July, 2003
About Orthodontic Bonding, An Interveiw with Dr. Michael Swartz
Dr. Larry White
Dr. Michael Swartz probably knows more about dental bonding from research, manufacturer and clinical perspectives than anyone in the world. His experiences provide orthodontic clinicians with the best information available regarding the use of bonded appliances. The Orthodontic CYBERjournal believes Dr. Swartz’s comments will help its readers achieve a new and possibly heretofore misunderstood appreciation about this intricate task.
You have been involved with dental bonding from its inception, e.g., as a chemist, then dentist and finally as an orthodontist. Could you fill us in with some of those early experiences and people you worked with?
Dr. Swartz: I first got involved with dental materials in 1964 while working on a research project for the National Institute of Dental Research. NIDR funded three companies to try and develop an adhesive restorative material. While this project failed to develop such a restorative material, it was during this time period that I worked on the first applications of enamel etching and bonding and on developing one of the first composite restorative materials. I worked for five years under contract with Johnson and Johnson to develop the first composite restorative material, Adaptec. I also worked with the U.S. Public Health Service in the clinical trials of the first pit and fissure sealants. It was during these years, 1964 – 1971, before going to dental school that I got to know and work with many of the people that pioneered the way to the materials and procedures that we enjoy today. For example, Dr. Ralph Phillips at the Univ. of Indiana, Dr. Ray Bowen, National Bureau of Standards (first proposed BIS-GMA resin), Dr. Michael Bounocore (first to propose etching enamel with phosphoric acid).
How have orthodontic bonding materials evolved over the years?
Dr. Swartz: Well in the bigger picture, little has changed since the first successful bonding materials in 1974-75. We now have more hydrophilic sealant/primer materials, which are an improvement. In the United States, nearly 80% of the offices are using a light initiated resin material. While these are definitely improved materials, it is interesting to note that in 1974 we were using ultra violet lights to cure light initiated resins. Some may recall Nuva Seal and Nuva Fil as light cured resins systems that were being used to bond brackets. The basic acrylic resin chemistry has changed little since the mid ‘70s.
What do you see as the most important limitations to orthodontic bonding materials currently?
Dr. Swartz: The orthodontist! Seriously Larry, the bonding materials that we have had for many years are reliable and relatively easy to use. The problem has been and will continue to be that we are working in a fairly hostile environment, the mouth. We have to contend with mucous, the pellicle, tight muscles, poor access, etc. – all on a squirming youngster. In my opinion, the technique and the individual variables of enamel will always be the major issue, not the bonding materials.
Do we need to develop stronger bonds between brackets and enamel?
Dr. Swartz: No, definitely not! The bond to etched enamel is already strong enough to fracture enamel. Witness the numerous enamel fractures that occurred with a ceramic bracket in the mid ‘80s when the bond at the resin/bracket was increased.
Because of the great variability in enamel solubility and etch patterns between individuals and between teeth, we cannot design a specific failure point at the enamel interface. We must therefore design the failure to occur at the bracket/resin interface, a cohesive failure. The mesh base, which has now been around for 28 years, has been very successful at retaining brackets well and yet not causing enamel damage. When debonding or when high loads are placed on the bracket, the resin fractures at the surface of the mesh, as a safety mechanism so that we do not overly stress the enamel.
Because of the brittle nature of enamel, the individual enamel variations and need to have an enamel safe failure mode, we should not expect to eliminate all bond failures. A failure rate of less than 5% appears to be practical and doable. Unfortunately the national average appears to be significantly higher, perhaps more than 15%. When an office experiences excessively high failure rates, the natural tendency is to search for a stronger bonding material when the cause for these excessive failures is not a weak material but the bonding techniques. The best of the bonding materials (not necessarily the one with the highest in vitro bond strengths) will not compensate for poor or inconsistent procedures.
I once read a remark by you that clinicians can clean the enamel sufficiently prior to bonding with acid etching alone and don’t need to clean the surfaces with pumice. Do you still advocate that?
Dr. Swartz: I am an advocate that the pumice prophy, as a routine procedure prior to etching, is a waste of time.
