and Armin Segarra1
University of the Philippines Manila College of Dentistry, Paranaque, Philippines
In indirect restorations such as crowns, bridges, inlays, onlays, and posts, cement plays a vital link between the restoration and the tooth. Although the retention of crowns, bridges, inlays, and onlays depends primarily on friction between the walls of the preparation and the internal surface of the restoration, the cement is still an integral part of the indirect restoration assembly.
Different types of cement have been used over the past 100 years. The earliest cements did not have any adhesive properties. They merely filled the microscopic space between the walls of the preparation and the internal surface of the indirect restoration, hence the term luting cements or conventional cements. Luting cements include the zinc phosphate cements, polycarboxylates and the glass ionomers. They have performed particularly well on restorations with long, almost parallel walls as retention mainly relied on the frictional forces between the walls of the preparation and the internal wall of the restoration.
Over the years, indirect restorations have evolved. Newer materials have been developed to fabricate indirect restorations such as composite-ceramic hybrid materials (ceromers) and high-strength ceramics among others. Retention of indirect restorations became more complicated because of the increased complexity of these materials. These tooth-colored materials require cements that have better physical and mechanical properties. In addition, indirect restorations with compromised retention such as short crowns, tooth preparations with too much divergence, and too little remaining tooth structures are not retained well with conventional cements.
To keep up with these developments, newer cements were developed such as the resin hybrid cements and resin cements. The newest of these cements, the resin cements, have adhesive properties and have true adhesion to the internal surface of the restoration and to the tooth structure. These cements are not merely luting but bonding cements.
Today, cements used for indirect restorations are divided into two main types: the older luting cements which do not require any pretreatment and the newer bonding cements.
1.2 Luting Cements
1.2.1 Zinc Phosphate Cements
The oldest of the luting cements, zinc phosphate, has been used for over 100 years. Zinc phosphate is a mixture of zinc oxide powder and phosphoric acid liquid. The initial acidity of the setting cement (less than 2.0 pH) may cause sensitivity during and after cementation. The pH however slowly rises to 5.9 within 24 h and is neutral (pH 7.0) by 48 h (Kendzoir and Leinfelder 1976). Zinc phosphate cements have been used successfully in all-metal and metal-supported restorations with very good mechanical retention (parallel walls, more than 4 mm of tooth height, good bulk of the remaining tooth structure). These cements however are not recommended for use in ceramic crowns and composite crowns because of their inferior compressive and flexural strength.
The disadvantages of zinc phosphate cements far outweigh their advantages. They are soluble in oral fluids; they discolor and have weak physical and mechanical properties. Although they can still be found in the dental market, their use in dentistry has become very limited.
1.2.2 Polycarboxylate Cements
Invented in 1968, the polycarboxylate cements use the same powder, zinc oxide, found in the zinc phosphate cements. The liquid however is replaced by a polyacrylic acid solution to improve its resistance to solubility in oral fluids. Because of the polyacrylic acid molecules, there is some form of chemical adhesion to the tooth structure. The polycarboxylates are the first cements to show chemical adhesion to the tooth (Burgess and Ghumann 2011). However, these cements have difficult handling, are tacky, and may be too viscous which makes seating of the restoration difficult.
The main application of polycarboxylate cements today is for the cementation of long-term provisional restorations.