Delving Into Reality: A Conversation With Dr. Michael Miller

Dr. Michael Miller

In an interview conducted by Dr. Damon Adams, Dentistry Today’s Editor-in-Chief, Dr. Michael B. Miller, Editor of REALITY, shares his opinions, knowledge, and vast clinical experience on testing a variety of products.

Dr. Adams: Why are there so many self-adhesive materials being introduced?
Dr. Miller: The appeal of self-adhesive materials is obvious. What could be easier than cutting a preparation, rinsing off the debris, and placing the restoration without having to bother with either etching or priming steps? Once dentists try a self-adhesive material, and have what seems to be success, they figure that this is the way to go.

Dr. Adams: These new materials are receiving accolades from our colleagues, but you seem a bit skeptical. Why?
Dr. Miller: Success means different things to different clinicians. Just because there are no immediate problems does not necessarily mean there will be long-term success. The tests we have run in our lab still indicate that self-adhesive materials do not perform up to the level of those products that are used in combination with primers, with and without etching first with phosphoric acid. When it comes to cements, the products with self-etch primers that had impressive 24-hour bond strength results on dentin include Clearfil Esthetic Cement (Kuraray) and Bistite II DC (Tokuyama).

Dr. Adams: Doesn’t this debate hearken back to the days when we were afraid to etch dentin? How can we move forward if we balk at the progress these materials represent?
Dr. Miller: There are 2 significant differences between the dentin etching fear and this latest hesitation to fully adopt self-adhesive materials. First, there was no credible evidence that etching dentin for only 15 seconds in and of itself was damaging to the dentin, or even the pulp. Second, the bond strength tests done in the laboratory showed etching dentin combined with the correct bonding agent resulted in much better adhesion compared to not etching. With self-adhesive materials…including cements, core materials, and flowables…the bond strength lab tests do not approach the adhesive values of the etched and/or primer groups. Of course, there are exceptions to this finding. G-Cem (GC America) in its capsule form scored quite well in our tests, although the paste-paste version did not fare as well.

Figure 1. Bond strength tests are done with operator and assistant working together to simulate the clinical procedure.

Figure 2. Fluorescence of materials is tested intraorally in a live subject in a custom-made black light cabinet. It is not possible to test fluorescence using extracted teeth or discs.

Figure 3. Bond strength is determined by placing specimens in an Instron universal testing device.

Figure 4. Specimens for bond strength testing are prepared on a dual-wheel model trimmer.

Figure 5. Thermocycler is used to simulate the temperature extremes that a restoration would experience intraorally.

Figure 6. Spectrophotometer determines the translucency/opacity of materials to allow more specific selection. This property is important when choosing materials to replace enamel and dentin and to simulate incisal effects.

Figure 7. Transilluminating and digitizing discs of composite quantifies porosity in composites. Materials with high levels of porosity can cause problems during the finishing/polishing phase of a restoration.

Dr. Adams: Bond strength doesn’t necessarily tell the whole story. Isn’t that correct?
Dr. Miller: It is correct that the absolute value of bond strength does not categorically indicate that a material will or will not be satisfactorily adhesive. The fact is that we are not really sure exactly how much bond strength is required to retain a restoration. However, if you had a choice in your mouth between 2 materials—with one of them having much better adhesion—which would you choose, especially since you are only adding an extra minute or so to the entire procedure?

Dr. Adams: With all in this mind, how did this entire trend get started?
Dr. Miller: Self-adhesive materials have been one of the Holy Grails when it comes to restorative dentistry. As a result, the manufacturers have been on a mission to provide this version of nirvana. Of course, the first truly adhesive material was glass ionomer. It is still highly popular in some areas of world. Personally, I particularly like FujiCem Automix and Fuji IX GP Extra, both GC products. However, the floodgates of self-adhesive resin-based materials were opened by 3M ESPE when it introduced the dual-cured resin cement RelyX Unicem 7 years ago (2003).

Dr. Adams: Well, since it has been 7 years since that popular product was introduced, if you are saying that these materials have a lower chance for long-term success, wouldn’t we now be seeing a surge with debonding problems?
Dr. Miller: Not necessarily, but I’ll be the first to admit that we have received very few complaints about RelyX Unicem. On the other hand, remember that crowns used to be luted with totally nonadhesive materials such as zinc phosphate. Not too many crowns dislodged even then. So it is not that surprising that a large number of crowns can be retained using a material with relatively low bond strengths. In addition, the original RelyX Unicem was a powder/liquid in a predosed capsule that was mechanically mixed in a triturator. Most of the current self-adhesive cements, including the latest version of RelyX Unicem, are paste-paste formulas mixed through one of the ubiquitous tips that mount on the end of the typical dual-barrel syringe. In the few short years since these paste-paste materials have been available, at least one product has already been replaced with a second generation version due to less-than-adequate performance; a second material was recalled due to stability issues; and a third product was taken off the market altogether. These actions do not exactly engender confidence in this category.
However, I would like to note that our tests with RelyX Unicem show the paste-paste version is actually stronger than the encapsulated, powder-liquid variety. So this particular material is bucking the trend.

