The search for a new dental amalgam

Materials World magazine
,
4 Jul 2016

The mixture of mercury and a silver-tin alloy known as dental silver amalgam has dominated dentistry as a filling in posterior tooth repairs for 200 years in the west, but its days are numbered. James Perkins found out why, and what its replacement might be.

English dentists Edward and Moses Crawcour quickly established a thriving practice in the USA after bringing amalgam across the Atlantic in the mid-19th Century, but within 20 years the 'Amalgam War' had begun.

Given the toxicity of mercury, which makes up about half the mass of the amalgam mixture, it had drawn the ire of many in the profession. Experts took their sides in the fiery debate, but the sheer usefulness of amalgam won through. Inexpensive, malleable and durable – it did its job well and, over the years, hundreds of scientific studies found in favour of its safety.

But the war never really ended. Today, new threats to this material have come on two fronts – the European Commission has advocated minimally-invasive dentistry, and the United Nations Minamata Convention, which seeks to phase out the use of mercury worldwide, singles out amalgam as a priority.

Despite the iconic status of amalgam, it is hard to find a pro-amalgam dental academic today, and that's because an amalgam filling is mechanically locked in place by removing healthy tissue. From this moment, the tooth's days are numbered. Soon there can be a second filling, then a crown, and then the tooth might be lost.

'It is crazy,' says Brian Darvell, an honourary professor in Dental Materials Science at the University of Birmingham, UK. 'You have to undercut the restoration in order to get it to stay in place, because there is no way of gluing these things.'

Brian Millar, Professor of Education at Kings College London, UK, and NHS Consultant said, 'We want to break that cycle and one way simply is not to cut teeth. It is a strong philosophy.'

Adhesive bonding of composite fillings requires the removal of less tooth, but these materials have their own problems related to polymer shrinkage and the release of monomers through hydrolysis. Meanwhile, zirconia-based ceramics are difficult to bond and other ceramics are too brittle, while glass ionomer cements are not strong enough to act as a load-bearing filling on their own. Research is also steaming ahead on bioactive materials that could mimic the structure of tooth itself and restore it through remineralisation.

The tooth - strong and tough but dissolves

The composite that is enamel is 'a remarkable material', says Darvell. 'It is extremely damage tolerant – it will accumulate cracks over a lifetime but still not fail and it requires a traumatic experience for tooth to break.' Research for John Hopkins Whiting School of Engineering showed that enamel reaches a hardness of 6GPa, with an elastic modulus of around 120GPa. The enamel is aided by its substructure. 'Crack growth is controlled by the interface between the dentine and the enamel. You may be getting cracks in the enamel, but they don't grow.' But teeth are not perfect and have a well-publicised weakness. 'Acid is a major challenge and so drinking Coke, lemonade, and so on will result in dissolution.' He continued, 'Localised acid attack through bacterial plaques leads to caries and that dissolution is a major problem'. The search for a material to replace amalgam comes as rates of caries, especially among young people, are skyrocketing and, just like the obesity epidemic, this rising health problem is being blamed on the proliferation of sugary food and drinks.

Composite resins and adhesive dentistry

Composite resins can be adhered to the tooth, therefore the healthy tooth does not have to be removed to install a restoration, making it a popular option among dentists operating under the new philosophy. Dentists etch the tooth, giving the adhesive more surface area to bond, but there are still problems with this method, namely polymer shrinkage and hydrolysis.

Dentists do their best to reduce shrinkage, through various techniques including incrementally polymerising the filling with the latest blue light technology, or inserting the restoration as a whole – a technique known as 'bulk filling' – but it seems shrinkage is not a problem that is going to be solved.

Robert Hill, Professor of Physical Sciences in Relation to Dentistry at Queen Mary University of London (QMUL), said, 'It is a big problem – a major cause of failure for conventional composite light-polymerised fillings is secondary caries due to polymerisation contraction that causes marginal gaps, and the decay-causing bacteria get into those gaps. You have to be careful in terms of the design of the material to minimise the shrinkage and to offset what shrinkage does occur by water absorption.'

Millar says, 'Dentists have to be very careful to polymerise restorations carefully, so that the shrinkage doesn't lead to problems such as bending the tooth, breaking the margins away and making gaps. The other problem with the polymer is that it has monomer in it and, of course, the monomer comes out and has an effect on human cells. So we have a good material but there is still a problem.'

