April 1, 2006
by George Freedman DDS, FAACD, FADFE
Over the past century, dentists have used a variety of cements for indirect procedures. A cement is defined as an agent that relates two or more materials such that they stay together in a specific relationship, incorporated as if they were a single entity. Dental cements are used to attach dental restorations onto or into the teeth; some have been in use for many decades, while others have just recently been introduced.
The earliest luting cements relied on mechanical properties (long axial walls, tapered preparation, and precise fit) for retention. These conventional cements did little more than fill the space between the restoration and the tooth. The newer, adhesive cements, however, are designed to bond to both the restoration and the tooth, stabilizing the entire system. Adhesive cements bond to all the restorative components as they fill the gap between the restoration and the tooth, creating a MONOBLOC. In addition, adhesive cements are required to be functional, color-matched, and bio-compatible.
The type of procedure and the materials used determine the choice of cement; no single cement is ideally suitable for all purposes. Clinical selection criteria include strength, reliability, predictability, aesthetics, and most importantly, ease-of-use. Since resin cements bond to both enamel and dentin, and can develop micromechanical attachments to restorative materials, they have rapidly increased in popularity. After all, zinc phosphate and polycarboxylate cements have no adhesion or attachment whatsoever to enamel, dentin, metal, or ceramic.
Unfortunately, a great deal of confusion has developed in the indications and utilization modalities of resin cements. Although their adhesive properties are far superior to those of the earlier luting cements (zinc phosphate, polycarboxylate), dentists have hesitated to incorporate these materials due to their complex protocol, and the clinical challenges associated with the many procedural steps required for applying them chairside.
The properties of an ideal cement include:
* Simple and effective adhesion to all dental and restorative surfaces (enamel, dentin, metal and porcelain).
* Little or no technique sensitivity (eliminating chairside mixing, multiple step applications, drying/wetting requirements, and a long self-cure setting time).
* Cementation must be easily performed by the dentist and the assistant together, or the dentist alone.
Light-cure cements are indicated for thin, metal-free restorations (porcelain veneers, metal-free orthodontic retainers, and periodontal splints). In order to assure complete polymerization, the curing light must reach every part of the adhesive. Overly thick luting resin or ceramic can hamper deep photo-initiator activation, preventing complete polymerization, and leading to restorative failure.
Dual-cure resin cements are indicated for metal-free inlays, onlays, crowns, and bridges (and metal and ceramo-metal restorations where adequate self-cure initiators are present). The curing light beam polymerizes the visible resin cement directly, while light-inaccessible areas are cured by secondary chemical initiation. Once the dual-cure resin has been photo-initiated, phosphenes will continue the polymerization reaction in the remaining non-illuminated cement to completion.
Self-cure resin cements are indicated for metal inlays, onlays, metal and ceramo-metal crowns and bridges, and endodontic posts. These cements are not light-reactive, polymerizing to completion by chemical reaction after the separate components are physically mixed together.
RESIN CEMENT PARAMETERS
The consistency of cements ranges from very viscous to very runny. The clinical choice is a matter of personal preference. Viscous cements may require ultrasonic vibration during the seating of the restoration. Runny cements may not fill the tooth-restoration interface effectively.
The film thickness measures the minimum thickness that a cement must have under loading and functional pressures while maintaining its optimal properties. Most current resin cements have film thicknesses from 10-30. This resin film thickness fits well into clinical reality; the typical tooth-restoration gap observed with a good technician is about 50 (film thickness of zinc phosphate is 25).
Resin cements are available in a variety of shades and opacities. Often, translucent resin cements offer the best aesthetic results. There is often a color discrepancy between tooth and restoration at the marginal interface. A resin gradient or color transition, can blend the colors.
Extraoral working time is no longer an issue with automix cements. Most four-handed practices utilizing automix cartridges prefer as short an extraoral working time as possible (a longer working time is appropriate for large cases or the solo dentist).
The setting time measures the polymerization phase of the cement after seating. This should always be as short as possible. Since the fluid pressure of the setting cement tends to extrude the restoration away from the preparation into high occlusion, constant and continuous occlusal pressure is needed until polymerization is complete.
