Factors Affecting etching of enamel
TIME
Increased Time Application
High fluoride content and primary teeth require longer etching time. The increased etching time is needed to enhance the etching pattern on enamel that is more a prismatic than that of permanent enamel. Currently 15 sec, a sufficient time to produce a bond equivalent to that produce by a 60 sec etching time is used routinely.
Shorter Etching Time
C.J. Guba et al (1994) highlighted that etching times and etchant consistency were not critical to enamel bond strengths. It yields acceptable bond strength. It conserves enamel and saves time.
They also found that on microscopic examination of a 10 secs etch versus a 60 sec etch showed that etching the enamel for 10 sec produced a very superficial etch compared to a very deep etch with a 60 sec etch. However this did not have significant impact on the tensile strength. Though some researchers suggested that the etching effect is reduced when the etching viscosity of the acid is high, this study showed no significant difference as related to their viscosities.
ACID CONCENTRATION
An interesting and important phenomenon is the existence of an inverse relationship between the etching effect of phosphoric acid and it’s concentration. The phenomenon was first observed and reported in 1965 and subsequently confirmed by others. The same etch time lower concentrations of acid tend to be more destructive of the enamel than higher concentrations
Concentrations of phosphoric acid over 65% tend to show minimal changes. The concentrations of acid, producing consistent, more or less evenly distributed relatively deep etch pattern, appear to be in the range of 30 to 50%. Bond strengths are greater with 30 to 50% acid concentration, the difference between their values and those obtained on surfaces etched with 10 to 70% acid were not as great. The higher concentration of acid may not produce a sufficient in depth etch to provide adequate resin penetration (tag formation) and / or sufficient bonding area to resist repeated long-term masticatory and other dislodging stresses encountered in the oral environment.
In an in vitro study carried out on bovine substrate by M.J.Shingi et al (2000) it was concluded that milder concentrations of phosphoric acid or less aggressive acids could be used to pretreated enamel for orthodontics adhesive systems and sealants if the diffusion potential of applied monomers is high enough.
According to Unos (1996) depths of demineralization increased by both acid concentration and conditioning times following a logarithmic relationship.
Chow and Brown (1973) demonstrated that the application of phosphoric acid solutions greater than 27% Phosphoric acid resulted in the formation of monocalcium phosphate monohydrate while dicalcium phosphate dehydrate was formed with phosphoric acid concentrations less than 27%. The former product is readily soluble and would be completely washed away in the clinical situations.
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