4 minutes reading time (829 words)

Material facts

Composite material

Back in the past, I used to hate dental materials lectures. It all seemed so irrelevant. I just wanted to know the material worked. I couldn’t get excited about the chemistry. Oh, I remember the important stuff. 

The catalyst in alginates is heavier than the filler, so you need to shake the tin to disperse the contents. Zinc phosphate cement sets by crystallisation, so when you mix, be slow and gentle with the spatula otherwise you create more seed crystals and it sets too fast (not forgetting to run the glass slab under the tap to cool it down to slow the setting reaction) – but who needs to know that it only takes one molecule of water to trigger the setting reaction of zinc oxide and eugenol?

I’ve probably lost some of the younger generation here – alginate? Yeah, yeah, I get it. You use a scanner now. Zinc whaaat? And zinc whatever …… oh, and that’s why there’s a drawer full of glass slabs in the spare surgery, waiting for some sort of Victorian time warp, when the world has run out of plastic to make those little squirty syringes all the materials come in these days and the nurse has to learn how to hand mix again. That’ll be the day when she says goodbye, that’ll be the day when you make her cry. Oops, sorry, bit of a Buddy Holly trigger there.

Back to the present. There was a request on a dentist’s chat group the other day for recommendations for a flowable composite – which is the strongest and which has the research to back it up. Thirty comments later, after multiple recommendations, the discussion comes to dead halt with the comment ‘what we like is one thing and hard facts is another.’ That, of course, is not a new comment. Indeed, it’s a comment which was made in the very first edition of the very first UK dental journal, the British Journal of Dental Science, in 1856. Mr Perkins of Prospect Place, Maida Hill urges the editor to ensure that any opinions posted in his journal have ‘a good foundation in fact.’ Evidence based dentistry in 1856 – who knew?

Back to the present. Where are the hard facts about materials? How long do they last? I had a browse around a couple of manufacturers websites. One has evidence that 50 dentists used their material 20 times each and gave it a 98% approval rating. Another had flexural strength graphs, comparing their product to others, the data being ‘unpublished’ but the ‘documentation available.’ The evidence base is not independently scrutinised. An opinion piece in the British Medical Journal discusses the illusion of evidence based medicine. The dominance of the market by a small number of large pharmaceutical companies and universities chasing commercial funding from Big Pharma has emasculated the possibility of independent scrutiny of research data. The same could presumably be said for the large dental materials companies.

Long term studies of composite restorations are not common. Examiner consistency over time is difficult to manage. Patient recall drops off over time. Operator and procedure consistency is hard to reproduce over the years. The results will never be perfect. But when someone tries to assess composites over time, the conclusion is interesting. A Brazilian practitioner recently put his head above the parapet and published a paper which evaluated the performance of his posterior composites after up to 33 years of clinical service. Clinical records were reviewed of composites placed between 1986 and 1992, using rubber dam and state of the art techniques of the time. Six hundred and eighty three restorations were assessed and over the time period, there was an annual failure rate of around 2.5%, which is in line with the few other similar in practice studies. The interesting comment to me in the paper was that

It is remarkable that most if not all materials used in the present and other longitudinal studies are not available in the market anymore. In addition, the techniques applied such as the use of calcium hydroxide liners, glass ionomer cement bases, and adhesives that are now considered obsolete have apparently not reduced the survival rates of restorations. This finding may raise questions on the importance of several material properties for restoration survival.

Manufacturers want to sell new stuff. Let’s face it, dentists like trying new stuff. But in the long term, are the new materials ‘better’ from a long term performance point of view? The evidence seems to be lacking.

Back to the future. Maybe hard facts about materials are not quite as important as we thought. Maybe what we prefer, what works in our hands, is just as good a way of deciding material preferences because for long term survival, that old stuff at the back of the cupboard actually worked quite well.

So, were I still in practice ……. ‘Zinc phosphate and 2 amalgams please, Joan and don’t forget to cool the slab.’

Glass Slab

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