LED

The selection of a curing light that fits your style of practicing remains one of the most important equipment purchases you will make. If you have an active restorative practice, it is a device that you use virtually every time you treat a patient. The right light can help you achieve success, while the converse is true -- the wrong light can make your efforts more tedious and your results less consistent.

Curing lights allow us to initiate the polymerization reaction "on demand" for a vast array of materials. However, there is, perhaps, more misinformation and hype regarding this type of equipment compared to just about anything else we use on a daily basis. Most of these controversies center on how long you have to cure specific types of restorations as well as how deep you can cure specific types of materials.

Manufacturers continue to make outlandish claims of their curing capabilities, most of which fall into the "too good to be true" category. An example is the claim that a new light can accomplish a "3mm depth of cure in one second". While the manufacturer of this light has ignored our attempts to receive an evaluation unit, our previous tests on other so-called high-powered lights have not come close to validating these types of ludicrous claims. Therefore, we urge you to "not be fooled by these ads" they can lure you into grossly undercuring your restorations.

If you undercure a restoration, for example, you may not even be aware of the negative sequelae for years. Therefore, selecting a curing light and using it properly can greatly affect the performance and longevity of your restorations.

Types of Curing Lights

Halogen Uses a halogen bulb as the source of light.

+ Reliable -- long track record

+ Cures all materials due to wide bandwidth (400nm-510nm)

- Requires a cord due to power consumption

- Cooling fans are necessary and can be noisy

Plasma Arc Bulb is really an aluminum oxide, high pressure vessel, which contains highly energized xenon gas (plasma) under 150psi. The inside shape is specific to reflect light arcing between two electrodes. Arc is only about 1mm long, enabling a very focused beam.

+ Very fast (when a small tip is used)

- Expensive

- Large base units

- May not cure all materials

- Requires a cord that may be liquid-filled, may be stiff, and can degenerate over time

Argon Laser Generates light when energy is applied to an atom raising an electron to a higher, unstable energy level. Electron will return to stable level by releasing light through a medium of argon gas.

+ Fast

- Very expensive

- Large base units

- Small tips

- May not cure all materials

- Require a cord due to power consumption

LED (Light Emitting Diode) Special diodes (electronic devices that restrict current flow chiefly to one direction) that emit light when connected in a circuit.

+ Cordless or corded

+ Lightweight

+ Small

+ Wide variety of designs, from wands to guns

+ Long battery life due to the low power usage

- May not cure all materials

- Some have poor and/or no selection of tips

- May shut down due to overheating during long curing intervals

This category covers LED lights. When it comes to LEDs in general, they are being touted as being on the leading edge of technology for numerous consumer and business applications. Along with nanotechnology, LEDs are blazing hot and getting hotter by the minute. Therefore, it is difficult to dispute the attraction of LED curing lights.

Although there is a surging tsunami of LEDs, this category of products has almost entered the commodity arena. This is evidenced by online marketers selling their lights for as low as $160. Virtually all of the new ones have plenty of power to cure your restorations, so choosing the right one for you may come down to their features, which we will describe in our reports.

Nevertheless, while we acknowledge the obvious appeal of these lights, some of them, especially the simpler versions, have significant drawbacks. The most significant ones are lack of timers, small tips, and the inability to cure beyond 3-4 minutes.

The lack of a timer may be viewed as just a nuisance, but it could lead to serious undercuring if you lose track of the beeps and/or how many times you have had to reactivate a light that automatically cuts off after 10 seconds.

In the same regard, small tips force you to overlap your curing area. Not only does this take longer, but you could inadvertently miss a section of the restoration, leaving an undercured area. And, since many of these lights do not have fans, their heat sinks may be only effective for a few minutes. This is not an issue if your service mix is typically single restorations, but could be a significant problem when you are seating multiple indirect restorations such as veneers. With each veneer requiring at least 40 seconds (and that is a bare minimum; 60 seconds is more reasonable), you would be out of luck trying to cure more than four veneers at a time.

Of course, then there is the issue of some LEDs not being able to cure all materials. There is no doubt that the vast majority of light-cured materials can be fully polymerized with an LED. However, the few materials that cannot be cured with an LED mandate that you still have a halogen around for these contingencies. This may be just a nuisance as long as you know which material falls into this category, but it will not compromise patient care. But what if you do not know that a material cannot be cured properly with an LED? More than likely, it will still get pretty hard, but its degree of cure will be compromised along with its long-term performance.

