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Light Emitting Diode
The unsung heroes

This article deals with the details of LED, Light Emitting Diode, covering all the topics from the evolution of LEDs to their future directions in relation to conservative dentistry and endodontics.

 

1. Evolution of LED


A Story of success

Light emitting diodes, commonly called LEDs, are real unsung heroes in the electronics world. Were we able to look into the future, we would see LEDs everywhere around us. In the past, LEDs were mainly used for advertisement signs and as background light, but LEDs are being used for an increasing number of applications, ranging from architecture and lighting design to traffic signals and even medicine. The full potential of this young and promising technology has not been realized yet. Given their superior technical properties, LEDs offer an immense potential for innovation and growth. LED lights are not only much more energy efficient than conventional halogen lamps, but they also offer an unmatched service life.

Basically, LEDs are just tiny light bulbs that fit easily into an electrical circuit. But unlike ordinary incandescent bulbs, they don't have a filament that will burn out, and they don't get especially hot. They are illuminated solely by the movement of electrons in a semiconductor material, and they last just as long as a standard transistor.

Russian Oleg Vladimirovich Losev independently created the first LED in the mid 1920s; his research was distributed in Russian, German and British scientific journals. But no practical use was made of the discovery for several decades. The first practical visible-spectrum (red) LED was developed in 1962 by Nick Holonyak Jr., while working at General Electric Company. Holonyak is seen as the "father of the light-emitting diode". M. George Craford, a former graduate student of Holonyak, invented the first yellow LED and improved the brightness of red and red-orange LEDs by a factor of ten in 1972. In 1976, T.P. Pearsall created the first high-brightness; high efficiency LEDs for optical fiber telecommunications by inventing new semiconductor materials specifically adapted to optical fiber transmission wavelengths. In February 2008, Bilkent university in Turkey reported 300 lumens of visible light per watt luminous efficacy (not per electrical watt) and warm light by using nanocrystals.

In January 2009, researchers from Cambridge University reported a process for growing gallium nitride (GaN) LEDs on silicon. Production costs could be reduced by 90% using six-inch silicon wafers instead of two-inch sapphire wafers. The team was led by Colin Humphreys.

 

2.Influence of evolution of material science on LED

UV light activation was first introduced to the dental community. This concept served as a major advance over conventional chemically cured composites by providing premixed, shelf-stable materials with infinite working time and cure-on-demand ability. For the first time, curing restorative materials required no mixing, provided adequate time for placement and preparation of anatomy, and delivered instantaneous results. This gave more control to the dentist and reduced the stress involved with restorative dentistry. UV photocuring dramatically changed the direction of dentistry; however, it still had several limitations including limited depth-of-cure due to light absorption of the resins, pigments and fillers in the composites, and also significant safety considerations due to the nature of higher energy UV light. The early 1980s led to further advances in the photocuring of dental materials. The key advance was in the area of visible light curing and the identification of CPQ as an ideal photoinitiator (Figure 1-Click to view). The advent of visible light curing and photoinitiator use in composite materials began changing the world of restorative dentistry. Blue light in the form of quartz-tungsten halogen curing lights became the standard for light curing, offering better depth of cure and a safer means of curing versus UV light. Restorative materials and curing light technologies continued to advance. Manufacturers continued to improve upon the physical and esthetic properties of composites while curing lights increased in intensity in an attempt to direct more light toward the restoratives to cure them faster and deeper. Plasma Arc and Laser curing lights entered the market, promising faster cures by producing more light. Light emitting diode (LED) curing lights are the latest advancement, coming to market with numerous benefits to traditional curing lights, including low energy consumption with the light output fine tuned to CPQ (Figure 2-Click to view).2,3,4.

 

3. Impact of changing clinical trends on LED

 

Sufficient curing is the prime concern of polymerization…

A restoration made of light-curing materials will only be a long-term success if discoloration and the formation of marginal gaps and increased abrasion. As the surface of light-curing composites appears hard after only short curing, it is impossible to determine the polymerization quality in practice by tactile means,using a probe or using other instruments. The physical values of the entire cured material are what matters. This includes the achieved hardness – in particular on the bottom side of the restoration, the flexural strength and elasticity as well as the abrasive values (Figure 3-Click to view). The specifications of the manufacturers regarding the gentle curing of the various materials are important guidelines. At least well-known suppliers base such recommendations regarding curing time and program on clinical tests with various materials and various layer thicknesses. Sufficient curing depends on many factors. The most important ones are, however, high light intensity and the activation of the photoinitiators used, i.e. the suitability of the curing light for the materials used. Light-curing materials, such as composites, are mainly composed of organic monomers and inorganic fillers.

