If tooth is very badly decayed and large amount of tooth structure is lost, it is not recommended to get composite filling done because composite filling gets retention from tooth structure and if adequate tooth structure is not present to support the filling, it will not retain and will come out easily. In such a case it is better to get an inlay, onlay or crown placed over the tooth.
If filling is to be done in the tooth subjected to high occlusal stresses. Although newer composites are having good strength but if occlusal forces are very high such as in patients of bruxism (unusual grinding of teeth), then is better not to go for a composite restoration.
Composites are not preferred on the surface of root because if composite is placed on root surface a V-shaped gap is created at the margin of restoration and tooth due to shrinkage of composite. So, glass ionomer cement is preferred if cavity is present on the surface of root.
Deciding for Composite Filling or Other Alternatives
Anterior (Front) Teeth
Composites are ideal restorative materials for small cavities and minor defects in anterior teeth with respect to strength as well as esthetics. If defect is large it is better to go for a crown because composite cannot fulfill the strength requirements. But crown is expensive as compared to composite restoration, so if cost factor is there you can opt for composite restoration keeping in mind there are more chances of failure of composite as compared to crown and you may have to get to repaired or replaced after sometime.
Posterior (Back) Teeth
Composites have become a good restorative material for back teeth also but for small cavities and in teeth not subjected to greater forces of mastication. Although amalgam(silver) filling has more strength as compared to composite filling, most people prefer to get composite filling done because of its esthetic properties. For a large defect crown is recommended but again due to financial reasons one may opt for a composite filling.