Biomedical Engineering Reference
In-Depth Information
7.4 Dentin/Adhesive Bond and the Hybrid Layer
The two fundamental processes involved in bonding an etch-and-rinse adhesive to
dentin are: removal of the mineral phase from the dentin substrate without altering
the collagen matrix and filling the voids left by the mineral with adhesive that
undergoes complete in situ polymerization, i.e., the formation of a resin-reinforced
or hybrid layer. The ideal hybrid layer would be characterized as a 3-dimensional
polymer/collagen network that provides both a continuous and stable link between
the bulk adhesive and dentin substrate. Numerous studies indicate that this ideal
objective has not been achieved [
25
,
45
-
55
].
The hybrid layer is formed when an adhesive resin penetrates a demineralized or
acid-etched dentin surface and infiltrates the exposed collagen fibrils. During acid
etching, the mineral phase is extracted from a zone that measures between 1 and
~10
m of the dentin surface [
35
,
56
,
57
]. The composition of the exposed substrate
differs radically from mineralized dentin. For example, mineralized dentin is 50%
mineral, 30% collagen, and 20% water by volume [
39
], whereas demineralized
dentin is 30% collagen and 70% water [
58
,
59
]. With removal of the mineral phase,
the collagen fibers are suspended in water. If there is a substantial zone of deminer-
alization and the water supporting the collagen network is removed either by air
drying or the action of an air syringe, the collagen will collapse [
59
,
60
].
A collapsed collagen network reduces the porosity and inhibits resin penetration
through the demineralized layer [
59
]. It forms a barrier between the demineralized
layer and the underlying intact or unreacted dentin surface [
61
,
62
]. A collapsed
collagen network severely compromises the dentin/adhesive (d/a) bond [
58
,
60
,
61
].
m
7.4.1 Wet Bonding
In the early 1990s, wet bonding was introduced to counteract the problems of
collagen collapse [
46
,
63
-
66
]. Wet bonding means that the dentin is kept fully
hydrated throughout the bonding procedure; the surface morphology of the
demineralized layer does not change because the water supporting the collagen
matrix is not removed [
67
]. Bond strength results [
46
,
63
-
66
] with “wet” bonding
support these findings, that is, the higher bond strengths with this technique reflect
the minimal collapse of “wet” vs. air-dried dentin collagen [
59
]. It is speculated that
moist dentin provides a more porous collagen network and that increased porosity
means more space for adhesive infiltration [
59
,
61
,
63
-
65
,
68
].
With wet bonding techniques, the channels between the demineralized dentin
collagen fibrils are filled with water, solvent, conditioner, and/or oral fluids [
59
].
The only mechanism available for adhesive resin infiltration is diffusion of the resin
into whatever fluid is in the spaces of the substrate and along the collagen fibrils.
Ideally, the solvent in combination with hydrophilic monomers (e.g., hydroxyethyl
methacrylate (HEMA)) conditions the collagen to remain expanded during
Search WWH ::
Custom Search