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First Impressions of APC™ Flash-Free Adhesive:What To Do with the Free Time?by Dr. Mohammad Razavi

Dr. Mohammad

Razavi received

his dental training

at Case Western

Reserve University

– DDS (‘02),

orthodontic certificate (‘05), and MSD (‘05).

Upon completion of his orthodontic training,

he was invited to join the department

as an assistant clinical professor, where

he founded and directed the Skeletal

Anchorage Clinic, and has integrated various

TAD systems into the training program.

He is a member of the craniofacial team at

the Cleveland Clinic Foundation, and has

served as the orthodontist for the Cleveland

Browns. Dr. Razavi is a diplomate of the

American Board of Orthodontists, a Fellow

of the Royal College of Dentists in Canada,

and an ad hoc reviewer for the American

Journal of Orthodontics, and the Journal of

Clinical Orthodontics. Dr. Razavi maintains

a private practice in Ottawa, Canada.

For years we accepted cleaning excess cement around orthodontic brackets as an unavoidable obstacle of orthodontic practice. Fearing the consequences of this excess flash of cement, such as white spot lesions, possible caries upon debonding the brackets, and staining around bracket margins, we employed the use of pre-pasted and color-changing adhesives to improve our ability to remove these bacteria traps. In fact, the obsession to remove all the flash surrounding each and every bracket led to significant inefficiencies during direct bond procedures, as we constantly knocked the bracket away from its ideal position. Yet, we all accepted this annoyance as a part of being a conscientious orthodontic professional, never questioning if there was a way to eliminate this excess cement. How else would we “cement” the brackets onto the enamel surface?

A New Era in BondingWhen my 3M Unitek representative handed me the first five brackets with new APC™ Flash-Free Adhesive for my evaluation, I quickly placed three of them onto extracted teeth. Then I heard her ask me, “What exactly are you doing?” I realized, based on a series of pre-programmed motions that had been engrained into my subconscious, I was running the tip of my scaler around the edges of each bracket, attempting to remove the non-existent excess cement, just as I had been doing for years, up to this day.

Immediately, I was hooked! My mind raced with the implications of incorporating this technology in our practice. Imagine the time saving during a bonding procedure using this new pre-pasted bracket, where the 28 positioned brackets had to be positioned only once. We would no longer repeatedly disturb their position in an attempt to remove the flash. But could we employ our existing bonding protocol and tooth preparation? Would we get the same bond strength?

We started our clinical evaluation of APC Flash-Free adhesive in the last quarter of 2012. I was pleased to see the brackets arrive in individualized packages similar to APC™ II Adhesive. We were able to employ our existing preparation protocols, and simply had to switch the bracket

blisters from our old bonding trays.

It took only a few minutes to adapt to the fact that there is no flash clean-up with the

APC Flash-Free adhesive coated brackets. Of all the products we have introduced into our

office in recent years, APC Flash-Free adhesive had the simplest learning curve by far.

We quickly noticed that patient chair-time in our office was reduced significantly. In our video-taped trials, we consistently had shortened times during the placement and positioning stage of the brackets. This time savings will allow us to reduce our allotted time for a direct full upper and lower bond procedure from 90 minutes to about 70 minutes. A 20 minute saving per patient, over the course of a full day, is a saving of 60-80 minutes.

The shorter initial bonding appointment works toward our goal of improving the patients’ overall experience, especially at the beginning of the treatment. In addition, the requirement for “Doctor Time” is reduced during this appointment, freeing up this resource for more productive use. The patient experience is also enhanced during the course of the treatment, as the adhesive agent does not stain (Figure 1A-E), unlike the excess cement that was often left on teeth with previous luting agents. Moreover, the patients have enjoyed the benefits of a strong, reliable bond strength, since only one of the nearly 200 brackets placed so far had unintentionally debonded.

My entire clinical team was blown away by the potential implications of using APC™ Flash-Free Adhesive as the bonding agent. Team members commented: “Testing them really spoiled me; now even pre-pasted brackets seem like a lot of extra work and time to clean up all the flash.” “Initial placement is great. The first contact between the adhesive and tooth sticks much better when compared to other adhesives.”

