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Department of Restorative Dentistry Assistant Professor, Camilo Castelo Branco University São Paulo, Brazil.
Abstract
Despite
its common use, the dental resins still have a lower durability when
compared to amalgam restorations and indirect restorations. Success in
adhesive dentistry involves long lasting restorations; however, the
resin-dentine interface degradation appears as the biggest obstacle to
achieve this goal. Independent of the adhesive strategy used for bonding
to tooth substrates (self-etch or etch-and-rinse adhesive systems)
degradation of the hybrid layer can be observed. It should be emphasized
that the lower durability of restorations results from degradation of
the bonded interfaces formed by the adhesive systems (Hashimoto, 2010),
being more apparent when dentin is the major bonding substrate involved.
According to the strategic plan on tooth-colored resin restorations
(NIDCR, 2009-2013), its durability is approximately 6 years. Considering
that the dentist spends about 50 to 70% of their clinical time only for
replacing this composite resin restorations (Ericson et al, 2007), this
yields an annual cost (US only) of approximately $5 billion (Jokstad et
al, 2001). What causes this reduction in the durability of
tooth-colored resin restorations? The reduction in the durability of
adhesive restorations is directly related with the balance between the
resinous components of the adhesive system and components from organic
substrate which can lead to dental degradation of adhesive interface.
This degradation occurs in two ways: Either by hydrolysis of the resin
components or by hydrolysis of the collagen matrix. Being the main
causes, the incomplete infiltration of resin monomers, the hydrolytic
degradation of adhesive system polymer by water sorption, and the
breakdown of collagen fibrils by MMP and cathepsin-cysteine. The
increased concentration of hydrophilic monomers (e.g. HEMA) in both
self-etch or etch-and-rinse adhesive systems leave the adhesive film
permeable to water. This water may arise from pulpal pressure of
dentinal tubules (Bresch et al, 2008) or remnants of water molecules
that do not evaporated with the solvent (alcohol or acetone). Aside from
the presence of water at the base of hybrid layer, the interfibrillar
spaces of collagen in apatite-depleted dentin also contain hydrated
negatively charged proteoglycans that form a hydrogel (Scott and
Thomlinson, 1998). If these hydrogels remain hydrated in interfibrillar
spaces, they may be responsible for 'molecular sieving' of larger
hydrophobic dimethacrylates (like BisGMA-bisphenol A-glycidyl
methacrylate), allowing only smaller hydrophilic molecules (like HEMA-
2-hydroxyethyl methacrylate) to permeate upon the bottom of the hybrid
layers. Hydrophilic resin monomers (like HEMA) are vulnerable to
hydrolysis, due to the presence of ester linkages (Ferracane, 2006).
Hydrolysis of monomer methacrylates ester bonds can be caused either by
the increase in acidity of monomer components in self-etch adhesive
systems (Aida et al, 2009) or by salivary esterases (Shokati et al,
2010) that can break covalent bonds between the methacrylate polymers by
the addition of water molecules to the ester bonds. Thus, the
plasticization and nano-phase separation of polymers decreases the
dynamic mechanical properties of the polymerized adhesives (Park et al,
2010) and increases their susceptibility to esterase-catalyzed
hydrolysis (Kostoryz et al, 2009). Mineralized dentin contains matrix
metalloproteinases (MMPs), such as MMP-2, 3, 8, and 9 (Birkedal-Hansen
et al, 1993). Due to dentin mineralization process, the MMPs are
retained in the collagen extracellular matrix as inactive proenzymes in a
latent state (Tjaderhane et al, 1998). However, MMPs can be activated
if, for some reason, the demineralized dentin is exposed to acid, such
as monomers of etch-and-rinse (Mazzoni et al, 2006) and self-etch
adhesive systems (Nishitani et al, 2006a). When activated, the MMPs act
in the degradation of collagen, elastin and extracellular matrix
components (Birkedal-Hansen et al, 1993). Thus, apatite-depleted,
resin-sparse collagen fibrils within the hybrid layers become
susceptible to degradation, compromising the longevity of resin-dentin
bonds (Breschi et al, 2008; Hashimoto, 2010). Another extracellular
enzyme present in dentin is the cysteine cathepsins. They have recently
been reported to be present in intact dentin (Tersariol et al, 2010) and
more abundantly (approximately 10-fold) in carious dentin (Nascimento
and Tjäderhane, unpublished observations). They are derived from the
dental pulp via the dentinal fluid (Tersariol et al, 2010) and, similar
to MMPs, may be activated in mildly acidic environments, produced by
both etch-and-rinse and self-etch adhesive systems. Adhesive technology
has evolved rapidly since it was introduced more than 60 years ago. The
main challenge for dental adhesives is to provide an equally effective
bond to two hard tissues of different nature. The contemporary dentin
adhesive is not as durable as we had assumed. The complete replacement
of free and loosely bound water within the apatite-depleted collagen
fibrils within the hybrid layers and the inactivation of collagenolytic
enzymes appear to be the main objectives to improve durability of
tooth-colored resin restorations.
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