Acid Conditioning of dentin
Any discussion of the effects of acid conditioning of dentin must begin with the acid etching of enamel. This was first proposed by Buonocore (1955) as an attempt to clean enamel, increase the microscopic surface area for bonding, and infiltrate unfilled resins into enamel porosities. Many investigators were alarmed at what was then regarded as an unconventional and even reckless ap¬proach to the problem.
Buonocore, Wilernan and Brudevold (1956) not only intro¬duced the acid etching of enamel to dentistry, they also were among the first to attempt to bond resins to acid-etched (7% HCL, one minute) dentin.
Their success with acid etching of enamel led them to try to acid etch dentin. Unlike enamel, when dentin is etched, its surface becomes mineral -poor protein rich; and it tends to become wetter (Brannstrom and Nordenvall 1977). Unfortunately, Buonocore and his colleagues' suc¬cess with dentin was never realized, because the relatively crude resin materials that were available at that time would not wet dentin very well. Buonocore, however, was very much aware or the requirements for good bonding.
For clinical success, the conditioned dentin must be sealed to prevent sensitivity and to prevent the pathology (Brannstrom, 1981) associated with the increased permeability of the dentinal tubules. Conditioning of dentin will be defined as any alteration of dentin done after the creation of dentin cutting debris, termed the smear layer.
The objective of dentin conditioning is to create a surface capable of micro-mechanical and possible chemical bonding to a dentin-bonding agent.
Goals of acid conditioning of dentin
• Remove the intrinsic weakness of the smear layer to permit bonding to underlying dentin.
• Demineralize the superficial dentin matrix to permit resin infiltration into surface.
• Uncover both intertubular and peritubular dentin.
• Clean the dentin surface free of any biofilms.
It is important to define the purpose of the acid etching of dentin so that once identified, these goals can be tested in a systematic scientific manner.
As the smear layer is intrinsically weak, the first goal is to loosen it or remove it so that subsequently placed adhesive resins can interact with solid dentin adhesive resins can interact with solid dentin matrix. Most smear layers are 1-2 µm thick; they are composed of the cutting debris of the materialized tissue on which they lie. (Ruse and Smith 1991)
The reason for acid etching is to demineralize the solid dentin matrix (both intertubular and peritubular dentin) to increase the porosity of the dentin. While this is analogous to why enamel is etched, the porosities that are produced are of the order of 0.05 – 1-3 µm in peritubular dentin rather than the 5-7 µm diameter of enamel prisms. Further, acid-etched enamel can be thoroughly dried, while that foal is much more difficult in vital, normal dentin. Enamel contains little protein that is at risk of being denatured by acid treatment. Dissolving away hydroxyapatite mineral crystallites from the collagen component of dentin matrix creates dentin porosities. The crystals tend to stabilize collagen and prevent its denaturation.
There is a risk that the acid used to demineralize the dentin may denature or weaken the collagen. As denatured proteins generally change their dimensions, the pores may become smaller if the collagen is denatured. This may interfere with subsequent resin infiltration and prevent the formation of a hybrid layer (Nakabayashi, Nakamura and Yasuda 1991). Another danger in the etching step is that the demineralized zone may extend, for instance, 5 µm into the dentin, while the resin infiltration may only extend 4 µm, leaving a 1 µm demineralized zone at the base of the hybrid layer that is unpro-tected by mineral or resin and that may be structurally weak. If the pulpodentin complex can re-mineralize this unprotected basal 1 µm of demineralized dentin (Tatsumi, 1989; Tatsumi and others 1992), then the layer may become as strong as normal dentin, rather than be a zone of debonding that has been seen in vitro (Nakabayashi and others 1991).
Another purpose of acid etching dentin is to clean the dentin surface. Often dentin is inadvertently contaminated with blood during the cavity preparation. Acid etchant, by dissolving most of the smear layer, tend to float these biofilms on the dentin when it is rinsed. The low pH of the etchant may also denature the plasma proteins and hemoglobin. The purpose of acid etching may vary depending upon the material. If the intention is simply to remove the smear layer but leave the smear plugs in place, as when one uses glass-ionomer cements, then short etching times with dilute acids would seem to be indicated (Bowen 1978; Pashley and Others 1981; Hamlin and Others 1990a).
However, if one wants to create a resin hybrid layer (Nakabayashi and Others 1991) in the dentin rather than on the dentin, then one must demineralized more deeply and in the process, removes smear plugs. This can still be accomplished using dilute acids, but the etching time may have to be extended.
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