 

The various photoinitiators

In order to transform a monomer into a polymer, photoinitiators, which break down into radicals when irradiated with light and thus cause a polymerization reaction of the monomers, are required. The most commonly used initiator is camphorquinone. Camphorquinone absorbs light in the wavelength range between approx. 390 and 510 nm and displays a yellow color, which unfortunately also affects the shade of the cured restoration. White photointiator which absorb light in the wavelength range between 380 and 430 nm (Figure 4-Click to view) such as phenylpropanedion (PPD) or Lucirin TPO are also used. These white photointiatiors are used in bleach shades of resin composite material. The ability of a curing light to cure all materials, therefore, decisively depends on the wavelength range that is emitted by the light. Given their broadband emission spectrum, halogen lights activated the various initiators without any problems. Because they typically have a narrow emission spectrum, conventional LED lights are not automatically suitable to universally cure all materials. Today, however, there are LED curing lights which generate blue light in the range between 380 and 515 nm and which are thus suitable for all light-curing materials (Figure 5-Click to view).2, 5

 

Wavelength range

LEDs in general have a more limited light spectrum than halogen lights. Therefore, the polymerization of materials which do not exclusively contain camphorquinone as photoinitiator might be problematic even when modern LED lights are used. Depending on the polymerization light used, this may be the case with very light composites (Bleach shades) and special adhesives. In such situations, lights with a spectrum which is not designed to match only the absorption maximum of camphorquinone, offer significant advantages. In order to safely use the materials in the own practice, a negative list of the incompatible materials is required from the manufacturer of conventional LED lights.

 

4. Physics behind LED

In contrast to halogen and plasma arc lamps, LEDs produce visible light by quantum-mechanic effects. LEDs comprise a combination of two different semiconductors, the ‘n-doped’ and the ‘p-doped’ type. N-doped semiconductors have an excess of electrons while p-doped semiconductors require electrons, resulting in creation of electron ‘holes’ (Figure 6-Click to view). When these two types of semiconductors are combined and a voltage is applied, electrons from the n-doped type connect with holes from the p-doped type. A characteristic light with a specific wavelength range is then emitted from the LED (Figure 7-Click to view). The color of an LED light, its most important characteristic, is determined by the chemical composition of the semiconductor combination. Semiconductors are in turn characterized by their band gap. In LEDs, this gap is directly utilized for light production. When electrons in the semiconductor combination move from higher to lower energy levels, the energy difference of the band gap is released in the form of a photon of light, In contrast to halogen and plasma arc lamps, LEDs produce light with a narrow spectral distribution. This is the main difference between light produced by LEDs and other light sources, as light of selected wavelengths can be preferentially produced using LEDs with appropriate band gap energies. This innovative method of light production therefore creates a more efficient way of converting an electric current into light.6,7

 

1,000 mW/cm2: the ideal value

Normal light is not enough to polymerize materials in the dental practice. For this process, energy-rich blue light is necessary. Already for direct restorations, an irradiance (commonly referred to as “light intensity”) of at least 400 mW/cm2 is required. The ideal value, however, is thought to be 1,000 mW/cm2, in order to ensure that indirect restorations are also sufficiently cured when the irradiation takes place through the ceramic restoration or tooth substance. According to the Total Energy Concept, light intensities of more than 1,000 mW/cm2 are necessary to provide adequate curing of composites in all suboptimal but routine conditions in only ten seconds. If the light intensity is lower, the curing time is accordingly longer. If the above prerequisites are not met, the composite or adhesive may be cured only insufficiently in deeper areas. Therefore, it is advisable to check the light intensity, which decreases in time, regularly. For that purpose, built-in or separately supplied radiometers or the integrating spheres are helpful tools.5

 

Total Energy Concept …

Or: “What curing time is required for successful curing?” The Total Energy Concept states that the light-curing process depends on the energy and is basically determined by the multiplication of light intensity and time (for example: 20 seconds at a light intensity of 800 mW/cm2 = 20 s x 800 mW/cm2 = 16,000 mWs/cm2). The required dose depends on the type, shade and translucency of the composite. As a general rule, a dose of maximum 16,000 mWs/cm2 is required to adequately cure an increment of 2 mm – in some cases, the value might be lower. Based on this maximum value, various curing times can be calculated depending on the light intensity of the polymerization light used.5,8.