In future months we will continue to document the treatment progress of patients we treated with the APC Flash-Free adhesive. We will be able to report bond failure rates, hygiene implications, and debonding and removing the cement. In addition, we will continue to determine the practice management impact of using APC Flash-Free adhesive. Look for future publications for more information on this product, which I feel is certain to revolutionize orthodontic bonding.

1A

1C

1E

1B

1D

Figure 1A-E

Case photos provided by Dr. Mohammad Razavi.

Evaluators Give APC™ Flash-Free Adhesive Rave ReviewsThe APC™ Flash-Free Adhesive Coated Appliance System was evaluated in vivo by orthodontists in North America, Europe, Canada, and Australia.* Overall, 92.5% of the evaluators were satisfied or very satisfied with APC Flash-Free Adhesive.

96% of the evaluators were satisfied with the ease of positioning; 89% were satisfied with the amount of resistance felt while seating the bracket onto the tooth; 95% were satisfied with the total amount of resin on each bracket; 84% mentioned that the bonding time with APC Flash-Free Adhesive was shorter than that with their current bonding system. The current observed bond failure rate is less than 2% within the first 3 months of bonding.

* Participants were doctors willing to bond patients with Clarity™ SL Self-Ligating Brackets or Clarity™ ADVANCED Ceramic Brackets. Although the majority of the evaluators were precoat users (APC™ II or APC™ PLUS Adhesive), some non precoat users were included in this evaluation as well. Evaluators were asked to complete a survey on handling properties such as ease of positioning, resistance while seating brackets, amount of resin and bonding time. They were also asked about their overall satisfaction of this product.

“ APC™ Flash-Free Adhesive Coated Brackets are very easy to work with. Not having to clean up flash is a real time saver during the bonding process. Plus, I have the peace of mind knowing that no flash was left behind.” – Dr. Scott Harper, USA

“ APC™ Flash-Free Adhesive exceeded my expectations. Bracket handling was very easy out of the blister. I was very impressed. There was zero drift. I inspected all corners of every bracket for undercuts, there were none. There was no excess adhesive notable for clean up. I am highly impressed with the amount and consistency of material from bracket to bracket and from one case to another.” – Dr. Joseph Gray, USA

– Dr. Lars Christensen, UK

“

”

Having tried APC™ Flash-Free Adhesive on several patients...

the product appears to take precoated technology not just one, but two or three steps

further. Well done guys!

– Dr. Jackie Berkowitz, USA

“ “

APC™ Flash-Free Adhesive Coated Brackets are easy to maneuver on the tooth, without sliding off. The biggest advantage is the “confidence” knowing no flash is left on tooth. This should increase overall aesthetics and, in my opinion, improve oral hygiene compared to when some flash remains on the tooth.”

”

APC™ Flash-Free Adhesive: A Technical Overviewby Dr. David K. Cinader, Mathazin Aung, Rachel Ugai and Alan Conley

Dr. David K. Cinader received a B.S. Degree in Chemical Engineering from Michigan Tech University in 1994 and a PhD in Chemical Engineering in

1999 from Northwestern University. He joined 3M Unitek Research and Development in September 1999 and has been involved in orthodontic bonding development including Transbond™ Plus Self-Etching Primer, APC™ II, APC™ PLUS and Transbond™ Supreme Low Viscosity Adhesives.

Rachel Ugai received a B.S. Degree in Chemistry from University of California, Irvine in 2002. She joined the 3M Unitek Research and Development in 2007 and

has been involved in many experiments in the laboratory. In 2011, she took part in APC™ Flash-Free Adhesive development team as an Advanced Technical Service Engineer.

Alan F. Conley is a Packaging Engineer for 3M Unitek, having joined the company in 2007. He received his B.S. and M.S. in Packaging

from Michigan State University, where he also researched the dynamics of package distribution systems.

Orthodontic adhesives, including previous APC™ Adhesives, have traditionally been highly-filled pastes descended from dental restoratives. Orthodontists usually consider removal of excess adhesive flash after appliance seating an inherent part of their bonding procedure. If not removed, exposed adhesive flash could discolor or stain. Further, flash removal is a time-consuming and stressful task for many orthodontists as their careful positioning of appliances on teeth, as well as the bond integrity, could be compromised by bumping the appliance. Furthermore, any reduction in the time that the patient spends in cheek retractors enhances comfort.

