Composites are extremely technique sensitive. The dentist’s technique must be 100% perfect otherwise the composite is likely to cause problems and fail. All dentists have seen posterior composite being placed and caries being evident sometime later. Composite is very technique sensitive and does not like any moisture at all.
This is extremely problematic for posterior teeth where there is moisture present and even from the patient’s own breath or the bleeding/crevicular fluids around the gingival margins.
Most composites now are light cured and therefore have to be placed in layers in order for the light to penetrate fully to cause polymerisation setting reactions. This means that composite takes longer to place and in addition if the light does not reach the full extent or deeper layer of the cavity, this will result in unset composite. Composite which is unset will cause the body of the filling to move resulting in micro leakage and secondary dental caries.
Polymerisation shrinkage is a major disadvantage of composite resin. This means that the unset material occupies a larger volume than the polymerised set material. This then results in a gap between the filling material and the cavity walls. This causes bacteria to get in there and cause secondary or recurrent caries.
In addition the gap picks up stains and every dentist has seen a brown line along the margins of a composite restoration. This is worse in avid smokers although also in people who have strong tea and coffee or red wine habits. Polymerisation shrinkage can also cause physical stress inside the cavity/restoration interface.
One of the ways in which polymerisation shrinkage effects can be reduced is to place the composite in layers. It is important however to understand that the amount of polymerisation shrinkage is the same whether the composite is light cured or whether it is chemically cured.
The difference is the manner in which the polymerisation shrinkage occurs. When composite is light cured, the polymerisation shrinkage occurs very rapidly but in chemically cured composite, the polymerisation shrinkage occurs more gradually. This is relevant when considering how the composite is to be bonded onto the enamel and dentine.
As composite does not have the inherent ability to bond chemically onto enamel or dentine, it is necessary to use extra steps for bonding. However, if the polymerisation shrinkage is too rapid, this can cause breakage of the bond or it can cause the composite to pull on the walls of the cavity. If the bond breaks, this leads to micro leakage but if the bond does not break, the pulling effect can even cause cracks within the enamel.
Patients will often come back to the dentist after a composite filling material has been used complaining of pain. One way to reduce the polymerisation shrinkage is to have good enamel and dentine bonding but also to cure/set the composite in layers rather than in one go. Newer composites have been formulated to have a reduced polymerisation shrinkage change which helps reduce microleakage.
Composite is nowhere near in getting a good contact point as amalgam. This is extremely important in class II cavities where a good contact point is required. However, modern posterior composite filling materials have fairly good contact ability but even so, a wedge is needed in conjunction with a clear matrix band.
The earlier composites were too stiff but difficult to pack and manipulate in order to meet this demand or they were too soft making them easier to pack and manipulate but difficult to press hard against the matrix band.
Composite does not inherently bond onto enamel or dentine and therefore extra steps are required for this to occur. There are two separate phenomena occurring here. Firstly, enamel bonding occurs by etching the enamel with 37% phosphoric acid for 20 seconds.
This acts on the enamel prisms and causes differential dissolving resulting in a roughened surface which acts to give micro mechanical retention. Dentine bonding is slightly different in that the acid attacks the mineral component of the dentinal tubules. Dentine is 70% mineral and 30% organic. This then causes a collagen mesh to remain and resin will then flow into the collagen mesh.
This is a form of micro mechanical retention. However, drilling into a tooth causes crushed hydroxyapatite debris and this is known as the smear layer which must be removed prior to etching and bonding. Creation of the smear layer prevents the usage of resin-tags.
Fortunately, bonding systems have dramatically transformed and now, one step bonding systems exist which etch both enamel and dentine, remove the smear layer and create micro mechanical retention simultaneously. Older dentists will remember the original systems which used 8 or 9 separate steps with lots of potential for errors at any point.
Composites according to the specific brand can sometimes can be difficult to discern on the radiographs due to their radiopacity similar to dentine and this can be a disadvantage when trying to assess for secondary or recurrent caries.