Author's response to reviews Title: New method for detection of complex 3D fracture motion--Verification of an optical motion analysis system for biomechanical studies Authors:

Changes in the manuscript according to reviewers comments (Reviewers comments are printed in bold letters): Minor Essential Revisions 1. Page 3 and 4. Background and methods. PONTOS 5M system reportedly used for crash testing and vibrational analysis, but maximum15Hz frame rate described. Please explain why system for crash testing does not have higher frame rate? Page 3. Optical measurement system. Role of different passive and active markers used needs better explanation. It is not made clear here why 200 passive markers plus light emitting diodes are required to make the relatively simple measurements presented. 10. Page 6. Discussion, line 23, please state size of " very small " passive markers, possible in the methods section. Background The optical measurement system PONTOS 5M (GOM-Optical Measuring Techniques, Braunschweig, Germany) is an established system for motion analysis in the automotive and aerospace industry (used for crash-tests and vibration-analysis of airplane wings). Methods Optical measurement system For measuring the 3D fracture motion we used the optical analysis system PONTOS 5M (GOM-Optical Measuring Techniques, Braunschweig, Germany). The system is offered in 4 different configurations (5M, 4M, 12M or High Speed) with several camera resolutions (up to 4096 x 3072 pixel) and frame rates (up to 5000 Hz) to cope for diverse applications. The individual system consists of two CCD cameras. For the detection of the motion passive markers are required. These passiv markers are simple white dots. The size is adapted to the object size and camera resolution. The points are recorded and tracked by the PONTOS software. The accuracy of the measurements is directly addicted by the resolution of the cameras. We used white self-adhesive dots with a diameter of 2mm. At least 3 points per object are needed. For a higher accuracy we used all in all about 200 passive markers. It is not necessary to add the points in a special pattern. But doing that, the system could learn to identify the objects with the help of the different patterns. A group of points functions like a six degrees of freedom (6DoF)-sensor. The high number of points allowed fitting two cylindrical geometry elements which represent the physical cylindrical fracture members. The 6DoF motion could therefore be directly represented at the location in the centre of the the fracture gap for each member. Relative motion in all six degrees of freedom where analysed respectively.