We recognized flash removal as a persistent pain point and developed a new class of APC adhesive that would not require this step by means of a compressible material and a lightly-filled resin formula.

A 3M SolutionAPC™ Flash-Free Adhesive is a unique composition of a compressible nonwoven mat (a 3M core technology), soaked with a relatively low viscosity adhesive resin (another 3M core technology). When an APC Flash-Free adhesive coated appliance is seated in place on a tooth, the compressible mat lets the resin seep out to fill the space between the appliance bonding base and the tooth.

The surface tension of the low viscosity resin allows it to wet the tooth surface well and form a fillet around the bonding base, instead of forming the irregular clumps of adhesive flash typically produced by paste adhesives. The fillet is typically smooth and similar to a meniscus seen at the interface of a liquid and laboratory glassware. In this application, the size of the fillet is constrained by the precise amount of resin determined for each type of tooth. As a result, APC Flash-Free appliances offer a bonding procedure with no need for the flash removal step so that orthodontists can focus on appliance positioning.

Specially Designed Nonwoven MatThe term “nonwoven” generally applies to fabric-like materials made of long, entangled fibers. Nonwovens differ from woven or knit fabrics in that they are randomly arranged much like a bowl of noodles. Examples of 3M nonwoven brand products range from Filtrete™ filters and Thinsulate™ insulators to Nomad™ doormats and Scotch-Brite™ abrasive pads. Depending on the needs of each application, material properties are typically tailored through selection of fiber composition and processing technique. For APC Flash-Free, the nonwoven material needs to provide a degree of compressibility in the direction (z) of seating the appliance in order to accommodate a range of tooth contours, yet not expand in the direction (x-y) as flash around the bonding base. Additionally, its thickness and loft need to provide similar adhesive bond line thickness to paste adhesives so that orthodontic treatment outcomes will be unaffected.

Mathazin Aung received a B.S. in Physics with Minor in Mathematics from University of Arizona in 2004. She worked in 3M Unitek Manufacturing from

2005-2006 and joined R&D in 2006. She was involved in product development of Clarity™ SL Brackets and Ortholux™ Luminous Curing Lights before joining the APC™ Flash-Free Adhesive project in 2009.

The required nonwoven material was developed internally within 3M and is a web of micron–sized polypropylene fibers, shown close-up in Figure 1. The web is made with 3M’s expertise in web processing so that the fibers are entangled enough so as not to unravel as small pieces of mats are cut and applied to the appliances while still providing the necessary thickness and loft.

The most important job of an adhesive is to provide a reliable bond. In adhesive joint design, well-formed fillets such as seen in Figures 3A, C and E are instrumental in distributing stress and contributing to a reliable bond. One indicator of the bond reliability of an orthodontic adhesive is the bond strength measured in laboratory tests. In Figure 4, the bond strength of APC Flash-Free adhesive is plotted alongside that of well-received Transbond™ XT Adhesive.

It can be seen that APC Flash-Free adhesive compares favorably. Further evidence of APC Flash-Free adhesive’s reliability is recounted in the discussion of customer evaluations (see page 6).

Figure 2A-B: The size and shape of the nonwoven mat is custom designed for each bonding base. The mat is attached to the bracket with a small amount of cured resin.

2A 2B

Figure 4: Bond Strength of APC™ Flash-Free Adhesive vs. Transbond™ XT Adhesive.

APC™ Flash-Free Adhesive Transbond™ XT Adhesive

25.0

30.0

35.0

40.0

20.0

15.0

10.0

5.0

0.0

Mea

n Bo

nd S

treng

th (l

b./b

kt.)

Bond Strength of APC™ Flash-Free vs. Transbond™ XT Adhesive

4

Figure 1: The nonwoven fabric used in APC™ Flash-Free Adhesive consists of randomly oriented, entangled fibers.

