Plasma Arc

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 plasma arc lights or, more precisely, light, since there is really only one significant product left, although it does have its strong advocates. However, just like with halogen versions, all the buzz these days is over LEDs (one estimate is that about 80% of all new lights being purchased in the U.S. are LEDs). Therefore, it remains to be seen how long plasma arc technology can hang on when it comes to curing lights.

Curing Modes

Unlike other types of lights, plasma arcs only have one mode.

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). And power is the main calling card of plasma arcs. However, power from these lights is highly dependent on the tip being used.

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 10 seconds.

2. Power from different tips Peak power produced by each tip. 

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 the largest tip available for a plasma arc is 12mm.

4. 2.5-minute curing test Power level is tested over 2.5 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 5 or 10 seconds. This hardness was measured on the proximal surface.

6. Heat generation We tested the heat produced using all available tips. This will tell you whether the light has a 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

Typically sits on the counter in the treatment room and includes the electronics that operate the light. It has the light generation module, timer, holder for the gun, and the power switch.

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.

Gun 

Houses the light bulb, fan, trigger, and portal for the tips. A gun should be comfortable to hold. Even though most are not excessively 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 still get very warm (even downright hard-to-handle hot) when they are activated for more than a minute or two.

Tips

Multiple tips increase the versatility of a curing light and access to hard-to-reach areas. However, most plasma arcs do not offer more than one tip and the remaining rated product only has two.

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 won't 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 - 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.

Testing and Maintenance

Measure the power baseline for your light when it is new using a radiometer and remeasure it on a weekly basis. If there is a significant decrease in output, you should send the light back to the manufacturer for a check-up.

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, 10 seconds seems to be the reasonable compromise between speedy 5 seconds and sluggish but optimally effective 20 seconds. This is due to the fact that an adequate depth-of-cure may not be achieved in 5 seconds with all shades and applications. A 10-second cure gives you a measure of safety. However, if you are curing a deep restoration, reverting to 20 seconds, at least for the first increment, seems prudent. And, if you are using a large tip, an even longer cure may be necessary.

4. With microfills, rapid curing appears to be very risky. Our results point toward no less than 20 seconds.

5. Don't cure right up to the edge of your tip. Make sure your tip overlaps the margins of the restoration.


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