 

Total Energy Concept:8

Maximum curing time = Dose / Intensity

 

Interesting facts about the light probe

If a polymerization light is designed without a light probe and instead is equipped with an LED mounted at the front of the light-emission window, much of the intensity is lost due to scattering at a certain distance from the object to be cured. Fibreglass rods have proven themselves very adequate to reduce this loss due to scattering. These fibreglass rods consist of many individual glass fibreswhich are embedded in a protective glass case with a precisely defined light transmission. This, however, does not eliminate the need to increase the curing times with increasing distance from the object to be cured. In the case of the popular turbo light probes, the effective energy decreases by up to 50% at a distance of 5 mm, which means that the curing time must be doubled according to the Total Energy Concept. Given their outstanding light-scattering characteristics, parallel-walled (standard) light probes offer an advantage in this regard. The available energy is reduced by 50 % only at a distance of 9 mm (Figure 8-Click to view) (Figure 9-Click to view)

 

To summarize

Determining the required curing time in three steps

 

1.Check material compatibility

Not every conventional LED polymerization light is suitable to cure all light curing materials. Ask the manufacturer of your LED curing light whether it is compatible with the materials camphorquinone, Lucirin TPO and phenylpropanedion (PPD) – or ask the manufacturer of the composite which photoinitiators are contained in the composite.

 

2.Calculate curing time

With the help of a simple equation you can easily calculate the curing time for composite restorations.

 

3.Take the distance from the object into account

The curing time must be doubled if only 50% of the original intensity is available to cure the composite.

 

5. Current concepts in LED

Table 2 -Click to view. Product features according to the manufacturer.

POWER DENSITY (commonly referred to as “Light Intensity”):

Results: The highest light intensities were achieved with blue phase and the L.E.Demetron II, which were both over 1000 mW/cm2. FLASH-lite 1401 with a power density of 439 mW/cm2 was the lowest.

 

Depth of cure

Results: The depth of cure achieved with Herculite (Kerr Dental), a microhybrid composite, using the L.E.Demetron II light was the highest of all the LEDs. The depths of cure achieved with Heliomolar (Ivoclar Vivadent), a microfilled composite, were highest using the Coltolux, FLASH-lite’s 1401,Freelight, L.E.Demetron II and Radii Plus lights.

Comments: Depths of cure are dependent on the characteristics of the curing light (intensity and spectral distribution), the composite (type, shade, opacity and photoinitiator) and clinical variables such as distance of the light tip from the composite surface. In our study we measured the depths of cure of two types of composites (microhybrid and microfilled) and one shade (A2) with the light tip placed close to the composite surface.

 

Temperature rise

Results: Ultra-Lume LED 5, blue phase and L.E.Demetron II lights caused the largest temperature rise (over 12°C) over the 40 second curing time. SmartLite iQ caused the smallest rise in temperature (7.4°C).

Comments: The experimental setup may tend to overestimate the temperature rise. This is because the clinical situation allows more efficient heat dissipation. Also, dentin has a low thermal conductivity, which protects the pulp tissue.

 

Spectral distribution

Measures the distribution of light across the spectral range

Results: Ultra-Lume LED 5 exhibited two spectral peaks, one at 404 nm and one at 460 nm. All the other LEDs had a single peak between 444 nm and 468 nm.

Comments: The spectral emissions of all the LEDs allow curing of composites that contain camphorquinone, the most popular photo-initiator, which has an absorption peak around 470 nm. The bimodal spectral emission of Ultra-Lume LED 5 purportedly allows it to cure every photo-initiated product on the market, although this claim was not. As noted above, blue phase and L.E.Demetron II had the highest intensities compared to the other lights. However, while use of L.E.Demetron II also resulted in higher depths of cure this was not the case for blue phase, which has a spectral peak around 444 nm – well below the absorption peak of camphorquinone (the photoinitiator in the composites used). This demonstrates the point that intensity alone does not dictate the effectiveness of a curing light.

