International Journal of Orthodontic Rehabilitation

REVIEW ARTICLE
Year
: 2019  |  Volume : 10  |  Issue : 3  |  Page : 121--133

Light-curing unit (devices)


Pooja Garbadrao Hadole, Suchita Sadashiv Daokar 
 Department of Orthodontics, CSMSS Dental College and Hospital, Aurangabad, Maharashtra, India

Correspondence Address:
Suchita Sadashiv Daokar
Department of Orthodontics, CSMSS Dental College and Hospital, Aurangabad - 431 002, Maharashtra
India

Abstract

Bonding is the most published and researched procedure in orthodontics. Since its inception in 1954 by Buonocore, bonding material and technique have undergone major innovations and upgrading. Self-cured bonding materials were truly replaced with light cure ones, which provide an added advantage of controlled curing time and ease of operation. The light cure bonding material needs a specific light cure device for its curing. These devices have also undergone major changes in the past years. Halogen light cure devices were replaced by plasma arc, and recently, market is now flooded with light emitting diode light cure devices. However, literature search failed to reveal any review on this aspect. Hence, the author felt the need to review this untrodden topic. This article deals in detail with the various light cure devices used in orthodontics.



How to cite this article:
Hadole PG, Daokar SS. Light-curing unit (devices).Int J Orthod Rehabil 2019;10:121-133


How to cite this URL:
Hadole PG, Daokar SS. Light-curing unit (devices). Int J Orthod Rehabil [serial online] 2019 [cited 2020 Sep 19 ];10:121-133
Available from: http://www.orthodrehab.org/text.asp?2019/10/3/121/267587


Full Text

 Introduction



Since the introduction of bonding by Buonocore (1954), there has been a continuous attempt to formulate a material and technique which fulfills the requirements of bonding along with the expected physical, mechanical, and biological properties. Self-curing adhesives were introduced first, but quickly discarded because they had limitations such as less porosity and discoloration, longer working time, ease of manipulation and increased hardness and wear resistance of superficial layer. To overcome these limitations, light-activated composite resin was introduced in 1960s according to Strassler.[1] These resins contain photosensitizer (Camphoroquinone [CQ]), which absorbs blue light with wavelengths between 400 and 500 nm. Light-activated resin system utilizes light energy to initiate free radicals; thus, introduction of light-curing resin led to the development of the first curing light.

Clinical efficiency of a light-curing unit is crucial for obtaining the optimal polymerization and a successful outcome.[2] With the advancing research in the field of orthodontic bonding, a need for an appropriate curing unit has always been felt. In this article, the author has attempted to review the history, advantages, and disadvantages of various light-curing units available in the market.

Visible light wavelength is between 400 and 700 nm. Most of the composites are sensitive within range of 400–520 nm wavelength (blue).

Photoinitiator like camphorquinone in the resin absorbs photon energy and then combines with activator Amine (DMAEMA) and creats free radicals which initiates polymerization. Process of formation of free radicals is described in [Figure 1].{Figure 1}

Other photoinitiators used are 1-phenyl 1,2-propanedione (PPD), Bis-acylphosphine oxide, and Tri-acyl phosphine.

 History



According to Strassler,[1] in the early 1960s, the first light-curing resin composites were introduced; this led to the development of the first curing light. The first dental-curing light was developed in the 1970s. It was the Nuva Light (developed by Dentsply/Caulk) that used ultraviolet light order to cure the material. This was discontinued because of the drawbacks of ultraviolet (UV) light used in the system. Furthermore, these lights were not very effective due to the shorter wavelengths that limited the depth of cure.

According to Rueggeberg,[3] during the early 1980s, advances in the area of visible light curing took place. Only a few years following the introduction of UV radiation for curing dental restoratives, the ability of using visible radiation was introduced: February 24, 1976. On that day, Dr. Mohammed Bassoiuny of the Turner School of Dentistry, Manchester, placed the first visible light-cured composite restoration on Dr. John Yearn, the then head of department. This advancement led to a curing device that now uses blue light. The next type of curing light that developed was the quartz-halogen bulb. This device had longer wavelengths of the visible light spectrum and allowed for greater penetrating curing light and light energy. The halogen curing light replaced the UV-curing light.

