Biomedical Engineering Reference
In-Depth Information
7.4.3 Composite-Dentin Interface
The long-term success and clinical safety of resin composites dental res-
torations depends, among other factors, on the integrity and durability of
tooth/restoration interface. Compromised interface integrity leads to inva-
sion of
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1
fluid and bacteria, postoperative sensitivity, pain,
tooth dis-
coloration, caries and finally failure of restorations.
For a durable composite restoration-tooth interface, several adhesive
systems, that depend on separate or combined applications of etching,
priming and bonding, are evolved.
168
Etching with
35% phosphoric acid
selectively removes hydroxyapatite crystals, that act as a 'natural shelter'
around collagen fibrils,
169,170
from dental tissues (Figure 7.4). Accordingly
demineralized collagen fibrils, of dentin in particular,
171,172
collapse upon
subsequent air drying. The application of a hydrophilic primer then expands
the collapsed fibrils to enable subsequent penetration of the bonding
monomer. Polymerization and entanglement of the bonding monomer
within the demineralized collagen fibrils network creates a hybrid layer or
inter-diffusion zone of resin-reinforced collagen. With some primers (self-
etch primers that combine etching and priming steps), a very thin (
B
1 mm)
acid-base resistant layer adjacent to the hybrid layer was observed. This layer
has been considered as a part of the hybrid layer and can prevent secondary
caries development.
173
The thickness of the hybrid layer affects the
durability of the resin-dentin interface.
45
Insucient resin infiltration/
encapsulation of collagen fibrils makes the hybrid layer vulnerable to en-
zymatic degradation.
174
The stability of the hybrid layer is therefore de-
pendant mainly on the effective coupling of the adhesive monomer with
the infiltrated substrate. In addition to the mechanical retention in the
B
.
Figure 7.4 Atomic force microscopy images showing dentin before (a) and after
(b) acid etching. Acid etching removes the hydroxyapatite crystals from
dentin leaving only the collagen network. Collapse of collagen network
could happen upon excessive air drying preventing further penetration
of adhesive; therefore maintaining wet collagen is essential for proper
penetration of adhesive resin into the collagen network to form the
hybrid layer of resin-reinforced collagen.
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