Bite - In

This month's Internals presents a very nice insight into the workings of one of the everyday equipment of the modern day dentist. Dr. Milind Karmarkar lucidly highlights the mechanisms of a light cure unit.

THE LIGHT CURE UNIT

The basic piece of dental equipment that has made esthetic and cosmetic dentistry possible, is the light cure unit. This device as we all know is used for photoactivation i.e. initiating polymerisation (curing) of light cure and dual cure resin based dental composite restorative materials and cements.

Principle of Working: The device uses visible light in the 460-470 nanometer(nm) range to which the photoinitiator, an alpha-diketone in the composite resin is sensitive and thus, initiates the light reaction of about 20 seconds wherein initial polymerisation occurs. This is taken to completion by the Dark Reaction, which begins when the light is switched off and continues for about 20 to 24 hours.

Types Light Cure Devices can be classified as Intra Oral or Extra Oral Types. In this communication only the Intra Oral type is being discussed. The Intra Oral Light Cure Device is used clinically and is of four basic types:

a) Wand Type - In this the bulb is in a remote unit connected to the wand with a fibre optic cable.

b) Gun Type - In this the bulb is in the gun with a fused quartz probe.

c) Cordless - This is battery operated and therefore unreliable

d) Unit Less - This does not have a base unit and all the components are included in the gun.

The most common type of unit is the gun type and this will now be discussed at length.

PARTS AND ACCESSORIES

A) The Light Source (Bulb) - The light source generally is housed in the gun. A variety of standard parabolic reflector halogen bulbs are used. Of late, specialised bulbs are used like the 14 Volt, 35 Watt which have more light output with less heat and are more efficient and have a better rated life. Faulty bulbs with damaged reflectors due to heat or aging are likely to give lesser light output and cause incomplete curing of the composite material.

B) The Filter - Visible light in the 460-470nm is achieved by a special coated filter that also cuts off harmful Ultraviolet (UV) and Infra Red wavelengths from the bulb. A damaged filter (pitted by excessive heat or where the coating has rubbed off or cracked glass) can cause severe pulp damage leading to tooth hypersensitivity due to overheating. Hence, the filter is the most important component of the LCU.

C) The FibreOptic Guide - It is an autoclavable, detachable curing tip made of fused quartz, image grade fibres and are available in a range of shapes and diameters for different clinical situations. They transmit light from the bulb, through the filter along a fixed path to the area of clinical interest. The standard is a curved sixty degree tip with 8mm diameter. But various tips are used from the 13 mm straight, short tip for laminate veneers to the turbo tip or focu-tip for better light concentration and utilisation in specific areas like Class II cavity fillings and periodontal splinting.

This enhanced intensity increases both the extent and the depth of composite resin polymerisation which can influence its physical and biological properties. An 8mm Dual/Twin tip is used for simultaneous curing from opposite directions. As the direction of light controls the direction of shrinkage towards it, by proper utilisation, one can have controlled shrinkage used to one's advantage. The cumulative light output of the gun LCU is a direct result of the interaction of the three parts.

D) Fan - Full lamp life, light intensity and safety from heat build up is ensured by a silent, powerful exhaust fan to extract and expel the heat from the bulb which is encased in a metal housing that also acts as a heat-sink.

THE CONTROL UNIT

This is connected to the gun by an electrical cable/card and consists of -

(a) A Transformer Unit : (from 220/110 volts AC to 12/14 volts DC) with voltage regulation and current surge suppression circuits for a high, constant light intensity and bulb safety.

(b) Audible Timer Unit : With an auto-reset variable timer activated by the gun trigger switch, for providing accurate curing time and

(c)A glass fuse with external housing for overload protection

LIGHT METERS

A variety of them are available from Analog to Digital. They are also stand alone or control unit incorporated. The digital ones are more reliable but expensive. They consist of a photocell, filter and indicator unit. They are used to check the intensity of light output in mw/cm2, over a certain range of wavelength of the same LCU over a period of time. With usage, the bulb intensity drops and it has to be replaced, when the reading is low.

The filter also needs changing, when it gets damaged/pitted with excessive heat build up when the reading is low. A damaged filter can cause excessive heat and light transmission, giving a very high reading (over 700 mw/cm2). This can cause irreversible hard and soft tissue damage.

Imp. Note: Light meters should not be used to compare two different Light Cure Units as their filters and therefore, wavelengths tend to differ between manufacturers of Light Cure Units and composite resins.

Reference: Curing Efficiency of the Turbo Tip - Curtis, Lee et al. General Dentistry - Sept-Oct, 1995, Page 428.

MAINTENANCE AND SERVICING

Simplicity is a big plus in serviceability and the simpler and less complicated the unit, the better it is to use, maintain and service. Periodic maintenance is better than repair. One should refer to the manufacturer's instructions for periodic maintenance and repair.

General Tips

1. The LCU should be used in a cool, dust free environment and kept free from dust by covering it when not in use.

2. The fibre-optic tips have to be periodically cleaned by using alcohol/spirit with a soft cloth after the debris of cured composite resin is removed with a blunt, scratch proof instrument. The tip should always have a clean shiny appearance on its surface.

3. Composite resin or other debris on the surface of the curing tip/probe can reduce light output considerably, (as much as 50-60 %) even on a new LCU.

HAZARDS & PRECAUTIONS

Eye protection for both the patient and the operator is essential in the form of photobloc orange spectacles or shields for visible light in the 400-500 mm range and UV light transmission from a damaged or cracked LCU filter, either directly or reflecting off a flat tooth surface is harmful to the eyes and has a cumulative effect over a period of time.

Retinal damage in the form of retinal burns leading to cataracts or macular degeneration can occur. Thus, one should never stare at the direct light output of an LCU. Excessive heat build up is another hazard and can damage the internal parts of the gun and control unit of the LCU or the operator's hand and patient's face.

Thus, Light Cure Units with too much light and too much heat should be generally avoided and newer and better models can be used for better efficiency.