The aim of the present study was to validate the efficacy of the optical measurement system PONTOS 5 M compared with a conventional tactile measurement system. The results show that both measurement systems were capable of analyzing interfragmentary movements with high accuracy (resolution of about ≤5 μm). However, the optical measurement system was able to analyze 3D motions whereas the tactile system used in this study only performed 2D measurements of the fracture motion. For this reason it was only possible to compare the 2D data obtained using the optical measurement system with the corresponding data of the tactile measurement system. This is one limitation of the present study.
In order to assess the accuracy of the optical measurement system we attached passive markers of the optical measurement system to the spindle of the dial indicators, analysed the collapse of the measuring spindle (Figure ) and compared the values of the dial indicators with those of the optical measurement system.
Additional measurement: Direct comparison of both Systemes. For this purpose, the passive markers of the optical measurement system were attached to the spindles of the 3 dial indicators.
The resolution of the dial indicators is known to be 1 μm. The accuracy between the tactile measurement system and optical measurement system showed a mean difference of -5.3 μm (dial indicator 1), 0.4 μm (dial indicator 2) and -1.8 μm (dial indicator 3). The variability of the differences between the two methods was between 5 μm and 9 μm. This was only little wider than the error limit of the dial indicators. The difference between the two methods was comparable with the precision within the dial indicators. With the optical measurement system interfragmentary movement can be detected directly at the fracture gap as the passive markers are very small and can be attached nearly anywhere. Contrariwise using the digital indicators the measurement of the interfragmentary motion was only possible indirectly by using the cranks. The average deviation in interfragmentary motion measured directly at the fracture gap (PONTOS) and indirectly using the tactile system and the cranks was 120 μm (y-displacement), -1310 μm (z-displacement) and -4.1 degree (α angle). These results are rather surprising. The large difference of the measured interfragmentary motion can be explained by the set up of the tactile system using the cranks. The cranks become deformed by the mechanical load during the test which was shown by attaching the passive markers on the cranks (Figure ). Using the cranks tends to result in a decreasing of accuracy. This effect was more pronounced the higher the axial load was.
Test set-up. Two cranks were fixed to position the 3 dial indicators. Passive markers were fixed directly to the fracture gap.
In addition to the high accuracy of the optical measurement system, this system is much easier to use in comparison to the dial indicator method, because the passive markers are self-adhesive and, as mentioned, can be attached nearly anywhere. The dial indicators, in contrast, each requires a specially produced crank for attachment. This setup is not only time-consuming, but also expensive, and the accuracy of the measurement depends directly on the cranks. In contrast, each passive marker of the optical measurement system functions like a 6-DOF-sensor, so that several different data sets for the object can be obtained. Due to its high accuracy, PONTOS 5 M is regularly used in the automobile and airplane industry for car crash-tests or vibration-analysis of airplane wings. Therefore, large amounts of data are available from different testing setups. In biomechanical setups for musculoskeletal research, diverse types of fracture models have been validated. We decided to use a simple model with a transverse fracture gap in our study in order to exclude measurement deviations as far as possible. There are some publications that deal with the application of optical measuring systems in biomechanics. Here also the accuracy of the systems was part of the research. A common system in the biomechanical field is the Vicon system [15
]. A study by Windolf et al showed an accuracy of 64 ± 5 microns using the Vicon-460 system [16
]. Arbitrary changes in camera arrangement revealed variations in mean accuracy between 76 and 129 μm. This is less accurate than measuring with the PONTOS System.