Dr. Michael L. Swartz has spent more than 38 years in the dental field as a dental technician, dental materials research chemist, general practitioner, director of research and development at Ormco, practicing orthodontist and worldwide lecturer. He has been instrumental in the development of composite restorative materials, pit and fissure sealants, enamel bonding and orthodontic bonding applications. After earning his D.D.S. from the University of Southern California School of Dentistry, Dr. Swartz maintained a private, general dental practice and served as the director of research and development at Ormco. Later he returned to school at the University of California, San Francisco to obtain his postdoctorate orthodontic certificate and then opened a private practice in Encino, California. Dr. Swartz is currently director of clinical affairs at Ormco, conducting numerous continuing education programs. He has given over 400 presentations around the world and been published extensively in both clinical and research journals.

As people all over the world are putting greater emphasis on self-improvement, and with recent advances in orthodontic technology and aesthetic materials, more adults are seeking orthodontic treatment. In fact, out of the 1.3 million people who begin orthodontic treatment each year, around 25% are over 18 years old.¹ As a result, orthodontists are faced with the challenge of bonding to porcelain restorations (crowns, veneers) (Figure 1). This has presented quite a challenge in the past, but using the proper materials and following the necessary procedure can ensure a reliable bond.

Essentially, there are two viable options when bonding to porcelain:
(1) bond it mechanically by etching the porcelain with hydrofluoric acid or
(2) bond it chemically using a silane coupling agent.

The disadvantages of a hydrofluoric acid etch are that it involves using a potentially dangerous acid and it creates a porous, roughened surface in the porcelain, much like etched enamel.

While you can obtain an excellent bond to etched porcelain, this process removes the outer glaze, which is extremely difficult to regain after treatment. For this reason, I recommend using a silane coupling agent to obtain a chemical bond. This involves following a meticulous procedure; however, it is a much safer method and provides bond strengths comparable to a hydrofluoric acid-etched surface without destroying the porcelain glaze (see Literature Review).

In dentistry, silane coupling agents are provided in the form of porcelain primers. There are various types of porcelain primers on the market today, each consisting of a different chemistry. Ormco’s Porcelain Primer consists of ethanol and a silane coupling agent, which chemically unites the silicon in the porcelain to the acrylic bonding material used. Because using a porcelain primer involves meticulously following a specific procedure, I am providing this procedure in detail in the following text, as well as other information I have found useful.

Instructions*

1. Obtain isolation and saliva control for the porcelain crown to be bonded.
2. Apply a liquid phosphoric acid solution to the glazed porcelain surface (Figure 2). (Because the acid must react with the silane in the Porcelain Primer at the surface of the porcelain, it is important to use a liquid solution rather than a gel. The gel form prevents Porcelain Primer from reaching and reacting with the porcelain surface.) Once applied, the phosphoric acid will clean (remove any biofilm present) and acidify the surface. Do not rinse off the acid! The acid solution must remain on the porcelain surface in order to react with the Porcelain Primer.
3. In the presence of the acid, apply the Porcelain Primer solution to the porcelain surface using a fresh cotton pellet. (Figure 3.) The Primer may be applied immediately after the phosphoric acid solution. Apply a second coating of Porcelain Primer with another fresh cotton pellet. Leave the combined solution of acid and Primer undisturbed on the porcelain surface for one minute. Note: Porcelain Primer has a one-year shelf life. Do not use after the expiration date because the material can polymerize over time and become ineffective. Keep the bottle tightly capped when not in use.
4. After one minute, thoroughly rinse and dry the porcelain surface (Figure 4). Maintain isolation and saliva control.
5. The porcelain surface has now been coated with a mono-molecular layer of acrylic that can be bonded with any orthodontic bonding resin. First apply the sealant/primer to the prepared porcelain crown and then apply the bracket and bonding paste (all according to the manufacturer’s instructions) (Figure 5).

