Clinical: Bulk-fill composite resin thermography and curing irradiance

Dr Chris Callahan explores the new landscape of composite resin curing on behalf of the ADA’s Dental Instruments, Materials & Equipment (DIME) committee, with the help of two recent scholarly articles.

Changes in composite resin curing technology have developed a new generation. The polymerisation chemistry of conventional polymer chain addition has been changed with the inclusion of addition fragmentation chain transfer (AFCT) components to improve crosslinking. Bulk-fill composite resins using this technology have the potential to achieve greater depth of cure.

Several variable factors need to be considered in evaluating this advance: Spectra emitted by curing lights; Irradiance intensity of different curing light technology; Duration of cure; Composite resin chemistry including monomers, filler content, viscosity and photoinitiators. The application of these factors may lead to different outcomes in total polymerisation (degree of conversion; DC), polymerisation shrinkage, microporosity inclusions inside material and estimation of material durability.

Degree of conversion should not be confused with depth of cure, despite the coincidence of the initial letters. 

Light curing systems have utilised several different technologies. Quartz halogen lights used filters (410-500nm) to eliminate infrared and pass blue spectra. Light emitting diodes (LED) produced a narrow spectrum (440nm) blue light. Multiple LED of different spectra in blue (420-500nm) and violet (380-420nm) may permit the use of different photointiator chemistry. Laser systems produce a high intensity single frequency colour 455nm.

Output irradiance intensity has evolved from Quartz halogen 100mW/cm2, LED 400mW/cm2 to now 1,000-3,000mW/cm2, depending on the system selected. Such intensity is mostly measured at the tip of the curing light, not the surface of the composite resin in a clinical setting.

The combination of irradiance intensity and curing time can produce a measure of the radiant exposure energy in Joules/cm2. Radiant exposure has been shown to be a better measure of DC than irradiance intensity. The necessary figure of Joules/cm2 will vary, dependent on the material selected.

The classic Camphorquinone (CQ) photoinitiator produces an additive polymerisation reaction, which has the disadvantage of leaving pockets of unreacted monomer within the cured material. The AFCT modification of monomer with Beta ally sulphone or other chemistry allows a higher DC, improved toughness, decreased shrinkage, and decreased shrinkage stress.

This inclusion of AFCT chemistry allowed shorter curing times with similar DC. However, it has become evident that too high an initial curing irradiance may lead to variation of DC in the deeper segments of a filling.

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A 4mm test sample has become the standard for in vitro testing depth of cure and variation of DC through the material. Two recent journal articles may help readers understand these issues. Excellent literature reviews of the topic and analysis of the materials are present in these articles:

- Thanoon H, Price RB, Watts DC. Thermography and conversion of fast-cure composite photocured with quad-wave and laser curing lights compared to a conventional curing light. Dental Materials 40:3 2024, pp. 546-556.

- The effect of high-irradiance rapid polymerization on degree of conversion, monomer elution, polymerization shrinkage and porosity of bulk-fill resin composites. Lempel E, Szebeni D, Őri Z, Kiss T, Szalma J, Lovász BV, Kunsági-Máté S, Böddi K. Dental Materials 39:4 2023, pp. 442-453.

The first study considered the thermographic analysis of heat generated within different curing modes of three different curing lights: a single-colour blue LED, a quad-colour LED and a diode laser.

The temperature change and DC were strongly correlated for all different curing systems. High irradiance for shorter cure times caused the least temperature change, and also the least DC. Low irradiance for longer cure times caused higher, but not excessive temperature change and greater DC. Temperature change was also proportionate to depth in the material.

Observed temperature change was not considered to be clinically significant or hazardous to pulpal tissues.

The second study compared the result of different curing modes from a 385-515nm spectrum LED light on two different bulk-fill composite resin materials.

Radiant exposure varied from 7.5 to 19 Joules/cm2 depending on the settings. Again, the highest irradiance settings produced the lowest DC and the greatest variation in DC throughout the 4mm test material depth.

In conclusion, while it would seem that higher intensity should lead to a greater depth of cure, this has not been proven to be so in several materials. The need to ensure the consistent polymerisation of material will require longer curing times at lesser intensity.

This article was first published in the ADA's News Bulletin, June 2024