The 1990s presented great improvements in light-curing devices. It improved previous devices as well as developing new devices. The main focus was to improve the intensity to be able to cure faster and deeper.

In 1998, the plasma arc curing light was introduced. It uses a high intensity light source, a fluorescent bulb containing plasma, to cure the resin-based composite. It claimed to be able to cure material in 3 s. However, on average, it took between 3 and 5 s.

 Different Aspects of Light-Curing Unit



Light-curing unit is an instrument capable of generating and transmitting a high-intensity blue light with a wavelength oscillating between 400 and 500 nm that is designed specifically to polymerize visible light sensitive dental material.

The ideal light-curing unit should have:

Broad emission spectrumSufficient light intensityMinimal drop-off of energy with distanceMultiple curing modesSufficient duration for multiple curing cyclesDurabilityLarge curing footprintEasily repairable.[4]

The light-curing units are classified into the following five generations:

1st Generation - Ultraviolet light2nd Generation - Visible light-curing units3rd Generation - Plasma arc units4th Generation - Light-emitting diodes (LEDs)5th Generation - Lasers.

The basic components of light-curing units are as follows [Figure 2]: handpiece, handpiece push button, nose cone, light guide, eye shield, power module, power cord, main switch, indicator light, fuse, plug, bulb, filter, and fan.[5],[6],[7] Some of the light-curing units have integrated curing meter, microprocessor, and battery charger.[5],[8]{Figure 2}

To lowest to highest intensity

LED lampsQuartz-tungsten halogen lampsPAC lampsArgon laser lamps.

 Ultraviolet Curing



UV-curing unit was introduced in dentistry in 1970. UV light-curing unit was the first to be used in curing light-cured composite. The technology came from other industry such as ink, paint, and coating materials that used the UV in photopolymerization process.[5],[9],[10] This unit utilized the polymerization process of a composite that can be accomplished by the energy derived from ultraviolet light. The wavelength is in the range of 364–367 nm.[5],[11] Ultraviolet curing units used benzoin ether type of compound as photoinitiator in sealant at that time.

Disadvantages

It was time-consuming, as a 90 s application must be given to each bracketIt has the potential to cause retinal damage and the possibility of selectivelyAltering the oral microflora through exposure of ionizing radiation [5],[12]Limited depth of cureCarcinogenicLoss of intensity over time.

 Quartz-Tungsten Halogen



QTH was used in 1990. These lights contain lamp with a tungsten filament in an inert gas with a small amount of halogen gas. An electric current passing through QTH heats the tungsten to 2700°C and creats visible light and infrared radiation [Figure 3]. The light is filtered to approximately 380–500 nm.[12]{Figure 3}

Disadvantages

Short-curing depthGradual loss of high energy wavelengths in their light outputVery high heat generation as most of its energy dissipated in the form of heat rather visible light; these lights is that they only use 9% of the total energy produced and majority is dissipated as heat and so requires cooling fan and filter [12]Furthermore, this light requires frequent monitoring and replacement of the actual curing light bulb because of the high temperatures that are reached. (For example, one model uses a bulb with an estimated life of 50 h which would require annual replacement, assuming 12 min use per day, 250 days per year)The time needed to fully cure the material is much more than the LED curing lightThis implicates a reduction of curing efficiency over time by aging of the components.[13]

 High-Performance Halogen-Curing Light



Advantage of high performance halogen-curing light is less curing time over conventional halogen light cure. This unit has a special tungsten quartz halogen optibulb whose performance does not degrade with time. It also has an 8 mm light guide, which emits a full spectrum light filtered as blue with a range of 40–505 nm. Curing time for metal is 8 s and ceramic bracket – 5 s. This light has boost mode, which increases the light output to 1000 mWatt/cm2 in 10-s cycles with a 5 s beep. This will allow the composite under metal bracket to be cured in 5 s. The light produced by this unit is intense, and the tip of the guide may occasionally cause some discomfort to the skin mucosa.

