Requirements of Direct Filling Materials and Historical Perspectives
Direct filling materials are used for chairside restoration of teeth. They differ from indirect restorations, such as crowns, bridges or inlays, because no laboratory stage is involved in the provision of the restoration.
Teeth may need restoring for a variety of reasons. Destruction of tooth substance caused by dental caries may result in the loss of considerable quantities of enamel and dentine. Trauma may cause fracture and loss of parts of teeth. In this case the anterior teeth are most vulnerable and those teeth affected may be otherwise sound and caries-free. A third factor causing loss of tooth substance is wear. This often arises due to over-zealous brushing using an abrasive dentifrice but may also arise due to a peculiarity of the diet, working environment or habits of the patient. High frequency of exposure of teeth to acids in food and drink or from regurgitated gastric juice is of growing concern in relation to the wear of teeth involved.
The parts of teeth which require replacement by a restorative material vary in size, shape and location in the mouth. Thus, at one extreme, it may be necessary to restore a large cavity which extends over the mesial, occlusal and distal surfaces of a molar tooth. An entirely different situation is the restoration of the corner of an incisor which has been lost in an accident. The requirements of materials used in these and other applications vary and it is not surprising that no single restorative material is suitable for all cases. For some situations the strength and abrasion resistance of the material may be the prime consideration. In other situations appearance and adhesive properties may become more important.
The factor which is generally used to assess the success or failure of a restorative material for any application is durability. In this context the term refers to the life expectancy of the restoration and the life expectancy of the surrounding tooth substance and how it may be affected by the presence of the restoration. Durability depends on the physical and biological properties of the restorative material.
The acceptance of the material by the profession also depends on the ease with which it can be handled in the surgery.
Logic dictates that materials used as restorations in ‘visible’ cavities in anterior teeth will be subjected to greater scrutiny of their appearance than materials used in occlusal cavities in posterior teeth. However, patients’ awareness of the variety of material options available to the dentist may result in requests for the use of tooth coloured materials even where appearance may seem to be of minor significance to the trained professional eye.
Many restorative materials are supplied as two or more components which require mixing. Thorough mixing should be easy to accomplish in a reasonable time. After mixing, the ease of handling depends on factors such as viscosity, tackiness and setting characteristics such as working time and setting time. Different techniques must often be adopted to handle different materials. Whereas some materials readily flow into the prepared cavity under little pressure some products require ‘packing’ under considerable pressure. When materials remain tacky for some time after mixing they may be difficult to handle because they adhere to instruments. Working time should be sufficiently long to enable manipulation and placement of materials before the setting reaction reaches the stage at which continued manipulation is either difficult or would adversely affect the structure and properties of the final set material. Setting times should, ideally, be short for the comfort and convenience of both the patient and clinician.