Light Curing in Restorative Dentistry

  Light Curing in Restorative Dentistry

Light Emitting Diode

INTRODUCTION

Light-activated dental materials have been available in dentistry since the 1970s. Today, most restorative practices utilize dental materials that are dependent on, or are aided by, photoinitiators when providing dental care to patients. It is estimated that more than 260 million direct resin composite restorations are placed worldwide each year.  These dental procedures are at risk of not performing at the highest levels if the techniques and equipment used to polymerize them are not understood, implemented properly, or routinely monitored.

Plasma arc light

Restorative materials that are aided by light curing may include sealants, dentin bonding agents, restorative composites, resin-modified glass ionomers (RMGIs), build-up materials, cements, and pulpal liners. Understanding the proper use and limitations of the dental light-curing unit can strongly influence the long-term outcome of our restorative procedures.

Argon Ion Laser

Dental curing units have evolved from ultraviolet lights to quartz tungsten halogen (QTH) lights to plasma arc lights to argon-ion laser lights and now to light-emitting diode (LED) lights. The goal of each new technology is to provide increased speed and depth of cure with less heat generation. Chemical changes in restorative materials have also been introduced with the same goals of faster and deeper depth of cure with less shrinkage and shrinkage stress while maintaining high physical properties.

Quartz tungsten halogen (QTH)

The most widely used light curnig technique , consist of quartz bulb with tungsten filament in it that emitbthe light in a halogen environment .

This type of curing light however has certain drawbacks, the first of which is the large amounts of heat that the filament generates. This requires that the curing light have a ventilating fan installed which results in a larger curing light. The fan generates a sound that may disturb some patients

Quartz Tungsten Halogen "QTH

laser ions travel in, phase with high energy high intensity but expensive so used in bleaching and sof tissue surgeries for the high intensity of it .

Argon Ion Laser

light-emitting diode (LED) lights

Light Emitting Diode LED

Basic concepts when using a curing unit 

* Measuring the Irradiance Levels 

Every dental office should have systems and equipment in place to routinely check the power levels of each unit in the office .  Measuring a larger-diameter-tip curing unit with a small-diameter sensor on the radiometer may give you a false value. Once you have confirmed that your light is emitting the manufacturer’s set irradiance level, record this value and use it as your control. If the value starts decreasing, locate the problem and correct it. Always confirm that your light tip is clean before each test or clinical use.

Concerns With High Irradiance Levels

The desire for shorter procedure times continue to drive the introduction of ultra-high-power lights. Concerns include increased shrinkage stress, pulp tissue damage due to heat, and potentially inadequate cures of bulk-cure materials. Shrinkage stress is one factor that is responsible for clinical failure of composite restorations. High-intensity lights can increase stress at the composite/tooth interface when the pre-gel phase time is reduced, causing less flow of the material during the initial moments of polymerization.

All light energy generates heat. Lights have been introduced with irradiance levels up to 7,000 mW/cm2. Many studies use a threshold temperature increase value of 5.5° C as the value when potential harmful effects may occur to the pulpal tissue.Many of the available light-curing units on the market can generate a heat increase of pulpal tissue that can exceed this value if care is not used. Techniques such as continuous air flow over the tooth during curing  and a spray of water following the final cure should be considered to lower any risk to the pulpal tissue.

When using ultra-high-powered lights to cure bulk-filled composites, short exposure times with high irradiance results in a significantly lower depth of cure and degree of polymerization compared to longer exposures and lower irradiance values. This would support that the previously discussed energy density formula is not always the complete answer. When working with light units designed with a selection of power settings, it may be preferred to keep the power in the 1,000 to 1,500 mW/cm2 range and calculate the required curing time from this setting.

 * Color/Composition of the Composite

Color and type of composite resin material can also change the required curing times and affect the degree of polymerization. Darker shades of composite resin material could be cured for double the time as a lighter shade of similar material and still not achieve the same degree of polymerization or depth of cure. Some Microfil and flowable composite resin materials have also been shown to require additional curing times. When working in deep areas of a preparation where color is less important, it is preferred to use lighter color composite resin material . Bulk-fill composites follow these same rules and should always be limited to a single increment of no more than 4.0 mm. Multiple smaller increments are always a better choice .

 * Tip Diameter

Practitioners should evaluate the internal light source diameter at the tip of the light guide. This diameter should allow full coverage of the length and width of the restoration to be cured . If not, multiple exposures should be used to cure a single restoration that is larger than the tip selected. It is also important that your light unit has adequate total power as the light-tip area increases. The irradiance equals power/tip area. Increasing the tip diameter will require more total power to maintain the same irradiance levels as a smaller tip. Obviously, a 12-mm tip diameter may seem better than a 7-mm one, but only if the power is uniform over the entire tip area. This may explain why we continue to see smaller tip light guides on many of the units available. A low-power unit can still generate a high irradiance value if the tip diameter is smaller.

 * The Dangers of Blue Light

It has been discussed how important it is to position and maintain the light tip in the ideal relationship to the material being cured. It is also well understood that all curing lights emit blue light in the hazard zone, 440 nm. When the retina has multiple exposures to low levels of blue light, it can be absorbed, leading to accelerated macular degeneration.  Many dentists may use the “turn away” method of protection. Looking at the light for only a second to confirm positioning during each curing cycle presents a great risk to your eyes when understanding that the damage is cumulative over years of practice. The use of unfiltered magnification/loupes can be of even greater concern when looking at an activated light. If you do not confirm placement, the light position can change, causing under-polymerization. There are many filter designs to help block blue light. These designs include glasses, oral filters, nose cones placed over the light guide, and loupes with built-in filters. 

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