An overview of light-curing within orthodontics

Quartz–tungsten halogen

While there has been a rise popularity of LED light-curing technology, QTH curing units remain on the market despite having a number of perceived disadvantages. Perhaps the most significant drawback relates to the energy output of QTH units which, while emitting visible light, also generate a great amount of heat. This heat necessitates the incorporation of cooling fans into the design of QTH units, adding bulk and impeding ergonomics. The lifespan of a halogen lamp has been shown to be as low as 50 hours, with significant degradation of the light output occurring over that time.⁷ The financial impact of these shortcomings over time must be factored into the overall cost of using a QTH unit.

Light emitting diodes

Limitations of early LED units were that they had a lower radiant output and emitted a narrow spectrum of light when compared to QTH units. However, advances in LED technology have resulted in more powerful LED chips (with a greater radiant output) and the use of multiple LEDs producing different wavelengths within the same unit to give a spectrum similar to that of QTH units.⁸ LED units are energy efficient, allowing the production of lightweight, battery-powered units that are easy to use and can be conveniently stored (Figure 1). When considering the longer-term costs of these units, a lifespan of >100,000 hours, with little degradation, can make them a reliable and economical choice.

Clinical effectiveness of different LCU types

When reviewing the evidence base for the effectiveness of light-curing units, it is important to consider that the specific orthodontic application of LCUs presents issues relating to the external validity of research that examines the effectiveness of LCUs in dentistry more generally. Orthodontic bonding poses specific challenges. While curing depth and aesthetics are less of a concern than when placing composite restorations, there is often the added obstacle of an opaque metal bracket base that impedes the transmission of light. Bond strength is a further unique consideration because this must be sufficient to retain an appliance for the duration of treatment while allowing the removal of brackets without enamel damage at debond. Within orthodontics, the duration of curing and incidence of bracket failure could be considered the most important outcomes when evaluating LCU effectiveness.

In vitro research has demonstrated no significant difference in bond strength between QTH and LED light-cure units when each is used for 20 seconds⁹.

Furthermore, LED units have been shown to achieve comparable bond strengths when used for shorter durations than QTH units¹⁰ with LED units requiring 50% less time to achieve a similar degree of hardness when curing orthodontic resin compared to a QTH device.¹¹ It must, however, be recognized that much of the research investigating curing duration has been lab based with inherent disadvantages relating to the external validity of results.

Many in vivo studies have looked at the relationship between bracket bond failures and LCU type. A meta-analysis from a systematic review comparing QTH and LED units failed to detect a statistically significant difference in bracket debond rate.¹² Therefore, factors such as cost and ease of use may be more important when considering which type of unit to use.

Direction of the beam

The unique nature of orthodontic bonding requires modifications to the use of LCUs relative to other areas of dentistry. Visible light-curing of orthodontic composite by transillumination through dental hard tissue is a technique, which proponents claim, allows clinicians to successfully cure thin layers of composite hidden behind opaque metal bracket bases. Evidence surrounding the efficacy of this technique remains equivocal. The technique was first demonstrated as effective in vitro in the 1970s,¹³ further in vivo studies concluded that, while transillumination resulted in a reduced degree of polymerization when compared to direct illumination, complete curing may not be necessary for an adequate bond.^14,15 More recent research has suggested that similar bond strengths can be achieved though small increases in curing duration if using a transillumination technique.¹⁶ It is generally recommended that the light-cure tip is positioned as close as possible and parallel to the surface of the resin composite being cured.¹⁷ Difficulties can be encountered when an assistant is tasked with using the LCU when seated to the side of the patient without a clear view of the bracket. The tip should be stabilized, with sufficient attention paid to prevent the LCU from drifting from the target tooth.

Curing duration versus irradiance

Curing duration and the irradiance of light are two further factors that influence bond strength. The advent of higher intensity LED units has raised the possibility of reducing the amount of time spent curing composite while achieving satisfactory bond failure rates. A randomized controlled trial comparing a high intensity (3200 mW/cm²) LED used for 6 seconds and a lower intensity (1200 mW/cm²) LED used for 20 seconds showed no significant difference in the percentage of bracket bond failures.¹⁸ Such reductions in the duration of curing may seem small at the level of an individual bracket; however, when considered in the context of bonding the entire dentition, even marginal gains can result in significant time saved. Resin-composite manufacturers state recommended light exposure times, and these should be followed by the operator.¹⁷

While increasing the irradiance of a unit may improve curing efficacy, caution must be used as the increased heat generated can have adverse effects on pulpal health. A lab-based study established that LED LCUs were unlikely to cause a critical change in pulp temperature of 5.5°C and described LED units as having less of an impact on pulp temperature than QTH units.¹⁹ However, the LED LCU tested had a power intensity of only 400 mW/cm², which is significantly lower than the 3200 mW/cm² described in the previous comparison of LED units. Practitioners should be aware of the risk of iatrogenic thermal trauma to pulp tissue, and are advised to air-cool the tooth when undertaking longer exposures or using high-energy units.¹⁷

Cleaning and sterilization

When choosing an LCU, consideration must be paid to local infection control policies and how any device would fit with such guidelines. Many LCUs are now cordless, battery-powered devices charged from a docking station that can be kept away from the immediate clinical area. Interchangeable tips are available that can be removed and sterilized between patients. It is important to note that the design of light-cure tip (in particular its diameter) can have an influence on irradiance and, therefore, curing ability.²⁰ Consideration should be given for tips designed for the curing of orthodontic resin composite. Plastic sheaths are available to reduce contamination of the LCU during use. While such barriers reduce the intensity of light from a curing unit,^21,22 especially if an excess of plastic is gathered around the tip, any reduction in irradiance is thought not to be clinically significant if barriers are applied as per the manufacturer's instructions.

Damage to light-cure tips

Deterioration of the light-cure tip can occur for several reasons, and well-used tips may appear clouded or slightly opaque after repeated cycles in an autoclave. Similarly, tips may become scratched during transportation to sterilization facilities, or if improperly stored alongside metal instruments. Individual housing cages, with a clip to securely fasten the removable tip, help to reduce wear and tear and can be a worthwhile investment (Figure 2).

Failure of power supply

As with any wireless device, the rechargeable battery within an LCU will deteriorate over time. Furthermore, damage to the base of the unit or the docking system can result in a failure of the unit to charge. Replacement batteries and docking systems are available from suppliers.

LED failure

Failure of the light emitting diode is an uncommon cause of LCU failure. LED units have a much greater lifespan than QTH devices, and other LCU components, such as the battery, are likely to fail long before the LED.

Visible inspection

It is good practice to regularly inspect the light-cure tip for signs of damage or for the residue of composite resin which can adhere to the tip surface if a protective barrier is not used.

Measurement of irradiance

The output of LCUs should be regularly measured using the same device to give a reliable comparison of readings. Any device which fails to meet the manufacturers guideline values should be investigated further and disposed of or repaired, as necessary.

Different types of light-meter are available to test LCUs, ranging from lab-based radiometers (held in laboratories for research purposes) to smaller and more affordable hand-held units (Figure 3). Despite having good reliability, the majority of hand-held radiometers have less validity than laboratory-grade power-meters with significant differences in output measurements when devices are compared.²³

For this reason, it is recommended that, rather than placing emphasis on the absolute measure of irradiance, clinicians should use hand-held radiometers to measure changes in irradiance over time.

Chairside curing of composite

Using an LCU to cure a sample of orthodontic composite at the chairside can act as a crude assessment of how well an LCU is functioning. However, testing the surface hardness of resin composite with a sharp instrument is not a valid assessment of curing depth.¹⁷