A New Method for Finding the Shoulder Complex Rotation Centre Using 3D Body Scanning

To avoid radiation exposure to the body, the rotation centre of the shoulder complex is often assessed using the Optotrak motion capture system instead of X-ray imaging. Recently, 3D body scanning techniques evolved from static to temporal scanning. This study investigated if a time series of 3D body scans may be an alternative for motion capture systems focussing on the shoulder complex rotation centre which is relevant for exoskeleton alignment to the user’s body. 13 Male participants participated in this study (age 24.8±2.4 years, stature 182.2±5.5 cm, body mass 80.7±7.3 kg). Motion capture data (Optotrak) and 3D body scans (3dMD) were collected while the participant performed predefined movements (abduction, anteflexion and a combination of the two). The helical axes method was used to calculate the rotation centre from motion capture data as reference. The 3D scans were processed modelling the upper arm as a cylinder or as a set of perpendicular slices to the centroid axis. Also, a point 30 millimetres below the acromioclavicular joint, assessed from the 3D scan, was defined as the conventional method. The mean distance from the rotation centre of each individual to the mean rotation centre of the cylinder (19 mm), centroid (18 mm), conventional (16 mm) and helical axis method (21 mm) were much smaller than the distances between the 3D scan derived methods and the helical axes method (43-49 mm). The shoulder complex rotation centre location differed considerably between the four methods. Therefore, X-rays of the shoulder are necessary as a golden standard to indicate which method is closest to the real rotation centre and to determine the correction factor for each method that removes the systematic error. 3D body scans contain valuable anthropometric information and have the potential for biomechanical modelling since the random error does not exceed the error of the Optotrak motion capture system.