The primary purpose of the pumice prophy prior to bonding is to remove the pellicle. It does this but so does phosphoric acid. If fact, phosphoric acid may do a much better job at removing the pellicle than the pumice prophy. Contact angle measurement of surface tension seems to indicate that the pellicle is removed in about the first five seconds of exposure to phosphoric acid.
It is also interesting that when we do perform a pumice prophy, with the intended purpose of removing the pellicle, that we do not first isolate the teeth and then following the prophy allow the patient to rinse out and allow the saliva to contact the teeth. The pellicle can almost immediately begin to redeposit on the cleaned teeth upon contact with the mucous. So for most, the pumice prophy might make you feel better but is likely doing little for its intended purpose. I have not routinely done a pumice prophy prior to bonding since 1980.
You can also find information on this in our literature, for example: Effect of pumice prophylaxis on the bond strength of orthodontic brackets. Lindauer, S.J., Browning, H., Shroff, B., Marshall, F., Anderson, R.H.B., Moon, P.C. Am J Orthod Dentofac Orthop 1997;111:599-605
Do you prefer auto-cure or light-cure bonding composites? Why?
Dr. Swartz: My personal preference is for a no-mix self cure for metal brackets and a light initiated resin for ceramic or plastic brackets. I prefer the no-mix material (System 1+) for its speed and its ease of excess clean up. However, it must be noted by your readers that I bond using a four-handed procedure with a chair-side assistant. I prefer to use a light initiated material for clear brackets as the light initiated systems will discolor significantly less than a self curing material.
I do however firmly believe that the selection criteria of a bonding material are highly individual and should be based on the individuals preferred handling properties and the desired working time. When working with a thin layer of bonding material (the glue line), one should not select a material based primarily on reported laboratory bond strengths. Within a reasonable range, all of the resin based bonding materials, from the weakest to the very strong, will provide more than adequate bracket bond strengths. If the majority of bond failures are caused by operator technique errors, as it seems to be, then this constant search for higher and higher bond strengths will not resolve technique problems. It may be interesting to note that the material that I prefer to use, System 1+, may test as one of the lowest in bonding strength and yet I enjoy a 3% failure rate bonding second molar to second molar.
L.E.D. lights seem to be one of the newest developments in bonding protocol. What advantages and limitations do these have vis a vis the other light sources.
Dr. Swartz: The light initiated bonding resins (acrylics) have a chemical (most often camphorquinone) that absorbs the light in a specific, narrow wave length and then converts that energy to a chemical reaction that initiates the polymerization of the resin. This wave length is at the lower range of visible light, down close to ultra violet. The older quartz-halogen lights put out light through the whole spectra from ultra violet all the way up to infra red. Most of that light is wasted as it is only the wave length of curing light that matches the absorption wave length that is usable. The new L.E.D. (light emitting diode) lights have an output wave length that very closely matches the absorption wave length so there is no wasted light, which is energy. Therefore the L.E.D. lights can initiate the polymerization of the bonding materials, every bit as efficiently as the more power hungry halogen light but with much less wasted energy. They are portable, rechargeable and less costly. I have been using the Demetron L.E.D. light for about six months and I am very pleased with its performance and handling. For those using light initiated bonding resins, an L.E.D. light is certainly the way to go.
Although indirect bonding offers orthodontists more bonding accuracy, only about 10% of them use an indirect technique. Why is that?
Dr. Swartz: Various forms of indirect bonding have been around since 1974. I was the one that proposed the indirect procedure using the caramel candy, Sugar Daddy. Yes, you are correct that in all those years indirect bonding has not increased in its use and remains at about 10-15% of all bondings. The primary justification for an indirect procedure should be for better bracket placement. To be successful it must therefore be able to transfer, from the laboratory to the teeth, with a high degree of the accuracy. It must also do so with a bond failure rate at least equal to that of direct bonding.
I suspect that the popularity of indirect bonding has not increased for several reasons. Indirect bonding is best used on cases with a full dentition and good clinical crown lengths. Many of our patients are young adults where we are bonding at a time when not all permanent teeth have erupted. When an indirect novice starts out he or she must overcome the nuances of the technique. The problem with this is that when a failure occurs, it usually involves multiple brackets and teeth. It only takes one or two of these early failures to sour one on the procedure. If there are minor errors along the way the bracket position can be thrown off (usually seen as brackets being bonded far to the occlusal/incisal) on several teeth. This of course defeats the objective of indirect and can also turn one off to the whole idea. Conversely, those that have managed to master the procedures are true converts.