Disadvantages

Bigger in sizeThe light performance degrades with timeIt generates more heat and requires filter and ventilating fan.[12]

 Adaptor Light Guide



Adaptor light guide is made by computer technology having maximum tapered optic fibers for better output compared to others. The surface area is about 28 mm2. The light output ranges from 880 to 1120 mW/cm2. The guides are currently available in various sizes and shapes [Figure 4].{Figure 4}

Advantages

It can be sterilized either chemically or in an autoclave; it can cure the composite with reduced time It is economical since the adaptor is cheaper than other light-curing units.[12]

Disadvantages

Its usage relies heavily on the halogen-curing unit. Therefore, whatever problems encounter by the halogen-curing unit may have an effect on its performance.

 Argon Laser



Argon laser was introduced in 1991 having 488 nm wavelength [Figure 5]. Dual-wavelength argon lasers are used in minor procedures such as gingival recontoring and coagulation. They operate at 488 nm for curing and 514 nm, respectively.[14] The time required to cure the orthodontic composite is 5 s.{Figure 5}

It has potential to cause retinal damage and the possibility of selectively altering the oral micro flora through exposure of ionizing radiation but it does not damage the pulp tissue.

Disadvantages

The curing depth is limited to 1.5–2 mmThe curing tip is small, so more time is needed to cure the red blood cells (RBCs)They have narrow spectral outputsThey are expensive.[12]

 Plasma Arc



In the mid-1990s, plasma arc were introduced as a more affordable, high-speed curing light. This unit has been developed after the technology used by the United States National Aeronautics and Space Association in aeronautical. This light uses xenon gas, distilled from liquid air, and then electric current is passed through the gas which ionizes it and produces negative and positive charged particles. High-powered light produced is then filtered to an effective curing wavelength of 450–500 nm. These lights have an energy level of 900 mV from 2000 mV [Figure 6].{Figure 6}

Advantage

It can cure the composite in 2 s.

Disadvantage

They are expensive and produce more heat so filters and

Ventilating fan are requiredMore bulky and heavy to use.[12]

 Light-Emitted Diode



LED were introduced by Mills in 1995.[12] They used junctions of doped semiconductors to generate visible light with no light filtration required. LEDs are highly efficient light sources that produce light within a narrow spectral range. Blue LEDs curing unit has an advantage over halogen light-curing unit in that it is inexpensive. The LED unit has no bulb or filter that requires maintenance. They do not require filters because they emit light at a specific wavelength within the 400–500 nm [Figure 7] and [Figure 8]. Overtime, only little degradation of light output is observed and they do not produce heat. This may be another advantage for avoiding any possible gingival or pulpal irritation – the light performance degrades with time. LED is very popular among pediatric dentists particularly, since less chair time and an adequate polymerization is the main goal. It has been suggested that even though the strength is inadequate, by far, it is the most reliable.[12]{Figure 7}{Figure 8}

Disadvantages

Cost is more than conventional halogen lightsThe curing time is more than plasmaNeed to recharge batteries.[12]

Basic specifications of light-curing units that are available in the market summarized in [Table 1].[5]{Table 1}

 Discussion



There are several factors related to light curing that can influence the polymerization process and the strength of the material such as intensity of the light, curing time, and depth of cure.[5],[15],[16],[17],[18]

Intensity of light

Lambert's Law – When a light beam hits an orthodontic adhesive surface, penetration of light into the relatively thin layer of material depends on many factors related to the light beam itself, the application mode, and the material characteristics.

First, the distance of the source from the surface and the path that the incident beam will have to travel to reach the adhesive has a large effect on the intensity of incident light. The well-cited Lambert Laws in this field describe the variation of intensity with distance as:

I = Io e− yd

Where I is the light intensity at distance d, I0 the intensity departing from the source, and y the absorption coefficient of the medium.

Curing time

If curing time increases, bond strength also increases, and if production of heat increases from increased curing time, there are more chances of irreversible pulpitis.

Depth of cure

Depth of curing depends on intensity of light.

