bond strength of an adhesive system is
mostly influenced by the hybrid layer, followed by resin tags in the
dentinal tubule and finally by chemical bonding 1,2. However,
porosities can be observed at the bottom
of hybrid layers. This phenomenon was termed “nanoleakage” and
was first described in 1994 by Sano et al. 3,
they noticed incomplete infiltration of hybrid layers following acid-etching
during adhesive bonding techniques
3. Silver nitrate was originally used to
detect microleakage around composite restorations 4.
The low molecular weight silver nitrate tracer diffusion is similar to that of
water and has been used to trace nanometer-sized water-filled spaces or
“nanoleakage” within bonded interfaces, examined under scanning or transmission
electron microscopy 5,6.
Several factors influence the development of nanoleakage, including the
type of solvent (water vs. acetone) 7,8,9,10,
the individual chemical constituents system (e.g. hydroxyethyle methacrylate
(HEMA), and bisphynol glycodal methacrylate (Bis-GMA)), and the different
molecular weights of constituents ranging from
130 (HEMA) to
513 (Bis-GMA) as well as other additives (among others maleine acid or
glutaraldehyde) 5. In addition, and the mode of application
when using these materials (moist bonding vs. dry bonding), as dry bonding of
etch and rinse adhesives increases the occurrence of nanoleakage 5. This phenomenon is explained by the collapse of acid-etched
dentin matrix that interferes with resin infiltration. The nanoleakage effect
has been discussed to be one factor negatively affecting the quality of
dentinal bonding 10,11,12.
Recently, developed resin adhesives
contain more acidic hydrophilic
monomers, and higher amounts of water to
improve monomer impregnation into wet dentin substrate, resulting in lower
degrees of polymerization of adhesive resin. This results in increased silver
uptake into the hybrid and
adhesive layers (i.e. increased nanoleakage).
Influence of nanoleakage on microtensile bond strength was determined
using different visualization techniques 5.
For instance, resin–dentin specimens were prepared and immersed in silver
nitrate that penetrated into nanoleakage. Subsequently, specimens were broken
at the adhesive interface (commonly using tensile testing or shear bond strength testing procedures), and
nanoleakage was visualized indirectly on the exposed surface using scanning
electron microscopy (SEM) 6.
It has been postulated that, between polymerization shrinkage and
microtensile bond strengths (?TBS) a
highly significant correlation was found because forces developed during
the polymerization of dental restorative composites placed in a restricted setting,
cause tension in the material, with possible subsequent distortion of the bond
to the tooth 13. Furthermore, the quality of the bond
between tooth and restorative materials could, also, be affected by the
incompatibility between adhesive and restorative material as well as by the
surface tensions of the two components coming into contact with each other 14.
The extent of this shrinkage influences the tension state generated at
the interface composite/dental structure and, commonly, compromises the bond
integrity at this region. In addition, the polymerization shrinkage of
composites is also influenced by the geometric form of the cavity. When the
ratio between the bounded to unbounded surfaces is higher than two, the stress
generated by the composite shrinkage may exceed the bond strength to the cavity walls and produce marginal gaps 15. When these problems are added to an incorrect placement
technique and finishing mistakes, marginal leakage, inadequate anatomic form
and proximal contacts occur clinically, which lead to a consequent reduction in
the longevity of the restoration 16,17.