Putting curing lights in the spotlight

Light curing units (LCU) are essential for the successful polymerization of direct composite restorations. However, they’re rarely in the spotlight and, many times, considered supporting cast for the features and benefits of the material itself.

Let’s take a closer look at why LCUs are vital for a direct composite restoration’s success, the types of curing lights available, and what you can do to get the most out of your LCU and keep it optimized for proper curing.

Polymerization is crucial for composite materials. When polymerization isn’t sufficient, it reduces the strength of the restoration. Insufficient polymerization also results in water absorption, increased solubility of the composite and reduced hardness of the cured material.¹

Related reading: Is your curing light 'bright' enough?

The curing light used has a vital role in polymerization. Per the Journal of Clinical and Experimental Dentistry, some ways the light affects the polymerization process include:²

• The duration of radiation

• Where the tip of the curing light is pointed in relation to the composite and how far it is from it

• The wavelength and bandwidth of the light’s output

• The intensity for the light

• The irradiation time

Your LCU works with the photoinitiator in the material to activate the polymerization process. According to Spear Education, there are three types of photoinitiators: camphorquinone (CPQ), phenyl-propanedione (PPD), and trimethylbenzoyl-diphenyl-phosphine oxide (TPO).³

Most composite materials use CPQ as the light-sensitive component.⁴ The activation and performance of CPQ require the LCU to have the proper intensity with the highest assimilation wavelength range of CPQ. If your LCU falls short on either intensity or wavelength, you can compromise the strength and longevity of your direct composite restoration.⁵

The ISO 4049 Standard is the International Organization for Standardization’s recommendation for the intensity and wavelength of curing lights. These standards are:⁶

• An intensity of 300 mW/cm²

• A wavelength bandwidth of 400-515 nm (on the tip of the device)

• A depth of cure minimum of 1.5 mm

• Five generations of curing lights

An LCU is a significant factor in the success of a direct composite restoration. Over the years, LCUs have changed a lot, improving power, reducing size and increasing efficiency.

Per the IOSR Journal of Dental and Medical Sciences, there have been five generations of curing lights since the 1970s, including:⁷

• Ultraviolet light (no longer available)

• Visible light curing units: quartz-tungsten halogen (QTH) lights fall into this category

• Plasma arc curing units (PAC)

• Light-emitting diodes (LED)

• Argon lasers

The first curing lights were ultraviolet lights. However, these weren’t effective enough for curing composite because of the short wavelength that put limitations on the depth of cure. Moreover, they used ultraviolet light, which has other hazards to the patient.⁸

The second generation of curing lights appearing in the early 1980s were the visible light curing units. These replaced the ultraviolet lights and led to a few developments over the next 20 years, such as using blue light to initiate polymerization, developing the quartz-tungsten halogen bulb (QTH) and increasing the intensity of the light to ensure a greater depth of cure.⁹

More from the author: 5 mistakes you might be making with composites

The late ‘90s presented the third generation of curing lights with the plasma arc curing light (PAC), which used a fluorescent bulb that contained plasma. PACs were fast, curing the composite material in three to five seconds.¹⁰

Light-emitting diodes (LED) emerged in the 2000s and were the fourth generation of curing lights. LEDs have long lifetimes and require no filters to produce the blue light. They also have a more concentrated bandwidth than their predecessors.¹¹

Finally, the fifth generation of curing lights are argon lasers. First introduced into dentistry in the early 1990s, dental professionals began using argon lasers to cure composite because the wavelength enhances the thorough cure and physical properties of the material when compared with the polymerization and physical properties produced by other types of curing lights.¹²

Dr. John Flucke, technology editor for Dental Products Report, explains that curing lights had significant advances in the 2000s. LCUs decreased in size and changed to “toothbrush-style” units, which Dr. Flucke says make it easier to access challenging areas in the oral cavity, particularly in the posterior.¹³

Dr. Flucke prefers the LED lights over all the others for a few reasons. First of all, their wavelength is efficient for CPQ photoinitiator. Second, they’re energy-efficient, and they use less electricity, which means they don’t need AC. These battery-powered units take up less space in the operatory, and they’re also cordless and environmentally friendly. In addition, many of the LED lights allow you to change the wavelength of the curing light, which means you can change the wavelength if you have a different photoinitiator than CPQ. 

Tips for getting the most out of your LCU

LED lights are smaller and located on the curing head, making them optimal LCUs for curing direct restorations at the proper distance. Dr. Flucke recommends keeping the light as close and parallel to the direct restoration as possible. Even moving it a few millimeters away from the composite decreases the efficiency the LCU, which can result in inadequate curing for the restoration.

Dr. Gary DeWood, a Spear Education faculty member, recommends when using an LED light to cure a direct composite restoration to move it continuously during the cure.¹⁴ LED lights don’t have a homogenous spread from the tip, which affects the cure pattern of the composite. Moving it ensures that the material will cure appropriately. However, since it changes the distance of the tip from the material, Dr. DeWood says it also means you should add additional time to the process to allow for an adequate cure.

Dr. Ed Kusek, a private practice dentist in Sioux Falls, South Dakota, encourages doctors to take care of their curing light. He also suggests putting a shield over it when in use.

