Pre-curing of composite to reduce the effects of contraction shrinkage
Contraction shrinkage effects of command-set composite materials is well documented and therefore the onus is placed on the operator to reduce these distorting shrinkage effects by a careful incremental placement technique.
In this way the composite shrinkage can be minimised as a smaller total proportion of the filling. Composite materials are said to shrink towards the light source and this is often away from the base of the cavity. In addition, curing lights may only have a penetration of approximately 2mm, although this does vary between different manufacturers.
The possible risks associated with contraction shrinkage are:
1. Poor bonding
3. Post-op pain (particularly in occlusal restorations), due to stresses set up between cusps.
Many composite manufacturers are now developing materials that have reduced contraction shrinkages associated with light curing. SDI ICE and SDI ROK were used in this case study (Figures 1 and 2).
Small increments of the composite material were made into spheres and light cured as recommended by the manufacturer’s instructions. The composite system chosen was a nano-composite with larger ceramic particles added for strength and durability. The aim of this was to pre-cure the composite spheres prior to placement within a dental cavity mixed with un-cured composite. This would potentially reduce the overall contraction amount upon curing.
SDI ROK is a composite material, which is specifically designed for placement into posterior teeth. The teeth are pre-etched and a resin layer is applied to line the entire cavity prior to placement and curing (30 seconds). An incremental method is advised. In addition, opposing cusps are not ideally cured to a single unset layer to avoid stresses which may lead to post-op pain and even cusp fracture long term.
Small irregular spheres of composite were created and light cured on a paper mixing pad. They were then placed within a small Dappens pot and the light curing repeated. This was to ensure complete curing and to also prevent the particles from bonding to each other (Figures 3, 4 & 5).
This patient presented with a fractured amalgam restoration on the upper premolar tooth. The palatal cusp had fractured and a small dentine pin was used to aid in the retention of the composite mechanically (Figures 6, 7 & 8).
Once the cavity had been fully prepared, etched and bonded with unfilled resin, the ROK composite was mixed with some unset composite of the same type and transferred to the cavity. Again, small increments were used of approximately 2mm and light cured (Figures 9 & 10).
This produced an aesthetic composite restoration where we had a greater deal of certainty that contraction shrinkage was less likely to produce detrimental effects to the tooth/restoration interface (Figure 11). The base of the cavity was lined with a layer of unset composite prior to addition of the first thin layer of set and unset mix composite. This example illustrates how simple pre-curing can be done chairside and is suitable for the stratified/lamellar multi-shade methods described for higher aesthetic composite cases.
The SDI ROK material has a high degree of finish due to the nano and micro particle mix and the preset composite incorporated does not have any apparent effect on the finishing of the material.