The amalgam debate!

There’s been a famous dental debate in the last decade: whether the mercury in amalgam fillings is toxic. At one extreme are those of the opinion that amalgam causes everything from ME to Alzheimer’s; and in the other corner are the hardened “amalgophiles” who point to the biggest informal case study of a dental material in human history - the millions upon millions of people who live with amalgam in their mouths and have no noticable side effects. We dentists should continue to use amalgam, they say, because it cannot be proven that it has any adverse health effects. Unfortunately for these stalwarts, our Western societies are becoming ever obsessed with safety and most patients have a “but what if…” bug implanted in the back of their minds. The practical reality is that it it doesn’t matter if it actually does or not - it’s the uncertainty about it in the minds of the patients that affects the daily practice of dentistry most.

An amalgam restoration

There’s also the wave of increased awareness on the part of our patients that they have the right to ask questions. ” Do I really need to have my new filling done with amalgam?” Is one of the more common ones. Here’s a hypothesis: if we answered that question with, “You can choose between a white filling and an amalgam one… they will both last equally as well, take the same length of time to place and cost the same amount”, what’s the decision likely to be? Virtually nobody is going to choose an amalgam filling over a white one in those circumstances. The question then becomes, “DOES a white filling last equally well, take the same length of time to place and cost the same amount as an amalgam?” The conservatives would say, “NO! Porcelain costs a lot more and takes longer; composite leaks in deep ginigval margins; and glass ionomer doesn’t have good enough wear resistance!” All good points. Case closed?

Composite wears well but doesn’t bond to dentine…

Sure porcelain is expensive and takes longer; and even with Cerec it doesn’t approach the cost-effectiveness or convenience of amalgam. But look at composite and glass ionomer again: Composite has a problem with marginal leakage in deep interproximal and cervical areas because it doesn’t bond chemically with dentine. It also shrinks a little when it sets. And it needs a very dry environment. It IS possible to get a good seal with composite in a deep proximal box - but it’s very difficult and not realistic for the majority of dentists to achieve. Some big advances have been made with composite in recent years, but even with high filler content, fluoride release and self-etching primers, composite simply doesn’t achieve a good chemical bond with dentine, and dentine does not have the rigid prismatic structure to allow the micro-mechanical bonding that enamel does and which composite relies upon. However… modern composites DO have very good wear resistance, especially high-filler hybrids that have been designed for posterior teeth. I have a 2mm thick composite covering the incisal and palatal portions of my upper left canine because I ground about 1/3 of the tooth away through bruxing on it; and it’s been there for 3 years with no noticeable wear. It guides my occlusion on that side and the material is amazingly durable - if it’s supported well.

Glass Ionomer seals well but doesn’t have good wear resistance…

And what about Glass Ionomer? Again, recent years have seen big improvements in its durability, but if you do a big restoration that includes load-bearing areas it will wear pretty fast and that can result in undesirable tooth movement in the longer term. I wouldn’t want to use it to restore the surface of my canine! It’s also rather opaque and doesn’t look like enamel; actually, it has very similar optical characteristics to dentine which is undesirable for a visible restoration. BUT glass ionomer doesn’t shrink when it sets. It bonds very strongly to dentine with a chemical (as opposed to a mechanical) bond; it releases fluoride, buffers the pH in its vicinity and resists recurrent caries… exactly what’s needed to prevent leakage in a deep proximal box or cervical restoration.

One material’s weakness is the other’s strength!

So you gotta wonder… composite has great aesthetics and wear resistance, but doesn’t bond well and tends to leak in deep proximal and cervical areas. Glass ionomer bonds really well to dentine, doesn’t mind moisture and achieves a great result in deep areas, but has poor aesthetics and wear resistance. One material’s strength is the other’s weakness, and vice-versa. WHAT IF we could come up with a technique that gets the best of both materials!? Then we’d have a restoration that rivals amalgam for durability and cost effectiveness, and beats the #$%! out of it for aesthetics. Who in their right mind would choose to have an amalgam over one of those?

