Oxygen is not always welcome

Anyone who works with light curable resin knows about the so-called oxygen inhibition layer. This layer forms on the surface of the resin after light curing and has caused many clinicians to wonder whether their material is expired.

That is not the case; the explanation is simple. Light curable resin only fully polymerises in the absence of oxygen. Whenever ambient air (and thus oxygen) has access to the resin – this is even the case on the cured surface – there remains a thin unpolymerised layer.

This layer can be simply wiped off, but sometimes it’s very useful. When composite is built in layers, it ensures that each layer binds to the next. This is because each successive layer seals off the previous one from ambient air, thus allowing the previous layer to be completely polymerised. Removing the oxygen inhibition layer would prevent a solid bond from forming between the layers.

There are, however, situations where the formation of the inhibition layer is bothersome; for instance, when using composite to bond tooth-coloured restorations such as porcelain inlays and crowns.

It is well known that the weak point with a crown or an inlay is the cement interface. Therefore, an excess or shortage of composite must be avoided in order to achieve a flush interface.

Because excess composite is so hard to remove after polymerisation, clinicians usually try to remove excess composite before light curing, and this is where the problem comes. Once the oxygen inhibition layer is wiped off, what looked like a flush interface is now concave and leaves a void.

What should we do? Leaving excess composite at the interface which has to be laboriously removed after polymerisation is not an acceptable solution.
Personally, I remove all excess composite before light curing and apply a protective gel called Deox to the surface to be cured. The syringe tip delivery, of course, makes the application of Deox practical.

Deox is almost as clear as glass and doesn’t hinder the penetration of the light from the polymerisation lamp. However, Deox provides a tight barrier against the influence of oxygen. After use, Deox is easily rinsed away. Under the Deox, we can achieve complete polymerisation and avoid voids in the interface.
Deox is also useful during a restoration when the light curing lamp must be brought into direct contact with the composite. In this case, we can use Deox to cover the tip of the curing lamp and prevent composite from hardening on the filter and decapacitating the light.

Once again, the Inspiral brush tip is an effective tool for applying Deox. The tip lowers the viscosity of the gel slightly so that it is easier to apply. The fine brush fibres also help in the application process.

Deox is a simple aid for a particular purpose, but I wouldn’t want to be without it in my practice. There is another group of materials that continues to prove useful in new ways. Bonding systems are no longer uniquely designed for enamel; they can now be used on dentine, composites, porcelain and metal.

Dan Fischer graduated with his BS from the University of Utah in 1971 and received his doctor of dental surgery degree from Loma Linda University in 1974. He has maintained a full-time private practice for 15 years. Since 1990, he has worked extensively in research and development but still maintains a part-time practice with an emphasis towards aesthetic dentistry. He is president and chief executive office of Ultradent Products, Inc. and extensively involved in the research and development of many products used widely in the dental profession.  Dr Fischer was presented with the Alumnus of the Year award at Loma Linda University in 1994, and the Governor’s Medal for Science Technology for the state of Utah in 1997.

Dr Dan Fischer will be presenting the ‘Future of dentistry’ on 17 September at the Royal College of Physicians in London. This one-day course will provide ideas and techniques to enhance your practice while accommodating the needs of all patients. It will demonstrate how the use of technology enables you to provide affordable, quality treatments so you can improve your profits while still offering high quality patient care. The seminar will also emphasise minimally invasive, repairable aesthetics and early childhood preventative intervention. For more information and to book call 0800 371652.

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