Written by Barry F. McArdle, DMD.
Although they may not be as aesthetically pleasing as direct composites, in many cases class I and class II amalgam restorations are functionally superior over the long term.1 The risk of cuspal fracture increases with restoration size for any tooth regardless of the restorative material used.2 Amalgam is a viable treatment option for direct restoration of teeth that require a material that meets certain functional demands. However, because of the thermally conductive nature of the material, temperature sensitivity continues to be a problem with extensive amalgam restorations, whether they are placed over conventional liners and bases or bonded with an adhesive.3,4 Since the latter method results in the occlusion of dentinal tubules, it would seem to indicate that this type of irritation cannot be totally explained by the hydrodynamic theory of pulpal sensitivity, even considering that the numbers of tubules increase deeper into dentin.5 Indeed, when more than 1 to 2 mm of dentin thickness remains between the floor of the cavity preparation and the pulp, thermal sensitivity seems to become a mute issue.6
Shortly after the introduction of flowable composites onto the dental market in 1996, I found in my practice of general dentistry that using flowable composites as a base with an adhesive liner under deep amalgam restorations greatly reduces postoperative thermal sensitivity for my patients. It may also have the added benefit of reinforcing enamel undermined by caries removal during the restorative process. I began using flowable composite in this manner because I had been looking for a more effective base for amalgam restorations among the materials in my existing restorative armamentarium, and I knew that composite resin is not an efficient thermal transmitter. I was aware that certain applications of glass ionomer, though less convenient, also served to reduce thermal sensitivity under amalgam restorations.7 Some clinicians advocate the use of a light-cured resin-modified glass ionomer as a base, as well.8 I use flowable composite instead of traditional composites or resin-reinforced glass ionomers because I find it superior for both its ease of application and absence of voids.
The procedure is quite simple and requires very little additional time if you are already routinely hybridizing the internal surfaces of your cavity preparations with an adhesive system. I have been doing so for a number of years because this procedure protects the pulp by forming a hybrid layer with vital dentin that mechanically reinforces the dentin.9 After your preparation is complete (Figure 1), line it with the bonding agent of your choice as you normally would (Figure 2). When you have accomplished this, syringe on a layer of flowable composite that covers all dentin and the seat of any unsupported enamel in the preparation to a thickness that visibly obscures the tooth structure beneath it (Figure 3). This is critical because all of the flowable composites that I have used have been of a low enough viscosity that thinner layers are transparent. Light cure the flowable composite and then condense, burnish, and carve your amalgam as usual.
I base my choice of a flowable composite on its flow characteristics. I prefer one with a lesser flow rate because I find that its placement within the preparation is easier to control. Flowable composites with a lesser flow also have mechanical properties that are more similar to traditional composites.10 This should make their performance with adhesives more reliable compared with higher-flow composite materials where greater polymerization shrinkage may be problematic.11 It is preferable for the properties of the flowable composite used to duplicate those of conventional resins as closely as possible since the latter, when placed under large amalgam restorations, have been shown to prevent cuspal fractures of the teeth so restored.12
I started using flowable composite as a base under extensive amalgam restorations over 3 years ago. Since then, I have not had a single patient complain of thermal sensitivity from a restoration so placed. I routinely call every patient on whom I perform any invasive procedure a day or two later to check on their progress. This appears to be a time frame during which any thermal sensitivity that may arise will manifest itself.4 A review of my patient records reveals no adverse consequences associated with this procedure, and I am not aware of any pulpal pathology associated with the restorations I have placed in this manner. I am likewise unaware of any cuspal fractures of the teeth that I have restored in this way, while I have had teeth with like-size amalgams suffer cuspal fractures after bonding alone over comparable time periods. I believe this to be the case because, while amalgam can be bonded with enamel as successfully as composite in theory,13 the composite-enamel interface is better adapted, and thus reinforces weakened cusps more efficiently.14
Flowable composite that is not of too low a viscosity is particularly well suited for this purpose, as its still lower modulus of elasticity in relation to conventional resin allows it to absorb polymerization shrinkage more effectively and flex with the tooth while in function. These properties also permit greater adaptation of flowable composite to tooth structure than its traditional counterpart which, when coupled with low microleakage, makes it an ideal base material for extensive restorations.15
My only difficulty with this method is in judging when the depth of preparation warrants its use. I do believe that using flowable composite as a base under every amalgam would significantly increase a practiceâ€™s overhead costs for these restorations. However, since I started using flowable composite as a base, I have encountered situations where I used no base over my hybrid layer, and it did result in a patientâ€™s complaint of sensitivity. This type of base under amalgam restorations has worked so well for me that I believe further research to confirm its validity would be worthwhile.