Light Energy Matters

Authors
Richard B. Price
Suffix
BDS, DDS, MS, PhD, FDS RCS (Edin), FRCD(C)
Cite this as: J Can Dent Assoc ;76:a63
Topics
materials
restorations

Dr. Richard Price

In 2005, 116 million resin restorations were placed in the United States.1 Although evidence suggests that the longevity of these restorations can equal amalgam,2 some studies have shown that resin restorations are not lasting as long as they could.3-7 For example, a recent study from Sweden reported that resin restorations lasted an average of only 6 years.4 In another recent study, replacement rates were significantly higher for resin restorations than for amalgam.7 Why, then, the reduced longevity?

The success of resin restorations depends upon many factors, including technique, moisture control, polymerization shrinkage, the resin used, porosity within the resin and adequate curing of the resin. Dental resins that receive inadequate amounts of energy from a curing light, or energy at the wrong wavelengths, are inadequately polymerized.8,9 This adversely affects their physical properties,10-16 reduces bond strengths,12,13,17 increases wear and breakdown at the margins,14,15 and increases bacterial colonization. 18 All of these outcomes may lead to secondary caries, the major cause of failure of resin restorations. In addition, inadequately polymerized resins are less biocompatible.19-23

A recent study using new curing lights tested the ability of 20 dental professionals to deliver energy to simulated restorations.24 There was a large variation between operators, with 27% delivering less than 10 J/cm2 to a Class I restoration and 82% delivering less than 10 J/cm2 to a posterior Class V restoration. Thus, even with new curing lights capable of delivering sufficient energy, operator technique often prevented sufficient useful light from reaching the resin.

Four variables determine how long a curing light should be used to adequately cure a resin.

  1. Curing light design and condition. Dentists rely on hand-held dental radiometers to test the output of curing lights, but these radiometers are inaccurate and unreliable.25,26 Consequently dentists have no way of knowing how much energy they are delivering to their restorations. Instead, dentists must rely on manufacturer recommended curing times, but many of these include "fine print" details (e.g., shade, increment thickness, distance from light source to the resin, light output), the implications of which may not always be fully understood. In addition, reports indicate that curing lights in many dental offices deliver insufficient light output. This can result in inadequate polymerization even if the light is used for the recommended curing times.27-31
  2. Technique used by the operator. Some dentists deliver as little as 20% of the energy achieved by others using the same curing light in the same location.24 This happens because they do not use protective glasses, do not stabilize the curing light at 90° to the restoration and do not pay attention when light curing. Due to potential temperature increases during light curing,32-36 dentists cannot arbitrarily increase curing times to ensure that sufficient energy is delivered. Dentists can get a sense of this temperature increase by using the light on the back of their hand for the same time they use the light in the mouth.
  3. Type and location of the restoration. When curing lights are used for the same amount of time, significantly less energy is delivered to hard-to-reach restorations compared to more accessible restorations.24 Furthermore, if the curing light is used for the same amount of time for all increments, the resin at the bottom of a 6-mm deep proximal box will receive much less energy than the final increment.
  4. Energy requirement of resins varies greatly. Depending on the brand and shade, as little as 6 J/cm2 or as much as 36 J/cm2 is required to adequately cure a 2-mm thick increment of resin.37

To manage these 4 variables, dental professionals need more specific, consistent and accurate information on the energy required to adequately cure a resin restoration and how much energy they are actually delivering to the restoration. Delivering the energy required to achieve the properties intended by the resin manufacturers should allow these restorations to last longer.

 

Richard B.T. Price, BDS, DDS, MS, PhD, FDS RCS (Edin), FRCD(C), is a professor in the department of dental clinical sciences, Dalhousie University, Halifax, Nova Scotia. Email: rbprice@dal.ca

References

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