Multiple laboratory studies have found definite advantages for bonded amalgam restorations including increased retention, fracture resistance, and marginal seal. Staninec found that the use of adhesives provided greater retention than grooves or dovetails. Oliveira and others found improved fracture resistance in large MOD preparations when bonding amalgam compared to the use of Copalite alone.
A study by Burgess and others found no difference in the strength of complex amalgam restorations using four TMS pins or bonding, but the combination of the two significantly increased the forces necessary for fracture. Studies have also shown increased retention of amalgam when bonding with resins containing filler particles. The more viscous bonding agent may improve penetration into the amalgam during condensation. Also, research has shown a reinforcement of remaining tooth structure with bonded amalgam restorations. However, the ability to maintain this reinforcement over time remains equivocal with some studies showing no increase in fracture resistance after aging and thermocycling. The use of an adhesive agent under amalgam has been shown in laboratory studies to decrease microleakage. Again, the long-term significance of this decrease is unknown.
Most of the clinical studies have found no decrease in post-operative sensitivity and no difference in the performance of bonded amalgam restorations compared with traditional mechanically-retained restorations. Contrary to popular belief, the preponderance of clinical investigations has demonstrated no difference in sensitivity reported by patients receiving amalgam restorations with or without resin adhesives.
Summitt and others published a clinical study comparing the performance of bonded versus pin-retained complex amalgam restorations and found no difference after five years between the two techniques. They concluded that bonding with a filled bonding resin (Amalgabond Plus, Parkell Inc., Farmingdale, NY) was a satisfactory method of retaining large amalgam restorations replacing entire cusps.
So, should you place an adhesive agent under all of your amalgam restorations? Given the added cost, time and technique sensitivity of using adhesive liners, there appears to be no clinically-demonstrated benefit in bonding conventional preparations which contain customary retentive features.
However, given the advantages of increased retention, strength and marginal seal found in laboratory studies, the bonding of amalgam may be justified adjunctively with traditional mechanical retention in large restorations replacing a cusp, when tooth structure may need some reinforcement, and for crown foundations.
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