Novel superimposition technique for in-vivo quantitative wear study

Accurate validation of techniques and reference points are problematic for in-vivo quantitative wear studies. Descriptions of instrument capability have rarely distinguished between precision (the level of instrument variability) and accuracy.

Objectives: This study reports the development of an accurate and a reproducible method for in-vivo wear measurement.

Methods: The impressions taken at different intervals were scanned using a UBM digitizer¹.  The digitized images were transferred into numerical data files using the UBSoft software².  These files were transferred into 3D cloud of points images using a mathematical software program2.  Using the Scan-Surf software³, the cloud of points images were transferred into a superimposible 3D modelled images.  The accuracy of measurement taken using this technique was compared, by measuring step-heights of objects with known heights, with measurements taken using UBM, a Digimatic Micrometer⁴ and the Tandem Scanning Microscope⁵. The measurements were taken by an operator and an assessor. The data for accuracy were analysed using Kruskal-Wallis non-parametric analysis. 

Results: This technique measures only one surface of the tooth with margin of errors ±3.5 µm from the UBM and 0.001-0.1 µm from selection of reference points.  Statistically, there was no significant difference (p< 0.05) between measurements of step-heights taken with the Digimatic Micrometer, UBM, surface modeling and TSM. Also no significant difference between the measurements taken by the operator and the assessor.     

Conclusions: Surface Modelling Technique reduced the number of errors related to impression technique, using additional features as reference points and the clinical factors related to patient variables. Within the limits of possible sources of error the procedure was reproducible. The accuracy tests described here have validity and are applicable as standard for three-dimensional measuring instruments and surface modelling technique.

1-UBM, Messtechnik GmbH, Germany. 2-Mathcad professional, MD, USA. 3-Scan-Surf, 3D Digital Corp, USA. 4-Digimatic Micrometer Mitutoyo Corp., Japan. 5-TSM, Noran Instruments, USA.