Curing-Light Dose in a Deep Class II Composite Restoration
A dentist is restoring a 4 mm deep Class II cavity in a mandibular molar with a light-cured composite. The LED curing unit has an 8 mm diameter light guide and the manufacturer states an irradiance of 1,200 mW/cm² at the tip. The composite's instructions state that 16 J/cm² is required for adequate conversion. The dentist holds the guide 2 mm from the occlusal surface, but for the gingival portion the guide is angled 30° from normal. The composite is shade A3.5, and the restoration is built in one 4 mm increment. The assistant suggests a 20 s cure and says 'the meter at the tip says 1,200, so it is fine.' A radiometer placed at 2 mm from the guide reads 800 mW/cm². The dentist asks whether the cure is adequate and what to change.
1. Calculate the radiant exposure delivered in 20 s at the tip and at the measured 2 mm distance, ignoring the angle. Is the assistant's 20 s adequate?
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At the tip: 1,200 mW/cm² = 1.2 W/cm². Radiant exposure in 20 s = 1.2 x 20 = 24 J/cm², above the 16 J/cm² requirement. At the measured 2 mm distance: 800 mW/cm² = 0.8 W/cm²; 20 s gives 16 J/cm², exactly at the threshold before angle and attenuation. With a 30° angle, effective irradiance = 800 x cos 30° = 693 mW/cm². Time needed = 16 / 0.693 = 23.1 s. So the assistant's 20 s is not adequate for the angled gingival portion, and it is marginal even before composite attenuation. The occlusal surface may be fine while the gingival floor is undercured. The correct decision is to increase time, improve angulation, and cure in smaller increments, not to trust the tip specification.
2. Explain why the 4 mm bulk increment is a problem even if the surface receives 16 J/cm². Include attenuation, shade, and C-factor.
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A 4 mm bulk increment is a problem for three reasons. First, composite attenuates and scatters light; irradiance falls roughly exponentially with depth, so the bottom may receive far less than the surface. Many composites have a depth of cure around 2 mm, and darker shades are worse. Second, shade A3.5 contains more pigment and photoinitiator absorbers, so it transmits less light than a lighter shade. Third, a large MOD cavity has a high C-factor: the composite is bonded to many walls, so polymerization shrinkage cannot flow and instead generates stress at the margins. That stress can open the gingival margin, cause fluid movement and cold sensitivity, and allow recurrent caries. The top can look well cured while the bottom is undercured. Use 2 mm increments, cure from occlusal and gingival directions, and consider a higher-output unit or longer exposure.
3. The dentist says 'the meter at the tip says 1,200, so it is fine.' What is wrong with using the tip reading, and what should be measured instead? Include the inverse-square approximation.
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The tip reading is the output at the light guide, not the irradiance at the restoration. The guide is an extended source with a diverging beam, so the inverse-square law (I proportional to 1/d²) is only an approximation; at 2 mm the measured value has already fallen from 1,200 to 800 mW/cm². Angle, distance, saliva, blood, and the matrix further reduce delivery. Composite itself attenuates light, so the bottom of a 4 mm increment receives much less than the surface. A radiometer at the tip cannot detect a damaged guide, debris, or poor coupling. Measure with a dental radiometer at the actual working distance and angle, check the central and peripheral output, inspect the guide for scratches and cured composite, and use a unit with feedback if available. The clinical question is the dose at the bottom of the cavity, not the label on the box.