Written by George Freedman, DDS, FAACD, FADFE Monday, 28 February 2005 19:00
Cement is a material that relates 2 or more materials so they stay together in a specific relationship, incorporated as if they were a single unit. In dentistry, we have a variety of cementation materials, some of which have been used for many decades and others that have just recently been developed.
The more traditional, older cements were luting materials that relied on mechanical retention, such as long axial walls, taper, and precise fit. These conventional cements filled the gap between the restoration and the tooth, and nothing more. The newer adhesive cements stabilize the entire system of components by adhesively bonding to both the restoration and the tooth. Adhesive cements bond the gap between the restoration and the tooth, creating a "monobloc." The adhesive cements have additional required properties: they must also be functional, color matched, and biocompatible.
The choice of cement is defined by the procedure and the materials that are involved; no one cement is necessarily ideal for all purposes. The selection of a particular cement in the dental practice should be based on its strength, reliability, predictability, aesthetics, and most importantly, ease of use. Over the past decade, adhesive resin cements have become increasingly popular among dentists. After all, resin cements bond to enamel and dentin and develop micromechanical attachments to restorative metals and ceramics. On the other hand, zinc phosphate and polycarboxylate cements have no adhesion or attachment to enamel, dentin, metal, or ceramic.
While the properties of resin cements are generally far better than the earlier luting cements such as zinc phosphate and polycarboxylate, their use has been hampered by the confusion that surrounds their indications and utilization. The main culprits responsible for this uncertain state of affairs are the number of steps typically involved in the cementation procedure and the associated clinical challenges involved in the chairside application of a long and complex restorative protocol.
Ideally, a cement should function simply and effectively by adhering to all dental and restorative surfaces (enamel and dentin, metal and porcelain). It should involve little or no technique sensitivity (such as chairside mixing, multiple sequential layer applications, drying or wetting requirements, or a long self-cure setting time). Cementation should be easily performed by the dentist and the assistant, or the dentist alone.
|Figure 1a. Rinse and leave moist.||Figure 1b. Automix cement directly into crown.|
|Figure 1c. Place crown onto preparation.||Figure 2a. Rinse tooth and damp dry.|
|Figure 2b. Mix primer and apply.||Figure 2c. Wait 30 seconds.|
|Figure 2d. Gently air dry.||Figure 2e. Apply metal primer.|
|Figure 2f. Place base and catalyst onto mixing pad.||Figure 2g. Mix cement.|
|Figure 2h. Load cement into crown.||Figure 2i. Place crown onto preparation.|
|Figure 3a. Rinse and damp dry.||Figure 3b. Activate capsule.|
|Figure 3c. Load capsule into triturator.||Figure 3d. Mix cement.|
|Figure 3e. Place capsule into dispenser.||Figure 3f. Load cement into coping.|
|Figure 3g. Place crown onto preparation.|
Resin cements fall into 3 major categories, which are distinguished by their curing modes:
(1) Light-cure resin cements are typically used for thin, metal-free restorations (such as porcelain veneers) and metal-free orthodontic retainers and periodontal splints. For these cements, it is essential that the curing light reach every part of the adhesive in order to ensure polymerization. If the resin is too deep, the ceramic too thick, and not enough light reaches the photoinitiators, then the luting material will not set completely. Ultimately, bonding and restorative failure are the result.
(2) Dual-cure resin cements can be used for metal-free inlays, onlays, crowns and bridges, and where enough self-cure initiators are included, for metal and ceramo-metal restorations as well. In these applications, the beam of the curing light may reach most of the resin cement, but the dentist wants the extra assurance that the material will set in questionable areas. Typically, once the dual-cure resin has been photoinitiated, the cement immediately adjacent to the light will cure in a matter of seconds while simultaneously initiating a self-cure reaction in the remaining cement that has not been illuminated.
(3) Self-cure resin cements are used for metal inlays and onlays, metal and ceramo-metal crowns and bridges, and endo-dontic posts. These cements are not reactive to light; they polymerize by chemical reaction only when the separate components are physically mixed together.
Following are some of the important parameters that should be considered in the selection of resin cement:
(1) Film thickness. This is a measure of the minimum thickness that a particular product can assume under loading and functional pressure while maintaining its strength and other properties. Most resin cements have film thicknesses from 10 um to 30 um. It may seem at first glance that this could hinder the complete seating of a restoration. However, the typical tooth-restoration gap that is seen with a good technician is about 50 um, and often a far greater space is observed. (Zinc phosphate has a film thickness of 25 um.)
(2) Radiopacity. The visibility of the cement in subsequent recall radiographs is very important. This is what allows the dentist to distinguish between cement lines and recurring decay.
(3) Consistency. Cements run the gamut from very viscous to very fluid. The choice is a matter of personal preference. Some of the very thick resins require ultrasonic vibration during the seating of the restoration. Some cements are so thin that they will not effectively fill the gap between the tooth and the restoration.
