Dentin/Adhesive Interface in Teeth - Structural Interfaces and Attachments in Biology - page 137

solubility enhancer [ 70 , 123 ], and water-soluble photoinitiators [ 81 , 83 ], have been
utilized to improve adhesive performance. Therefore, future adhesive systems need
to be designed carefully to achieve a more homogeneous monomer distribution and
conversion within the hybrid layer to overcome the defects associated with phase
separation.
Any change in the chemical structure intended to increase esterase resistance
and water compatibility will likely alter other chemical and physical properties of
the adhesive. Under clinical function, the methacrylate-based dentin adhesives are
subjected to both chemical and mechanical stresses. The interplay between the two
forms of stress is expected to result in an alteration of the adhesive mechanical
properties with time [ 124 , 125 ].
The mechanical property change results from a variety of mechanisms, includ-
ing: (1) change in the chemical nature of the polymer in the form of either
plasticization, strain hardening, embrittlement, or crystallization, and (2) prolifera-
tion of surface and subsurface flaws due to combined effect of mechanical loads
and chemical stress, e.g., biologic fouling, exposure to lactic acid (produced by
S. mutans). The change in adhesive properties has a significant effect on the
mechanical performance and durability of the d/a interface, which is a complex
construct of different material phases at the micro-scale. Based on micro-scale
structure-property measurements, our group has developed an idealized micro-
structural representation of the d/a interface [ 126 ] that can be utilized to perform
micromechanical stress analyses using 3D micro-scale finite element (

FE) models
[ 127 ]. Figure 7.2 shows a 3D micro-scale finite element model based on the
idealized representation. We can see from Fig. 7.2 that the different material phases
at the d/a interface will experience different stress amplitudes under functional load
[ 126 , 128 ]. Therefore, they reach their failure strength at different overall stress
levels. As a consequence, the overall failure behavior of the d/a interface is not
necessarily determined by the weakest component. Instead, the component whose
stress concentration is closest to its failure strength determines the failure.
We have used micromechanical stress-analysis to show the effect of such stress
concentrations on the mechanisms that govern the overall fatigue failure behavior
of the d/a interface [ 44 ]. Fatigue-life (durability) curves were obtained for a number
of d/a interface conditions as shown in Fig. 7.3 . It was found that d/a interfaces with
graded adhesive infiltration and thick hybrid layer exhibit lower durability.
Predictions were compared to experimental data [ 129 ] to illustrate the predictive
power of our methodology [ 44 ].

m

7.7 Dentin/Adhesive Interface: The Weak Link
in Composite Restorations

In summary, the d/a bond can be the first defense against substances that may
penetrate and ultimately undermine the composite restoration in vivo . However, as
indicated in a recent review of dental composite, the properties of the materials are