Dr. Adams: In addition to cements, what other types of resin-based self-adhesive materials are available?
Dr. Miller: At least one core material and 2 flowable composites are being marketed as self-adhesive. Of these, we have tested the core material and one of the flowables. Both performed below what we (at REALITY) would consider to be acceptable in bond strength tests. However, I recently used one of the flowable composite resins (Fusio [Pentron]) to restore a difficult wrap-around Class V carious lesion. Along with mechanical retention created during the removal of the carious tooth structure, this material demonstrated enough adhesive strength to resist displacement during the finishing procedures and its handling characteristics were impressive.
While a mediocre bond strength result may not produce failure when luting crowns, many dentists have experienced direct restorations popping out; especially when the restorative material is placed in a totally nonretentive lesion such as an abfraction where the axial wall is often sclerotic dentin. So, it still remains to be seen whether these self-adhesive restorative materials will stand the proverbial test of time.

Dr. Adams: So, before we leave this topic, what is your practical advice to our readers concerning the use of self-adhesive materials at this point in time?
Dr. Miller: For crown cementation, they will probably perform adequately, but using a resin cement with an accompanying self-etch primer, in my opinion, is still better. For other uses such as core build-ups and definitive restorations, I would personally urge caution, based on our testing. Remember, no manufacturer is going to show up at your office, compensate you for having to replace a failed restoration, and then tell the patient that it was not your fault.

Dr. Adams: It seems more manufacturers today are marketing their restoratives as being low-shrink or low-stress materials. Can you address this trend?
Dr. Miller: All dental polymers shrink to some degree. Until relatively recently, this shrinkage for highly-filled sculptable restorative composites averaged about 4% to 5% volumetrically, with flowables usually being slightly higher. The main problem with shrinkage is the stress it puts on the bond between the material and the tooth structure.
It’s easy to picture this in your mind’s eye: You have just applied your bonding agent to the preparation. Immediately after light-curing, when the bond is still not particularly stable, you place the restorative material and light-cure. As the composite is curing, it is also shrinking. The bonding agent is trying hard to hold onto the composite so gaps do not develop. This is a tough job for a bonding agent to accomplish, especially if the shrinkage is 4% to 5%, or even higher. However, if the shrinkage is lower, then it is logical to assume that the bonding agent would not have to work so hard to minimize the aforementioned marginal gaps. There are also other issues involved in this tug-of-war between the bonding agent and composite to prevent gaps from forming, such as how well the composite actually wets out on the bonding agent. This wetting-out property can be enhanced if a composite is reasonably thixotropic, which means it will flow when manipulated (packed) with an instrument. With this increased flow, there is a more intimate interface with the bonding agent, which usually is better in minimizing gap formation at the margin compared to thicker, “drier” composites.
So, with everything else being equal, this means that choosing a low shrinking material over one with higher shrinkage would be a prudent decision. The universal composites that we have measured with 2% volumetric shrinkage or lower include: Premise (Kerr), Clearfil Majesty Esthetic (Kuraray), Tetric EvoCeram (Ivoclar Vivadent), Gradia Direct X (GC America), and Estelite Sigma (Tokuyama). But some of the so-called “packables” for posterior teeth are even better, with Filtek LS (3M ESPE), Clearfil Majesty Posterior (Kuraray), and Aelite LS Packable (Bisco) demonstrating volumetric shrinkage of 1.3% or less.
Unfortunately, other properties can suffer as the shrinkage goes down. For example, Filtek LS (3M ESPE), the material with shrinkage lower than any other material we have ever tested (1.1%), has to be used with its own bonding agent due to the unique chemistry used to lower the shrinkage. This means that if you are comfortable using another bonding agent, you have to switch to the corresponding 3M ESPE bonding agent when using this low-shrink material. In addition, our bond strength tests indicate that it takes about one hour for this adhesive to mature to the level achieved immediately by other bonding agents. We are not sure how clinically relevant this finding is, but it certainly raises a red flag. Furthermore, the material itself has a high exotherm during curing. In other words, it gets pretty hot; more so than virtually any other light-cured material. Again, this high exotherm may not be clinically relevant–3M ESPE has data showing this high exotherm has no effect on the pulp. However, if you find yourself restoring a tooth in a patient who decided to forego local anesthesia, the heat will definitely cause a painful reaction from the patient. Finally, the depth of cure of this material is not so great. If you are using it at the bottom of the proximal box in a Class II situation, our tests show it doesn’t reach the 80% cure goal compared to the occlusal surface.