The obvious solution to this problem is to buy an LED light that is capable of curing all materials. While more full spectrum LEDs have come on the market recently, there are still many that do not have this capability. Therefore, it is still somewhat of a guessing game and you just have to hope that you do not guess wrong.

Interference with pacemakers?

However, one factor that has not received much publicity concerning cordless lights is their possible interference with devices such as cardiac pacemakers. As described in a study published in JADA in May 2010, one cordless LED curing light was found to disrupt the function of a pacemaker using an in vitro protocol. The lead author in the study, Dr. Craig Miller, replying to an inquiry from us, stated that if a clinician used the curing light with the battery end pointing upward toward the patient's head, the distance would probably be too great to affect the pacemaker.

On the other hand, does this tactic apply to all cordless curing lights? Maybe, but maybe not. While other devices such as ultrasonic scalers can also interrupt the function of devices such as pacemakers, you can always switch to hand scaling if a patient is at high risk. With a curing light, you do not have that type of option.

Nevertheless, we do not want to sound the alarm too loudly, since we are unaware of reports linking pacemaker failures to curing lights. We think it is just prudent to be aware of this linkage and to take proper precautions as suggested previously.

With this topic in mind, we were curious whether the manufacturers of the lights even mentioned this potential adverse effect in their directions. As it turns out, some lights are stated to have no effect whatsoever, some have a warning against using them in patients with pacemakers, and the some do not even broach the subject. 

Curing Modes

High/Boost/Fast Usually the highest power the light will generate. Typically synchronized to a timer that has a 10-second (or even less) curing interval, which may not be adequate for many restorations.

Regular/Normal/Standard If a light has a high/boost/fast mode, the regular/normal/standard mode will be set at a power level somewhat lower. If a light does not have different power levels, then the regular mode will be the highest power level.

Adhesive or Low Touted as a safer energy level for curing bonding agents that presumably do not require the high power used for composites and cements. It presumably is safer since this mode produces less heat.

Step Cure at low power (usually about 150mW/cm2) for 10 seconds, followed by an instant step up to a much higher power (usually maximum of light) for the rest of the curing interval.

Ramp Start curing at low power (usually about 150mW/cm2), followed by a linear increase to a higher power (usually maximum of light) for 10 seconds, and then stay at that high level for the rest of the curing interval.

Pulse Has different meanings for different lights, but usually means either the power cycles between high and low every second or so or the power cycles on and off every second or so from the beginning of curing.

NOTE: Our tests were unable to detect any significant differences in microleakage in Class II restorations from the so-called "low stress" modes, such as step, ramp, or pulse. And, after bulk curing a posterior (packable) composite in a Class I preparation using two "low stress modes", one regular mode from a halogen light, and one regular mode from a plasma arc light, we were not able to detect any differences in marginal integrity, stain uptake, enamel crazing, or the infamous "white line" formation at enamel margins, as viewed under a stereomicroscope at 50x.

Curing Power, Cure Times, and Radiometers

More power, as measured by a radiometer, presumably means we can cure materials in less time, more deeply, or both. Since no one likes to sit at the chair holding the light for at least 40 seconds per increment, for example, high-powered lights that presumably permit fast curing have generated enthusiastic interest within the profession. In addition, the less time you spend curing a restoration, the more income you can presumably realize (although some of the increased revenue predictions are very difficult to believe).

However, the marketing of power being emitted by curing lights is becoming just as frenzied as the horsepower race in cars or the bond strength wars with adhesives. Unfortunately, unless a light is capable of an extremely high power output, relatively small differences in power output will not significantly increase its true performance. This again is similar to cars, where big boosts in horsepower only allow vehicles to drop their 0-60 times fractions of a second, which may be important on the race track, but has no relevance to everyday driving practices. This also applies to  "turbo tips"  that may not perform superiorly to conventional tips, despite their higher radiometer readings.

Then there is the issue of the accuracy of the radiometers being used today, many of which are calibrated differently (see LIGHT METERS). The peak power readings in this edition were recorded on the LED Radiometer, which we found provides consistent power readings for all types of lights despite its name.

