At the end of the 19th century, a famous dentist, J.V. Black, formulated the familiar silver filling material that has become so very common. It was an amalgamation or blending of silver, mercury and other metals. Black discovered that when mercury was mixed violently with metals it combined to form a mass that could be molded and shaped before it finally hardened. This made the mixture an ideal dental filling material. As the formulation gained popularity, it was simply referred to as Amalgam.
Modern Amalgam formulations include: silver, mercury, copper, tin and sometimes other trace metals in varying quantities. The solid metals are ground into fine shards or made into tiny spheres. Depending on the shape of the solid metals, the character of the soft mass is altered which helps in packing it into the cavity.
Amalgam is placed into a cavity after all the decay is removed. A liner, or varnish, is usually used to coat the walls of the cavity. This prevents the silver and mercury from staining the tooth. The cavity must be shaped to retain the amalgam. To do this the bottom of the cavity is made slightly larger than the top. This creates undercuts that the material locks into. It also requires the removal of more tooth structure to achieve this.
When the silver filling sets up and is hard, it begins a long, slow corrosion process. This corrosion is the magic that helps the filling seal to the tooth. This corrosion seal can take six months or more to occur. During this time before the corrosion seal is complete, the filling can experience leakage about its edges and cause sensitivity in the tooth.
Amalgam, since it is metal, conducts thermal changes very well. Hot and cold in the mouth that come in contact with the filling can transmit this stimulus to the inner portion of the tooth which can cause pulpal pain. This becomes a significant problem when the filling is deep. Pain can be significant and continued painful thermal changes can even damage the pulp of the tooth, causing it to die. As such, deep fillings that will be restored with Amalgam, often need an insulating base beneath them. These non-metallic, insulating bases are usually cements which harden quickly. In addition to thermal conduction, amalgam also conducts electricity. This can create problems when a dissimilar metal comes in contact with it--like gold. The saliva and the two different metals act as a battery. This scenario can create something referred to as electro-galvanic shock--which is uncomfortable, even painful.
Amalgam is a good material because it is strong, its easy to manipulate, it is inexpensive (as compared to Gold) and it has good resistance to abrasive wear. For these reasons it became the "king" of restorations in dentistry for almost a century.
While amalgam had many good properties, it had one significant drawback--it was silver-gray in color and was unsightly when placed in front teeth. As such, dentistry had to find a white filling material that would be more friendly to smiles. A few different formulations were developed, but they had many problems.
In the second half of the 20th century, a new material called composite resin was developed for restoring anterior teeth. This new restorative was greeted with much applause. It was unique in several ways. Composite had a resin matrix filled with tiny hard particles. The resin portion, could be cured, either chemically or by the use of light. In addition, the resin matrix could be chemically attached to the tooth structure by a process known as "bonding."
Composite resin was non-metallic and it did not conduct electricity or thermal changes like amalgam. Therefore composite did not need bases like the silver required. It was also able to be placed in cavities that didn't have undercuts because it relied on bonding technology to hold it in place. This allows for more conservative cavity shaping. When light curing became a reality for this material, it then allowed the dentist as much time as he needed to shape and form the filling before making it hard on command with an intense blue light. Unlike amalgam which required months to corrode and seal, composite sealed to the tooth instantly upon curing because of the bonding.
First and foremost, composite could be polished to a high shine and it was produced in several different hues and shades which allowed the dentist to match the patient's tooth.
Composite was adapted rapidly for the front teeth and by the early 1980s, it was the most common restoration placed in front teeth. Many dentists saw the desirability of using this material on the back or posterior teeth, but they quickly realized it was not strong enough to stand up to the chewing and grinding that occurred on these teeth. Composite wore at a rate that was much greater than amalgam, and as it wore, its bonding broke down.
After years of research and trials, new posterior composite came onto the market in the late 1990's. These new posterior materials were highly resistant to wear--as good as amalgam or better, and they were unlike the sticky, soft composite for front teeth in that they could be packed into cavities like amalgam. A stiff consistency to the material allows for this. With the advent of the posterior, condensable composite, a slow shift in dentistry began. More and more dentists left amalgam behind for the many beneficial properties of the new composite. Esthetics was a huge driving force. No longer would people be doomed to unsightly silver fillings on their back teeth. In addition, the elimination of liquid mercury from the dental office was greatly welcome as this heavy metal was a potential health risk.
Today composite is quickly replacing amalgam as the filling material of choice. This change, however, is accompanied by some other changes that dentists need to be aware of. The most significant of these is the limitation of filling size for composite.
All matter expands and contracts when it is heated or cooled. In the mouth, thermo-cycling refers to the repeated heating and cooling the mouth experiences with hot and cold food. In this hostile environment, filling materials must not be too different in their expansion and contraction rates than that of the teeth, otherwise the seal of the filling to the tooth will breakdown and cause leakage.
As fillings get larger, their mass increases. The larger the mass, the more expansion and contraction that occurs with the same temperature changes. This is dramatically illustrated when we consider a roadway. A bridge's expansion joints--interwoven teeth-like devices that allow for the expansion and contraction of the road bed--allow for many inches or feet of size change. This significant size change occurs on such a large span, in the heat of summer and in the cold of winter.
Amalgam had the unique ability to form very large, multi-surfaced fillings with some success, because if a margin opened up due to expansion or contraction that was too great, it would corrode again and then finally seal. Composite resin, however, has an instant, solid, bonded seal. When a composite restoration gets too large, the differential between the tooth and restoration size change in expansion and contraction, causes the margins to break open. In a nutshell, a dentist must place only medium to small conservative fillings when using composite.
Large amalgam restorations were never considered good dentistry in that leakage could occur, the tooth was weak and the depth could cause pulp problems. With composite, similar size fillings would fail because of the restorative's bonding. Even though that bonding was a superior, instantaneous seal.
As such, teeth that need crowns due to heavy decay, cannot be patched successfully with this type of white restorative material as it was once was (with limited success) with amalgam. In cosmetic practices, amalgam has been eliminated, and as such, an appropriate esthetic restoration must be selected for teeth with heavy damage.
Composite is a terrific new filling material that has many advantages over amalgam. Its few short comings are, however, significant and it must be used in accordance with those shortcomings.
