DOH124 Lecture Notes - Lecture 6: Polyacrylic Acid, Wetting, Ionomer
ADHESION TO TOOTH STRUCTURE
Bond to mineralised tissue
One of the most important characteristics of glass ionomer materials is their ability to
adhere chemically to mineralised tissues. The probable mechanism of adhesion is
based upon both diffusion and adsorption phenomena. Adhesion is initiated by the
polyalkenoic acid when freshly mixed material contacts the tooth surface. Phosphate
ions are displaced from apatite by carboxyl groups, each phosphate ion taking a
calcium ion with it to retain electrical neutrality. The setting of the material and
dissolution of the enamel or dentine surface result in buffering of the polyacid, a rise in
local pH and reprecipitation of minerals at the cement-tooth interface. Therefore, it
appears that chemical bonding is achieved by a calcium phosphate-polyalkenoate
crystalline structure acting as an interface between enamel or dentine and the set
material The significance of this method of bonding lies in the fact that there cannot be
adhesive failure between the two materials, tooth structure and glass ionomer, but, if a
restoration is to be lost, there will need to be cohesive failure in one or the other. As the
tensile strength of glass ionomer is notably lower than that of tooth structure, an
ion-exchange layer will generally be found still attached to the dentine or enamel. There
may, of course, be occasions where there are already micro-cracks in the enamel,
resulting in failure in tooth structure.
Bond to collagen
Adhesion to the organic component of the dentine may also occur through either
hydrogen bonding or metallic ion bridging between the carboxyl groups on the polyacid
and the collagen molecules of the dentine. The strength of the union has not yet been
measured because failure, under normal circumstances, is cohesive in the cement. The
ion-exchange layer is always present, so tests purporting to reveal the strength of
adhesion will reveal the tensile strength of the relevant cement rather than the
adhesion.
PLACEMENT OF THE GLASS IONOMER
Conditioning the tooth surface
Logically, adhesion will take place best in a clean environment. Various agents have
been proposed:
i) to remove some or all of the smear layer and
ii) possibly, to pre-activate the enamel or dentine.
Low molecular weight acids such as citric acid or hydrogen peroxide were
recommended initially.
Document Summary
One of the most important characteristics of glass ionomer materials is their ability to adhere chemically to mineralised tissues. The probable mechanism of adhesion is based upon both diffusion and adsorption phenomena. Adhesion is initiated by the polyalkenoic acid when freshly mixed material contacts the tooth surface. As the tensile strength of glass ionomer is notably lower than that of tooth structure, an ion-exchange layer will generally be found still attached to the dentine or enamel. There may, of course, be occasions where there are already micro-cracks in the enamel, resulting in failure in tooth structure. Adhesion to the organic component of the dentine may also occur through either hydrogen bonding or metallic ion bridging between the carboxyl groups on the polyacid and the collagen molecules of the dentine. The strength of the union has not yet been measured because failure, under normal circumstances, is cohesive in the cement.