1

A Unique Resin DesignThe resin used in APC Flash-Free adhesive is unique among orthodontic adhesives. It is of a much lower viscosity and has a surface tension designed to wet the enamel and form the filleted edge. Additionally, the resin amount is critical to creating a well-formed fillet. Multiple in vitro studies were conducted with doctors to define an acceptable fillet and define the resin loading for each bracket. The results can be seen in Figure 3A-E, in which well-formed fillets are contrasted to irregular edges obtained when bonding paste adhesives.

3A 3B

3C 3D

3E

Figure 3A-E: Images of brackets bonded with APC™ Flash-Free Adhesive (left, 3A, C, E) versus a typical paste adhesive (right, 3B, D).

APC™ Flash-Free Adhesive mats are precisely cut from the web to fit and align with the shape of each appliance’s bonding base. Figure 2A-B shows a mat as assembled on a bonding base. When attached to the appliance, the defined mat configuration helps ensure that the resin reaches the edges of the bonding base, creating a well-formed fillet.

Reprinted from Orthodontic Perspectives Innova Vol. XX No. 1. © 2013, 3M. All rights reserved.

Figure 6: APC™ Flash-Free Adhesive Coated Brackets are held in place by structures that grip the occlusal and gingival edges of the bonding base.

6

Naturally, with any bonding system one is concerned with discoloration/staining of the bonding material. Extrinsic staining occurs mainly as a result of materials dissolved in the water that an adhesive may absorb while in use. APC™ Flash-Free Adhesive has been formulated with very hydrophobic monomers to minimize the amount of material that will be absorbed during treatment. Its performance in resisting the staining effects of common foods and beverages is shown in Figure 5A-H. It is observed that APC Flash-Free adhesive performs as well as APC™ II Adhesive, which is generally considered to be a very stain-resistant adhesive.

7A 7B 7C

Figure 7A-C: Suggested method for bracket removal. Open lid with peeling motion. Grasp bracket tie-wings and gently rock side to side. Remove from blister by lifting.

5A

5E

5B

5F

5C

5G

5D

5H

Figure 5A-H: Bonded samples exposed to curry broth for 240 minutes at 40°C. Figure 5A-D: APC™ Flash-Free Adhesive; Figure 5E-H: APC™ II Adhesive.

An Integrated Delivery SystemDelivering a precoated bracket to the customer is a unique challenge. A new package was designed which minimally contacts the adhesive to ensure that the correct amount of resin remains on the bracket. It is shown in Figure 6 and consists of two tapered bars which touch the bracket bases on the occlusal and gingival edges only.

The package was designed to retain the same ease of use that customers have come to expect from the APC™ Adhesive System blister. Brackets may be gripped in the same manner in which users are accustomed and pulled in a direction perpendicular to the blister bottom. Alternatively, a gentle rocking motion in the mesial-distal directions can be employed to ease removal (Figure 7A-C).

Figure 8: Inventory Dispensing System including back-up storage unit, stackable dispensing drawers, and set-up trays.

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Ease of removal from the package must be balanced against surviving the rigors of shipping through a global supply chain. To this end, a comprehensive test was conducted to ensure that the bracket would not come loose during exposure to prolonged harsh vibration and multiple sharp shocks.

Ease of use extends to the manner in which the product fits into the orthodontic practice. The original APC™ Adhesive System was designed to provide a dedicated “inventory to chairside” solution. The APC Adhesive Inventory Dispensing System (IDS) includes a back-up storage unit, stackable dispensing drawers, and set-up trays (Figure 8). The outside of the APC Flash-Free adhesive blister package is identical to that of APC PLUS adhesive packaging so that it fits into the existing IDS components, providing both new and existing users the convenience of the APC system.

ConclusionThe APC Flash-Free adhesive coated appliance system combines multiple 3M core technologies to create an innovative bonding system. The resin, nonwoven mat, and unique package come together to deliver an easy to use solution to flash clean-up.

Dr. Mohammad

Razavi received

his dental training

at Case Western

Reserve University

– DDS (‘02),

orthodontic certificate (‘05), and MSD (‘05).

Upon completion of his orthodontic training,

he was invited to join the department

as an assistant clinical professor, where

he founded and directed the Skeletal

Anchorage Clinic, and has integrated various

TAD systems into the training program.

He served as the orthodontist for

the Cleveland Browns, and is currently a

member of the clinical staff at the University

of Alberta.

