Ask the Experts

Previously submitted questions with replies:

Q: When using the new Smartcem2, does this have to be initiated by a light? If not, is this ok to use when cementing posts?

A: SmartCem™2 Self-Adhesive Cement is a dual-cure material. When mixed, the material will self-cure without the use of a light. SmartCem™2 cement is an excellent choice for cementing posts. Following excess cement removal, the post must not be moved, torque, or otherwise disturbed during the 6 minute set time. Use of the light on accessible margins, while not required for initiation, can assist in stabilizing the post during this 6 minute setting time. As with all products, always consult the manufacturer’s Directions for Use for the product’s recommended indications and proper usage techniques.

Q: I would like to know which cement works better for cementing orthodontic bands: polycarboxilate, glass ionomer or zinc phosphate?

A: Glass Ionomer works better for cementing bands, as it is the most popular band cement and has a good track record of success while being easy to use. As for zinc phosphate, that is an older style cement that requires mixing, which can sometimes prove technique sensitive.

Q: Why can’t I use Aquasil Ultra Fast Set Impression Material for more than one preparation? I can syringe several preps within a minute.

A: As impression materials set, they go from their initial fluid consistency, through a gel phase, finally achieving the dimensionally stable “set.” This process is greatly influenced by temperature, and is accelerated by the warmth of the mouth.

The work time is the time between the start of mix until the material begins to gel, and begins having some elastic memory. That work time, stated as 1’30” at room temperature for Fast Set, is reduced to 35” at 37ºC. While it is possible to syringe multiple preparations within the room temperature work time of 1’30”, it is likely that the very first material syringed intraorally is already beginning to gel. Inserting the tray over this slightly gelled wash material may lead to imperfections.

We recommend intraoral syringing of one preparation only by the operator, with simultaneous tray loading by an assistant, then tray insertion immediately upon completion of syringing. This procedure typically takes less than 30”, and assures that both wash and tray materials are well within their environmental work times, allowing seamless co-adaptation and intimate capture of detail.

Q: What exactly is a flowable compared to the composite and why would I want to use it?

A: Typically, a flowable composite has similar chemistry to a universal composite. The ratio of the more fluid resin matrix to the filler particles is adjusted to reduce its viscosity. The lower viscosity “flowable” then can be precisely placed into a cavity directly from a narrow gauge syringe tip. The rheology of the lower viscosity “flowable” composite allows intimate adaptation to cavity walls with little or no instrumentation.

This adaptation and minimal manipulation make flowable composites ideal for cavity liners placed under higher density, more wear resistant composite restoratives, and for restoration of minimal class I, III and V cavities. Some formulas may also be used to repair small margin defects, and as pit and fissure sealants. Flowable composites are generally not indicated for large and/or stress bearing restorations, as the modification to improve the flow also reduces some phyical properties compared to traditional universal and packable composites. As with all products, always consult the manufacturer’s Directions for Use for the product’s recommended indications.

Q: Where would you use a Self Etch vs a Total Etch and why?

A: There is no single rule for use of one over the other. Above all, always confirm that the manufacturer indicates that product for the application where you intend to use it. Some operators believe the elimination of separate etching steps reduces potential contamination or other technique errors that might contribute to post-operative sensitivity.

They choose a self-etch adhesive where hypersensitivity might be anticipated. Some believe the separate phosphoric acid etch of enamel makes total-etch adhesive their choice for restorations predominately in enamel. When used properly, both systems provide clinically acceptable bonds to enamel and dentin. When misused, both systems can lead to sensitivity, bond failure and restoration loss. Newer formulations of both systems are available that are more robust, and more forgiving than their predecessors. No matter whether ease and speed of use lead you to a self-etch system, or clinical history of durability and universal use lead you to a total-etch system, the manufacturer’s directions for use must be followed explicitly for success. For further information, DENTSPLY Caulk has prepared the Younique™ Portfolio of Adhesive Capabilities (Form #530920). Please contact your local DENTSPLY Caulk representative or call 1-800-532-2855 to request your copy.

Q: What exactly are white lines?

A: This is a subject that has stirred much debate among key opinion leaders. The prevailing theory is that white lines along the enamel margin of a restoration are representative of microfractures of the enamel.

Theories about why they form include tensile stress resulting from rapid polymerization of composite material to overzealous rotary finishing of margins. Regardless of the etiology, their clinical significance is not universally accepted. Many advocate the routine use of a composite surface sealer, which might mitigate any deleterious effects, and, at the least, provide a high gloss finish to composite restorations.

Q: When using the Self Cure Activator, how long does it take for the resin to fully cure without the light?

A: The Self Cure Activator is designed to be mixed with adhesive and applied before the application of a two-component (base + catalyst mixed) self cure or dual cure resin restorative material or cement that chemically cures by itself without light. When using any dual cure or self cure resin system, always allow at least the set (cure) time specified in the manufacturer’s Directions For Use provided with that resin material.

However, the use of a Dual Cure Adhesive System (Adhesive plus Self Cure Activator mixture) can shorten the work time and accelerate the chemical cure/no-light setting (cure) time. It’s a good idea to investigate whether there is an accelerating effect in the laboratory prior to clinical use.