Tests

1. Time to reach peak power If you are only curing for 10 seconds, but it takes your light five seconds to achieve high power, then you are really only curing for five seconds. We tested this peak power over 20 seconds.

2. Power from different tips Peak power produced by each tip in the most commonly used modes for each light.

3. Tip mapping The power emitted from the face of curing tips is typically highest in the center and decreases as you get closer to the edge. To show this effect, we mapped the power of most tips >7mm, using three setpoints: exactly in the middle, midway between the middle and the edge, and 0.5mm from the edge. This mapping was done by testing the hardness of a composite disk at the three setpoints.

If you are curing a large restoration and you are depending on the edge of the tip to cure critical areas like a veneer margin, you may be unknowingly undercuring. For example, the mesiodistal width of a MOD preparation in a mandibular first molar may be 11mm. If you are using an 11mm tip, the power at its edges may not be strong enough to fully cure the marginal ridges. So, if you see fractures in these peripheral areas, it may be due to the restorative material not being cured properly to maximize its physical properties.

Using a tip too small could also cause brown lines at margins of veneers due to undercured resin cement. Large restorations would be better served in most instances by curing with a 13mm tip, which overlaps the restoration margins by several millimeters. However, the power output by a 13mm tip may be lower compared to smaller tips and may require longer curing times. And it is rare for an LED light to have a 13mm anyway.

4. 10-minute curing test Power level is tested over 10 minutes of continuous curing (reactivating the lights as necessary). This is roughly the amount of time you would cure if you were luting 6 - 10 veneers at one time and were curing them from both the facial and lingual. If your light cannot maintain a steady output over this time period, then the restorations at the end of the line may not be fully cured.

5. Hardness @ 2mm This test will show you how well the composite is cured at the depth of a typical increment using the different curing times and tips. Knoop hardness of a standardized universal composite at 2mm depth is tested after it has been placed in a modified Class II preparation in a real tooth model, using a conventional metal matrix, and cured for 10, 20, and 40 seconds. Five seconds was also tested for any light claiming to be effective at this interval. This hardness was measured on the proximal surface.

6. Does it cure all materials? Resin glazes are cured for 40 seconds after being applied to an unpolished composite surface. After wiping off the uncured material, the gloss created by the resin glazes is compared to that when the glaze is cured with a halogen light. If the gloss created by the LED light is less than that created by the halogen, it can then be assumed that the LED has a narrow bandwidth and will not be capable of curing all materials.

7. Heat generation We tested the heat produced using the highest power mode and several tips (including turbo versions if available) for all the lights and compared this reading with the heat from lower power modes (if available). This will tell you which lights have a higher potential to cause pulpal problems due to heat. This heat generation is measured directly on the tip to simulate when you are stabilizing the tip in contact with the tooth.

Are there Negative Effects of Fast Curing?

Fast curing has been accused of putting too much stress on the bond of a restoration to the tooth. If you apply too much light to a restorative material, it will presumably shrink more quickly, opening gaps at the tooth-restoration interface, causing white lines and microleakage. High power has also been accused of inducing cracks in thin porcelain veneers. To test these issues, we performed Class I & II microleakage studies, plus one with porcelain veneers:

Class I White Lines and Microleakage Eleven different curing protocols using five different lights and four different restorative materials were investigated as to whether any variables could be isolated to predict the incidence of white lines at the margins and/or microleakage. We found that, while there is a general association between white lines and microleakage, it is not consistent across composite materials and curing protocols. In other words, there are too many other variables to merely conclude that if you eliminate the white lines, you will also eliminate microleakage.

Class II Microleakage The same 11 different curing protocols and five different lights were used as in the Class I study, but with this project, we used three different flowables on the gingival wall and investigated as to whether any variables could be isolated to predict the incidence of microleakage. We found that neither the curing light nor the curing protocol produced any statistically significant differences in microleakage.

Veneer Crazing and Microleakage Porcelain veneers, standardized to 0.7mm in thickness, were bonded to teeth using either a halogen light for 60 seconds or a plasma arc light for 15 or 30 seconds. The results showed no craze lines in any veneers when viewed under the stereomicroscope at 10x, both before and after thermocycling and staining. In addition, with margins at the CEJ, all the microleakage scores were very low, signifying no differences between the lights.