Composite / Glass Ionomer Hybrids

OK, well just to spice things up a bit more, let’s get really far-fetched. Amalgam stains dentine. And doesn’t support remaining cusps. In other words, it lasts well but the tooth gradually deteriorates around it (Note: I’m aware that it is possible to create tight, bonded amalgam restorations that are highly polished and don’t leak; but again we’re talking about what is realistic for the average dentist to achieve in an average appointment. The majority of amalgam restorations in the real world leak, stain the tooth and don’t support the remaining structure.) WHAT IF our new hybrid restoration was actually faster to place than amalgam, and also supported the remaining structure without dehydrating or staining it? If such a restoration was possible, it’d be a bit of no-brainer really. Well, in my opinion anyway.

And the amalgam debate would cease to be relevant.

Assuming that you agree with me, read on….

It’s not only possible to place such a restoration, it’s ridiculously easy…

Here are several basic principles: replace dentine with glass ionomer; replace enamel with composite; place all materials before any of them have set; apply pressure before the materials have set.

The handling properties of the composite and glass inomer are particularly important. You want a fairly viscous glass ionomer and a very low-viscosity composite that does not slump. You also need a compomer (a hybrid of glass ionomer and composite) to place between the two materials and bond them together. I use GC’s Fuji IX for the glass ionomer; FUJI LC for the compomer and Filtec P60 for the composite. P60 is very firm to handle (a bit strange when you first use it) which is ideal for this technique.

I’ll use an MOD in a lower 4 as an example since it has deep proximal areas, occlusal load-bearing areas and a need for good aesthetics. I’ll also overlay the cusps to illustrate a couple of trimming techniques. The prep is important but not terribly special. You just have to make sure that all your internal line angles are rounded (use a round bur to prepare them…) and that the cavity margins are bevelled. I use a slow-speed round bur to finish the interior of the cavity (the concave bits) and a fine tapered high-speed diamond to finish the cavity margins (the convex bits). You want smooth, continuous curves - NOT sharp angles.

Now you need a matrix. A degree of skill is involved in placing the matrix well, and I’m not going to try to give pointers on that here. We will assume you have some good techniques for placing matrices… the only stipulation is that it has to be have a curved shape and preferably be made of clear plastic. Make sure you wedge it well, and don’t blame the materials for the resulting periodontal problems if you create overhangs!

Once the matrix is in place, check that it contacts the adjacent teeth firmly (consider a contact matrix and bi-tine ring if it doesn’t) and that it’s higher than the marginal ridges of the teeth next door. You don’t want the composite to flow over it and stick to the neighbours!

Etch the whole cavity, but only for 10 seconds

The placement of the restoration is really fast, and requires good co-ordination between the dentist and assistant. I strongly suggest practicing it a few times on an extracted tooth or similar to get it fine-tuned before using it on patients.

The materials are prepared in the reverse order to that in which they are placed. So first, cut off a piece of composite with a spatula and pat it into approximately the shape of the cavity with your fingers (clean, powderless gloves essential, preferably vinyl) For an MOD, I’d roll it into a small sausage, then squeeze it into an elongated pancake, and then bend the ends of the pancake around my thumbnail to give it a magnet shape which will fit into the cavity. It takes a bit of practice to estimate the amount of composite and shape it right, but it’s not difficult.

Shaping the composite into an “inlay”

Now pick up the composite carefully with college tweezers. Don’t squash it. When you open them, it’ll cling to one of the beaks. Lie them on the tray with the beaks upwards so that the composite is ready to go into the cavity.

The composite will hold on to the beak of the tweezers

Next the assistant mixes the compomer and hands it to you on a pad with a microbrush. Then (s)he immediately mixes the glass ionomer and hands it to you in a syringe. You apply the glass ionomer to the floor of the cavity, about 1-2mm thick. Don’t overdo it or you’ll end up with a mess. Again, practice it first on an extracted tooth to get a feel for the right amount. You really just want to cover the floor of the cavity.

Applying the glass ionomer

Pass back the glass ionomer applicator and pick up the microbrush. Paint the compomer over the glass ionomer and the entire surface of the cavity. You can do it quickly, as long as the coating is even and coverage is complete it doesn’t require too much finesse. Try not to push the microbrush into the glass ionomer though!