Dr. Adams: With these types of issues, should a clinician take a chance on a low-shrink material?
Dr. Miller: With the aforementioned small handful of materials now testing out at below 2% volumetric shrinkage, I think it would be helpful to pick a material from this group when you are doing a Class I or II restoration. We do not know if lowering the shrinkage additionally to the level of Filtek LS is significant or not.

Dr. Adams: How is “low shrink” different from “low stress”?
Dr. Miller: Although both properties seek to achieve the same end result,─namely to place less pressure on a bonding agent to keep the marginal gap as minimal as possible, low stress materials make the point that even though a product may be low shrink,─that does not necessarily mean it still doesn’t challenge the bond propensity of the adhesive. I’m not sure if I totally buy into that proposition, since logic tells me that a low-shrink material will be less abusive to a bonding agent performing its duty, compared to a high-shrink product. Regardless, a low-stress flowable restorative base, such as the recently introduced SureFil SDR Flow (DENTSPLY Caulk), holds tremendous promise. In addition to its low-stress property, it also cures to a depth of 4.0 mm. However, our tests found that you need 40 seconds of curing to achieve this depth, not the 10 seconds stated by the manufacturer. Nevertheless, a 4.0 mm depth of cure is a noteworthy achievement, since our tests show most restoratives will only cure to a depth of 2.0 mm.
This means that you could literally fill the entire cavity with this product, especially if you are treating children, although the manufacturer does recommend veneering the occlusal surface with a more conventional sculptable composite. It is debatable whether this is necessary when restoring a deciduous tooth.

Dr. Adams: You mentioned depth of cure during the previous discussion. Won’t one of the new high-powered LED lights increase the depth of cure with most composites?
Dr. Miller: I like to make an analogy between the power of a curing light and the horsepower of a car. If you are a car enthusiast, you know that more horsepower usually translates into propelling a vehicle more quickly, especially in its off-the-line, 0 to 60 MPH time. However, the incremental speed increases, in real world situations, are miniscule. For example, if you buy a car with 300 horsepower, it may run from 0 to 60 in 6 seconds. Increase the horsepower to 400 and your 0 to 60 time may decrease to 5.5 seconds. That’s a 33% increase in power, but all you get is a vehicle that reaches 60 mph in a blink-of-an-eye faster. Maybe in NASCAR something like that matters, but it does not in real life.
The same applies to curing lights. While there are a few very high-powered lights that will cure composites faster, such as the Swiss Master from EMS with its 340 Watt halogen bulb, the amount of heat that is produced along with a higher cost makes them, in my opinion, a less-than-desirable choice unless speed is your top priority.
For most clinicians at this time, purchasing a curing light that emits 1,000 to 1,500 mW/cm² is perfectly acceptable. As a matter of fact, we have been using curing lights over the years that only emit as little as 350 mW/cm² with what appears to be a reasonable level of success. As long as you stick with the time-tested 40 seconds of curing for each 2 mm of composite, virtually any curing light that you buy today will work. Some of the lights that had impressive performance tests in our laboratory include the DEMI (Kerr), Fusion (DentLight), FLASHlite Magna (Discus), Elipar S10 (3M ESPE), and VALO (Ultradent).

Dr. Adams: Would you please comment on the claims of numerous manufacturers that their lights will cure composite in 5 to 10 seconds?
Dr. Miller: One manufacturer is claiming it has the “world’s first wireless 3-second LED curing light.” This claim is even more ludicrous than the 5- and 10-second claims. The problem with all these claims is the manner in which the lights are tested. Our tests are done using extracted human teeth, not the metal mold used by most manufacturers.
We have shown repeatedly that you cannot adequately cure a composite in 10 seconds, or even 20 seconds; unless you are using a very translucent material and you can virtually place the light tip in contact with the surface of the material. On the other hand, if you are trying to cure composite placed on the gingival wall of a Class II proximal box that is 6 to 8 mm away from your light tip, there is no way you are going to cure that material in less than 40 seconds. So I would say that clinicians should view these fast-curing claims with a very jaundiced eye.
Nevertheless, the current generation of LED curing lights, as mentioned previously, are very handy and some of them are a pleasure to use. VALO, in particular, while being corded and not offering the cordless convenience of the other units, is the only one that mimics the wavelength spread of a halogen light. This means that it will cure all materials regardless of the type of photoinitiator in the product. The other lights can’t make this claim.