 

Corded or Cordless?

The advantages of a cordless light are obvious, but the disadvantages are not so apparent. When selecting a light, there are a few things to consider with respect to the type of battery and maintaining cordless operation. Consider how often and for how long you use your curing light. More frequent, light intensive procedures will require a battery that has a higher energy capacity. The lights tested used the Nickel Metal-Hydride (Ni-MH) or Lithium-Ion (Li-ion) batteries. These batteries cost about $100 to replace when they can no longer hold a charge. Two of the lights used a non-removable rechargeable battery. When they can no longer hold a charge, the light has to be replaced. To get the longest life out of a new Li-ion battery (the newest battery type), perform an initial conditioning of the battery. For the first three charge cycles, fully charge the battery overnight and allow it to fully discharge before recharging. Ni-MH batteries must also be conditioned before use and then again every 3-5 charge cycles. Li-ion batteries have a higher power density than Ni-based batteries. This allows longer battery life in a lighter weight battery. You can also recharge a Li-ion battery whenever convenient, without the full charge or discharge cycle required to keep Ni-MH batteries operating at peak performance. Lithium-ion batteries need to be used for maximum performance. If you don’t use your light very often, make sure you complete a charge cycle at least once per month.

L.E.Demetron II had a significantly higher average depth of cure associated with its use at 20 seconds; however, in the clinical forum, the dentists consistently rated this light lowest because of its bulky size. Dentists participating in the forum gave the highest scores to blue phase and FLASH-lite 1401.

Ultra-Lume LED 5 offers the advantage of a broader spectral range, which allows more efficient curing of materials containing photoinitiators with absorption peaks below 400 nm. Although camphorquinone is the most popular photoinitiator in dental materials, some materials contain other compounds like monoacylphosphine oxide (MAPO), bisacylphosphine oxide (BAPO) or phenylpropanedione (PPD)

.Table 3. Buyer's Summary for LEDs. .

Table 4. Summary of Led attachments available. .

6. Consideration for use

 

Power sources

The current/voltage characteristics of an LED are similar to other diodes, in that the current is dependent exponentially on the voltage. This means that a small change in voltage can lead to a large change in current. If the maximum voltage rating is exceeded by a small amount, the current rating may be exceeded by a large amount, potentially damaging or destroying the LED. The typical solution is therefore to use constant current power supplies, or driving the LED at a voltage much below the maximum rating. Since most household power sources (batteries, mains) are not constant current sources, most LED fixtures must include a power converter.

 

Electrical polarity

As with all diodes, current flows easily from p-type to n-type material. However, when a small voltage is applied in reverse direction, there will be no flow of current and hence no light production. If the reverse voltage becomes large enough to exceed the breakdown voltage, a large current flows and the LED may be damaged.

While all diodes release light, most don't do it very effectively. In an ordinary diode, the semiconductor material itself ends up absorbing a lot of the light energy. LEDs are specially constructed to release a large number of photons outward. Additionally, they are housed in a plastic bulb that concentrates the light in a particular direction.

 

Merits

Efficiency: LEDs produce more light per watt than incandescent bulbs.

Color: LEDs can emit light of an intended color without the use of color filters that traditional lighting methods require. This is more efficient and can lower initial costs.

Size: LEDs can be very small (smaller than 2 mm2)

On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in microseconds. LEDs used in communications devices can have even faster response times.

Cycling: LEDs are ideal for use in applications that are subject to frequent on-off cycling, unlike fluorescent lamps that burn out more quickly when cycled frequently, or HID lamps that require a long time before restarting.

Dimming: LEDs can very easily be dimmed either by Pulse-width modulation or lowering the forward current.

Cool light: In contrast to most light sources, LEDs radiate very little heat in the form of IR that can cause damage to sensitive objects or fabrics. Wasted energy is dispersed as heat through the base of the LED.

Slow failure: LEDs mostly fail by dimming over time, rather than the abrupt burn-out of incandescent bulbs.

Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer.

Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.

Focus: The solid package of the LED can be designed to focus its light. Incandescent and fluorescent sources often require an external reflector to collect light and direct it in a usable manner.

