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(Background information)
Polymers are a part of everyday life and examples can be found almost anywhere. Many people
think of polymers simply as plastics used for packaging, in household objects and for making
fibres, but this is just the tip of the iceberg.
Polymers are used in all sorts of applications you might not have thought much about before.
Polymers and composites (materials made by combining two or more materials) are vital to
modern dentistry, for example.
Teeth
Bones and teeth, the hard tissues in the human body, are made partly of organic and partly of
inorganic material. The inorganic component mainly consists of a substance called hydroxyapatite.
The simplest formula of hydroxyapatite is Ca5(PO4)
3(OH).
The outer layer of your teeth is the hardest material in your body and is called enamel. Enamel
consists of approximately 92% hydroxyapatite. Enamel is a ceramic material.
Beneath the enamel, the bulk of a tooth is made of dentin. This is a composite material and
contains a mixture of hydroxyapatite, collagen, water, and salts. Collagen is an organic substance.
Teeth function in one of the most inhospitable environments in the human body. They are subject
to larger temperature variations than most other body parts and can cope with exposure to ice at
0 °C and to hot tea and coffee. Teeth also encounter pH changes in the range 0.5 to 8, as well as
large mechanical stresses during chewing.
Tooth decay, called caries, occurs when teeth are frequently exposed to foods containing
carbohydrates (starches and sugars). These foods include milk, some soft drinks, ice cream, cakes
and even some fruits, vegetables and juices. Bacteria that live in the mouth form plaque. Plaque is
a film on the teeth where bacteria reproduce. The plaque interacts with deposits left on your
teeth from sugary and starchy foods to produce acids. Over time, these acids damage tooth
enamel because they dissolve the hydroxyapatite present in your teeth. The acids formed by
plaque can be partly counteracted by saliva in your mouth.
The acids in plaque can eventually dissolve the enamel surface of the tooth and create holes
(cavities) in the tooth. Cavities are usually painless until they grow very large and destroy the nerve
and blood vessels inside the tooth. It is important that any holes that form in our teeth are filled
as soon as possible.
Fillings
Amalgam fillings
When we visit the dentist for a filling, he or she may use a material that looks like a silvery metal
to fill up the tooth cavity. This filling material is often called ‘amalgam’. Dental amalgam is an
alloy of mercury (50%), silver (30%), tin, copper and zinc. It is made by dissolving the solid metals
in the liquid mercury. Amalgam has been used to fill teeth for about 160 years.
Dental amalgam and the ‘mercury issue’
In 1995 a news programme ran a story about dental amalgam. A researcher claimed that
amalgam is poisonous because of its mercury content and is responsible for most of the diseases
that have not yet been cured by medical science. Many people who watched this programme
were appalled and contacted their dentists immediately to have their amalgam fillings removed, or
even to have their teeth extracted!
When other scientists looked at the evidence they discovered that the research was poorly done.
The results were misinterpreted and often contradictory.
Scientists have evidence that the mercury metal used in amalgam fillings is very inactive. The
American Food and Drug Administration has concluded that amalgam causes no demonstrated
clinical harm to patients if it is properly placed and that removing existing amalgam fillings will not
prevent ill health or reverse the effects of existing diseases. The use of amalgam is still strongly
supported by the American Dental Association.
It is certainly not disputed that mercury is a poison if it enters the body in large quantities.
Although the mercury in amalgam fillings is bound to the other metals in the amalgam, it can be
freed during chewing or when the teeth are brushed. When this happens, the mercury is released
from the filling as a vapour. Mercury vapour is also present when amalgam fillings are placed into
and removed from teeth. To remove a filling, the dentist has to drill into it. The friction between
the drill and the amalgam can vaporise some of the mercury in the filling. It is thought that the
amount of mercury vapour that enters the body by inhalation during this process is small enough
to be safe.
New research is being carried out to investigate the relationship between amalgam fillings and
some diseases, such as multiple sclerosis and Alzheimer’s disease. No link has yet been proven.
The UK Department of Health and the US and Canadian authorities generally favour the
continued use of amalgam fillings on the basis of current scientific advice.
The Department of Health advises that pregnant women avoid having amalgam fillings put in or
removed as a precautionary measure. This is because mercury has been shown to cross the
placenta from mother to unborn baby.
White fillings
White fillings are routinely used on front teeth and are increasingly being used on back teeth.
There are two main types of white filling materials currently used by dentists: composite fillings
and glass ionomer cements.
Composite fillings
Composites used for filling teeth are generally made of silica or glass particles bound with a
polymer resin.
The polymers that are used as the resin in composites for fillings are based on a monomer called
methacrylic acid.
The polymer resin is usually filled with between 35 to 85% glass filler.
The procedure used to place a composite filling in a tooth involves several steps. First, the tooth
must be prepared. It is etched with acid to remove debris and an adhesive is applied. The solvent
in the adhesive is then evaporated.
Next, the cavity is filled with a layer of composite. This layer is hardened by shining a light on it –
a process called photocuring. The light causes the monomer molecules to react with one another
and link together to form a solid resin.
The resin shrinks a little during polymerisation so several successive composite layers are added
and photocured. Photocuring is useful because it allows the dentist time to work with the
material, building and shaping it correctly before it is exposed to light. When the dentist is ready,
the filling can be hardened immediately by shining light on it. Finally, the filling is polished.
Composite fillings have advantages and disadvantages. The table below summarises these.
Glass ionomer fillings
Glass ionomer fillings are similar to composite fillings because they also use a polymer resin.
However, the filling material contains strontium, phosphate and fluoride ions. The big advantage
of this filling material is that it interacts with the enamel and dentin in the tooth and forms an
excellent seal between the filling and the tooth. A true biological and chemical link is formed with
the tooth and this reduces the sensitivity of the filled tooth.
Another advantage of glass ionomers is that the fluoride ions from the filling material are
continually released by reaction with saliva. These fluoride ions are next to the teeth and can react
with the enamel. This helps strengthen the teeth and prevent further decay.
The disadvantages of glass ionomer materials are that they are not as aesthetic as composites and
they are weak under normal chewing forces.