Surface form analysis on complex freeform organic structures – measuring erosive wear on human teeth in vitro

Abstract

Natural human enamel (NHE) is a complex freeform surface which has presented significant difficulties in measuring surface form change using non-contacting laser profilometry (NCLP). Measuring surface form change on NHE is a metrology proxy for measuring dental tooth structure loss, and characterising this using non-ISO parameters (volume, surface area, and normalised lesion depth) has been seldom studied due surface complexity and undetermined measurement errors. This study determines NCLP measurement errors (instrument repeatability and method reproducibility) for non-ISO parameters, characterises change in surface form on NHE following a dietary pH-cycling model. NHE (n = 1) was scanned consecutively twenty-times using NCLP with/without sample replacement producing consecutive surface profile data. Residual data was created after subtracting consecutive filtered profile data (80 μm, Gaussian filter), and mean (SD) volume, surface area, and normalised lesion depth was determined within a 1.5 mm circular region of interest (ROI). Volume error (expressed as height variation across ROI surface area) was 0.022 μm (instrument repeatability) and 0.149 μm (method reproducibility), whilst surface area error (expressed as percentage change of the surface area deviation across the entire surface area) was 0.034% (repeatability error) and 0.081% (reproducibility error). Sixty-four natural enamel surfaces taped with polyvinyl-chloride tape leaving 1.5 mm exposed ROI underwent dietary erosion cycling (three 5-min cycles, 0.3% citric acid w/v, pH 3.2) generating artificial erosion lesions. Samples were scanned with NCLP before/after each erosion cycle, scans filtered for microtexture, and after-erosion scans were subtracted from beforeerosion scans. NCLP results show mean (SD) volume, surface area, normalised depth, and 3D stepheight of the eroded area increased significantly after each erosion cycle, with no significant difference in calcium and phosphate release after each cycle. We demonstrate a robust and valid dental model with analysis workflow to measure surface form change in NHE using NCLP, improving understanding of measuring surface form change in complex freeform surfaces.

Superimposition of sequential scans to measure erosion on unpolished and curved human enamel

“…This process is analogous, but more complex, to the simple form removal techniques used in studies on polished enamel. It overcomes the issues of mathematically trying to use form-removal which would not work on eroded natural enamel samples, and allows the analysis of 3D step height using ISO 5436-1 [1,10,24].…”