Components

Base Unit/Battery Charger

Typically sits on the counter in the treatment room and includes the electronics that operate the light. For cordless LEDs, its function may be as the recharger. It may have the timer, some type of holder for the gun or wand, and the power switch (unless it is functioning as a battery charger, in which case it would not have a power switch since it would always be on).

Since counter space in treatment rooms is usually at a premium, the smaller base units are favored. Timers should be easily seen and accessible for changing. The gun or wand holder should keep these items secure, but allow easy placement and retrieval at the same time. Built-in radiometers are also featured in some base units.

Gun or Wand

Houses the LEDs, fan (in some models), trigger, and portal for the tips. A gun or wand should be comfortable to hold. Even though most are not heavy, some assistants may not be able to take the gun from you with their "pinky finger", so instrument transfer can be difficult. Some guns or wands can get very warm (even downright hard-to-handle hot) when they are activated for more than a minute or two.

To try to compensate for this heat generation, some models have fans that can be noisy. Some lights even cut off after a certain period for cooling.

CAUTION: Never turn off a curing light while the fan is still running as it will overheat. Always allow the fan to cool the light. Once the fan stops running, the light can safely be turned off.

Many LED guns and wands also include the timer, battery charge indicator, curing mode adjustment button, and other controls. For most models, these controls are located on the top of the gun or wand so they can viewed by both right-handed and left-handed operators. However, in some instances, the controls are located on the side of the handle visible only to right-handed operators. These lights would not be a good choice for the left-handed minority.

Tips

Multiple tips increase the versatility of a curing light and access to hard-to-reach areas. Four tips, all curved at roughly 60 degrees, should be sufficient for the vast majority of procedures.

2mm Useful for tacking down indirect bonded restorations such as veneers, inlays, onlays, and crowns. Some 2mm tips can even fit into proximal boxes for curing closer to the gingival wall. Unfortunately, this may not be of much value unless you overlap the cure areas, taking as much or even more time than if you used a conventional tip and just extended the cure time.

8mm Routine, small to moderate-sized restorations.

11mm Moderate to large posterior restorations.

13mm Veneers, onlays, and crowns. However, this size of tip is rarely available with an LED.

The key in tip selection is to make sure that it actually extends beyond the outline of the entire restoration, so that multiple cures overlapping each other will not be necessary.

Note that the size of the tips as listed by the manufacturer is not necessarily the diameter of the light curing portion. For the most part, the diameter of tips as stated by the manufacturer is usually the external dimension. But this can be misleading on tips that have a protective covering that reduces their useable area by about 1mm.

Tips should swivel to allow positioning the light for maximal curing, but not be overly loose so they will not stay in the intended position.

They should also be autoclavable for optimal sterility or adaptable for barrier use. It is especially important to keep the tips clean and free of adherents. Composite sticking to tips is a common problem. Any adherents will interfere with the light's curing ability, so the face of the tip should be checked after each use. Be careful when cleaning the tips as they are easily scratched.

Protective Shields

Most lights (but not all) come with different types of protective shields that fit over the end of the tip or mount on various locations of the tips. These shields are meant to protect our eyes from blue wavelength light being emitted by these devices. While these shields can be convenient and do not require any additional hands to hold them, they can also be cumbersome to use and difficult to switch from tip to tip. In addition, they are not universal in their protection.

For example, the larger shields may interfere with getting your light tip close to a second molar. In addition, they provide no protection when curing the linguals of anterior teeth. We recommend the use of handheld shields to protect your eyes from the light generated by these units.

Barriers

Some of the wand-type lights come with plastic barriers, which is definitely the asepsis method of choice. These barriers can usually only be used with lights that do not have fans, although they can have cutouts to accommodate those with fans. Unfortunately, some barriers do not fit the lights very precisely and can be a nuisance if they move around excessively. On the other hand, barriers typically have minimal effect on the power output, but it is a good idea to get a radiometer reading with and without a barrier to be sure it is not going to interfere with curing effectiveness.

