BY: DR PALLAVI MADANSHETTY

DENTAL CURING LIGHTS- (PART I)

QTH LAMP

INDEXHistoryUV light curingTypes of light curing unitsComponents of QTH lampPhysics behind QTH lampRadiometer Calculation of curing timeProperties of QTHAlternative photoinitiatorsAdvantages and disadvantages of QTH curing unit.

HISTORYInitially, resin based composites were chemically activated. They were bulk-filled with the direction of polymerization shrinkage towards the center of the mass.

During mixing it was almost impossible to avoid incorporating air into the mix.

Provided no control with the working time.

Increased finishing time.

Had less color stability due to breakdown of tertiary amines.

To overcome these problems, UV light activation was first introduced to the dentistry in 1970s.

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.

UV photocuring dramatically changed the direction of dentistry. These curing units were designed to emit ultraviolet light(about 365nm)through a quartz rod from a high pressure mercury source.The photo initiating system relied on benzoine- ether type compounds which broke down into multiple radicals, without need of an intermediary component.

But UV curing lights had several drawbacks like;Limited ability of the light to penetrate deep within the material (1-1.5mm of penetration)Altered the oral micro flora.Units needed warm up time of 5 minutes.

If human (operator and patient’s eyes) are exposed to short wavelength UV energy , there is danger of formation of corneal burns and cataract.

Therefore they were replaced by visible blue light-activated systems

The early 1980s led to further advances in the photocuring of dental materials. The key progress was identification of Camphoquinone (CQ) as an ideal photoinitiator. In the process of photocuring, in order to transform a monomer into a polymer, photoinitiators, which break down into radicals when irradiated with light, are required. The advented CQ’s absorption range was found to be between 370-500 nm with a peak at 468 nm, and therefore allowed the use of visible blue light for photocuring of composite resins.

THE INNOVATION

ABSORPTION SPECTRUM OF CQ

CQ

VISIBLE LIGHT SPECTRUM

CQ

There are Four main types of polymerization sources:

1. Tungsten –halogen(QTH)

3. Plasma arc4. Light –emitting

diode(LED)

2. LASER

QTH LAMP

COMPONENTS OF QUARTZ- TUNGSTEN –HALOGEN (QTH)LAMP

QTH

Halogen lamps use an incandescent light bulb consisting of a tungsten filament surrounded by the halogen gases that give them their name.

Basis for light production heated objects emit electromagnetic radiation.

In the case of halogen lamps, light is produced when an electric current flows through a thin tungsten filament.

PHYSICS BEHIND QTH CURING UNIT

When a tungsten filament is heated to approximately 100 ºC, it gives off heat energy in the form of infrared radiation.

When the temperature is increased between 2000C and 3000 ºC, a significant portion of the radiation is emitted in the visible light spectrum.

100ºC

2000-3000ºC

NOTE : To provide visible light for photo polymerization, halogen lamps must be heated to a very high temperatures. Therefore, the QTH lamps are not energy efficient.

When a tungsten filament in the QTH bulb is heated, 5% of the total energy is visible light, 12% heat, and 80% light emitted in the infrared spectrum and some amount of UV light is also produced.

Visible light Heat

Infra-red

RANGE OF WAVELENGTHS COVERED BY QTH LAMPS

As, the QTH unit produces long band width spectrum, preferential production of blue light is not possible with this kind of technology.In order to produce the blue light required for the activation of Camphorquinone, unwanted portions of the spectrum have to be filtered out. As a result, the largest part of the radio active power of this light source is wasted.

An entire Visible light spectrum, IR radiation and UV light is emitted than the preferred blue light spectrum when a tungsten filament is heated.

NOTE: Since QTH lamps emit a rather wide range of wavelengths, band-pass filters are required to limit the wavelength between 370 and 550 nm in order to fit the peak absorption of Camphorquinone. This approach yields an efficiency rate of only 0.5%; other 99.5% of the energy is given out as heat.

HEAT

UV light

IR

Visible light

QTH LampBand pass filters

Blue light

So, the central drawback of the halogen lights is the need to overcome waste heat produced during a wide spectrum light production. Therefore, a cooling fan is required for heat dissipation, which makes the curing unit noisy and produces vibrations.

NOTE: Halogen curing lights must not be turned off until the fan has stopped running, as it will overheat.

Because of the degradation of the bulb, reflector and filter over time, caused by high operating temperatures and considerable quantity of heat being produced during operating cycles, QTH lamps have a limited lifespan of 100 hours.The lamp reflector may lose its reflective properties because of loss of reflective material, or deposition of surface impurities.

Filter coatings may become pitted, chipped, or flaky and the filters themselves may crack or break. This implicates a reduction of curing efficiency over time by aging of the components. The power output may also deteriorate over time due to insufficient maintenance of the light sources and especially the light tips. Resin contamination on the curing unit tip tends to scatter the light, considerably reducing the effective output.

NOTE: The light guide must be kept free of resin buildup either by using a sleeve cover to protect the light guide or using acetone to remove any residue that has formed. When necessary, an appropriate rubber wheel on a slow speed hand piece can be utilized.

MAINTENANCE OF QTH UNITS

Many QTH lamps used in dental offices operate beneath the minimum power output specified by the manufacturers.