Paint on the compomer

Drop the microbrush and pick up the tweezers, and place the unset composite “inlay” into the cavity. If you estimated the shape right, the two bent bits will fit into the proximal boxes and the composite will fill the rest of the cavity with little glass ionomer visible.

Place the “inlay” in the cavity. Note that the composite is still not cured!

Now this bit is important: place a thumb or finger over the matrix and apply gentle but firm pressure to the composite, and keep the pressure on for a few seconds. This causes the composite to acts like a plunger and force the glass ionomer into all the nooks and crannies, so there will be no air bubbles or porosities when you remove the matrix band.

Applying digital pressure

If you use a transparent band you can check that the visible margins are free of bubbles; if not, apply more pressure. If necessary, you can burnish the composite to remove any excess glass ionomer that has extruded from the margins; wipe the burnisher after each stroke to avoid plunging GIC from the burnisher into the composite on the next stroke.

Now you have a little time to shape the occlusal surface. Use a flat plastic to simultaneously create cusps, planes and fissures; marginal ridges will result automatically. Use a large ball burnisher to reduce the marginal ridges to the same height as their neighbours, and give some thought as to how high you want the cusps to be. It’s not critical that you finish this part before the glass ionomer sets, but it is preferable. I dip the instruments in a very small amount of unfilled resin to improve their ability to slide over the surface. Try to use as few strokes as possible to create the shape you want, if you can create each cusp plane with a single final stroke it will not require polishing.

Work up the occlusal surface with a flat plastic

Check that the contacts with the adjacent teeth are good (they should be if you placed the matrix well), grab the curing light and cure the whole restoration. I usually give it 10 seconds from each of the occlusal, lingual and buccal angles.

You don’t need to wait any longer; the glass ionomer will be set by now and you can remove the matrix as soon as you’ve finished curing. I leave the wedges in place until I’m completely finished to avoid any bleeding while I’m trimming the restoration.

The matrix is ready to remove immediately after curing

Next, grab a diamond bur and trim the excess away from the lingual and buccal sides. It shouldn’t take much if you’ve estimated the amounts of your materials well. Stop short of touching the enamel with the bur (if you can see the difference between the enamel and the composite!). Then use a sof-lex disc to finish trimming the margins. You should not need to trim near the gingivae if you have placed your matrix and applied the materials and pressure well. Unless there is a notable irregularity or overhang, don’t trim.

Trimming with a sof-lex disc

The restoration does NOT have to have a butt margin with the tooth. It can be feathered, the glass ionomer bonds extremely well with dentine, especially when pressure has been applied before setting.

So now you just have to check the occlusion with some bite paper and adjust. I use an ordinary cylindrical diamond to reduce cusp tips and planes, and a rugby-ball shaped diamond to reduce marginal ridges. Make your fissures reasonably deep and the adjustment should rarely extend to them.

Finally, polish any surfaces that you’ve adjusted. Usually the marginal ridges, occlusal contact planes and the lingual and buccal margins are the only areas that need polishing, and they are conveniently the most accessible. I use a fine plastic polishing strip to finish the proximal surfaces.

Polishing should be easy

One point of note - most of the trimming will be in composite and you can trim without water; but with a cusp restoration or other restorations where you might trim glass ionomer around the margins, be sure to keep it wet. Glass ionomer actually wears well on lateral surfaces, but it does not like to be dehydrated! Cover it with a thin layer of unfilled resin wherever it is exposed once you have finished with it.

The finished restoration

And there you have it: a restoration that looks great, has great wear resistance, great caries resistance, great marginal seal, great support for the remaining tooth structure, that won’t stain the tooth and that doesn’t have any mercury in it (which matters to the patient even if not to you!) In short, it’s a great restoration - and it’s faster to do and at least as cost effective as an amalgam. In my opinion that makes the amalgam debate a big waste of time.

How the restoration should look (ideally) in cross section

How the restoration really looks in cross section.
Note the similarity in appearance of the secondary dentine (arrow) to the glass ionomer.