The light-curing unit should be able to cure the composite to the optimum bond strength. Curing lights all generate heat and require a cooling fan, especially halogen which generates noise and so bulb life reduces to only 100 h and minimum is generated by LED.[19] Halogen lights do significantly increase the pulpal temperature more than other light cures. Because LED uses minimal energy and produces less heat, they are marketed as cordless units with a rechargeable battery and with no other parts or light filaments present so they better resist vibrations and shock. Therefore, these are effective for more than 10,000 h.[12],[20]

The manufactures go to another photoinitiators rather than CQ because one of the main problem of CQ initiator is their yellow color rather than their need to prolonged light curing, which give the RBC undesirable yellow color after polymerization [Figure 9].{Figure 9}

From this Graph [Figure 10], we should see:{Figure 10}

The peak of wavelength of LED units is perfectly matching the wavelength needed to activate CQ initiatorsThe initiators like Lucerin TPO and PPD their peak near UV wave length away from LED wavelength zone.

If increased light exposure, there is increased depth of cure, increased conversion i.e., polymerization and increased hardness upto threshold level [Figure 11]. If decreased light exposure, there is inadequate polymerization. Due to inadequate polymerization, there is lack of retention, increased wear, color instability, and microleakage, and due to microleakage, postoperative sensitivity and secondary caries occur [Figure 12].{Figure 11}{Figure 12}

There are two types of light-curing techniques:

Continuous curing techniques:

Uniform continuous curingStep cureRamp cureHigh-energy pulse cure.

Discontinous cure techniques:

Pulse delay cure.

1. Uniform continuous cure [Figure 13]{Figure 13}

In uniform continuous curing technique, light of medium constant intensity is used and applied to composite for period of time. It is the most familiar method that is currently used. In QTH and LED curing units uniform continuous curing technique is used.

2. Step cure [Figure 14]{Figure 14}

In step cure technique, first, we used low energy and then stepped up to high energy. The purpose for step cure is decreasing the degree of polymerization shrinkage and polymerization stresses by allowing the composite to flow while it is in gel state. Step cure cannot be carried out by plasma arc or laser.

3. Ramp cure [Figure 15]{Figure 15}

In ramp cure technique, light is applied in low intensity and then gradually increased over the time. It decreases initial stresses and polymerization shrinkage. It cannot be carried out by plasma arc or Laser curing.

4. High energy pulse cure [Figure 16]{Figure 16}

High energy (1000–2800 mW/cm2) is 3 or 6 times more than the normal power. High energy pulse cure technique is used in bonding of ortho brackets or sealents. In argon laser, plasma arc, 3rd generation of LED high energy pulse curing technique is used.

5. Pulse delay cure [Figure 17]{Figure 17}

In pulse delay cure technique, single pulse of light applied to restoration then followed by pause then a second pulse with higher intensity and longer duration. The first low intensity pulse slows the rate of polymerization and decreases the rate of shrinkage and stresses in the composite whereas the second high-intense pulse allows the composite to reach the final state of polymerization. It is carried out by QTH light cure.

When pulse delay cure technique compared to uniform continuous cure technique more amount of shrinkage take place in uniform continuous cure technique [Figure 18].{Figure 18}

Irradiance is the Power (mW) incident on an surface area of the tip of the light guide (cm2) [Figure 19].{Figure 19}

[INLINE:1]

If surface area of the tip of the light guide is larger, then irradiance is lower. If surface area of the tip of the light guide is smaller, then irradiance is higher. Radiometer is used to check irradiance [Figure 20].{Figure 20}

Factors affecting the bond are as follows:[12]

From orthodontic point of view, increase in thickness of resin reduces the shear strength of bonding enamel–bracket interface [21]Penetration of light depends on shade and opacity of composite. Translucent, very light shades will have easier penetration than dark ones. Light translucent shades may cure about 3 mm below the surface, while darker ones may be only 1 or 2 mmBulk of material– Bulk filling should only be done on shallow preparations to make certain that the deepest layer is polymerizedDepth of cure and time:

A standard time of 20 s is usually required to cure to a depth of 2.0–2.5 mm by most curing-light units having a power density of 800 mW/cm2 in clinical practice. The battery and for a unit emitting 400 mW/cm2, an exposure time of 40 s is importantWith standard metal brackets, recommended curing times for a complete cure are 15–20 s on the mesial and distal of each bracket using a halogen light, 10 s mesial and distal for LED lights, 4 s mesial and distal using an argon laser, and 2 s mesial and distal with a plasma arc lamp while ceramic brackets require only half of the total time. Bondable molar tubes require about 150% longer curing times on each of the mesial and distal aspect. Latest introduced light curing devices bonds the metal brackets within 6s time.