Toxicity: LEDs do not contain mercury, unlike fluorescent lamps.

 

Demerits

High initial price: LEDs are currently more expensive, price per lumen, on an initial capital cost basis, than most conventional lighting technologies. The additional expense partially stems from the relatively low lumen output and the drive circuitry and power supplies needed. However, when considering the total cost of ownership (including energy and maintenance costs), LEDs far surpass incandescent or halogen sources and begin to threaten compact fluorescent lamps.

Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment. Over-driving the LED in high ambient temperatures may result in overheating of the LED package, eventually leading to device failure. Adequate heat-sinking is required to maintain long life.

Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.

Light quality: the color rendering properties of common halogen lamps are often inferior to what is now available in state-of-art white LEDs.

Area light source: LEDs do not approximate a “point source” of light, but rather a lambertian distribution. So LEDs are difficult to use in applications requiring a spherical light field. LEDs are not capable of providing divergence below a few degrees. This is contrasted with lasers, which can produce beams with divergences of 0.2 degrees or less.

Blue Hazard: There is increasing concern that blue LEDs and cool-white LEDs are now capable of exceeding safe limits of the so-called blue-light hazard as defined in eye safety specifications such as ANSI/IESNA RP-27.1-05: Recommended Practice for Photobiological Safety for Lamp and Lamp Systems.

Blue pollution: LEDs can cause more light pollution than other light sources.

7. Curing Techniques

Two categories of technique are commonly used in curing polymers: continuous and discontinuous. The continuous cure refers to a light cure sequence in which the light is on continuously. There are four types of continuous curing: uniform continuous cure, step cure, ramp cure, and high-energy pulse. Continuous curing is conducted with halogen, arc, and laser lamps. The discontinuous cure is also called soft cure, which commonly uses a pulse delay.

 

A. Continuous curing techniques

Uniform continuous cure

In the uniform continuous cure technique, a light of constant intensity is applied to a composite for a specific period of time. This is the most familiar method of curing currently used.

 

Step cure

In the step cure technique, the composite is first cured at low energy, and then stepped up to high energy, each for a set duration. The purpose is to reduce polymerization stress by inducing the composite to flow in the gel state during the first application. Theoretically, this practice reduces the overall polymerization shrinkage at the margin of the final restoration. The reduction in shrinkage, however, is small and results in less composite polymerization because the lower intensity light yields lower energy levels. In addition, this technique results in an uneven cure, since the top layer is more saturated with light and thus more highly cured. Step curing is possible only with halogen lamps; arc lamps and lasers cannot be used because they work by applying large amounts of energy over short periods of time.

 

Ramp cure

In the ramp cure, light is initially applied at low intensity and gradually increased over time to high intensity. This allows the composite to cure slowly, thereby reducing initial stress, because the composite can flow during polymerization. Ramp curing is an attempt to pass through all of the different intensities in hopes of optimizing a composite’s polymerization. Some studies indicate ramp curing causes polymerization with longer chains, resulting in a more stable composite. In theory, very high energy applied over a short period tends to cause dimethacrylate monomers to attach to themselves, resulting in shorter polymer chains and a more brittle material with higher polymerization shrinkage and more marginal gaps. Ramp curing, with its dependence on low intensity, is possible only with halogen lamps; arc and laser lamps can generate only large, nonvariable amounts of energy. It is possible to ramp cure manually by holding a conventional curing lamp at a distance from a tooth and slowly bringing it closer to increase intensity.

 

High-energy pulse cure

The high-energy pulse cure technique uses a brief (10 second) pulse of extremely high energy (1000–2800 mW per cm2), which is three to six times the normal power density. This type of polymerization has not yet been adequately examined, and there are three areas of potential concern: (1) the rapid application of energy might result in a weaker resin restoration owing to the formation of shorter polymers; (2) it is possible that rapid applications of energy could reduce diametrical tensile strength; and (3) there may be a threshold level at which a resin has good properties, and thus, higher energies would result in more brittle resin.

 

B. Discontinuous curing techniques

In the discontinuous or soft-cure technique, a low intensity or soft light is used to initiate a slow polymerization that allows a composite resin to flow from the free (unbound) restoration surface toward the (bound) tooth structure. This reduces polymerization stress at the margins and could reduce “white line” or other marginal openings or defects. To complete the polymerization process, the intensity of the next curing cycle is greatly increased, to produce the needed energy for optimal polymerization.

