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Update on LED Curing Lights (part II, Q&A)


Questions and Answers
Q: What is unique about high-intensity LED units?
A: High-intensity LED units increase driving power to the chip to obtain higher output levels. These units commonly utilize an internal fan to cool the chip or use metal-coated unit bodies to help dissipate internal heat.
Q: Is it still important to use a radiometer?
A: Periodic evaluation of LED light performance with a radiometer is recommended. Typically, LED chips do not fail instantaneously, but have reduced output with prolonged use. There is no clinically relevant difference between radiometers for LED and QTH units, and absolute values from radiometers cannot be used to predict the potential for curing. Tip diameter can also greatly affect measurements. Use of these devices should be restricted to evaluating operating performance over time of a specified curing light/light guide combination. 
Q: What new features are available in LED units?
A: Some new units generate light at two or more different wavelengths (poly-wave lights) to provide radiant energy to polymerize materials having CQ as well as other photo-initiators, where a higher frequency (around 400 nm) is needed. Also some lights have two or three intensity levels available.
Q: Are the temperatures generated by LED units important?
A: With the advent of high-intensity lights, temperatures from exposure to LED units have become a significant clinical issue. Although LED light-curing units have been advertised as being “cool” and not raising tooth temperature, temperatures are raised. Heat emitted from LED tips may affect pulpal vitality, patient comfort and curing of the resin.
Clinical Notes
Openings or Gaps in Light Unit Housing
* Openings may allow disinfectant fluidsto enter the unit and possibly degrade the electronics over time – clean with care.
Weight (Exclusive of Battery)
* Indicator of an internal heat-sink capability – the greatest detriment to diode life is overheating.
* Increased diode temperature can result in decreased output.
* More expensive units use internal fans or large metallic components to draw heat away from the diode as the light is used. They also incorporate more sophisticated electronics that monitor and adjust changes in chip-driving current to attempt to maintain constant levels of output.
* Less expensive units shut down to avoid overheating, tend to have inadequate heat-sinking features, and use rudimentary electronics.
* High-power units get hot internally, having the potential to damage a patient’s lip, tongue, or cheek. 
* Run the unit through a number of sequential, repeated exposures and sense the temperature to gauge amount of heat at housing and tip.
* Lithium-ion - longer charge, no memory effect, 40% more capacity than Ni-Cad.
* Nickel-metal-hydride (Ni-MH) - less memory effect and greater capacity than Ni-Cad.
* Nickel Cadmium (Ni-Cad) – older technology, must use battery until drained before recharge or it develops a “memory effect.”
* Pencil grips - allow easy use of many controls with one finger.
* Gun – familiar, requires adjusting controls with two hands.
* Cordless units - battery size, weight, and positioning affect unit balance and ease of use.
* Corded units – balance between unit and cord weights affects holding comfort.
* Investigate the ability to place the tipeasily into difficult-to-reach regions.
Tip End Configuration of Conventional, Glass-fibered Guides
* Turbo - increased power output, but decreased exposure area - should not be used for distances greater than 4mm.
* Curved.
* Sheathed (non-glare) or unsheathed (light-sabers) - opaque, sheathed, blackened guide provides more throughput to the target and is easier on the eyes.
Tip End Configuration of Pencil-grip Models
* No guide - chips located at pencil end.
* Covered with glass or plastic lens.
* Covered by clear glass or plastic plate - tends to provide the worst uniformity of beam irradiance.
High-intensity LED Light-curing Units - The Heat They Produce
A primary goal for improvement of LED light-curing units has been to increase the light intensity - the thought being, the higher the intensity, the more rapid and more thorough the cure. Unfortunately, heat generation is a typical consequence of higher light intensity. The amount ofheat that develops when light interacts with tooth tissue, resin composite or soft tissues is dependent on: the intensity of the light, how much of the light is reflected or absorbed by the particular material, and how fast the heat is conducted away.
A recent survey of 160 Clinical Consultants of THE DENTAL ADVISOR indicated three of their principal concerns about the highintensity light-curing units coming into the marketplace were:
1. Is there a risk of damaging the pulp with a high-intensity light?
2. How much does the temperature of the composite restoration increase during light curing?
3. Can a patient’s oral soft tissues be burned with the tip of the light?
To answer these concerns, experiments were conducted using several highintensity (1,000-1,600mW/cm2) and ultrahigh- intensity (4,000-5,000mW/cm2) lights to determine the increase in temperature of the dental pulp and composite restoration during a typical curing cycle and the temperature increase of soft tissue in direct contact with the tip of the curing lights. That temperature increase of the dental pulp ranged from 0.0 to 1.2°C, with many of the light-curing units showing no increase in pulp temperature. This insignificant temperature rise can be explained by the fact that the time the light was on was short enough that the insulating qualities of the tissues and composite restoration protected the pulp. Therefore, following the recommended curing times should protect the pulp tissue from excessive heating.
That temperature increase of the composite restoration was higher, ranging from 8to 27°C for a single curing cycle of the light. There are times, however, when a clinician will use three curing cycles in succession to ensure thorough curing. When three cycles were simulated in the experiment, temperature increases were as high as 40°C. However, when each pulse was followed by a 6-8 second delay, the temperature increase was similar to that of a single curing cycle. A short delay in between pulses can, therefore, protect the restoration from increasing in temperature with each consecutive pulse.
Direct contact with soft tissue caused temperatures in the range of 42 to 58°C. Intolerable pain is produced with a contact temperature of 50°C. Every one of the lights tested can produce a painful burning sensation if the light-curing tip is inadvertently placed in contact with the lip, tongue, cheek, or gingiva over the length of the curing cycle. Therefore, care must be taken to avoid direct contact with these tissues especially when consecutive pulses are used.
Clinicians may be concerned that the light produced by their curing light is too intense and is creating high temperatures. The intensity of the light is generally a function of the inverse of the distance to the lighted surface. So, for most lights, moving the light further from the subject decreases the resulting heat. However, some lights focus the light at a point away from the tip. The best way to tell if you could produce pain in a patient’s soft tissue is to experiment with the light on your fingertip.
All lights are safe when manufacturers’instructions are followed. The clinician should be careful to not direct the light at soft tissues due to the potential of high temperatures being generated.
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