Erosion Lesion Detection using Photothermal Radiometry and Modulated Luminescence

Methods: Extracted caries-free human third molars (n=4) were collected and individually fixed on LEGO® bricks. The experimental set-up consisted of a semiconductor laser (659nm, 120mW), a mercury-cadmium-telluride IR detector for PTR, photodiode for LUM, two hardware lock-in-amplifiers, and computer for data processing. Smooth-surface erosion lesions were created using 37% phosphoric acid gel (Scotchbond, 3M) and freshly squeezed orange juice (OJ). PTR-LUM frequency and line scans (5Hz and 200Hz) were performed at 0, 5, 10 and 20 seconds for acid-etch samples and 0.5, 1, 3, 6 and 24 hours following immersion for OJ treated samples. Orange juice samples were coated in acid-resistant nail-varnish, excluding a 2x4mm window extending from enamel to root. Following erosion treatments, lesions were validated with transverse microradiography (TMR) and micro-computed tomography (MicroCT).

Results: With both erosive agents, PTR-LUM signals (amplitude and phase) showed gradual and consistent changes with treatment time. As early as 5s following acid-etch and 30min following OJ exposure, an initial depression in PTR amplitude and phase signals was consistent with the onset of surface mineral loss which continued in unidirectional fashion for the duration of treatment. A more pronounced curve separation in the high-frequency range (100Hz-1kHz) is consistent with mineral loss beginning at the anatomical tooth surface. LUM signals exhibited high sensitivity, however, less contrast than PTR due to baseline shifts. Surface wear of erosion lesions was identified with TMR. Mineral loss and lesion depth parameters and linear attenuation coefficient of microCT validated lesion presence.

Conclusion: This proof-of-concept study demonstrated the sensitivity of PTR-LUM in detecting and monitoring early erosion lesions on enamel and root surfaces.