Dentin/Adhesive Interface in Teeth - Structural Interfaces and Attachments in Biology - page 135

Current strategies to promote bonding of the resinous materials to intrinsically
wet substrates also include the incorporation of ionic and hydrophilic monomers
into the adhesive [ 96 ]. These adhesives etch and prime simultaneously, thus
addressing the problems of collagen collapse and simplifying the bonding protocol.
Unfortunately, the hydrophilic nature of these components enhances water sorption
and hydrolytic breakdown in the mouth [ 91 , 96 - 99 ]. With these systems, the
bonded interface lacks a nonsolvated hydrophobic resin coating and thus,
the resultant hybrid layers behave as semipermeable membranes permitting water
movement throughout the bonded interface even after adhesive polymerization
[ 93 ]. The higher concentration of hydrophilic monomers in these systems
is associated with decreased structural integrity at the d/a interface [ 93 , 100 ].
In vivo aging studies have reported degradation of the d/a bond at 1-year even
when the bonded dentin was protected by enamel from direct exposure to the oral
environment [ 101 ]. These results suggest that hydrophilicity and hydrolytic
stability of resin monomers are generally antagonistic [ 91 ].
7.5 Hybrid Layer Degradation
It has been hypothesized that the in vivo degradation of the hybrid layer follows a
cascade of events that begins when the dentin is acid-etched [ 102 ]. Disruption of the
tooth structure by drilling stimulates proteolytic enzymes such as matrix metall-
oproteinases (MMPs), which can degrade the exposed collagen component of the
hybrid layer [ 103 ]. Degradation by MMPs is expected to be most important acutely
in the period following adhesive application. Chronic deterioration of the hybrid
layer involves hydrolysis and leaching of the adhesive that has infiltrated the
demineralized dentin matrix [ 25 , 72 ]. Leaching is facilitated by water ingress into
the loosely cross-linked or hydrophilic domains of the adhesive. The hydrophilic
domain exhibits limited monomer/polymer conversion because of adhesive phase
separation [ 71 ] and lack of compatibility between the photoinitiator and hydrophilic
phase [ 83 ]. The poorly polymerized hydrophilic phase degrades rapidly in the
aqueous environment. The previously resin-infiltrated collagen matrix is exposed
and vulnerable to attack by proteolytic enzymes [ 103 , 104 ].
The structure of methacrylate adhesives suggests a general mechanism for their
chemical and enzymatic degradation in oral fluids. Water initially enters the
adhesive matrix by diffusion into loosely cross-linked or hydrophilic domains or
may be trapped within the matrix during photopolymerization in the moist oral
environment [ 105 , 106 ]. Portions of the matrix may be directly exposed to oral
fluids, e.g., the gingival margin of Class II and V composite restorations. The
presence of water promotes the chemical hydrolysis of ester bonds in methacrylate
materials. This reaction is expected to be relatively slow at the neutral pH typical of
saliva, but excursions in pH caused by food or cariogenic bacteria may lead to
transient acid or base catalysis. The carboxylate and alcohol degradation products
of ester hydrolysis are more hydrophilic than the parent ester, further enhancing the