 

Pulse-delay cure

In pulse-delay curing, a single pulse of light is applied to a restoration, followed by a pause and then by a second pulse cure of greater intensity and longer duration. It is best thought of as an interrupted step increase. The lower-intensity light slows the rate of polymerization, which allows shrinkage to occur until the material becomes rigid, and is purported to result in fewer problems at the margins. The second, more intense pulse brings the composite to the final state of polymerization. Pulse curing is usuallydone with halogen lamps.

8. Clinical considerations of LED

 

LEDs hot stuff!

Apart from sufficient curing, preventing damage to the tissue and the pulp is also a factor that needs to be considered. The blue light that is emitted is primarily energy. However, a part of the light energy directed at the tooth and gingiva is converted to heat. This holds true for any curing light, regardless of the type of light source. Whether laser, plasma or LED lights are in use, they should always be used carefully and with the clinical common sense and knowledge. Under unfavorable circumstances, the irradiated tissue can be damaged if the light intensity is too high or if the tissue has been cured for too long.

 

Some like it hot – some DON’T

Heat is generated by the exothermic reaction and the irradiation energy during every polymerization; as a general rule, it can be stated that the higher the light intensity, the higher is the released energy and the perceived heat (and the shorter the required curing time). In order to prevent possible damage to the pulp or the soft tissue, polymerization lights should therefore always be used cautiously. The consensus in the scientific community is that the temperature of the pulp should not increase by more than 5.5°C (approx. 9°F). Manufacturers should be able to present the corresponding data regarding their product. More sophisticated LED lights additionally offer a choice between the full performance and a program with reduced intensity for sensitive areas.

 

The required equipment for routine use

In the routine application, many other factors also play an important role for dentists. The polymerization light is the most-used product in the dental practice. Given this background, ergonomic factors are also relevant. Low weight and a balanced handpiece that allows a good grip are equally important. A cordless light offers maximum freedom of movement, and both dentist and patient are no longer bothered by the power cord. This, however, raises questions regarding the capacity and reliability of the battery. One of the main factors is the duration of continuous operation of the light. Nothing is more annoying than interrupting the treatment of a patient and therefore the procedure of the entire practice just because the battery is empty or the curing light needs to cool down for several minutes. In addition, special programs to reduce the shrinkage stress or heat development in areas near the pulp offer convincing advantages.

 

Design should suit your needs

LED lights are generally available in two versions: the pistol shape that is well-known from the halogen lights and the pen or bar shape. It is your choice which one you prefer. The pistol shape is not as futuristic and is often more comfortable to handle due to the balanced weight distribution as compared with pen-shaped lights. Generally, the weight and the angle of the light probe including the handpiece should suit the individual needs of the user. All of this has an influence on an ergonomic and comfortable use. If there are any selection keys, they should be easy to reach and easily readable.

 

Can Modern batteries live longer?

Due to their design, many LED lights are smaller and lighter than their halogen counterparts. As the power consumption is lower, LED polymerization lights can be efficiently supplied with power by a battery. The current standard is the lithium technology, which is very widely applied and used in billions of mobile telephones. Lithium-ion batteries are small and light, they have a long service life while the self-discharge rate is low, and they can be recharged in a short time. More than 500 charging cycles are possible – even many more if the battery is only partially discharged. Moreover, special protective circuits provide a high level of security. The charger technology of lithium-ion batteries makes sure that there is no lazy battery or memory effect. This phenomenon causes older nickel metal hydride and nickel cadmium batteries to lose part of their capacity prematurely and irreversibly. In other words: The lights still available today that use nickel-metal hydride batteries should be completely discharged before they are (completely) charged again. To prevent that time and capacity is lost, it is therefore recommended to completely discharge nickel-metal hydride batteries daily, if possible, and to recharge it overnight. Due to the comparably long charging times of several hours, the main switch must not be turned off. This requires a certain amount of discipline and entails regular monitoring – otherwise, it may be that the treatment of a patient may be delayed. It is only a matter of time until all manufacturers will change over to the newest state of battery technology. We therefore recommend that you pay attention to the type of battery used.

