Seeing caries before the radiograph: transillumination and laser fluorescence

Early interproximal caries is not visible to the naked eye, and the radiograph shows it once demineralisation has already involved a substantial portion of tissue.

Optical methods sit in the gap between those two moments, detecting earlier changes without using ionising radiation.

Transillumination rests on a simple principle: sound enamel transmits light, demineralised tissue scatters it. The lesion therefore appears as a dark shadow against a bright background.

The classic application concerns the interproximal surfaces of anterior teeth; devices working in the near infrared extend the assessment to posterior sectors as well.

The absence of radiation has one concrete operational consequence: the examination can be repeated as often as needed, which makes it suitable for close monitoring of a lesion that has deliberately not been treated.

Laser fluorescence works differently. Light of a specific wavelength excites porphyrins produced by bacteria; altered tissue emits more fluorescence than sound tissue, and the device converts this into a numerical value.

That number is where most mistakes are made, because it gives the impression of being an objective measure of caries when it is a measure of fluorescence.

False positives are numerous and predictable: calculus, exogenous staining, prophylaxis paste residue, plaque and some restorative materials all produce elevated values in the absence of caries.

One non-negotiable operating rule follows: the tooth must be cleaned and dried before measurement. Measuring a dirty surface means quantifying the dirt.

The main clinical risk is overtreatment. A high value read as an indication to drill leads to opening early lesions that, with risk control and fluoride, would have remineralised.

Correct use is as an additional element within a picture that includes visual inspection, assessment of the individual patient's risk, and radiography, never as data that decides on its own.

The greatest value emerges in comparison over time: on the same site, with the same cleaning procedure, a value rising across several recalls says far more than a single number read once. The bitewing remains the reference for interproximal surfaces, and these tools mainly help in deciding between monitoring and intervening.

In summary: transillumination exploits light scattering in demineralised tissue, fluorescence measures emission from bacterial porphyrins, the number is not a direct measure of caries, calculus and residue produce false positives if cleaning is skipped, and the main risk of uncritical use is overtreatment.