Dr. Razavi is a diplomate of the American

Board of Orthodontists, a Fellow of the

Royal College of Dentists in Canada, and

an ad hoc reviewer for the American

Journal of Orthodontics, and the Journal of

Clinical Orthodontics.

Dr. Razavi maintains a private practice in

Ottawa, Canada.

My journey to using the latest edition of APC™ Adhesive started in the early days of my orthodontic training, as I spent a significant part of clinic time buttering cement onto brackets. It was during three-hour initial bonding appointments that I realized that most patients did not appreciate sitting in a dental chair with their cheeks stretched and jaws apart for long periods of time. It became obvious to me that if I wanted to have a thriving orthodontic practice, I would have to incorporate products and techniques to make this “initial bonding” appointment a more pleasant one.

Picking the BracketsThe first step I took was the elimination of “sorting” through large containers of brackets, with the risk of getting the wrong bracket, and the need for the staff member to butter the adhesive onto the back of the bracket. I started using APC™ Adhesive Coated Brackets once I transitioned to a small boutique practice at the Cleveland Clinic, and I carried those efficiencies to the larger practice I purchased in 2009.

We used the pre-packaged brackets and their set-up tray as a marketing tool, since patients could see the level of quality control that went into their treatment. Patients noted that the brackets were individually sealed in blisters and specifically set aside for the treatment of their specific malocclusion.1 These pre-coated brackets were a necessity when we progressed to the 3M Unitek Variable Prescription Orthodontics (VPO) system, as the blister colors allowed for the identification of the specific bracket prescription.

All the while we enjoyed the benefits of the APC Adhesive System, we never questioned how the patient and doctor experiences could be even further improved during the bonding appointment. Lucky for us, the engineers at 3M Unitek were asking this question.

The APC™ Flash-Free Adhesive ExperienceIn the latter months of 2012, I was asked to trial a new line of APC™ Flash-Free Adhesive pre-coated brackets. At first, I could not comprehend the concept of a flash-free adhesive. In my defense, the only orthodontic adhesives that I had been exposed to were derivatives of dental restorative materials, requiring flash clean-up to avoid staining and potential decalcification and caries.

The APC Flash-Free Adhesive system incorporates a nonwoven mat, soaked with a relatively low viscosity adhesive resin, which is attached to the bracket. The nonwoven material allows for a slight compressibility without bounce-back as the bracket is pressed onto the tooth surface. The surface tension of the resin allows for wetting the tooth surface and forming a fillet around the bracket base, instead of extrusion of paste adhesive with traditional bonding agents.2 Once the mat is compressed, a small amount of resin seeps out to fill the space between the bracket base and the tooth (Figure 1).

APC™ Flash-Free Adhesive: The Game Changer in Orthodontic Bonding – Part IIby Dr. Mohammad Razavi

Figure 1: Bracket bonded with APC™ Flash-Free Adhesive.

1

One challenge with this product is becoming accustomed to this compressibility and seepage of resin, as we are trained to press firmly on a bracket to extrude all the excessive paste adhesive and allow for a uniform layer of resin between the bracket and tooth. APC™ Flash-Free Adhesive simply requires a slight press onto the tooth surface during the final positioning of the bracket, only enough to extrude a small amount of resin around the edges of the bracket to seal this tooth-bracket interface.

Once my clinical team and I adapted to the compressibility of the new adhesive, we quickly became “hooked” on APC Flash-Free Adhesive, especially since we documented a significant reduction in bonding time. It has also been pointed out by one of my clinical assistants that as we are no longer pressing brackets firmly against teeth, we are not causing discomfort for patients in the attempt to extrude all the paste adhesive (as we had done for many years up to now).

Figure 2 displays a patient in month 13 of a 21 month comprehensive treatment for the correction of a Class II Division 1 malocclusion, using Forsus™ Class II Correctors. The patient displays exceptional hygiene despite the presence of chain modules. In my opinion, the fillet of resin that extrudes around the bracket is a uniform and smooth surface that does not allow for plaque adhesion at the same level as paste adhesives.