Unless they are properly maintained and the output monitored, they could be a source of sensitivity due to uncured resin.

Therefore, the power output must be constantly checked with the help of a radiometer. Newer models house a radiometer right in the unit.

CAUTION : A study by Friedman showed that polymerization units used in dental practices have lost 45-89% of their initial light intensity

EXTERNAL RADIOMETER

BUILT IN RADIOMETER

HOW TO CALCULATE THE CURING TIME?

• TOTAL ENERGY CONCEPTAs a general rule, a dose of maximum 16,000 mW/cm² is required to adequately cure an increment of 2 mm.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: 40 seconds at a light intensity of 400 mW/cm²=

40 s x 400 mW/cm² = 16,000 mW/cm²).Therefore, QTH lamp which produces an average power density of 400 mW/cm² has a minimum of 40 sec curing time.

NOTE : Based on this maximum value, various curing times can be calculated depending on the light intensity of the polymerization light used.It is estimated by Ruggeberg that 280

mW/cm²is the minimal intensity necessary to adequately polymerize a 2 mm thick increment of universal shade composite. This minimally acceptable intensity is very similar to the 300 mW/cm² value reported by Bayne.

QTH

Wave length emitted 400-500nm

Band width Broad

Time of curing in seconds

40-60 sec

Performance Low energy performance

Power density 500 mW/cm²

PROPERTIES

VITAL TOOTH BLEACHING AND INTRODUCTION OF THE “ALTERNATIVE INITIATORS”Vital tooth bleaching was becoming an overnight success in the early 1990s. However, following bleaching, manufacturers were not able to provide direct esthetic restorative materials that were of high enough value to match the newly bleached teeth.

LASER CURING LAMP

This situation arose because the photo initiating system of used CQ, which is bright canary yellow, and photo bleaches only slightly upon exposures within clinically relevant times.Thus after curing, the restoration tended to have a yellow, residual tinge. To overcome this problem, manufacturers have incorporated a variety of other photoinitiators such as phenylpropanedion (PPD) or Lucirin TPO into light-activated restorative materials to minimize the yellowing effect and inefficiency of CQ.

LASER CURING LAMP

These class of photo initiators directly break

into multiple radicals without need for any co-

initiator, and although they are pale yellow in

color, get photo bleached to clear, once utilized.

These “alternative” initiators are much more efficient

than CQ, but require light of a much shorter wavelength

(between 350 and 430 nm in the violet range)

Their downside is that violet light will not penetrate

deeply into restorative materials, and thus CQ must always

be present.

Therefore, to activate a product containing both of these

photo-initiators, the light curing unit must be capable of

emitting in both the violet and blue regions.

A QTH lamp that generates photons continuously between 400

nm (violet) and 500 nm (just past the blue region), will

work well with these composites.

Curing light emission and photoinitiator absorption

spectra

QTH lamps cover the whole radiation absorption spectrum of CQ and part of the spectra for PPD and Lucerin.

ADVANTAGES DISADVANTAGES Longest history in dentistry. Low cost technology.Broad bandwidth allows curing of composites containing photo-initiators other than Camphoroquinone

Continuous spectrum must be narrowed by filter systems.Low energy performance, since use of the filter wastes (99%) of radiation.Generation of high temperatures (need for ventilating fan to cool the lamp) so, they are loud.May deteriorate over time due to insufficient maintenance of the light sources.Long exposure time of 40-60sec.Greater pulpal heat if prolonged curing times are used.

HALOGEN LAMPS

WARNING The future for QTH light is not “bright”, as many governmental agencies are banning this tremendously inefficient incandescent light sources from general usage soon. US govt. has stipulated deadlines by which incandescent light sources will be stopped being marketed: elimination of the 100-W bulb in 2012 and removal of 40-W source by 2014.State-of –the –art: Dental photo curing- a review Frederick A. Rueggeberg dental materials 27 (2011) 39-52

Visit https://manipaluniv.academia.edu/pallavimadanshetty for other curing units.

REFERENCES Developments in polymerization lamps Amparo Jimenez-

Planas, Juan Martín, Camilo Abalos, Rafael Llamas

(Quintessence Int 2008;39;180.e74–84).

Light curing of resin-based composites in the LED era

Norbert Kramer, Ulrich Lohbauer, Franklin García-

Godoy, and Roland Frankenberger, Am J Dent 2008;21:135-142 .

Exposure Times for Contemporary Composites Frederick

A. Rueggeberg, 2013 ;Vol 25: 2 82–84 Journal of

Esthetic and Restorative Dentistry.

Effect of light intensity and exposure duration on cure of

resin composites. Rueggeberg FA, Caughman WF, Curtis JW Jr.

Oper Dent 1994;19(1):26-32.

State-of –the –art: Dental photo curing- a review Frederick

A. Rueggeberg Dental materials 27 (2011) 39-52

Light Emitting Diode - The unsung heroes Dr.Deepak.J.Parekh

Monday, 14 December 2009http://www.onlinedentallearning.com

Generated: 2 November, 2011, 17:58

Light Curing Devices-A Clinical Review Dr.Tarun Kumar Singh,

Dr. Ida Ataide, Dr. Marina Fernandes, Dr. Rajan T. Lambor

Journal of Orofacial Research Volume 1: Issue 1 : 2011

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