Distance between the light-curing tip and composite:

However, the decrease in light intensity of the light-curing unit was found not to obey the inverse square law for the distances 0–15 mm [22],[23],[24]Ideally, tip of curing light should be within 3 mm of composite to be effective.

When long wavelength of light is used, there will be more penetration of light and better curing.[25]Size of light-curing unit tip.

A light-curing unit standard diameter tip (11 mm) energy is more scattered, whereas in a light-curing unit with a smaller tip (3 mm), it is more focused and so less time to cure but at the same time more temperature can be dangerous to tooth pulp.

Which of them do you think the most appropriate technique to use??

To answer this question, we need to know some points:

Process of light curing is variable process with different factors affecting itThere is no single curing protocol that we can depend on it completely in curing all types of composite.

The ideal results from light-curing RBC:

No negative effects such as marginal staining and restoration fractureNo microleakage, debonding, recurrent caries, or postoperative painHowever, no clear correlation between contraction stress in dental composites and the success of composite restoration was found clinically.

How long does it take to adequately cure a composite?

Depends on energy density, distance from composite, collimation of light, wavelength, and composite type.

So, how long should I cure composite?

For this, refer to the manufacturer's instructions for guidance.

Factors influencing curing time as shown in [Table 2].{Table 2}

General considerations:

A good rule of thumb is that the minimum power density output should never drop below 300 mW/cm2Shifting from a standard 11 mm diameter tip to a small 3 mm diameter increases the light output eightfoldIdeally, the fiber optic tip should be adjacent to the surface being cured but this will lead to tip contaminationIntensity of light is inversely proportional to the distance from the fiber optic tip to the composite surfaceTherefore, the tip should be within 2 mm of composite to be effectiveLight transmitting wedges for interproximal curing and light focusing tips for access into proximal boxes are availableIntensity of the tip output falls off from the center to the edges. Hence, bulk curing of the composite produces bullet-shaped curing patternMost light-curing techniques require minimum of 20 s for adequate curingTo guarantee adequate curing, it is a common practice to postcuring for 20–60 s. Postcuring improves the surface properties slightlyMore intense curing units have been developed to hasten the curing cycles. For example, PAC and laser unitsRapid polymerization may produce excessive polymerization stress and weaken the bonding system layer against tooth structure.

For maintenance of light guide, do periodic visual inspection of unit such as filters and bulb [Figure 21], check irradiance using radiometer.{Figure 21}

If light tip contaminated, it reduces passage of light, reflects light increases heat build-up shortens bulb life, remove debris using polishing kit and blade [Figure 22].{Figure 22}

 How Can the Performance of the Light-Curing Unit Be Measured?



The light produced by the light-curing unit can be measured either directly or indirectly. It can be measured directly using curing radiometer and indirectly, in terms of the bond strength of the materials cured by each unit in clinical trials or laboratory studies.

Dental radiometer is specialized light meter that quantifies blue light output, to measure the effectiveness of the curing unit. It may be built in or small handled device [Figure 23].{Figure 23}

For optical safety do not look directly at light, wear eyes glasses and shields [Figure 24] and [Figure 25].{Figure 24}{Figure 25}

Comparison between shear bond strength of Halogen light and LED for bracket bonding.[26]

The halogen lamp provided the highest mean shear bond strength of brackets, but without statistical significance in relation to three other protocols performed with LED devicesThe 3M/ESPE LED device had shear bond strength of brackets similar to that obtained by halogen source, even with the protocol with 10 s of activationThe Gnatus LED device showed shear bond strength of brackets similar to the one obtained by halogen source, only with the activation protocol of 40 s, being significantly lower when used for 10 s.

