresh amalgam, immediately postcondensation, implying that bonding is taking place prior to setting.[10]

The aim of this in vitro study was to evaluate and compare the effect of using three adhesive systems (universal bonding, universal bonding with flowable composite, and self-adhesive dual-cure resin cement) on the shear bond strength (SBS) between composite resin and freshly mixed amalgam.

 Materials and Methods

The study protocol was reviewed and approved by research committee of Mustansiriyah University (SA: 1015). Especially designed device as done by previous studies,[10],[11],[12],[13],[14],[15] 30 standard cubic acrylic blocks of 20 mm dimension with a central hole of 3 mm depth and 5 mm diameter. The spherical amalgam capsule (World-Cap Non-gamma-2, Ivoclar Vivadent, Liechtenstein) was triturated according to the manufacturer’s instructions and then manually condensed to the cylindrical hole; excess amalgam was immediately removed by carver leaving the amalgam surface continuous with the acrylic cubic surface.

Two plastic pieces of equal dimension were made for each cubic block, and were fixed over the block by two screws, forming a hole in the center of 4 mm diameter and 2 mm depth, that directly centered over the amalgam surface, to confine the composite resin to the amalgam surface only, without touching the acrylic block [Figure S1].[16]

The amalgam samples were randomly allocated into three groups according to the adhesive systems by one of the author (BS) using the Google random number generator. Group A (n = 10): universal self-etch bonding system (Single Bond Universal Adhesive, 3M, Neuss, Germany) was applied according to the manufacturer’s instructions and light cured for 20 seconds. Group B (n = 10): universal self-etch bonding was applied and light cured, followed by a thin layer (0.5 mm) of nanohybrid flowable composite (Tetric N-Flow, Ivoclar Vivadent, Liechtenstein); the thickness was checked by periodontal probe, then light cured for 20 seconds. Group C (n = 10): self-adhesive dual-cure luting resin cement (Calibra Universal, Dentsply Sirona, Charlotte, NC) was applied in equal quantity for each sample through the mixing nozzle tip and painted by microbrush over the amalgam surface.

Composite resin (Tetric N-Ceram, Ivoclar Vivadent, Liechtenstein) was applied by plastic instrument in a single increment to the height of the hole (2 mm), and then celluloid strip was placed and light cured for 40 seconds using light cure unit (Elipar S10, 3M ESPE, Saint Paul, Minnesota, USA) with light intensity of 1200 mW/cm2 and wave length peak 455 nm. With an intimate contact, upper two plastic halves were removed from the mold 24 hours later, to permit initial setting of the amalgam in order not to disturb the bonding interface with the amalgam.

All the specimens were then stored in 37°C distilled water for 48 hours, and then were subjected to thermocycling (5°C–55°C) for 5000 cycles with a dwelling time of 30 seconds at each temperature.

Shear bond test was conducted on a mechanical testing machine (Laryee Technology, Beijing, China) at crosshead speed of 0.5 mm/minute until failure; the specimens were fitted in the base of the machine with the amalgam surface parallel to the direction of the force produced by the machine. A thin steel wire (0.4 mm) suspended from the movable crosshead was looped around each composite cylinder and aligned with bonded interface.

The force then was recorded in newton which then divided by surface area to obtain SBS calculated in MPa by one of the authors blinded to the samples allocation (JH).

All the debonding surfaces were examined using a digital stereomicroscope (Koolertron, Shenzhen, China) at 40× magnification to identify the failure pattern.

Data were analyzed using the Python 3.10.4 (The Python Software Foundation, The level of significance was set at 0.05. The one-way analysis of variance (ANOVA) test was used for overall comparison, and the Tukey test was used for individual pairwise comparisons.


Results related to SBS test among the three study groups are presented in [Figure 1] and failure modes data are showed in [Figure 2] and S2.[16]{Figure 1}{Figure 2}


Despite the availability of different techniques, application of composite resin alone in the posterior region in certain cases has problems such as high technical sensitivity, polymerization shrinkage, and inadequate seal. One strategy to minimize such complications is to apply a base under the composite restoration, which referred to as combined amalgam–composite restoration.[17] The decreased amount of composite resin will reduce the contraction stress and give more chance for curing.[18]

All of the universal adhesives use phosphate esters as their primary adhesive monomer, they have potential to bond chemically to most of the restorative surfaces.[19]

The lowest mean of SBS was for group A (4.15 MPa) in which composite resin was bonded to amalgam using universal bonding agent, followed by group B (4.87 MPa), where composite resin was bonded to amalgam using universal bonding agent with intermediate flowable composite layer. The mean SBS for group C was 8.86 MPa, where composite resin was bonded to amalgam using self-adhesive dual-setting resin luting cement showed a significant SBS than that of group A and B. The null hypothesis was partially rejected for the resin luting cement adhesive protocol.

