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The ever changing world of cements

October 12th, 2005 · No Comments

leinfelder_small_small.jpgThe Ever Changing World of Cements

The introduction of polymers has caused a dramatic change in nearly every facet of life. Certainly, dentistry is no exception. While the resin system offered so much potential for better dentistry, they got off to a slow and less than exciting start. One of the first uses was as a luting agent. This first material was identified as Grip (LD Caulk). While the immediate post cementation was impressive, short-term clinical results were discouraging. Consisting of a spherical acrylic resin powder particle and a liquid monomer of the same composition, the material exhibited excellent wetting of the prepared crown surface. However, in a relative short period of time water from the salivary fluids penetrated into the interface between the cement and the prepared tooth structure. This unfortunatel resulted in a high incidence of crown separation. The first major step forward came about with the introduction of composite resin as a luting agent. Unlike its acrylic resin predecessor, the physical and mechanical characteristics were considerably superior. As a result, the potential for appreciable fluid penetrate and loosening of the casting were considerably minimized. The union with bonding procedures brought the composite resin-luting agent to the levels we experience today.

In fact, composite resin luting agents (Lute-It and Cement-It, Pentron clinical Technologies and Calibra, Dentsply/Caulk) are the materials of choice when inserting all-ceramic crowns (Optec 3G, and Empress, Ivoclar). By developing a strong bond along all the t interfacial zones (tooth-cement and cement-porcelain) the potential for fracture of the restoration is reduced appreciably. It is interesting to note that when all ceramic crowns (Dicor, Dentsply Ceramco) were used in conjunction with the traditional zinc phosphate cement, the failure rate due to fracture was unacceptable. Interestingly, the fracture rate of yesterday would have been considerably lower today if adhesion techniques would have been available.

Glass ionomers and resin modified glass ionomers have fared very well particularly well in association with metal-based crowns. Hybridizing in conjunction with glass ionomer luting agents serve excellently to minimize the potential of secondary caries when longevity is expected for 20 to 30 years. Unfortunately, alumina or zirconia based ceramic crowns (Procera and All Ceram) are difficult to bond to the prepared tooth structure because they are not susceptible to etching with hydrofluoric acid and silanation. Many clinicians therefore opt to use Unicem (3M ESPE) or Maxcem (Sybron/Kerr) for example since the adhesion to the crown is minimal to non-existent.

Unicem and Maxcem have become quite popular for both metal-based and all-ceramic crowns. Three reasons can be offered for their popularity: 1) ease of use, 2) no treatment of the preparation is necessary and 3) adhesion ranges between six and nine (6-9) MPa. It is also speculated that theses relatively new luting agents actually generate a superficial hybrid zone on the preparation surface.

Individually, Unicem and Maxcem are not composite resins as bases upon Bowen’s formulation. Rather they are methacrylate based and contain particles of glass ionomer, barium glass for radiopacity, fumed silica for handling characteristics and fluoro-alumino-silicate for lowered levels of fluoride release. Although they have been growing in popularity, this new class of materials is a bit of a compromise. In essence, there is a trade-off of bond strength for simplification of use. On the positive side they are being recommended for all types of crowns, regardless of their composition.

One of the more exciting resin cements is Calibra (Dentsply/Caulk) which was introduced to the profession several years ago. It is one of the few types of cement that can be formatted for the clinician to be either light-cured or chemically cured. Furthermore, the viscosity can be modified to the clinician’s needs. Finally, it is an excellent cement for bonding veneers. Unlike most other cements, it does not oissess any of the elastic memory in the preset state. Consequently, it does not move away from the prepared surfaced during seating and cementation. This unusual characteristic laminates the need for immediate curing with the light.

Finally, it is important for the clinician to realize that incompatibility commonly does exist between the dual cured or self-cured luting agents and the 5th and 6th generation dentin bonding agents. Before using any combination of cement and adhesive, the clinician should first bench-test the materials for compatibility prior to intraoral use.

Last comment: before accepting the new methacrylate based cements (Unicem and Maxcem) it should be recognized that these agents, while simpler to use than conventional composite resin cements, exhibit bond strengths that are considerably less. As a rule their adhesion is only 50& or less than those that are bonded through the conventional hybridization process.

Karl Leinfelder

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