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As readers are increasingly ditching (pardon the pun!) amalgam, as I did over 20 years ago, in favour of resin composite in posterior teeth, some may ask themselves, why did we stick with it for so long? Daresay some of the answers are found in Louis Mackenzie' s superb article on dental amalgam published 2 years ago, which may well go down in history as the last major review on this material.1 In this, he published a table with 19 advantages relating to dental amalgam. These included relatively ‘quick and easy to use, compared with direct adhesive procedures’, ‘optimal use can provide restorations lasting 30 years’, ‘suitable for use in posterior teeth where aesthetic demands are low’, and, ‘comparatively inexpensive’. But, the overriding property is its ease of use, and the fact that the clinician does not need to carry out an intermediate bonding procedure, given that the margins of amalgam restorations corrode, thereby creating a marginal seal of sorts.1
Who wakens in the morning and cries, ‘hooray, I'll be etching and bonding today?’ Not many! However, this is a necessary evil with direct resin composite-based restorations. We have had restorative materials for 50 years that do not require an intermediate bonding agent, namely glass ionomers, but it is only in the recent past that we have a glass ionomer that might stand up to the rigours needed in stress-bearing restorations in posterior teeth, as discussed in the 50th anniversary issue earlier this year.2 In that regard, devising a resin-based restorative material that does not require an intermediate bonding agent has been a goal that has eluded manufacturers for some time and was something that I was hoping to use before I hang up my dental gloves. The first example of this, in 2001, was with 3M's RelyX Unicem: this, however, was a self-adhesive resin-based luting agent, as opposed to a restorative material. It had an acidic monomer, which provided etching and bonding like a self-etch dentine bonding agent, and a filler similar to that in glass ionomer. The obvious step to producing a self-adhesive restorative material might have been to increase the filler loading in order to beef up the material's physical properties, but that then dilutes the etching and bonding potential of the acidic monomer, hence not a solution. It was therefore with interest that I read of the introduction of Surefill One from Dentsply in 2020. This contained a ‘modified polyacid’, stated to be an adhesion promoter, a ‘reactive glass filler’. The material was designed to be light cured, but, according to the manufacturer, ‘areas that are not reached by the light will cure chemically within 6 minutes after activation of the capsule, ensuring a reliable cure of the restoration’. If this seemed too good to be true, that might indeed have been the case, as it was withdrawn by the manufacturer earlier this year.
I have therefore been following with interest the research progress of a novel 3M self-adhesive composite. I first came across this at a research meeting in Brussels in 2021, when the novel self-adhesive bulk fill composite (SABF) was compared with a ‘conventional’ bulk-fill composite in a scientifically robust, randomized split-mouth design study.3 Thirty patients and 60 restorations were involved, and 29 patients were recalled at 2 years. One might have expected more marginal staining in the SABF restorations, given that the margins were not etched as they were for the control restorations, but this was not the case. The main difference in the evaluation related to less favourable aesthetics of the SABF restorations, with some bulk discolouration (which was present at the baseline examination), but the restorations remained clinically acceptable, with the conclusion being that ‘SABF showed promising results and may be recommended for clinical use’. Roll on another 2 years, and the 4-year results were presented in July 2023 at the ORCA (Organisation for Caries Research) research meeting.4 Twenty-six of the original 30 patients attended for recall, with survival of the SABF restorations being 96% compared with 92% of the bulk fill controls, when assessed by two calibrated, blinded examiners. Again, SABF exhibited less favourable, but clinically acceptable, aesthetic properties, and the conclusion was the same as at 2 years.
A remark attributed to Aristotle's ‘Nicomachean Ethics’ is ‘one swallow does not a summer make, nor one fine day’, so, similarly, one research project with positive findings does not constitute a world-changing event. Nevertheless, it represents a step towards what is possible. Given that ease of use was one of the reasons why amalgam survived for well over a century, the avoidance of etching/bonding with a resin-composite material and the time saving that will accrue, must be considered an advantage over the resin composite materials that most of us use today. Our dentine bonding agents have improved immeasurably in the past decade, with the universal bonding agents being considered to represent a big step forward,5 but perhaps the most user-friendly bond is…the one that we don't need at all!