Batteries

Lithium-ion (Li-ion) is the type powering most LEDs today, but some older models may still have Nickel Metal Hydride (NiMH). Lithium-ion are typically smaller or lighter, have a higher voltage, and hold a charge much longer, but they are more expensive than other types of batteries. NiMH are less expensive than Li-ion, but are larger, require conditioning, and suffer from the memory effect.

In some units, the batteries are easily removable and can be charged independently of the curing light. This means you will always have a fully charged battery ready to go. The batteries in other units are not removable and the entire wand or gun must be placed on the charger. This is not an issue if you are in the habit of always placing the light back on its charger, but could lead to your using a partially charged light. In addition, constantly charging a NiMH battery can damage it.

Testing and Maintenance

Measure the power baseline for your light when it is new using a radiometer and remeasure it on a weekly basis. And there is a lifetime to batteries, which will need to be replaced occasionally.

WARNING There are numerous manufacturers that have followed the lead of Kerr in providing some type of hardness disc to verify that a curing light will polymerize a specific thickness of composite in a specified amount of time. Most of these discs have a small hole in the center. For this test, you fill the hole in the disc with the composite, cure it for a specified time period, and then turn over the disk to check whether the bottom surface of the cured composite "feels" like the disc when scratched with an explorer or other sharp instrument. If it does, then this presumably indicates the composite is adequately cured for intraoral use.

However, this is a dangerous test that could give you false and misleading information. Consider what we found with the Demetron Hardness Tester, which is essentially a round white plastic disc with three holes. We filled the three holes in the disc with our test composite and cured each composite specimen 5 seconds, 10 seconds, or 40 seconds. We then turned over the disk and tested the bottom of each cured composite disc as well as the Hardness Tester itself for Knoop hardness. Finally, we asked three of the RRL staff to scratch the bottoms of the specimens with a sharp explorer and compare the feel to that of the Hardness Tester.

The results of this test were:

1. None of the RRL staff were able to distinguish a difference in hardness between the cured composite and the plastic surface of the Hardness Tester. This should have indicated that all the specimens were cured to the same hardness, which matched that of the Hardness Tester.

2. The Knoop hardness scores were:

Hardness Tester (disc) 35.4

Composite: 5 sec cure 15.5

Composite:10 sec cure 23.3

Composite:40 sec cure 42.6

These results show that it is not possible to determine the polymerization level of a composite merely by scratching the surface of its cured bottom surface and then comparing it to some known standard, which may not be applicable to the test in any event. We advise using these types of tests as a screening device, but do not rely on them for definitive polymerization guidance.

Cautions

Many directions include some strange safety measures such as using the light for 20 seconds and letting it rest for 60 seconds. Another one tells you not to use the light if the patient is being sedated with nitrous oxide. These stipulations are mandated by various government regulations and manufacturers must comply if they want to sell the product internationally. Do not let these warnings stop you from using the lights in a normal manner.

Basic Curing Guidelines

While the trend to shorten curing times seems to be unstoppable, our results and those from other researchers point toward some inevitable conclusions:

1. With all the various types of lights and materials on the market, it is virtually impossible to come up with one protocol, especially one featuring reduced curing times, across the board.

2. It is still prudent to limit the thickness of your increments to 2mm unless you are using a verified, deep-curing material.

3. With universal, flowable, and posterior composites, 20 seconds seems to be the reasonable compromise between speedy 10 seconds and sluggish but optimally effective 40 seconds. This is due to the fact that an adequate depth-of-cure may not be achieved in 10 seconds with all shades and applications. A 20-second cure gives you a measure of safety. However, if you are curing a deep restoration, reverting to 40 seconds, at least for the first increment, seems prudent.

4. With microfills, rapid curing appears to be very risky. Our results point toward no less than 40 seconds.

5. Do not cure right up to the edge of your tip. Make sure your tip overlaps the margins of the restoration.

6. Even though some studies show alternate curing modes to be helpful, our tests and the anecdotal clinical results from our Editorial Team suggest straight curing is still the mode of choice.

Many lights are basically retooled versions of the original units. Due to the still challenging economy, it seems that dental manufacturers are following other industries such as autos by introducing new models that are not drastically different than the ones they are replacing. The reports will give you insight into whether it makes sense to upgrade or stand put. After all, unlike autos, it is the rare dental supplier who will take your old light in trade for a new model. Maybe that is a concept some enterprising dealer will initiate!


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