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“We always put a shield over the top of it. If we’re putting it close to the mouth or the restoration, they don't get any bonding agent on the lamp portion,” Dr. Kusek says.

Dr. Kusek also advises that if the assistant has the light, the doctor should ensure he or she gets in the right spot. If the light is shaking or too far away, the LCU can’t do a good job of curing.

For Class II restorations, Dr. Kusek says you need to cure it on the buccal and lingual molar side to ensure the cure is adequate. When the cure isn’t suitable, you begin to have problems with your restoration.

“It will start breaking down much faster. If the inner portion doesn't cure as it should, the margins will break away, and then you have recurrent decay, and the restoration will fail,” Dr. Kusek explains.

To avoid failure, Dr. Kusek thinks the bond is essential. He always puts two layers of the bonding agent. He says he was lucky enough to do some studies with bonding and although there are plenty of decent options available, the study proved to him that Peak™ Universal Bond (Ultradent) is the best bonding agent.

“Peak has a filler agent to it. The bond strengths are one of the highest on the market, so it will take the pounding in posterior areas or the anterior,” he says. 

Because of the strength requirements, Dr. Kusek says he puts two layers on and then a thin layer of flowable to reduce the shrinkage value. Finally, he either uses a bulk fill composite or layered composite.  

Maintenance is crucial

Today, LEDs and QTH lights are the most common types of curing lights in an operatory. When you have either a QTH or an LED, light intensity decreases with the age of the light itself.¹⁵

The intensity of the LCU is also affected by many factors, including:¹⁶

• Poor or reduced performance of the lamp or filter

• Broken or dirty device tip

• Blurred bulb

• Electrical failure

• Fiber defects

You might not know you have these problems with your curing light. You can’t see when there’s a fiber defect affecting the wavelength or if the lamp isn’t “bright enough” to cure the material properly anymore. Feeling the composite after curing isn’t going to work either; the surface can be hard even if the material deeper in the preparation suffers from inadequate curing.

Related reading: Covering a larger area in one cure

Regular maintenance will lower the risks of an inadequate cure resulting from a loss of intensity. In Canada, the CDSPI, a nonprofit organization dedicated to providing insurance and investment solutions, says curing light maintenance is essential to protect against malpractice claims.¹⁷

The Canadian Dental Association (CDA) also suggests testing the LCU output regularly and always ensuring that the LCU is in good working order before using it.¹⁸ The Journal of Clinical and Experimental Dentistry suggests using a digital radiometer to measure the intensity of the LCU, especially as it ages.¹⁹ The readings will reveal whether the light needs maintenance or a repair.

In addition to regular maintenance, the CDA also recommends regular cleaning, disinfection and sterilization of the units before every use as well as an infection control barrier. As always, you should follow the manufacturer’s instructions for cleaning and maintenance of the LCU.

References

1. Omidi, Baharan-Ranjbar et al. “Intensity output and effectiveness of light curing units in dental offices.” Journal of clinical and experimental dentistry vol. 10,6 e555-e560. 1 Jun. 2018, doi:10.4317/jced.54756
2. Ibid.
3. DeWood, DDS, MS, Gary. “The Evolution of Composites and Light Curing.” www.speareducation.com. 17 December 2018. Web. 28 April 2019. < https://www.speareducation.com/spear-review/2016/04/the-evolution-of-composites-and-light-curing>.
4. Ibid.
5. Omidi, Baharan-Ranjbar et al. “Intensity output and effectiveness of light curing units in dental offices.” Journal of clinical and experimental dentistry vol. 10,6 e555-e560. 1 Jun. 2018, doi:10.4317/jced.54756
6. Ibid.
7. Mangat, Dr. Panna, Dhingra, Dr. Anil., Bhardwah, Dr. Garurav. “Curing Lights and the science behind them - An Overview.” IOSR Journal of Dental and Medical Sciences. (2014) 13:12; pp 35-39. From Web. Pdfs. Semantic Scholar 28 April 2019. < https://pdfs.semanticscholar.org/f679/6f3c916c6ed0104719f4a53b6316a71dbced.pdf>.
8. Ibid.
9. Ibid.
10. Ibid.
11. Ibid.
12. Ibid.
13. Flucke, Dr. John. “The absolute best in curing light technologies.” www.dentalproductsreport.com. 20 September 2013. Web. 29 April 2019. < http://www.dentalproductsreport.com/dental/article/absolute-best-curing-light-technologies>.
14. DeWood, DDS, MS, Gary. “The Evolution of Composites and Light Curing.” www.speareducation.com. 17 December 2018. Web. 28 April 2019. < https://www.speareducation.com/spear-review/2016/04/the-evolution-of-composites-and-light-curing>. 
15. Ibid.
16. Omidi, Baharan-Ranjbar et al. “Intensity output and effectiveness of light curing units in dental offices.” Journal of clinical and experimental dentistry vol. 10,6 e555-e560. 1 Jun. 2018, doi:10.4317/jced.54756.
17. “Practical How To: How do you clean your light curing unit?” oasisdiscussions.ca. 3 July 2014. Web. 29 April 2019. < http://oasisdiscussions.ca/2014/07/03/htcl/>.
18. Ibid.
19. Ibid.