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Why Do Fillings Wear Out?

Usually when someone talks of a “filling”, what is being referred to is something that fills in structure of a tooth lost to tooth decay or fracture. Probably the most significant factor in predicting how long a filling will last is how large the filling is.

A tooth has five surfaces that can be filled. It has the top (occlusal surface), the front (mesial surface), the back (distal surface), the inside by the tongue (lingual surface), and the outside by the cheek or lips (facial surface). Generally speaking, when a filling takes up parts of four or more surfaces, a filling is considered to be only a temporary solution. For teeth with this kind of damage, a crown is usually needed to last for a long time. Today, the subject is fillings, but at another time, I will write about crowns.

A filling can be placed either in one sitting, or it can be placed in two sittings. A filling placed in two sittings is called an inlay or an onlay. Again, I will caver these types later. For now, suffice it to say, a filling placed in two sittings is more expensive, and nearly always lasts longer than a filling placed in one sitting. Insurance usually only covers fillings placed in one sitting. For this reason, and because the cost is lower for the fillings placed in one sitting, this type of filling makes up the vast majority of fillings placed currently.

Filling materials placed in one sitting are required to be placed at room temperature in a usually wet environment. These are significantly complicating factors which drastically reduce the list of possible materials that can be used. A filling material has to be moldable at room temperature and then when wanted has to set up quickly also at room temperature, and not significantly change its volume when it sets. The different materials available today have differing characteristics.

One of the common materials today for fillings is silver amalgam. It is made up of silver, mercury, tin, and copper. The silver, mercury, and tin when mixed in just the right proportions are moldable for a few minutes at room temperature, and then set. Final strength is about half the strength of tooth structure. Final strength is not achieved for about 24 hours. This material lasts a long time (up to 20+ years) and dissolves only slightly in the mouth. The most significant factor causing silver amalgam to break down over time is corrosion. Little by little, the edge where the tooth meets the metal (the margin) develops small holes which cause leakage to become significant over time and leads to a cavity underneath the filling. Most of these fillings are not bonded into place. What holds them in place is the shape of the hole in the tooth where they are placed. The hole has grooves and other retentive features. There are other alleged health problems with silver amalgam that have emerged in recent years. I should stress that these are alleged health problems and are not as of yet proven to the extent that Canadian Health authorities consider them to be significant. At any rate, silver amalgam has served us well for over 100 years. The relatively recent addition of a small amount of copper to the silver mercury and tin mix has reduced the corrosion potential of the material significantly.

The most common other material to fill teeth is composite resin. This material is a mix (composite means mix) between epoxy resin and glass beads. This material has been around for over 35 years in one form or another. In the beginning, it was only 1/10 as strong as tooth structure. This is why it was not offered as a posterior (back tooth) filling material years ago. There is a dizzying array of brands of composite, and it is fair to say they all have quite different characteristics. These days, they are set by light, with a special blue light setting them quickly. An expensive and bulky light called an arc light will set the material about ten times as fast as a conventional light. Setting (also called curing) has to be done in small increments because the material shrinks slightly on setting and the light does not penetrate a thick mass of the material. If the dentist is careful in choosing the type of composite, this material is just as strong as silver amalgam (half as strong as tooth structure). This type of material is bonded to the tooth which greatly reduces sensitivity when compared to materials that are not bonded. As well, this material reaches full strength as soon as the light cures it. This reduces the chance of breaking the filling right after leaving the dentist’s office. The factor that breaks down composite fillings is dissolution. Little by little, they dissolve. Some composite materials dissolve ten times as much every year as other types. This is why the dentist must be careful in choosing the composite type. As well, certain types of composite can be polished to a mirror shine and actually become more shiny over time, while others brands of composite become rough over time.

There are other types of materials to fill teeth, but silver amalgam and composite are by far the most common. The other types have very specialized uses. If you want to find out more about your individual case, ask your dentist what he or she suggests for the type of cavity you are having filled and why.

- This article was written by Dr. Mike Christensen and published in the Daily Miner and News, and Enterprise. Local Kenora News Publicatons (1998-2006)

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