Are there Negative Effects of Fast Curing light?

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 Portalbe Dental 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 many base units.

Gun Houses the light bulb (in almost all halogen types), fan (in most halogens and some LEDs), 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.

To try to compensate for this heat generation, most halogen lights have extra powerful and sometimes noisy fans. Some lights even cut off after a certain period for cooling. In addition, some of the fans blow hot air into your face and/or make the immediate treatment area uncomfortably hot.

CAUTION
Never turn off a dental curing light while the fan is still running – 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 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 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.

Wand Typical pencil-thin wands were usually found with argon lasers and the original plasma arcs, but these types were corded. The cordless wands of LED lights have more bulk, but are still slimmer and lighter than the guns. Their activation mechanism using a pen grasp, however, may be somewhat awkward, especially if you are used to the triggers on guns.

Tips The power emitted from the face of curing tips is typically highest in the center and decreases as you get closer to the edge. 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.

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

2mm is 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 is for routine, small to moderate-sized restorations.

11mm is for moderate to large posterior restorations.

13mm is for veneers, onlays, and crowns.

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.

Barriers Some of the wand-type lights come with plastic barriers, which is definitely the asepsis method of choice. These barriers can only be used with lights that do not have 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.

Simple Tips on How to Choose a Gearmotor Combination

{C}What is the typical power you need? In other words, what is a typical speed and torque operating point for your system? Choose a marathon micro motor that is capable of providing this much mechanical power. (Most motors are classified according to their power.) Usually you will choose the smallest, lightest, most inexpensive motor that meets your specifications. You may also have constraints on the nominal voltage of the motor (e.g., you have a 12V supply available, so you want the motor to run at around 12V). Since the input electrical power is the current times the voltage, low-voltage-rated motors of the same power draw larger amounts of current.
·{C}Once you have chosen a motor, choose a gearbox for the motor so that the speed+torque combinations you want from it (including the maximum speed and maximum torque you need) are under the speed-torque curve of the motor+gearbox combination. Also make sure the gearbox output shaft maximum torque specification is sufficient.
·{C}Motors should not be run for long periods of time at stall, as they are likely to overheat, as explained in Brushed DC Motor Theory. It is fine if they intermittently stall. The allowable continuous operation region of the speed-torque curve depends on the thermal characteristics of your motor, but typically you don't want continuous operation at less than 1/4 or 1/2 of the maximum speed of the motor. You may need to choose a larger micro motor to meet this specification.
·{C}You can further optimize your design for maximum efficiency in converting electrical power to mechanical power, to save on electrical power for battery-powered robots. Motors usually are most efficient at converting electrical power to mechanical power at high speeds. On the other hand, gearboxes with larger gear ratios generally have lower efficiency than gearboxes with smaller gear ratios. Don't worry about efficiency in converting electrical to mechanical power unless it is a critical issue for you.
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