At the rock-hard set time, the cement is sufficiently hard that a sharp explorer cannot penetrate. At this stage, the exposed marginal cement can be routinely polished.
The radiopacity of the cement allows the dentist to distinguish between cement lines and recurring decay on radiographs during recall examinations.
The post-cementation expansion of resin cements will not affect metal or ceramo-metal crowns and bridges. An expansion that is too great or too rapid can be problematic for all-ceramic restorations, however. It is generally accepted that cements with less than 4% linear expansion are not likely to cause restorative failures (CRA Newsletter, Oct 2004).
The ease-of-use of a resin cement is often overlooked. The crucial questions include:
* How many steps are needed to prepare the tooth for cementation?
* How many steps are required to condition the internal surface of the crown?
* How many steps are required to prepare the cement components?
There are several steps that are common to all cementation procedures, and as such need not be considered in a comparison (removing the provisional crown and cement, pumicing the tooth, microabrading the internal surface of the crown, etc.).
Also common are the post-seating procedures for each of the cementation techniques: the margins are partially light-cured to initiate polymerization, and the excess cement removed. The margins are then fully light-cured, and polished once the cement is set.
Cementation is a well-documented protocol. Tooth surface preparation often involves several steps: etch, wash, condition, bond, and wait. Crown preparation includes internal surface activation, adhesion, and loading of the resin. Cement preparation can require material dispensing, measurement, and pad mixing. Add to these the absolute necessity for keeping the prepared tooth free of salivary (and other) contamination, and it is easy to see that traditional resin cementation can require six or eight hands for efficiency. Since most dentists practice in a 4-handed environment, this protocol is not realistic.
Dental practice is stressful at the best of times. There is no need to complicate it further, with techniques sensitive or clinically difficult procedures. The easier and faster a procedure is, the more readily it is adapted into the daily clinical routine.
Technique efficiency is also important consideration. Chairside time is the only commodity that a dentist has to offer, and given the operating overhead of a practice, it is expensive, costing at least $5-10 per minute. Thus, a more efficient cementation procedure is more valuable
to the practitioner (assuming, of course, that all other clinical parameters remain the same). Another consideration is that each additional step (particularly in long, involved, multi-phase procedures) brings with it an additional risk of clinical error or technique sensitivity; the more the steps, the greater the risk.
Significant advances in the resin cementation techniques listed below include the elimination of all tooth preparation steps (etching and priming and bonding) for Monocem, Embrace and RelyX Unicem. Both Monocem and Embrace are simply auto-mixed with a dual-barrel syringe through a mixing tip, and dispensed directly into the crown. RelyX Unicem capsule activation requires a triturator with appropriate settings for intensity and timing. Panavia F2.0 is manually pad-mixed and then spatulated into the crown. Monocem and Embrace are loaded directly into the crown, while RelyX Unicem requires a dedicated dispenser.
For all of these resin cements, the setting of the cement and the subsequent marginal curing and clean up are similarly straightforward.
The clinical steps required for PFM cementation with four popular cements: MonoCem, Embrace, Panavia F2.0, and RelyX Unicem are adapted from the CRA Newsletter, August 2004, are shown in Table 1.
Today’s resin cements offer a variety of clinical options. They are predictable and easy to use clinically. Resin cements adhere to dentin and enamel and bond to metals and ceramics. These materials are so effectively related that they function together as a monobloc, much like the original healthy tooth.
Material technology advances have eliminated the need to etch or prime the tooth surface, or to manually mix cements. Dispensing has been simplified, eliminating technique sensitivity. With the simplification of cementation protocol, clinical confusion has been replaced by user-friendliness and predictability.
Dr. Freedman is past president of the American Academy of Cosmetic Dentistry and a founder of the Canadian Academy for Esthetic Dentistry. He is the Chairman of the Clinical Innovations Conference (London, United Kingdom) as well as the Dental Innovations Forum (Singapore). Dr. Freedman maintains a private practice limited to esthetic dentistry in Toronto, Canada, and can be reached at email@example.com.
Oral Health welcomes this original article.
Special thanks to Len Lim, RDT of Progenic Dental Lab for the excellent laboratory work.
Click here to view the table from this article