The main reason for adhesive staining is the uptake of colored solutes by the dental materials. The APC Flash Free Adhesive resin is a hydrophobic monomer that significantly reduces stain uptake compared to hydrophilic resins. To date, patients in treatment in excess of 14 months have not demonstrated any signs of staining, discoloration or decalcification around the bracket margins.

Documented Time SavingsThe reduction in chair-time we and patients enjoy has allowed us to reduce our bonding appointments to 70 minutes, as flash clean up, the rate-limiting step during the bonding appointment, has been eliminated. In the past, the schedule allotted for 90 minutes for a full direct bond appointment (Chart 1). This newly “found” time, has allowed for an additional 60-80 minutes per day, which we routinely use for an additional bonding appointment, or appliance deliveries; procedures that allow for increased productivity during the same workdays. We look forward to further improving our clinical times, with the future introduction of APC Flash-Free Adhesive on molar tubes.

2A

2B 2C

Figure 2A-C: Patient bonded with APC™ Flash-Free Adhesive coated brackets after 14 months of treatment.

Chart 1: Scheduled full case direct bonding appointment times in Dr. Razavi's office.

Reducing the amount of time for the initial bonding appointment has not only improved our clinical productivity, but also has improved the patients’ overall clinical experience, as the amount of time in a dental chair with cheek retractors is significantly reduced. In fact, a closer look at bonding time reveals that patients were in a cheek retractor for a range of only 12-33 minutes during the entire bonding appointment.

Bond Strength and ReliabilityOur analysis of bond strength and bond reliability has been shocking, as we have documented the lowest bond failure rates since starting to track this statistic in 2010. Over the last 14 months, we have had eight bracket failures, five of which were on the same patient,

But how would this resin stand the test of time? Would it discolor? Would it attract cariogenic bacterial plaque leading to white spot lesions?

Reprinted from Orthodontic Perspectives Innova Vol. XX No. 2. © 2013, 3M. All rights reserved.

and are all likely due to variables other than the APC™ Flash-Free Adhesive. To date we have a record low bond failure rate of 0.75%, three bracket failures out of a total 398 brackets bonded with APC Flash-Free Adhesive. This average is significantly less than our previous bond failure rate of 2.91% with the APC™ II Adhesive System, averaged over the past two years (Chart 2).

We have only debonded a handful of patients so far, but we have noticed yet another benefit of this adhesive system. Cement removal from the tooth no longer requires the use of a high-speed handpiece and firm pressure on the tooth. Often, cleaning cement using a high-speed handpiece results in patient discomfort and sensitivity, not to mention the inherent fear of dental drills most orthodontic patients possess.

Chart 2: Bond Failure Rate tracked by Dr. Razavi. Rate for APC™ Flash-Free Adhesive System tracked over 14 months.

With APC Flash-Free Adhesive, cement removal can be simply accomplished using a slow-speed handpiece. We routinely use an 8-fluted carbide bur to clean the adhesive during the debond appointment, hence improving the patient experience during the last appointment of active treatment. Let’s not forget that a happy patient is a great referral source for future patients. The carbide

bur pulverizes the nonwoven mat with minimal effort and pressure (Figure 3), making cement clean up a shorter procedure, and one that in most North American jurisdictions is a delegable procedure.

Figure 3: Debonding of APC™ Flash-Free Adhesive bracket using 8-fluted carbide bur.

3

ConclusionOver the past eight years, I’ve had the pleasure to practice in the most exciting times of a new orthodontic revolution. The advances of self-ligating brackets, TADs, lasers, and compliance-free Class II Correctors have all helped our patients receive a more pleasant treatment experience. With all the combined benefits of these appliances, in my opinion, none has single-handedly improved the patient experience to the extent that APC Flash-Free Adhesive has done in our practice to date.

And as a final note, who among us would not love to eliminate hygiene concerns, staining of teeth and possible decay at the bracket-tooth interface? There will always be patients who will demonstrate poor hygiene compliance during orthodontic treatment; let’s give them a fighting chance by eliminating the clumps of adhesive left behind even by the best of us.

Case photos provided by Dr. Mohammad Razavi.

References1. Christensen LR. The APC™ Adhesive Coated Appliance System. Orthodontic

Perspectives. 2006; Volume XIII No. 2:15-17.

