Full coverage restorations today are still made of classic materials like all gold and porcelain fused to metal, as well as our more recent high strength all ceramic systems. Metal based restorations, Zirconia and Lithium Disilicate (IPS e.max) have adequate inherent strength to be cemented without risk of fracture of the material. For the all ceramic systems this assumes you are following the recommendations for tooth reduction from the manufacturer. Occlusal reduction recommendations to ensure fracture resistance for Zirconia suggest 1-1.25mm, although .5mm is considered the absolute minimum. Lithium disilicate requires a minimum of 1.5mm of thickness on the occlusal surface when cemented, and this can be reduced to 1mm if a true bonding process is followed.
Cementation of these high strength ceramic restorations is accomplished with glass ionomer, resin modified glass ionomer and self-adhesive resin cements. Glass ionomer cement has been a mainstay in cementation over the years due to ease of use and mechanical and physical properties that result in longevity in the oral environment. They attach chemically to tooth structure, effectively seal dentinal tubules minimizing sensitivity, release fluoride and have good expansion and contraction characteristics. It’s these features that motivate me to utilize a resin modified glass ionomer for cementation of a large number of my full coverage restorations.
Traditional glass ionomer cements set chemically through a reaction of a silicate glass powder and polykenoic acid. Resin modified glass ionomers are dual cure materials through the addition of HEMA and photoinitaitors, to the glass iononer components. Resin modified glass ionomers can be light cured from the buccal and lingual allowing cleaning of the cement in a gel phase. The addition of the resin also increases the flexural and tensile strength of the material and lowers the modulus of elasticity.
All glass ionomer based materials contain fluoride as it is a byproduct of the manufacturing process. The cement once set, can recharge with fluoride ions, when they are present in the oral environment in a high enough percentage, and then will slowly release these back when the salivary fluoride levels decrease. This can be a very advantageous characteristic for patients at high caries risk. Glass ionomer cements are convenient and efficient to use in the oral environment. They can still be hand mixed, dispensed through a self mixing tip, or mixed in a capsule using a mixing machine. The glass ionomer should be protected from moisture contamination in the early setting phase. Once set the physical properties of the glass ionomer allow it to be very resilient in the oral environment, resulting in longevity of the restoration. Glass ionomer and resin modified glass ionomer cements tend toward the opaque and this impact on translucent all ceramic restorations must be considered in anterior esthetics when working with translucent materials.