2: Clinical manipulation of materials

Chapter 2 Clinical manipulation of materials

It is well accepted that unless the moisture control is carefully controlled, the clinical behaviour of the restoration and its longevity may be significantly compromised. Poor moisture control is a major cause of adhesive bonding failure and also leads to decreased mechanical properties of the material. There are many methods of moisture control, namely cotton wool rolls, dry guards, high volume aspiration, use of saliva ejectors (Figure 2.1) and some clinicians have even advocated the use of systemic medications to reduce salivary flow. This latter approach is not recommended because there can be systemic side effects which are undesirable. The most effective and predictable method of moisture control, however, is rubber dam (Figure 2.2).

The many advantages of this technique will be readily found in operative dentistry textbooks but from a material’s perspective, it can provide the closest conditions to an ‘ideal’ environment for material placement. Its use will allow the clinician to control the operating field more precisely and with respect to humidity and moisture control. It also greatly improves the dentist’s access when placing the material. The different types of rubber dam and other materials used in connection with it are discussed in Chapter 13.

Shade Taking

Shade taking is an important aspect of restorative dentistry where the dentist attempts to match the colour of the adjacent teeth or tooth with the restorative material, be it a directly placed restorative material such as resin-based composite or one constructed in the dental laboratory such as ceramic. Often the patient will judge the success of the restoration by its appearance, with an imperceptible restoration being considered ideal by them.

Shade taking is renowned for being one of the most challenging aspects of restorative dentistry for the dental team. Although the techniques for shade taking are beyond the scope of this book, advice relevant to dental materials is included here.

Matching like with like

It is obvious that to most easily achieve a consistent and predictable result when selecting a shade of restorative material to match the tooth tissue being restored, the material of the shade guide should be the same as the restorative material. Unfortunately this may not be achieved as many manufacturers’ shade guides are made from a dissimilar material to the restorative material being used.

One of the most commonly used shade guides is the Vita Shade Guide (Figure 2.3), which is used to shade match for ceramics. Unfortunately, the tabs of the shade guide are made of a bulk of material whereas the crown is usually in much thinner sections, altering translucency and shade substantially. It is also possible to get small tabs of the various stains that are available to characterize the ceramic (Figure 2.4).

Some shade guides are designed such that a dentist can select both a shade for ceramic and acrylic when crowns and dentures are being provided for the same patient as the shades for the two materials correspond (Figure 2.5). Several manufacturers have recommended using the Vita Shade Guide to shade match for the resin-based composites, and a form of recipe wheel (Figure 2.6) guides the dentist in using the shades in the kit.

To reduce the risk of error, some manufacturers have produced shade guides which are made of the same resin-based composite that is in the kit. This has the benefit that similar types of material are being matched. There is a greater chance that a good shade match will be achieved. Figure 2.7 illustrates two shade guides. One is a simple tab system composed of the resin composite in the kit and the other involves inserting a core of the ‘dentine’ shade in the shell of ‘enamel’. If a small drop of glycerine (or water) is placed to remove the air barrier between the two shade tabs, the dentist can see the final shade result with some certainty.

The other and probably most reliable method of correctly ascertaining shade is applying a small piece of the composite to be used to the tooth without any bond and curing it for 10 seconds. This must be cured as there is a change in shade (shade shift) on curing.

Similarly when matching ceramic, ceramic should be used. Furthermore different batches of ceramic can differ slightly in their appearance and so ideally the technician should supply the clinician a shade guide manufactured using the same batch of ceramic which will be used to construct the restoration being fitted. For obvious financial reasons this rarely happens.

Light Polymerization

Many polymeric materials are prepared in an unset form in the factory and delivered to the dentist in a sealed container. The unset material is then dispensed at the site where it is to be used. The setting reaction is initiated by applying an external energy source – which is usually light. This section explains the mechanics of the light polymerization process.

Modern dental polymers generally cure or set into a solid mass, and this process is catalysed either chemically or by the application of external energy such as light. This light polymerization process has been used in dentistry since the early 1970s, the idea originally being developed in the car industry to speed up the set of car paint. The original intention was to save substantial time on the production line by irradiating the painted surface in a light chamber, instead of having to wait for it to dry by a chemical reaction. Although the idea has fallen into disuse in the car industry, its potential for dentistry was realized and as a result there has been a major trend in recent years towards the production of polymeric dental materials which are cured by the application of light.

Advantages and disadvantages of light curing

Light curing offers a number of advantages:

• The material has an extended working time so it can be manipulated for a long time without setting. This means that the unset material may be sculpted into a form without time pressure. This is particularly useful if complex aesthetic restorations are being provided. Only when the dentist is happy with the desired shape is the light applied and the material set. This is known as a command set.

• It delivers a more consistent means of polymerization as a more even distribution of the chemicals within the paste can be obtained because the blending of the paste is optimized by the manufacturer in the factory.

• The amount and concentration of amine required in the material can be lowered. A tertiary amine is one of the essential components in the (chemical or light) curing process. Unfortunately the presence of any residual amine after curing leads to darkening and yellowing of the set material with time. Restorations in the anterior sextants of the mouth which are subsequently exposed to sunlight are particularly affected, which is clearly not desirable. As the bulk of the amine is used up during the light-curing polymerization process, a decrease in the amount and concentration of amine reduces colour change and enhances colour stability.

• There is a saving in clinic time as there is no need to wait for a chemical reaction to go to completion before moving on to the next stage of the operative procedure, such as finishing and polishing.

• The quality of cure is improved. The level of conversion of the monomeric component to the polymer varies with the method of initiation of the reaction. The level of conversion is lower for two paste (chemical cured) materials compared to light activated materials. Most light activated materials convert between 50% and 70% monomer to polymer. However, this still leaves some unconverted monomer, which can lead to leaching out of material in the long term and degradation of the restoration with time. Conversion of up to 95–97% of the monomer can be achieved in the laboratory. This increases the mechanical properties but makes the set material more brittle. To achieve this optimum conversion, heat, light and pressure are required and this can only be achieved extraorally.

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Jan 31, 2015 | Posted by in Dental Materials | Comments Off on 2: Clinical manipulation of materials
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