Here is a quick, easy test:
Use two glass microscope slides (or other impervious, disposable material). Mix and apply the adhesive/activator mixture to one side of each slide and air dry and/or light cure according to the Directions for Use. Then mix some base and catalyst of your dual cure or self cure resin material and apply a small amount of the mixture to one of the treated slides, face up. Then quickly cover with the other slide, treated side down, and squeeze to a thin layer “sandwich.” Every 30 seconds, try to gently wiggle the pair of slides. When the slides do not move against each other anymore, the work time of the resin has passed. You can compare the time you observe to the manufacturer’s stated room temperature work time to see if there is an accelerating effect of the adhesive/activator mixture on your resin system.

Remember that the times you see in the laboratory will be shortened by the warmth of the mouth, as the resin’s setting reaction is accelerated by heat. So, knowing that both the Self Cure Activator and the warmth of the mouth may speed up (shorten) the actual work time and curing time, if you allow the amount of setting time the resin manufacturer recommends, you can be sure the material will be adequately cured, and it is safe to finish, polish and dismiss the patient.

Q: I’m confused about the use of the Dual Cure resin materials with “one-bottle” adhesives. Why do I need to mix in another component? Isn’t the adhesive fully cured if I shine the curing light on it?

A: In order to understand the need for adding another compound (typically “Self Cure Activator”), it is important to understand two things
about the tooth/adhesive/restorative complex. First, the adhesive functions as the “link” between the tooth substrate and the overlying
restorative material. Second, resin restoratives known as “light cured” (cured only by visible light) “self-cured” (cured by mixing, for chemical
activation) and “dual-cured” (cured by either or both visible light and chemical activation) use different initiator systems to cause “cure”
or “set.”

The primary function of the Self Cure Activator is not to cause the adhesive to cure, but rather to provide a chemical surface that promotes
curing of the interface between the adhesive and the self cure or dual cure resin restorative material. Without the Self Cure Activator
mixed in with the adhesive, the interface will cure much more slowly.
As always, please follow the Directions For Use for both the adhesive and the restorative materials chosen. A simple rule to follow is
“Match the Chemistries.” When using a chemically activated dual-cured or self-cured restorative material, use of the Self Cure Activator
mixed with the adhesive is recommended. If using a pure light-cured restorative material, the light-cured adhesive alone does the job
best.

Q: What measures can the clinician take to ensure the clinical success of metal-free (all-ceramic) crowns and bridges?

A: Cementable all-ceramic crowns and bridges are high-strength, metal-free restorations comprising a ceramic substructure (coping or
framework) veneered with porcelain, for an esthetic result. The strongest and toughest cementable ceramic used in dentistry today is
yttria-stabilized zirconia1. Cercon (Dentsply Prosthetics, York, PA) was the first such zirconia. Cercon has been tracked clinically in
Europe for more than 5 years, and its success rivals that of porcelain-fused-to-metal (PFM) restorations2. To ensure similar success,
clinicians should receive training in case selection, tooth preparation, handling of the material and requirements for follow-up care. Attention
to clinical detail and close communication with trained laboratory staff are essential. Some critical clinical issues are:

1. Design the Preparation to avoid High Stresses
Adhere to the recommended marginal designs of heavy chamfer (0.8-1.0mm) or a shoulder
Create rounded (> 0.4 mm radius) axial, occlusal and incisal line angles with a minimum axial convergence of 6º to 8º for optimal scanning
and CAD.

2. Design the Copings and Frameworks to recommendations
A coping thickness of 0.3 mm is required
The thickness of the veneering porcelain should be at least 0.5 mm but should never exceed 2 mm. It is essential that the zirconia framework
support the veneering porcelain, especially in high-stress areas.
Bridge connector thicknesses should be at least 9mm2, and larger in higher stress areas such as the molar region.
An occlusal guard can protect against excessive biting forces as well.

3. Precementation Adjustments – Treat this case like the All-Ceramic it is!
Remember that the porcelains used to veneer zirconia are characteristically low in leucite or have no leucite at all.
The surface of veneering porcelains is clinically acceptable if adjusted with medium, fine and extra-fine sandpaper disks (Sof-lex, 3M, St.
Paul, Minn.) or fine (< 15 μm) diamonds, followed by polishing with diamond paste3.
Similarly, a sequence of diamond ceramic polishers (Dialite, Brasseler USA, Savannah, Ga.) produces clinically acceptable surfaces4.
Machined, polished surfaces approach the smoothness of glazed surfaces, but machining cannot equal glazing. Grinding adjustments that
leave a roughened surface predispose the surface to chipping later in the life of the restoration.

References:
1: Erdelt K et al., Quintessenz Zahntech. 2004; 30(9): 942–54.
2: Ewoldsen N et al., J. Can. Dent. Assoc. 2007; 73(1): 47-8.
3: Curtis AR, et al., J Dent 2006; 34(3):195–206.
4: Sasahara RM, et al., J. Oper Dent 2006; 31(5):577–83.

AUTHORS
Dr. Nels Ewoldsen is an Adjunct Associate Professor in Adult Restorative Dentistry, UNMC College of Dentistry, Lincoln, Nebraska. He is
also director, clinical research and education, DENTSPLY Prosthetics, York, PA..
Dr. Veeraraghavan Sundar is Manager, Professional Services and Clinical Education, DENTSPLY Prosthetics, York, PA.
Email: vsundar@Dentsply.com