Accuracy and repeatability of an optical motion analysis system for measuring small deformations of biological tissues

Optical motion analysis techniques have been widely used in biomechanics for measuring large-scale motions such as gait, but have not yet been significantly explored for measuring smaller movements such as the tooth displacements under load. In principle, very accurate measurements could be possible and this could provide a valuable tool in many engineering applications. The aim of this study was to evaluate accuracy and repeatability of the Qualisys ProReflex-MCU120 system when measuring small displacements, as a step towards measuring tooth displacements to characterise the properties of the periodontal ligament. Accuracy and repeatability of the system was evaluated using a wedge comparator with a resolution of 0.25 μm to provide measured marker displacements in three orthogonal directions. The marker was moved in ten steps in each direction, for each of seven step sizes (0.5, 1, 2, 3, 5, 10, and 20 μm), repeated five times. Spherical and diamond markers were tested. The system accuracy (i.e. percentage of maximum absolute error in range/measurement range), in the 20–200 μm ranges, was ±1.17%, ±1.67% and ±1.31% for the diamond marker in x, y and z directions, while the system accuracy for the spherical marker was ±1.81%, ±2.37% and ±1.39%. The system repeatability (i.e. maximum standard deviation in the measurement range) measured under the different days, light intensity and temperatures for five times, carried out step up and then step down measurements for the same step size, was ±1.7, ±2.3 and ±1.9 μm for the diamond marker, and ±2.6, ±3.9 and ±1.9 μm for the spherical marker in x, y and z directions, respectively. These results demonstrate that the system suffices accuracy for measuring tooth displacements and could potentially be useful in many other applications.