 

Battery

The lithium-ion or lithium-polymer batteries that are used in high-quality LED lights do not need to be entirely discharged. Frequent charging even increases the battery life considerably. As a general rule it is therefore advisable to place the light back in the charging base after every use. Modern lights display the currently available capacity, ideally using symbols on a display or at least with a light-emitting diode. A warning signal is sounded before the light switches off.

 

Safety net for emergency operation

Nothing is more unpleasant than having to interrupt the treatment of a patient and therefore the entire work of the practice because the battery is empty. This raises the question of an “emergency mode” for situations when the practice team does not have time to recharge the battery and would like to carry out the treatment immediately. In such situations, there are generally two possibilities: First, an additional battery is supplied. Such an additional battery costs, and you must be sure that it is always charged and that it can be retrieved at any time. Secondly, there are innovative polymerization lights which offer the possibility to attach the handpiece to the power cord of the charging base. This allows the clinician to carry out treatments independently from the battery and at any time according to his/her requirements.

 

Proper maintenance of the battery

Rechargeable batteries, being small power plants, require careful handling. In order to use a battery for possibly several years, the following tips might be helpful: To prevent an irreversible deep discharge, nickel-metal hydride batteries must be recharged after three months at the latest and lithium-polymer or lithium-ion batteries after six months, if they have not been used in a long time.

Nickel-metal hydride batteries must be completely discharged before they are completely recharged. Lithium-ion batteries, however, can be discharged and recharged at any time. In order to prolong the battery life of lithium-ion and lithium-polymer batteries, it is even recommended to place the light in the charging base after every use. Clean battery contacts free of dust and composite residue ensure good conductivity and therefore charging capacity at any time. The electric contacts should therefore be cleaned regularly – for example using a wipe dipped in alcohol or a cotton swab. Ageing occurs in every type of battery, so that a decrease in performance is to be expected. Typically, lithium-polymer and lithium-ion batteries have lost about 30% of their original capacity after three years. This means that, instead of the 60-minute capacity, for instance, the battery can be used only for approximately 40 minutes.

 

Always with a fan

LED lights use up little space; they are energy-saving and have a long life span. These properties make them highly interesting also for use in dentistry. Like all electric parts, however, LEDs generate heat, which must be discharged, in order to prevent damage due to high temperatures or to prevent that the light prematurely fails to operate. The developing heat depends mainly on the light intensity of the LED light used. Modern lights offer an intensity of 1,000 mW/cm2 and more. Given this high performance, the best solution is to discharge the heat by means of a fan. If the light is not equipped with a fan, the housing or a special cooling unit must absorb this heat. During extended operation – when luting composites are polymerized for several minutes in the case of indirect restorations – the housing develops a perceivable heat. If a certain temperature is exceeded, it may happen that the light may be automatically switched off to prevent damage. If this happens, it takes several minutes until the light can be used again. If a fan is integrated, however, even high performance lights can be operated without restrictions.

 

Programs

If areas with only little residual dentin or areas close to the pulp are cured, polymerization should be carried out very cautiously, i.e., a reduced light intensity should be applied in order to prevent an excessive heat accumulation in the pulp or the soft tissue. If high-performance lights featuring intensities of 1,000 mW/cm2 are used, it is advisable to cure restorations in the cervical area, adhesives and liners using a so-called “Low Power” program. High performance also means increased polymerization stress in the composite. In this case, it is helpful to use a light which offers a special program including various levels of intensity or a “Soft Start” program which increases the intensity during curing.

9. Future directions of LED

By lowering the initial investment required and the operating costs associated with the use of light cure technology, these systems are allowing more manufacturers to take advantage of visible light cure adhesives.

The economic advantages of LED technology will continue to drive innovation in this area as light curing equipment suppliers find ways to satisfy clinical needs. On the horizon are LED systems that can cure larger areas, expanding the number of appropriate bonding processes. Increased irradiance may also allow LED systems to cure a broader range of resins.

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6. Elipar Free Light 2. Technical product profile, accessed on 03-10-2009.
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8. Koran P, Kürschner R. Effect of sequential versus continuous irradiation of a light cured resin composite on shrinkage, viscosity, adhesion, and degree of polymerization; Am J Dent 1998:10,17–22.
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