(4) Extraoral working time. This is typically not an issue for the newest automix cement materials. Also, dentists who work with assistants have fewer working time-related problems. If the dentist is working alone or trying to seat multiple restorations concurrently, then a longer working time is appropriate. However, in most 4-handed practices that use automix cartridges or devices, a shorter extraoral working time is appropriate.
(5) Set time. As a cement is setting, it is a good idea to have constant and continuous finger or occlusal pressure on the restoration to prevent its displacement from the cavity. The fluid pressure of the unset cement tends to extrude the inlay, onlay, or crown away from the preparation into high occlusion unless there is a barrier to this movement. At the set time, the pressure is no longer required. The final steps of excess cement removal can be completed at this time.
(6) Rock-hard set time. Within a few minutes after the set time is reached, the cement becomes sufficiently hard that it cannot be penetrated with a sharp explorer. At this stage, the marginal cements can be polished with conventional procedures.
(7) Expansion. The post-cementation expansion of resin cements is unlikely to affect metal or ceramo-metal crowns and bridges. But it can be problematic for all-ceramic restorations if the rate of expansion is too great or too rapid. It is generally accepted that cements that have less than 4% linear expansion are unlikely to cause restorative failures.1
(8) Translucence. While cements come in a variety of shades and opacities, often the best aesthetic results are created by translucent or relatively translucent resins. At the marginal interface, a color discrepancy often exists between the restoration and the tooth. An aesthetic gradient or color transition is best accomplished by an adhesive luting material that can blend into both.
Having considered the clinical and functional parameters of cements, the next major issue of concern is the ease of use. How many steps are involved in readying the prepared tooth for cementation? How many steps are involved in the preparation of the internal surface of the crown? And finally, how many separate steps are required to prepare the cement components for loading into the crown?
The clinical steps required for PFM cementation with 3 popular cements2 are described in the Chart. (The common preparation steps of removing the provisional crown and cement and pumicing the tooth are common to all the cements and not included in the comparison. The common crown preparation steps of microabrading or etching the internal surface of the coping are also not listed separately.)
After seating for each of the cementation techniques described, the margins should be partially light cured and excess cement removed. Then the margins can be fully light cured and then polished after the cement is set.
Given chairside stress levels, the more straightforward a procedure, the more readily it is adapted into daily clinical use. Since chairside time is an expensive commodity (running from $5 to $10 per minute), the more efficient a cementation procedure, the more valuable to the practitioner (assuming, of course, that all other clinical parameters are comparable). Each additional step (particularly in long, involved, multiphase procedures) introduces the risk of clinical error or technique sensitivity; the greater the number of steps, the greater the overall risk.
The significant advances in the techniques listed in the Chart include the elimination of the tooth preparation steps (etching and priming and bonding) for Embrace WetBond Universal Resin Cement (Pulpdent) and RelyX Unicem (3M ESPE). Capsule activation and a triturator are required for RelyX Unicem; Panavia F2.0 (Kuraray) simply is pad-mixed; and Embrace is auto-mixed with a dual-barrel syringe through a mixing tip. Panavia F2.0 is spatulated into the crown, while RelyX Unicem and Embrace are loaded directly. In each case, setting of the cement and cleanup are similarly straightforward.
|Chart. Resin Cement Technique Summaries.|
Today's resin cements offer a variety of clinical options. They are clinically easy to use and predictable, and continuing development is simplifying the cementation procedure on a regular basis. Resin cements are adhering to dental surfaces (dentin and enamel) and bonding to our commonly used restorative materials (metals and ceramics). They relate these substrates so effectively that for all intents and purposes, they behave as a monobloc, incorporated into a single, functional unit.
Material technology advances have eliminated the need to etch or prime the tooth surface and have simplified mixing and dispensing tremendously. As a result, the technique sensitivities that existed with resin cementation have mostly been overcome. With protocol simplification, the confusion that surrounded cementation has been eliminated.
1.CRA Newsletter. October, 2004;28(10):1.
2.CRA Newsletter. August, 2004;28(8):1-2.
Dr. Freedman is past president of the American Academy of Cosmetic Dentistry and associate director of the Esthetic Dentistry Education Center at the State University of New York at Buffalo. He is also director of postgraduate programs in aesthetic dentistry at the Eastman Dental Center in Rochester, NY, and university programs in Seoul, South Korea, and Schaan, Liechtenstein, and chairman of the Clinical Innovations Conference in London. He is the author or coauthor of 9 textbooks, more than 200 dental articles, and numerous CDs, videotapes, and audiotapes. He is a team member of REALITY and a past director of CE programs in aesthetic dentistry at the Universities of California at San Francisco, Florida, UMKC, and Baylor College. A diplomate of the American Board of Aesthetic Dentistry, he lectures internationally on dental aesthetics, dental technology, and photography. Dr. Freedman maintains a private practice limited to aesthetic dentistry in Toronto, Ontario, and can be reached at (905) 513-9191.
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