Darvell admits that he is sceptical of composite restorations. 'The hope is that these will be permanent restorations and I think that is a joke – that is not physically possible. Even with the infiltrated etched interface, mechanically these must break down because there is a modulus difference between the tooth enamel and the filling. Secondly, these are all acrylic resins, one way or another, that are capable of hydrolysis, and that hydrolysis may be slow, but it does occur. There are some systems which use polyurethanes, or urethane monomers in the mixture and as far as I can see – despite the fact there are some that are less prone to hydrolysis than others – this is still a route for chemical breakdown.'

Glass ionomer cements and bioglass in resins

There is a material that does not shrink appreciably, does not contain monomers, and can actively remineralise teeth – glass ionomer cement. This material is well-known and has been in use since the late 1970s. It forms an ionic bond with existing tooth material – however, it is simply not strong enough.

Hill says the remineralisation qualities of the cement could allow dentists to leave some of the diseased tooth in place. 'The only material that will do that at the moment is glass ionomer cement and we have shown that they can do this, but they don't actually do it very well – the actual physical and chemical properties of the glass ionomer are not that good and they are not that easy to handle by the dentist, in terms of placement.'

Efforts to improve glass ionomer cements, such as using them as a filler material in composite resins, have been met with some success, but even there the material is still too weak. At QMUL, Hill and his colleagues have been working on composite resin and bioactive glass materials, and in April launched spin-out company BioMin Technologies to commercialise their approach. The team is getting some promising results in therms of fluoride release, but more study needs to be done on strength. 'We won't see a filling quickly,' Hill said, when asked about the material's potential in restorations, 'we might well see similar technology used as a varnish.'

Alongside bioactive glasses remineralising teeth, they can also reduce polymer shrinkage in resins, as Hill explains. 'One of the advantages of having a bioglass is that, to a certain extent, you can offset the polymerisation contraction by subsequent water uptake. If your material contracts by one or two percent, but subsequently absorbs water by one or two percent, you can offset the contraction.'

Despite these exciting characteristics, bioactive glasses still need more strength. Some products of this kind need a strong composite layer to cover the restoration. As Millar states, 'Sadly, a lot of these materials are not hard enough to bite on, so they have to be covered with another layer. We are working towards better materials but we still have a long way to go.'

Darvell is cautious, 'There is a great deal of optimism about the ability to remineralise teeth. That is, in one sense, a faint hope, because you cannot rebuild the structure of tooth enamel externally. It is a built biological system and you cannot make that through precipitation processes. What you can do, it seems, if you have got a porous structure and can precipitate material in there, is make it a little more solid. People are too enthusiastic about the implications of remineralisation.'

The rise of zirconia

Ceramics seem a natural fit as a tooth restoration material, given their strength and hardness. But, again, these materials have weaknesses. Porcelain, for example, is too weak even with alumina added to the matrix, and zirconia is so unreactive it is difficult to bond to the tooth and cover with veneer, even if cut to the perfect shape using precision CAD/CAM technology. Both zirconia and aluminous porcelain are only substructure materials due to their colour.

'The zirconia is so unreactive that you cannot use normal adhesion systems to get these things to adhere to tooth tissue,' said Darvell. 'There is an awful lot of work being done to get them glued in place properly. But, after toughening of ceramics, the principal requirement is for a good way of getting permanent retention, to get that union up to scratch. That seems to me to be the most worthwhile centre of focus.'

Hill added, 'They are using zirconia where they would have used metal alloys previously. It has the advantage of having a high toughness and strength and, unlike metals, it is white or semi-translucent. But you still need to veneer over the top of it to make it look tooth-like and attractive – one of the problems is keeping that. Generally, you will be veneering over the top with a dental porcelain and it is quite difficult to keep that glass on the surface of the zirconia. Delamination is the big problem and that is where the emphasis on zirconia implants and zirconia crowns is going.'

What is the future?

Millar, sitting high up at Guys Hospital, location of the King College Dental Institute, says, 'It is an exciting time to be in dentistry,' That's because after 200 years, amalgam is finally on its way out, ushering in new dental practices and materials. But there is still no clear-cut replacement that provides the same all-round value. It is not an easy task. 'We have a lot of materials out there that we can't use in dentistry because physically they can't be put into a cavity, or they are toxic, or they won't withstand the oral environment.' For some time yet, we will catch glimpses of silver in people's mouths, but for how long?