Structural Characterization of the Eroded Enamel Subsurface Layers

Objectives: This study aimed to distinguish subsurface eroded enamel zones associated with dietary erosion by quantifying their response to indentation under varying loads, and investigating their temporal evolution during acid erosion.
Methods: 24 unrestored caries free, facial enamel samples were randomly assigned to three groups (n=8) and subjected to a single erosion cycle with 0.3% citric acid (pH 2.7) for 1, 10, or 60min. Samples underwent Vickers microhardness testing at 10 different loads (0.01–2kgF), followed by 30min of ultrasonication and repeat testing. Subsequently 8 further samples were exposed to sequential erosion cycles at intervals of 20s, 40s, 1, 2, 3, 4, 5, 10, 20, 40, and 60min, with microhardness testing at four loads (0.01–0.5kgF) after each time point. Indentation depths were estimated from the indenter geometry. Statistical analyses included two and one-way ANOVA with Tukey's HSD at α=0.05.
Results: Microhardness testing under multiple loads (accounting for indentation size effects) revealed distinct stratification of the mechanical response. Depth estimation suggested the effect of erosion extended to ~16µm. A notably soft superficial layer (1.27, 1.45 ± 0.72, 0.76µm) formed within 10 to 60min of erosion and was entirely removed after ultrasonication (p<0.01) Lower loads (0.01–0.02kgF) significantly discriminated the microhardness of this thin layer from deeper, less affected regions. Beyond 3min of erosion, microhardness values (at each load) were not significantly modified by increasing erosion time (p>0.99).
Conclusions: Varying micro-indentation loads successfully delineated the mechanical properties of eroded enamel layers and demonstrated that after a short period, further erosion produced minimal changes to the subsurface. Understanding the thickness and mechanical properties of these distinct subsurface zones can inform targeted remineralization strategies aimed at preserving or restoring vulnerable enamel regions.