[Application of atomic force microscopy in evaluation of three-dimensional morphology of eroded human enamel]

To compare the three dimensional morphology and surface roughness changes of enamel eroded for different etching time.

Fifteen freshly extracted sound human pre-molars for orthodontic purpose were collected. The buccal surface of teeth were prepared into smooth enamel slices, and then were randomly divided into 5 groups based on their etching time 0 s (control group), 5 s, 10 s, 20 s, 30 s, respectively by 37% phosphoric acid. The three dimensional morphology was observed under atomic force microscope (AFM). The profile was analyzed, and the value of Ra, Rq, Rz and the surface area and volume were measured.

The AFM photograph showed that with the etching time from 0 s to 20 s the enamel surface demineralised gradually, the top structure of enamel rod and the fish scaled structure became obvious. But the morphology only changed a bit after 20 s. The surrounding inter-rod enamel eroded first, the depth increased to 2.8 µm at 20 s but decreased to 1.8 µm at 30 s. The value of Ra increased from (19.69 ± 3.42) nm to (359.51 ± 75.79) nm, and Rq from (22.02 ± 5.57) nm to (431.02 ± 83.09) nm, Rz from (0.24 ± 0.08) µm to (2.38 ± 0.26) µm. Except for groups 20 s and 30 s, the difference among other groups was statistically significant (P < 0.05). The surface area expanded from (406.77 ± 3.88) µm(2) to (546.69 ± 84.02) µm(2), and surface volume from (65.73 ± 14.46) µm(3) to (474.63 ± 52.50) µm(3).

The depth, surface roughness, surface area and volume caused by erosion increased with etching time. The three dimensional morphology greatly changed by acid-etching process.