Polycarboxylates, Glass Ionomers and Resin-modified Glass Ionomers for Luting and Lining
Before reading this chapter, the attention of the reader is drawn to Chapters 24 and 25. Here, the glass ionomers and resin-modified glass ionomers used for restorative purposes are discussed in some detail. Much of the information given in these previous chapters is relevant to our discussion of luting and lining cements.
Two types of cement based on polyacids are in common use for both luting and cavity lining applications. The first products to be developed were the polycarboxylate cements which rely on the reaction between zinc oxide and a polyacid. The second group of products are described as glass ionomer or polyalkenoate cements. The setting reaction takes place between a polyacid molecule and cations released from an ionleachable glass.
A new family of cements has been developed by producing cements which have a nature which lies somewhere between that of the purely salt matrix glass ionomers and that of the purely resin matrix composite systems. These newer cements may be viewed as hybrids of the two parent groups from which they are derived.
30.2 Polycarboxylate cements
These materials may be supplied as a powder and liquid or as a powder which is mixed with water (Fig. 30.1). For powder/liquid materials, the powder is finely ground zinc oxide which sometimes contains minor quantities of other oxides such as magnesium oxide. The liquid is an aqueous solution of polyacrylic acid of about 40% concentration. In the powder/water materials the powder contains zinc oxide and freeze-dried polyacrylic acid. On mixing the powder with water, the polyacrylic acid dissolves and starts to react with zinc oxide. The setting reaction is similar to that reported in Chapter 24 for glass ionomers. Zinc oxide behaves as a basic oxide and undergoes an acid-base reaction with acid groups in the poly-acid to form a reaction product which consists of cores of unreacted zinc oxide bound together by a salt matrix in which polyacrylic acid chains are cross-linked through divalent zinc ions (Fig. 30.2). Many recently developed products contain fluoride salts which may exert an anticariogenic effect on surrounding tooth substance.
The cements have sufficiently early strength to resist amalgam condensation and, allowing for product variations, have an ultimate compressive strength of about 80 MPa, a similar value to that recorded for the EBA materials.
The polycarboxylate materials are acidic, though not as irritant as phosphate cements, for two reasons. Polyacrylic acid is a weaker acid than phosphoric acid and the polyacid chains are too large and lack the mobility required to penetrate dentinal tubules. Despite the more biocompatible nature of these materials they are not widely used as linings in very deep cavities unless a sublining of a calcium hydroxide or zinc oxide/eugenol material is used. One reason for this is that they are difficult to handle well in a clinical setting. They tend to be rubbery during their setting reaction and they also adhere to stainless steel instruments, making their placement complex.
Laboratory tests show that the solubility values of polycarboxylate cements are greater than those for the zinc phosphate, silicophosphate and glass ionomer materials (Section 28.2). Despite this apparent disadvantage, the materials are widely used for luting without appearing to display an unduly high failure rate.
The materials form an adhesive bond with enamel and dentine but only a weak bond with gold and no perceptible bond with porcelain. Hence, the adhesive nature of the materials when used for the luting of gold or porcelain crowns is only utilized to a limited degree and cannot be considered an overwhelming advantage for such applications. They will, however, bond to non-precious metal alloys that are being used increasingly for porcelain fused-to-metal crowns. The materials form a strong bond with stainless steel which makes them useful for attaching orthodontic bands. Care must be taken when using steel instruments for mixing and placing. Excess material should be removed from such instruments before it sets, otherwise a tenacious bond will form.