These are the materials used to permanently restore the tooth to its full function and appearance, and they must all have the following properties.
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Set to a hard enough degree to allow normal masticatory function to occur, without fracture of the material.•
Not to dissolve or otherwise deteriorate in saliva over time.•
To be biologically safe, by not reacting with the body’s tissues or giving off any harmful chemicals.•
Capable of being applied to the tooth using normal conservation instruments, in a straightfor- ward manner.•
Have a reasonable working lifespan of years, rather than of months.•
Ideally they should be aesthetically acceptable, although this limits the use of amalgam.The three commonly used materials are amalgam, composite and glass ionomer.
Figure 15.17 Calcium hydroxide liner with applicator.
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Amalgam
Amalgam has been in use for over 150 years and is probably still the most widely used permanent restorative material for posterior teeth in the UK. Despite advances in dental material science, it is still often cheaper to buy, more durable and easier to use than its tooth-coloured competitors – composite and glass ionomer.
Amalgam is prepared by mixing a powdered alloy with liquid mercury, usually provided as a preloaded capsule as illustrated in Figure 15.18. The two constituents are kept apart by a rubber separator diaphragm until mixing occurs, when the mixing vibration dislodges the separator and allows the powder and liquid to come into contact with each other. The mixture produced forms a plastic mass, which is packed into the tooth cavity and sets hard in a few minutes. The main constituents of amalgam alloy powder are:
•
silver – up to 74%•
copper – up to 30%, in high copper alloys•
tin – variable quantities•
zinc – small quantities.The preloaded disposable capsules are inserted into a special machine for automatic mixing, called an amalgamator (Figure 15.19).
Figure 15.19 Amalgamator.
Figure 15.18 Amalgam capsule.
Ball of liquid mercury
Rubber diaphragm
Alloy powder
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Varying the alloy powder constituents produces an amalgam mix with different levels of marginal ditching and discolouration. Modern amalgams tend to have a ‘high copper’ content (up to 30%
copper), to reduce these unwanted effects as much as possible.
As amalgam is a plastic filling and a good conductor, cavities are made retentive, lined to insulate the pulp against thermal injury, and the entire cavity may be varnished to give a good marginal seal before inserting the amalgam material.
Very large cavities may have too little crown structure left for adequate retention to be provided using undercuts and dovetails, and in these cases modern bonding agents (see later) or self- tapping dentine pins are used to provide retention instead.
Recommendations for providing the best long-term results of amalgam restorations are as follows.
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In shallow cavities, a calcium hydroxide lining or three coats of cavity varnish will suffice as a lining and marginal seal.•
Medium cavities are lined with either a zinc oxide and eugenol base or glass ionomer cement, and may also be sealed with three coats of varnish.•
Calcium hydroxide is used as a sublining in deep cavities, especially where zinc phosphate cement is used as a base.Amalgam restoration procedure
The general technique for amalgam restoration of a tooth, and the instruments used, are as follows.
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All dental personnel and the patient wear the correct personal protective equipment throughout the procedure; in particular, the patient must be given safety glasses to wear.•
All caries is removed from the cavity using burs and excavators as described previously, and without breaching the pulp chamber.•
The cavity is undercut so that the amalgam restoration does not fall out.•
Moisture control techniques are used so that all fluids and debris are removed from the mouth, and so that the cavity remains dry during material placement.•
Adequate soft tissue retraction with aspirators or mouth mirrors is applied, without causing trauma to the patient.•
A lining or base is placed on the floor of the dry cavity to protect the pulp, if required.•
A metal matrix band in its holder will be adapted to the tooth to prevent amalgam spillage during placement, whenever a class II cavity is involved. This will either be a Siqveland (Figure 15.20) or a Tofflemire matrix outfit.Figure 15.20 Siqveland matrix system.
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•
To ensure full adaptation of the band to the tooth in the interproximal area, a wedge may be pushed between the tooth and its neighbour to give a tight fit.•
The alloy and mercury are mixed in the amalgamator and the amalgam produced is inserted into the cavity in increments, using the amalgam carrier (Figure 15.21).•
Each increment load is fully pushed and condensed into the cavity, using the amalgam plugger.•
Once filled, any excess amalgam is carved off the tooth and the surface of the restoration is shaped so that food debris is naturally directed away from the interproximal areas, mimicking the normal occlusal fissure pattern of the tooth.•
The edges of the amalgam are adapted fully to the tooth surface by use of a burnisher instru- ment, so that no gaps or ridges remain.•
All excess amalgam and mercury are removed from the oral cavity through the high-speed suction.•
The matrix band is removed and the restoration is checked for overhangs.•
The occlusion is checked and adjusted as necessary.A summary of the materials and instruments involved is shown in Table 15.3.
Figure 15.21 Plastic amalgam carrier.