Future development in light-curing system:

Organic LEDs [Figure 26] and [Figure 27] are flexible and extremely thin video display to be made, but at current technology, their output level remains below LED chips. Its utilized in impression tray with walls and floor lined with these emitting films which designed to evenly irradiate all surface of photo curable impression material. Organic LED used in vital bleaching and cementation of veneers.{Figure 26}{Figure 27}

 Recent Advances in Light-Curing Unit



Recently, Scanwave is been introduced by MiniLed™ (Aceton) which could be considered as the first fourth-generation LED light to come to the market [Figure 28], [Figure 29], [Figure 30]. Along with incorporating many of the ideal features of the best third-generation lights, other significant improvements have been incorporated into its design. It is the first of its type and hence discussed in detail. It features patented wavelength scanning technology incorporated into its mode selection. This enables the dentist to choose the most appropriate spectral output mode and radiation time for any possible material and clinical situation. It has four different diode wavelengths, the most of any dental LED LAU to date, offering broad spectrum curing in “Full Scan” mode for all resin-based materials, irrespective of their photoinitiator chemistry. The diodes are spaced off center which helps distributes the energy across the light guide face and prevents “central hot spots,” which can occur with high irradiance third-generation single blue diode LED units [Figure 31]a and [Figure 31]b. Preliminary investigations on a prototype Scanwave unit have revealed that by sequentially activating different diode wavelength combinations throughout the irradiation cycle in “Full Scan” mode, it allows good conversion in depth while minimizing heating effects, which are common with high irradiance second- and third-generation LED LAUs [Figure 8]. Beam profile imaging has revealed the sequential on/off nature of the different diode wavelengths in full and “soft” scan menus [Figure 31]c and [Figure 32]. Scanwave has dedicated bonding and orthodontic menus, allowing customization of irradiation time and wavelength selection for curing adhesives and restoratives in a timely manner, thus minimizing heating and associated polymerization stress events. By sequencing the activation of the different wavelength diodes in scan modes, the manufacturer has integrated broad-spectrum-curing capability for universal curing of all materials while eliminating overheating issues, which challenge unit stability. The soft scan menu allows advocates of “soft” polymerization to use ramp, pulse, and “soft stop” concepts in a single sequence, optimizing cure while negating high stresses possible with bulk polymerization of fast-setting high modulus materials and thermal stressing caused by sudden light cessation. Scanwave's dual button activation system, coupled with its modified pen-style handpiece, allows improved ergonomics by allowing either pen or gun style grasps. It has also been designed to meet best practice from a cross-infection risk viewpoint. The intraoral optical guide is removable for autoclaving, thus meeting the gold standard and eliminating the need for barrier protection, which may reduce light delivery significantly. The grasping part of the handpiece has a metal casing for efficient disinfection, and its exclusive cooling system obviates the need for a fan, thus avoiding stagnation of microorganisms within the unit body, which may be a cross-infection risk for patients and the dental team.[27],[28] The charging base of this cordless unit features a drain to avoid trapping cleaning fluids. Scanwave is also available in an OEM-corded version for integration into a dental unit. The award-winning inbuilt Laser target ring feature allows the operator to view and control the zone to be irradiated, maximizing light delivery [Figure 28]. This innovative unit sets the standard for the next generation of LED LAUs.{Figure 28}{Figure 29}{Figure 30}{Figure 31}{Figure 32}

 Conclusion



The commonly used term of irradiance measured at the light tip should no longer be used to describe the output of curing lights as it implies that this is the irradiance the specimen is receiving and takes no account of distance between the LCU and the RBC or the effects of beam inhomogeneityIdeally, both manufactures and researchers should include the following information about the LCU:

Radiant power output throughout the exposure cycle and the spectral radiant power as a function of wavelengthAnalysis of the light beam profile and spectral emission across the light beamMeasurement and reporting of the light the RBC specimen received as well as the output measured at the light tip.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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