Regarding groups that used universal bonding agent as intermediate interface, the SBS of group B was significantly greater than group A. Flowable composite application over the bonded amalgam surface in group B slightly increase the SBS in comparison with group A, this could be related to better wettability of flowable composite to the cured bonding surface.[20] Moreover, shrinkage stress is also related to the elastic modulus of the material; low modulus flowable composite was used as intermediate layer under composite resin to reduce stress, resulting in slightly higher SBS.[14]

The results of this study, regarding universal bonding groups, were in agreement with previous studies.[10] Bichacho et al.[10] assessed the SBS of four adhesive bonding between fresh amalgam and composite. Their results were close to the present study, but the bond strengths dropped sharply after 100 days’ immersion in water, the bond was hydrolytically degraded during long-term immersion. In the present study, the measurements were done only after thermocycling for 5000 cycles; 10-Methacryloyloxydecyl dihydrogen phosphate (MDP) is the functional monomer that enhances the bond strength to base metal alloy. Long-term hydrolytic durability of such adhesive promoter needs further investigation. Ozcan et al.[21] investigated the SBS of composite resin to amalgam using scotchbond multipurpose adhesive bond in dry and thermocycled conditions. Mean SBS was 14.8 MPa and 5 MPa, respectively, in them. They used a woven fiber sheet of 0.006 mm thickness on the bonding interface to increase the interfacial strength of the adhesive joint.

Self-adhesive resin cement was the strongest adhesive system in this study. The application of composite resin over unset luting resin cement could led to more penetration to porosity and irregularities in amalgam surface, resulting in macro and micromechanical interlocking, where it polymerized and locked in by amalgam setting.[15] Nevertheless, the resin cement had inorganic filler content of 48.7% by volume that made it a strong adhesion interface in comparison to universal bonding agent.[20]

Regarding the failure mode, since in the current study, no cohesive failure was found, the resulted SBS values can be considered as the representative of amalgam–composite interfacial bond strength.

The fractured specimens exhibited adhesive failures (100%) in groups A and B, suggesting that the adhesive strength of the universal bonding agent to amalgam surface was greater than that to composite filling. Wettability of the universal bonding agent to porosity and irregularities of the fresh amalgam surface resulted in micromechanical bond in addition to chemical bond. Nevertheless, the wettability of composite filling to the cured bonding surface could be less, due to high viscosity of the composite resin.

Photographic analysis revealed a white patch covering the amalgam surface in digital microscope, which indicated that separation mostly occurred between universal bonding surface and composite resin surface. In fact, it is difficult to point out the exact separation site even with scanning electron microscope, since both bonding agent and composite resin were identical in organic composition and color; contrast coloring addition could be helpful in future investigation.

All the specimens of group C showed a combination of adhesive and cohesive failure, suggesting that adhesive strength approached the cohesive strength of resin cement.

Previous studies showed that combined amalgam–composite restoration had excellent marginal seal and adaptation at the interface, even mechanical loading did not affect the marginal adaptation at the interface.[12],[18],[22]

Upper and lower shear bond strength readings of group C, ranging between 9.21 and 8.49 MPa and exceeding the recommended ISO standard,[23] stated that the bond strength to the metal used for substructure should not be <5 MPa. Therefore, self-adhesive resin cement could be used as intermediate adhesive layer between unset amalgam surface and composite resin.

Readers must note, to the in vitro design of the study and limited sample size. Hence, the results cannot fully replicate oral conditions, thus further studies with cycling loading, and increased water storage aging is needed, in addition to in vivo studies.


The authors would like to thank Mustansiriyah University (www., Baghdad, Iraq, for its support in the present work.

Financial support and sponsorship


Conflicts of interest

There are no conflicts of interest.


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