Can Dietary Acid Effect Simulated Post Eruptive Maturation in vitro?

Objectives: To determine if dietary acids can effect simulated post eruptive maturation (PEM) using an in vitro pH-cycling model.
Methods: 48 ground and polished bovine enamel samples were divided into six treatment groups (n=8/ group) so mean microhardness (SMH) values were not significantly different. Samples subsequently underwent a 5 day pH cycling regime of 3([F] 0, 57, 228ppm)*2(Overnight Storage artificial saliva (AS)/ humid chamber) factorial design. Samples were subjected daily to four erosive challenges (1min, 1% w/v, pH 3.8) with two treatments (2mins, NaF) after the first and last erosive challenge, for 5 days, and stored in AS after each erosive or treatment phase (1h, 16.0mM KCl, 4.0mM KH₂PO₄, 1.0mM CaCl₂.2H₂O, 0.2mM MgCl₂, 4.5mM NH₄Cl, 20.0mM C₈H₁₈N₂O₈HEPES buffer, pH 7). Samples were stored overnight in either a humid chamber or AS. SMH was remeasured and bulk tissue loss determined by profilometry.
Results: Humid chamber samples had a ΔSMH of 55.7, 47.3, 70.7kgmm^-2 for 0, 57 and 228 ppmF respectively. Bulk tissue loss occurred with step height formation of 2.67, 2.10, 1.63μm for 0, 57 and 228ppmF respectively. 228ppmF was significantly different to 0 and 57 ppmF (P<0.05). The increase in softening and reduction in bulk tissue loss of the 228ppmF may have occurred through the deposition of a precipitate layer. Overnight AS samples had a ΔSMH of 66.5, 60.9 and 58.7kgmm^-2 for 0, 57 and 228ppmF, no significance was detected. No net bulk tissue loss was observed. No significant dose response was seen for either pH-cycle due to the very mild erosive challenge.
Conclusions: The current in vitro conditions do not provide evidence of dietary acid mediated PEM. Possible CaF₂ precipitate layers have formed which would require other techniques to verify. The extension of the pH-cycling model may allow for PEM re-hardening to occur.