What is your experience and expectations regarding the self-etchant sealants?
Dr. Swartz: The self-etch sealants are relatively new to enamel bonding but have been around for many years for dentin bonding. There mechanism relies more on a chemical bond (which is necessary for adhering to dentin) and less on a penetration of resin into etched enamel. Previous attempts to accomplish long term enamel bonds with a minimal etch or chemical bonds have eventually failed. Only time will tell with these newer materials. My concern is that a phosphoric acid etch (of about 30 seconds) is sufficient to neutralize the individual differences in enamel acid solubility. That is to say, a conventional etch takes care of that portion of the population whose enamel is more acid resistant. A minimal etch may be sufficient for many but is inadequate for those with more acid resistant enamel. The difficulty with a more chemical bond is saliva. Saliva is a powerful surfactant. It can migrate into a bond interface and gradually break the bonds. We can see a degradation of chemical bonds over time and we have no good means to test for this in the laboratory.
What would be your requirements for an ideal composite? What are our chances of having this material in the near future?
Dr. Swartz: An ideal orthodontic bonding material would; set on demand, adhere to all variations of enamel without etching, adhere to porcelain, gold, stainless steel, titanium and instantly dissolve in papaya juice. It would be non-toxic and would release free fluoride ions for two years.
What are the chances of ever developing a material with some of these attributes? Slim to none! I may very well be short sighted on this, but I do not see that the present materials have any great deficiencies. They work wonderfully under adverse conditions. As you may have detected in this interview, I firmly believe that most bond failures result from technique errors not material failure.
What have you found to cause most bond failures?
Dr. Swartz: The likely cause of most technique (doctor/staff) induced bond failures is moving the bracket while the resin is beginning to polymerize. This occurs even with the light initiated systems. In fact it may be occurring more often with the light initiated materials. We have seen, in the U.S., a great increase in the use of these light initiated materials over self-curing systems. One possible reason for this increase is the use of auxiliaries to place the brackets. The doctor then re-positions the brackets followed by exposure to the curing light. If any time has elapsed from the time the bracket was placed until it is re-positioned, it is likely that the bonding resin has already begun to polymerize just from the ambient light or the dental operatory light. Moving the bracket during the resins initial gel stage can disrupt the formation of the polymer chains causing a weakening of the cohesive strength of the material.
What is the best advice you could give orthodontists who want to improve their bonding success?
Dr. Swartz: Stop searching for the magic adhesive, it doesn’t exist. Spend the time on developing a controlled, routine, reliable bonding procedure.
Could you briefly describe your bonding technique for our readers?
Dr. Swartz: Yes, briefly:
- Place Dri-Angles in the buccal vestibule and place the cheek retractor.
- Etch for 30 seconds.
- Rinse for 5 seconds per tooth and dry.
- Apply the unfilled resin sealant/primer.
- Place the bracket, press it firmly, remove the excess resin, and re-position the bracket.
I notice that you use the words “light initiated” rather than light cured. Why is that?
Dr. Swartz: It is a commonly held belief that curing lights instantly or nearly instantly cure (polymerize) the bonding resins. This may be true under clear brackets but is not the case under metal brackets. A good portion of the polymerization of acrylic resins is via a chain reaction of one activated molecule activating its neighbor and so on. The light is absorbed by the camphorquinone when initiates this chain reaction. Since the light does not fully penetrate all the way under metal brackets, much of the polymerization, after the initial exposure to the light, still remains. At the time that we usually place our initial archwire and dismiss the patient, the resin is only about 50% of its total strength (50% polymerized), under metal brackets. Under metal brackets, the light source initiates the polymerization, it does not fully cure the material. This is significant, as many believe that the bonding resin is fully set and they can go ahead and apply force.
Dr. Swartz, I and readers of Orthodontic CYBERjournal thank you for these historical, practical and clinically useful comments about bonding and hope they result in all of us having greater success in the near future with this important feature of our professional lives.
Interview by:
Dr. Larry White