Debonding Considerations

Care must be taken when removing bonded attachments from porcelain. Porcelain is a brittle material and the chemical bond strengths obtained with Porcelain Primer can be very high. When debonding metal and plastic brackets, the best method to use is a peel-type force, which is applied by distorting the bracket. With a twin-type bracket, gently squeeze the bracket wings together with a plier. This will separate the bracket from the adhesive underneath, leaving an adhesive layer on the porcelain crown, which is then removed with a finishing bur, sanding discs and polishing cups.

Take additional care when removing a ceramic bracket that has been bonded to porcelain. If the force necessary to debond a ceramic bracket is high, it can cause a fracture in the porcelain crown or veneer. To avoid stressing the porcelain, I recommend grinding down the ceramic bracket with a diamond bur and then polishing any remaining bonding material.

Conclusion

As more adults seek orthodontic treatment, we are often faced with the challenge of bonding to porcelain crowns and veneers. Though this requires following a detailed procedure and using both safe and effective bonding materials, the result is a reliable bond that does not compromise the integrity of the porcelain surface.

A Review of the Literature

An in-vitro study² comparing bond strengths of glazed and unglazed porcelain samples found no statistical difference in the bond strengths between the glazed and unglazed specimens when bonding with System 1®+ and Porcelain Primer (Ormco).

Another study measured the shear/peel bond strength (using a silane primer) to 100 glazed and 100 stone-ground porcelain specimens.³ Bond strengths were the same or greater with glazed surfaces but the incidence of porcelain fractures were greater with deglazed samples (71% vs. 36%).

In 1996, Zachrisson reported on laboratory bond strengths to porcelain.⁴ The specimens were sandblasted and then treated with either a silane coupling agent (i.e., Porcelain Primer) or etched with hydrofluoric acid. The sandblasted and silane treated samples had a mean bond strength of 11.6 MPa (S.D. 2.9) and the sandblasted and hydrofluoric acid etched samples had a mean of 11.5 MPa (S.D. 2.8). The conclusion stated that there was no significant difference in bond strength between the two.

More recently, an in-vitro study⁵ compared porcelain samples that were sandblasted and silane treated, silane treated only, etched with hydrofluoric acid or combined hydrofluoric acid etch with silane treatment. They concluded that “...the bond strengths for silane alone and for silane with sandblasting were not statistically different....The use of silanes without mechanically removing the glaze from the porcelain surface results in the least damage to the porcelain and still demonstrates acceptable bond strengths.... Clinically, the method of choice will probably be the one that provides a sufficient bond without having to roughen the porcelain surface.”

*Instructions provided in this article are for porcelain bonding using Ormco’s Porcelain Primer only. When using another material, please refer to the individual manufacturer’s instructions.

Figure 1. As more adults seek orthodontic treatment, clinicians are faced with the challenge of bonding to porcelain restorations (crowns, veneers).

Figure 3. In the presence of a phosphoric acid solution, apply two coats of Porcelain Primer to the porcelain surface and allow them to stay on for one minute.

Figure 4. After one minute, thoroughly rinse and dry the porcelain surface.

Figure 5. Brush a sealant/primer on the prepared porcelain surface and then apply the bracket using any orthodontic adhesive.

REFERENCES:

1. Adult Orthodontics, Articles: Dental Health, California Dental Association Online, February 25, 2002.
2. Eustaquio, R.; Garner, L.D.; Moore, B.K.: Comparative tensile strengths of brackets bonded to porcelain with orthodontic adhesive and porcelain repair systems, Am. J. Orthod. Dentofac. Orthop., 94:421-425, 1988.
3. Nebbe, B.; Stein, E.: Johannesburg, So Africa, Orthodontic brackets bonded to glazed and deglazed porcelain surfaces, Am. J. Orthod. Dentofac. Orthop. 109:431-436, 1996.
4. Zachrisson, Y.Ø.; Zacharisson, B.U.; Büyükyilmaz, T.: Surface preparation for orthodontic bonding to porcelain, Am. J. Orthod. Dentofac. Orthop. 109:420-430, 1996.
5. Kocaderell, I.; Canay, S.; Akça, K.: Tensile bond strength of ceramic orthodontic brackets bonded to porcelain surfaces, Am. J. Orthod. Dentofac. Orthop. 119:617-620, 2001.