Beyond the Surface: Mechanical and Porosity Gradients in Eroded Enamel

Enamel erosion alters the structural integrity of the tooth surface, which can be measured using indentation techniques. However, traditional single-load indentation methods assume homogeneity within the eroded enamel, overlooking potential stratification within the subsurface lesion. This study investigates the presence of mechanical and porosity gradients within the enamel following simulated dietary acid exposure and examines how lesion depth and structure change with continued erosion. We applied varying-load micro-indentation to human enamel subjected to citric acid challenge, revealing a distinct stratification of mechanical properties. A soft superficial layer (~1- to 2-µm thick) exhibited significantly reduced hardness and was easily removed by ultrasonication, indicating its fragility. Beneath this layer, mechanical properties stabilized despite prolonged acid exposure (~3 min), suggesting a saturation point in lesion development. Profilometric analysis confirmed that although material loss increased with erosion time, the depth of the altered subsurface zone remained constant. To explore the porosity distribution, we used a novel gold nanoparticle labeling technique coupled with synchrotron-based X-ray fluorescence imaging. Nanoparticles (~20 nm) penetrated to depths of 15 to 20 µm, aligning closely with mechanical gradients inferred from indentation measurements. These findings indicate that subsurface enamel exhibits not only mechanical stratification but also corresponding variations in porosity. Our results demonstrate the limitations of single-load indentation in characterizing erosion-affected enamel and highlight the utility of multiload approaches in detecting structural heterogeneity. The correlation between mechanical softening and increased porosity suggests that the enamel subsurfaces are differentially affected. These findings raise important implications for therapeutic intervention: should remineralization strategies shift from bulk mineral delivery to layer-specific, functionally informed repair?