Key to the long-term success of a posterior composite is the marginal seal and integrity. The margins are the area most likely to break down, evidenced by leakage, staining or recurrent decay. Interproximal margins are perhaps more subject to these issues than the margins we see easily on the occlusal surface. So how do we assure proper seal that will last over time. In the prior post of posterior composites I mentioned convenience form. I think this is a critical step in marginal integrity. Having adequate convenience form allows us to see, feel and finish the margins interproximally. In this way we can be certain the day of placement we have created sealed the margins.
Sealing the margins takes multiple steps. First is proper placement of the matrix band to ensure we seal the composite against the preparation. With this accomplished we now have to make certain the composite is adequately condensed. Many practitioners will use a first step of flowable along the exterior margins. This layer is placed into the depth of the box, and teased along the margin line with a perio probe. It can be left uncured during placement of the first increment of composite or cured first.
Some of our newer bulk fill composites are designed to increase the ability to properly fill a box form and seal the margins. Regardless of the material used we need to place the composite and than make certain it either flows or is condensed into the preparation to eliminate voids and create marginal seal. When using a layering technique, place increments less than 1.5mm in thickness and condense. Using instruments designed to minimize stick of the composite, materials with low stickiness and the addition of a small amount of dentin adhesive to the condenser will reduce pull back. Layering and the use of composites that have been shown to have low shrinkage stress will also reduce the strain at the margins that can result in breakdown after the final cure is complete.
I am aware of polymerization shrinkage and I’m aware of the concept when you say “shrinkage stress”; however, what should I be looking for in shrinkage stress. For example, what is an acceptable range (and where would I find that info–I plead ignorance). I am currently sing Z250 for posterior composites. Where would this product rank on shrinkage stress and do you have a link any articles, etc that may discuss other potential posterior composite materials?
Shrinkage stress is a more accurate number to look at when we talk about consequences clinically like open margins and sensitivity. I don’t think anyone has defined an “Acceptable range”. With that said, the lower the number the less stress the material undergoes during polymerization shrinkage, which means the less it is pulling on the bond at the dentin adhesive layer and the less of these consequences we will see initially and over time. Dr. Jack Farracane has done some incredible comparative studies on shrinkage stress if you want to compare materials. Another piece to remember is placement technique still counts when we have challenges.