Dr. Adams: Let’s touch on ergonomics. Since more clinicians than ever before are practicing into their 60s and 70 years of age, what innovations have been introduced to aid the more “mature” dentist?
Dr. Miller: Actually, I am officially one of those “mature” practitioners, although I would never call our younger colleagues “immature!” Nevertheless, even though 60 is supposed to be the “new 40,” 20 years does make a difference no matter how fit you are. Ergonomic breakthroughs that I believe every dentist should be looking into are lighter and smaller loupes. One example would be ErgoVision prism loupes from SurgiTel, which scored the highest in our tests. Also important is optimal illumination; Cordless LED headlights such as Solaris from Perioptix and Twin Beam LED DayLite from Designs for Vision, both of which have amazing power, are a couple of examples. Just be aware that the LED headlights do not feature a focused, collimated beam that is common with corded halogen and metal halide models. This lack of focused beam is noticeable, but the convenience of being cordless will be more important for most operators.

Dr. Adams: What about the effect of the extra weight of electric handpieces on repetitive use syndromes such as carpal tunnel? Should we be concerned about that issue?
Dr. Miller: This is certainly a topic that demands more study. Most electric handpieces weigh twice that of air-powered versions. Any operator who has used an electric handpiece can feel the difference immediately. For clinicians who have already experienced musculoskeletal problems, the extra weight of electric handpieces could potentially exacerbate the problem. On the other hand, the higher torque with more efficient cutting provided by an electric handpiece could be a plus for those with a repetitive use issue. My advice would be to try to find a manufacturer who will allow you to use a loaner handpiece for at least 30 days. Even if you have to pay a rental, it would be worth it. We have found the all-titanium Ti-Max NL400 from NSK (sold by Brasseler USA) to be the lightest of the electric handpieces and the most reasonably priced. Other models to consider are the Electrotorque TLC from KaVo and Optima MX from Bien Air, although both are heavier than the Ti-Max.

Dr. Adams: At least 2 manufacturers have announced a “hybrid” type of handpiece, featuring the torque of electric handpieces and the convenience of air power. Would you please comment on this new technology?
Dr. Miller: The information I have seen on these handpieces certainly indicates that they could be the best of all worlds. However, the 2 companies of which I am aware that have products in this niche have not yet sent us units to evaluate so I can’t really comment on them. So, although they appear very promising, we can’t verify their claims at this time.

Dr. Adams: Lasers always strike a chord with practitioners and patients alike. Is there anything new out there that would make these instruments more appealing?
Dr. Miller: The cordless diode laser, Styla (Zap) sold by Discus Dental is very impressive. It is really easy to operate, has extremely intuitive controls, and the cordless feature speaks for itself. However, it does command a higher price compared to the new corded units on the market. When it comes to corded units, the SoftLase Pro, which is Styla’s sibling also being sold by Discus, is small and attractive. There are other compact units on the market, but no other manufacturer has been brave enough to submit a unit for our perusal.

Dr. Adams: That last comment is intriguing. Are you saying some manufacturers refuse to send you products out of fear that you will find something wrong with them?
Dr. Miller: For some manufacturers, the answer to your question is definitely yes. Our mission is to protect patients by informing dentists. If a product doesn’t live up to its promotional marketing claims, we are going to publicize the glitches along with the triumphs. Sometimes the truth hurts, but that’s the “reality!”

Dr. Adams: As a final question, are you optimistic about the future of dentistry?
Dr. Miller: No doubt about it—dentistry has a bright outlook. Even though I am very aware that many dentists have seen holes in their schedules, I honestly believe that we will bounce back stronger than ever. On the other hand, prudent selection of products, materials, and equipment will always be the sensible mantra.

Dr. Miller received his DDS degree from the University of Maryland School of Dentistry in 1974 and completed a general practice residency at the Veterans Administration Hospital in Houston in 1975. He is a Fellow of the Academy of General Dentistry and is a Fellow and Founding and Accredited Member of the American Academy of Cosmetic Dentistry. He is also a founder of the National Children’s Oral Health Foundation. Also, Dr. Miller is the co-founder, president, and editor-in-chief of REALITY and maintains a dental practice in Houston, Tex. Dr. Miller is on the editorial boards of Practical Procedures and Aesthetic Dentistry and Spectrum, authors a regular column in General Dentistry, and has lectured extensively internationally. He can be reached at mm@realityesthetics.com.

Disclosure: Dr. Miller reports no conflicts of interest.