Dental fillings (so-called direct restorations) are made of soft materials that harden (or are actively cured) only after they are introduced into the prepared cavity and brought into their final form. Fillings are the repair method of choice for small defects in the tooth substance. The era of amalgam fillings is almost over. The classic dental filling material fell out of favour due to concerns about risks related to its mercury content – but its conspicuous silvery color is probably just as responsible for the steep drop in popularity amalgam has experienced. Today, plastic fillings are a widespread alternative. Dentists use resin cements – mostly only for temporary solutions – and the number one restorative filling material: the very versatile composite resins.
What are composites?
Composite resin is a mixture of a fluid, light-curable plastic resin matrix and small filler particles (ceramics, silica or glass) that serve to reinforce the material, plus a number of chemical additives. Size and amount of the filler particles may vary, adapting hardness and viscosity of the composites to the requirements of different purposes. The light color and excellent workability of composites make it possible to create fillings that blend in well with the tooth substance and can even be shaped into functional and natural looking occlusal surfaces.
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This is how we place a composite filling
Under local anaesthesia (if necessary), caries-afflicted enamel and dentin are removed and the cavity is prepared into a simple, regular shape without unnecessary sacrifice of tooth substance. Then, a rubber sheet is placed over the oral cavity, with only the tooth that we are working on poking through. This so-called Cofferdam keeps our “work area” dry to ensure good bonding between filling and tooth substance. Then, the surface of the cavity is roughened with an etching gel and primed with a liquid resin. Now, the composite resin is brought in.
For larger fillings, this is done in several thin layers that are each cured with UV-light before the next layer is added. The surface of the filling is then reshaped, should that be necessary to create optimum occlusion with the opposite tooth, and polished to give it a nice, smooth feel. The result of that procedure, which lasts, all included, barely 45 minutes even when the filling is painstakingly layered, is an inconspicuous, fairly durable tooth filling – a low-cost, quick fix for little caries defects anywhere in the tooth arch.
A material with some little weaknesses
Composite fillings don’t only get good press. The plastic resin does not contain mercury like amalgam does, but it is suspected to release small amounts of chemicals like formaldehyde or Bisphenol A. However, no damages caused by plastic resin dental fillings have ever been reported despite the fact that these materials have almost completely replaced amalgam by now, increasingly being used for posterior teeth, too.
If you handle plastic on a daily basis, have plastic materials in your environment, eat and drink from plastic containers, there is really no particular reason to worry about the tiny additional risk composite filling may or may not pose for you.
However, with composites, there are some typical problems of more immediate concern. Accurate work can minimize these shortcomings of the material, though.
1. Composites shrink during curing
This produces a tension inside the filling that may pull it away from the dental surface, causing the formation of a tiny gap between tooth and filling. This gap can harbor bacteria and thus puts the tooth at a significant risk to develop secondary caries. And, unfortunately, the continuing decay is often noticed too late, as the intact surface of the filling doesn’t betray what is going on under it. This is why it is good practice to build the filling layer by layer, curing each layer separately, and thus minimizing shrinkage and risk of gap formation.
2. Moisture weakens the bond between composite resin and tooth
Dry working conditions during the placing of the filling are an important requirement for long-lasting, durable composite fillings. Two cotton rolls under the lips just don’t cut it: Only the Kofferdam can truly guarantee dry surroundings. Unfortunately not a matter of common practice in every dental office…
3. Composite is slightly softer than dental enamel
Composite fillings are thus subject to abrasion by the forces of chewing.
4. The plastic resin matrix of the composite is only able to form a tight bond with dental enamel
The dentin below the enamel is a little more moist by nature, and there is no really strong adhesion between dentin and a composite filling.
Layering the filling and working under dry conditions improve the bond between composite and tooth. Composite fillings that have been created according to these rules of good practice can last many years – provided they are not strained beyond their capacity, e.g. by too much masticatory pressure.
Sometimes, a ceramic inlay is the better choice
Composite fillings have a limited functional capacity. Even the most accurately placed filling will fail when it is subjected to demands that exceed the limits of the material. Composite fillings are not suitable for deep cavities, as the composite resin does not adhere well to dentin. Composite fillings are a good choice for shallow to medium cavities in the occlusal surfaces. In the upper tooth necks, they should only be used for shallow cavities: The enamel there is less substantial than that of the occlusal surfaces.
The quality of the fillings suffers if these limits are ignored. Fillings simply fall out, or they might appear to be solidly bonded to the tooth, all the while hiding a gap between tooth and filling that gives bacteria the chance to get to work on some secondary caries without being bothered by the tooth brush. It is quite likely that this secondary caries will only be noticed after the tooth is starting to hurt, and a root canal treatment may well be the only remaining option.
Large cavities and decay in regions of the molars that do most of the chewing work are much better and far more durably repaired with a ceramic inlay. The somewhat costly treatment will amply repay itself: Risk of secondary caries and loss rate are considerably lower for inlays than for composite fillings. Ceramic inlays we have placed in our office during the 1990s are still intact today. Considering some further refinements of the technique since those days, ceramic inlays we put in today will likely last you a lifetime.