Polymerization Shrinkage with Light-Initiated Dental Composites

The present work addressed the determination and visualization of
the direction and extent of polymerization shrinkage in the
light-initiated composite. Hypotheses about the light-cured
composite contraction patterns are controversial. With high
resolution µCT images, the displacement vector fields are examined
and calculated two-dimensionally via an elastic registration
algorithm using vector-spline regularization and
three-dimensionally with a local rigid registration (block
matching) following images segmentation (corresponding traceable
fillers in composite). It appears that the light-initiated resin
composites do not always shrink toward the light source. Two major
contraction patterns were observed: either shrink toward the
top-surface (free surface), or toward one side of the cavity wall,
in which the bonding was stronger or remained intact. With the
proposed methods, it is possible to describe the contraction
patterns in great detail. We could demonstrate that the bonding
quality to the tooth affects the material movement more than
described so far. In addition, the geometry of the cavity also acts
as a factor. The continuation of the studies into the interaction
of tooth-adhesive-composite indicated the shortcomings and
limitations of the current FEA simulation studies. This meant that
the assumption of FEA, especially in adhesive systems (i.e.,
bonding situations and hybridizations), is too perfect and
simplificative to interpret the real condition in clinical. The
qualitative and quantitative analysis of the shrinkage vector field
along with the µCT datasets supply more insight into the shrinkage
behavior in real teeth with all their variations of the boundary
conditions than with any currently available method. This new
approach has the potential to reevaluate and hopefully unify all
the currently available hypotheses concerning the extent and
orientation of polymerization shrinkage.