Table 15.3 Amalgam restoration procedure: materials and instruments
Item Function
Liner and/or
base material To protect the pulp from thermal shock
Various available including calcium hydroxide and some of the temporary cements, all suitable under amalgam restorations
Matrix system To prevent overspill in cavities of two or more surfaces
Usual systems are Siqveland or Tofflemire; the bands are single use Wedges Placed interdentally with matrix system, to tightly adapt the band to the tooth
Both wooden wedges and plastic wedges are available Amalgam
carrier Autoclavable ‘gun’ used to pick up and carry the amalgam to the cavity, where it is squeezed out
Amalgam
plugger Instrument to pack and condense the amalgam into the cavity so that no air spaces remain
Finishing
instruments To ensure the restoration is adapted to the tooth and is not high in occlusion Various items used – Ward’s carver, flat plastic instrument, burnisher, greenstone drills
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Although the initial set of the amalgam takes only a few minutes to occur, it is not complete for several hours. The patient is therefore instructed not to attempt to eat or drink until the effects of the local anaesthetic have worn off, or for at least 2 h post treatment if no anaesthetic was used. If considered necessary, the filling may be polished at a subsequent visit using water-cooled finishing burs, brushes and pumice paste. Amalgam finishing burs are made of steel for use in low-speed handpieces, and come in a variety of shapes but are distinguished from other burs by having far more cutting blades than usual (Figure 15.22).
The advantages and disadvantages of amalgam are shown in Table 15.4.
Mercury poisoning
Despite the many advantages of amalgam over other permanent restorative materials, its one big disadvantage is the fact that it contains mercury, which is known to be toxic. It was formerly believed that mercury poisoning could only occur after several years of mishandling. However, it is now known that it can occur within a few months if a large quantity of mercury is spilled. There is also debate about the safety of the material within the oral cavity, to patients. The risks are considered so great that some countries are currently considering a ban on the use of amalgam as a dental restorative material, although this does not include the UK yet. In the meantime, every dental nurse must therefore understand the risks involved and the methods of preventing hazards associated with the use of mercury and amalgam.
Table 15.4 The advantages and disadvantages of amalgam
Advantages Disadvantages
Easy to use Mercury is toxic
Relatively cheap, compared to composites
and glass ionomers Not retentive to tooth, so cavities have to be undercut
Good set strength Can transmit thermal shocks, so liners and bases are required in deeper cavities Able to withstand normal occlusal forces Has to be mixed very accurately to be
dimensionally stable Excellent longevity, lasts for many years
under normal conditions in well-maintained mouths
Aesthetics are poor, so its use is limited to posterior teeth
(a) (b) (c) (d)
Figure 15.22 Finishing bur shapes. (a) Flame. (b) Pear. (c) Round. (d) Oval.
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Mercury poisoning can occur in the following ways.
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Inhalation of the vapours.•
Absorption through the skin, nailbeds, eyes and wounds on the hands.•
Ingestion by being swallowed.Although the possibility of skin contamination is obvious when handling mercury or amalgam, the risk of inhaling mercury vapour is not. Both mercury and amalgam release mercury vapour at ordi- nary room temperature – and the higher the temperature, the more vapour is released. Mercury vapour is odourless and invisible, so it is of the utmost importance to keep all mercury and waste amalgam in sealed containers in a cool, well-ventilated place – not near a hot steriliser or radiator, or even in sunlight. In particular, amalgam carriers must be dismantled and fully emptied of any residual amalgam before they are autoclaved, not only to prevent the release of mercury vapour but also to prevent blockage of the carrier by hardened amalgam residue.
Another source of mercury poisoning is the removal of old amalgam fillings. This releases a cloud of minute amalgam particles which can be inhaled or contaminate eyes and skin. It can be prevented by combining the use of copious water spray and an efficient aspirator, which is sealed to prevent the release of vapour in the clinical area. The use of a rubber dam and safety glasses is the best protection available for patients during filling removal.
Apart from very rare cases of allergy, there is currently no evidence of danger to patients from the presence of their amalgam fillings, as a well-placed restoration should have had all excess mercury removed during the procedure. However, it has been advised that removal or insertion of amalgam fillings in pregnant patients should be deferred until after the baby is born, if clini- cally reasonable to do so. Pregnant chairside staff involved in such procedures may also be concerned, but regular urine tests for mercury contamination of staff can be carried out to show if any risk is present.
The symptoms of mercury poisoning are as follows.
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Early symptoms may include headache, fatigue, irritability, nausea and diarrhoea.•
At this stage it is unlikely that mercury poisoning would be suspected.•
Later symptoms are hand tremors and visual defects such as double vision.•
The final stage is kidney failure, and then death.Precautions to be followed by all staff
The routine use of personal protective equipment (PPE), such as gloves, mask and safety glasses, or visors worn for protection against cross-infection, will provide protection against mercury haz- ards. Dental nurses can be reassured that no danger exists if the following precautions are taken.
However, they are so important that they are repeated here, having been previously covered with other occupational hazards in Chapter 4.
To avoid absorption of mercury through the skin, the basic rules of cross-infection control should be followed.
