PILP Remineralized Dentin Lesions Have Two Zones of Distinct Properties

We evaluated structural differences by SEM/EDS and TEM in the two remineralized zones that resulted from PILP remineralization of artificial dentin caries.

Method:

140 µm deep artificial dentin lesions were produced with  0.05M acetic acid buffer at pH=5.0, and  remineralized using a polymer-induced-liquid-precursor (PILP) process  with calcium phosphate solution containing  27 KDa Poly-L-aspartate as the precursor inducing agent. Remineralization occurred at pH=7.4 and 37°C for 0,7,14, or 28d.   Mechanical property profiles were obtained by nanoindentation and mineral profiles by micro x-ray computed tomography (MicroXCT™).  Cross sections were studied using SEM/ EDS at differing depths and  times. TEM/SAED (selected area electron diffraction)  was used to determine matrix ultrastructure, mineral orientation and crystallinity.

Result:

Demineralization produced two distinct zones: an “Outer Zone”(OZ) of 60 µm  from the surface with modulus values below 0.2 GPa and a “Graded Zone”(GZ) over 80 µm that  exhibited increasing modulus from 0.2 to 20 GPa at the  normal dentin. Remineralization occurred from the bottom of the GZ toward the surface with significant recovery in properties within 7d. The OZ recovered 50% of its properties after 28d with complete recovery in total mineral. Both zones were microstructurally different than normal dentin with mineralization within each dentin tubule in the OZ, and around the periphery of the tubules in both zones. At 7d,  OZ gained intrafibrillar mineral.  After 14d, plate-like hydroxyapatite completely filled collagen fibrils in the GZ and displayed characteristic 67-nm type-I collagen D-bands. Crystallinity and alignment of intrafibrillar minerals increased with growth time.  The OZ had total mineral recovery but intrafibrillar mineralization was incomplete within 28d, accounting for the incomplete recovery of mechanical properties.

Conclusion:

The PILP method can effectively restore mechanical properties of dentin lesions, but achieved different outcomes within two zones: the GZ regained full intrafibrillar mineralization, while the OZ did not within 28d.