2. Cinader DK, Aung M, Ugai R, Conley A. APC™ Flash-Free Adhesive: A Technical Overview. Orthodontic Perspectives, Vol. XX, No.1:7-9

Technical lab STUDY

APC™ Flash-Free Adhesive: What of the Excess?by Peiwang Zhu, Kevin nordine, and David K. cinader, 3M Unitek

Dr. Peiwang Zhu received a b.S. Degree in chemical engineering in 1991 and a PhD in applied chemistry in

1996 from beijing institute of Technology. he worked in the 3M central Research laboratory for five years, mainly on Pressure Sensitive adhesives, before joining 3M Unitek R&D in 2010. he has been focused on dental adhesives since then.

Kevin nordine has been a laboratory technician at 3M Unitek for four years. he brings a 30-year background

from the motion picture and film industry and has considerable experience with digital imaging, color measurement, and image analysis done in conjunction with technical laboratory work. he has been involved in many of the product development projects at 3M Unitek.

Dr. David K. cinader received a b.S. Degree in chemical engineering from Michigan Tech University in 1994 and

a PhD in chemical engineering in 1999 from northwestern University. he joined 3M Unitek Research and Development in September 1999 and has been involved in orthodontic bonding development including Transbond™ Plus Self-etching Primer, aPc™ ii, aPc™ PlUS and Transbond™ Supreme low Viscosity adhesives.

IntroductionaPc™ Flash-Free adhesive was launched in 2013 by 3M Unitek. a distinct feature of this product is that there is no need to clean the flash from the periphery when a bracket pre-coated with this adhesive is bonded onto teeth. The adhesive resin actually extends about 6-8 thousandths of an inch beyond the bracket edge when it is set in place, but removal of the resin is not necessary before cure. in fact, removal of the resin fillet is not recommended.

how will this fillet affect treatment? Will the fillet remain throughout the course of treatment? Will it protect the underlying enamel? To answer these questions, a set of experiments was designed and conducted, which are outlined in this article.

The results show that the resin does in fact resist toothbrush abrasion for up to a simulated 3.3 years. in addition, the resin protects underlying enamel from exposure to lactic acid, similar to the environment found in the plaque of the oral environment. Therefore, aPc Flash-Free adhesive not only reduces bonding time, it has the additional beneficial effect of being durable enough to protect the enamel during the course of treatment.

Abrasion ResistanceUpon viewing aPc Flash-Free adhesive, orthodontists have been curious about the durability of the excess resin contained in the fillet. Will it brush away during the course of treatment to expose the enamel beneath, or will it remain to serve as a protective layer?

We first approached this by examining the effect of toothbrush abrasion on exposed resin. Specifically, we constructed 15×1 mm disks of cured resin and applied strips of tape to expose only a 5 mm strip. We then subjected it to repeated “tooth brushing” with a popular toothpaste under a load of 200 grams. after regimens of 6,000, 12,000, and 20,000 strokes simulating 1, 2, and 3.3 years of brushing, we employed a profilometer to measure the amount of resin worn away. Figure 1 shows the results in terms of trough depth versus position on the sample.

Figure 1: Measurements of resin wear by profilometry.

1

it can be seen that, over the course of 3.3 years, a fairly lengthy treatment, only 2.2 microns of resin is worn away. For reference, a human hair is 17 to 180 microns in diameter. another comparison is that an orthodontic bracket archwire slot is approximately 500 microns across.

having established this baseline behavior, we then wondered what happens with the complicating effect of a bracket on the tooth. To answer this, we bonded an aPc™ Flash-Free adhesive coated bracket to a bovine tooth and photographed the resin fillet with a microscope. again, we subjected the sample to tooth brushing for 6,000, 12,000, and 20,000 strokes. after each regimen, we photographed the sample at the same position under the microscope. The outline of the resin fillet as well as that of the bracket bonding base was traced following each tooth brushing regimen. Figure 2a-b shows the resin fillet profile, before and after 20,000 tooth brushing strokes.

Therefore, for a typical aPc Flash-Free adhesive bonding, where the fillet is in the range of 6-8 mils, it can be expected that about 4-6 mils of resin covering the enamel surrounding the aPc Flash-Free adhesive pre-coated bracket will remain after a typical two-year orthodontic treatment.