•
Always wear disposable gloves when handling mercury, mixing amalgam and cleaning amalgam instruments.•
Do not wear open-toed shoes in the clinical area, as the floor may be contaminated by spilled mercury or dropped amalgam.•
Do not wear jewellery or a wrist watch as they may harbour particles of amalgam.•
Incidentally, gold jewellery can be spoiled by contact with mercury or amalgam.Chapter 15 Levison’s Textbook for Dental Nurses
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To avoid pollution of the air by mercury vapour, the following precautions must be taken.
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Ideally, a preloaded capsule system should be used, rather than the old-fashioned system of bottled mercury and alloy powder being manually loaded into the amalgamator.•
If the latter system is still in use, containers of mercury must be tightly sealed,. and stored in a cool, well-ventilated place.•
When transferring mercury from a stock bottle, great care must be taken not to spill any. It is very difficult to find and recover mercury which has dropped on the floor or working surface as it is a liquid metal and rolls away easily (see Figure 4.24).•
For removal of old amalgam fillings, the use of a high-speed handpiece with diamond or tung- sten carbide burs, water spray and efficient aspiration helps to reduce the aerosol of amalgam dust and mercury vapour while the use of a rubber dam will protect the patient.•
Surgery staff must wear full PPE throughout such procedures, as they should for all chairside procedures.•
All traces of amalgam must be removed from instruments before autoclaving, otherwise vapour will be released as the autoclave heats up – this is especially pertinent with amalgam carriers.•
Keep the surgery well ventilated.•
Amalgamators and the capsules used therein should be checked after use as cases have been reported of mercury leakage from capsules during mixing.•
Amalgamators must be stood on a tray lined with aluminium foil so that any droplets can be easily collected and disposed of as hazardous waste, using a disposable syringe (see Figure 4.25).•
The machines must also have a lid over the capsule holder, so that leaking capsules do not throw their dangerous contents into the surgery.•
All premises using amalgam must have a mercury spillage kit so that any accidents can be dealt with swiftly and correctly (see Figure 4.27).Surgery hygiene
Much can be done to minimise any dangers of working with mercury by adopting the following rules, many of which would be common sense anyway.
•
Smoking, eating, drinking and the application of cosmetics must not take place in the surgery.Any of these actions could permit absorption of mercury – from mercury vapour in the air or from contaminated hands.
•
The storage and handling of mercury must be confined to one particular part of the surgery, away from all sources of heat.•
Any spillage of mercury must be reported to the dentist or other senior staff member.•
Mercury spillage kits must be used for the safe recovery of all spillages greater than a few droplets.•
Vacuum cleaners must never be used for this purpose as they vaporise any mercury they pick up and discharge it back into the surgery.•
Floor coverings must not have any cracks or gaps in which mercury or amalgam can be trapped, and carpets must not be used as a surgery floor covering.•
Surgery equipment and plumbing must have easily accessible filter traps to collect particles of waste amalgam flushed through spittoons, aspirators or other suction apparatus. This waste must be collected and transferred to the surgery waste amalgam containers.•
Modern aspirators must be fitted with an amalgam trap, so that no waste material enters the drains (Figure 15.23).443
•
Waste amalgam must be saved in sealed tubs containing a mercury absorption chemical, and taken for collection by specialist waste contractors for recycling (see Figure 4.18).•
Efficient ventilation is essential at all times of the year, and high surgery temperatures should be avoided.•
The Environmental Protection Agency must be notified of any large spillage that may result in mercury poisoning, under RIDDOR.Safe disposal of waste amalgam
All amalgam waste and extracted teeth with amalgam fillings present must only be collected for disposal by authorised hazardous waste contractors (see Chapter 4). The reason for this is that most other hazardous waste is incinerated, and if that containing any amalgam waste was included, the incineration process would pollute the air with mercury vapour. Before collection by the authorised contractor, the amalgam waste must be stored in special containers, which they supply and which prevent the escape of mercury vapour (see Figure 4.19). The contrac- tors may also arrange periodic testing of the workplace mercury vapour levels and the checking of amalgamators for mercury leakage. If these tests show an unexpectedly high concentration of mercury vapour in a workplace with no report of a spillage occurring previ- ously, expert advice can be sought from the Environmental Health Agency in tracing the source and resolving the problem. As stated previously, urine tests can also be carried out on staff to ensure that they have not been exposed to high levels of mercury vapour, although unfortunately these tests are not routinely carried out by occupational health departments at the moment.
Mercury spillage
Accidental spillage of mercury or waste amalgam must always be reported to the dentist or other senior staff member. If a spillage occurs, globules of mercury can be drawn up into a disposable intravenous syringe or bulb aspirator and transferred to a mercury container (see Figures 4.24, 4.25) while small globules can be collected by adhering to the lead foil from x-ray film packets.
Waste amalgam can be gathered with a damp paper towel. For larger spillages, the following protocol should be undertaken.
Figure 15.23 Waste amalgam separator trap.
Chapter 15 Levison’s Textbook for Dental Nurses