Microhardness of Natural Unpolished Enamel Surface for Assessment of Dental Erosion

Objectives: Enamel microhardness testing is usually conducted using a Knoop indenter on a polished enamel surface. However, for erosion research models, the polished surface does not provide a true representation of how a natural tooth surface responds to erosion. The Vickers ISO standard specifically states curvature correction factors for Vickers microhardness, therefore it may be more appropriate to use this method on an unpolished enamel surface. This study aimed to validate the use of Vickers microhardness indenter on an unpolished enamel surface.
Methods: Polished and unpolished enamel samples were indented using Knoop and Vickers indenters in the central (zenith) area and imaged both using a 40x objective on the microhardness machine and also with a Tandem Scanning Microscope (TSM) with z-stacking reconstruction prior to measurement. Surface profilometry was used to calculate the diameter of curvature of the natural enamel samples in order to apply correction factors for convex Vickers indentation measurements using surface metrology software. Knoop and Vickers measurements of polished and natural unpolished enamel surfaces were measured and the variability of the baseline microhardness data were compared using F tests.
Results: For the polished samples, both the TSM and the 40x objective were able to produce analysable images for Knoop and Vickers, however for the unpolished only the TSM was able to produce analysable images for both Vickers and Knoop. The diameter of curvature varied between 4 mm to 26 mm which represented a correction factor ranging from 0.99 to 0.995 (i.e. there was only a ≤1% difference between the uncorrected and the corrected hardness values). The Knoop indentations in curved enamel showed significantly greater variation when compared with the Vickers (P<0.05).
Conclusions: For a polished enamel surface, measurement of Knoop microhardness indentations with standard optical microscopy is appropriate, however for an unpolished sample it is more appropriate to measure Vickers microhardness indentations using an imaging technique with an increased depth of field and applied correction factors.