Protection from Acidic Environmentsafter seeing the durability of the resin fillet, it is natural to wonder what protection it will offer to the enamel. To this end, we subjected bovine teeth bonded with aPc Flash-Free adhesive pre-coated brackets to immersion in an acidic environment, and measured the effects (Alsaffar et al., Pediatric Dentistry, 2011, vol.33, p.491).

Ten potted bovine teeth were cleaned with a pumice slurry and rinsed with tap water. They were then etched and bonded with aPc Flash-Free adhesive pre-coated brackets according to the recommended procedure.

Then, we immersed them in a lactic acid aqueous solution of ph 5.1. after immersion in this acid solution at 37°c for 21 days, we rinsed the teeth with tap water, and bisected the bracket and tooth in half vertically. We then placed the sliced halves cut-face-down and potted them in epoxy for polishing purposes. Figure 4 displays a specimen thus prepared.

2A

2B

Figure 2A-B: a) Resin fillet prior to 20,000 tooth brushing strokes b) Resin fillet after 20,000 tooth brushing strokes.

Qualitatively, it can be seen that the fillet recedes. To quantify the effect, we compared the two pictures. in Figure 3, the yellow highlighted area represents the resin retaining area after 20,000 strokes of tooth brushing; and the green highlighted area is that previously covered with resin. The ratio of the remaining resin area to the total resin area before wearing was 77% (calculated by counting the pixels of the respective areas). considering the resin extends to 420 µm beyond the bracket edge for this particular sample, the fillet boundary was receding at a rate of 32 µm/year.

We chose a cross-sectional micro-hardness technique to evaluate the extent of enamel demineralization, if any, underneath the coating. We made measurements underneath the bracket at 0.5 mm in from the resin margin and exposed area away from the aPc Flash-Free adhesive margin line (refer to Figure 5 for those locations).

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Figure 5: cross-sectional micro-hardness measurement of bovine tooth bonded with aPc™ Flash-Free adhesive coated bracket.

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Figure 3: illustration of resin fillet before and after tooth brushing, used to calculate coverage.

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Figure 4: bovine tooth bonded with aPc™ Flash-Free adhesive coated bracket, sliced and potted in epoxy.

Reprinted from Orthodontic Perspectives Innova Vol. XX no. 2. © 2013, 3M. all rights reserved.

however, the polishing quality of these samples was not as good as desired. The tooth enamel in the exposed area looked soft, causing the enamel material to be shed during polishing. consequently, we had difficulty reading the indentation. To overcome this, we adopted a technique called “difference matting” commonly used in adobe® Photoshop® software. Figures 6a-c illustrate the technique in this study. Picture 6a is an enamel surface taken before the indentation was made. another picture (6b) was taken after the indentation, illustrating the difficulty in distinguishing the indentation on this rough surface. Using the difference matting technique in Photoshop, we subtracted the background (6a) from (6b) to obtain a better image (6c), clearly showing the indentation.

ConclusionsThe experiments described above demonstrate that the aPc Flash-Free adhesive resin fillet is durable enough to survive at least 3.3 years of tooth brushing in typical orthodontic treatment. Furthermore, the resin fillet has the benefit of protecting the enamel underneath from acid attack of the sort encountered in the oral environment.

Disclaimer: This study is NOT intended to deduce any inference about the clinical outcome on the white spot lesion beyond the resin covered areas. This study only shows that the extruded resin at the edges is durable during treatment and that it does protect the enamel underneath against the acid challenge by acting as a barrier and seal against any micro leakage.

among ten samples, two brackets were lost during the bisecting step, leaving eight samples for us to measure. Figure 7 presents the results. in the exposed area, we observe demineralization as deep as 200-250 µm. Under the bracket and under aPc Flash-Free adhesive covered areas, the enamel remained intact after 21 days of acid challenge. These results confirm that the resin of the aPc Flash-Free adhesive protects the covered enamel against acid.

6A

6B

6C

Figure 6A-C: a) Poor enamel surface. b) indent on poor surface. c) Difference matting reveals the indent.

7

Figure 7: Demineralization comparison of protected and unprotected tooth enamel in acid aqueous.