Assessment of Soft Tissue Movement to Develop a Kinematic Model of the Lumbar Spine

Motion analysis of skeletal structures through surface methods is generally affected by errors due to marker positioning on the subject and to the unavoidable soft tissue movement (STM).This study aims to quantitative evaluation of such errors in the lumbar spine motion analysis by applying an innovative protocol, joining MRI measures with optoelectronic techniques. Twelve healthy subjects participated to this study: six predefined postures were acquired (from maximum extension to maximum flexion). The 3D position of a suitable number of skin markers and the orientation of the related vertebrae were extracted by a specifically developed software. Thus, the relative position of markers with respect to the corresponding vertebrae was computed and the effect introduced by STM was estimated through the analysis of the data taken from all postures assumed by the subjects. The coming out results are consistent in terms of accuracy and repeatability. The accuracy of measures made on the MRI images, suitably calibrated through a grid of markers to compute the distortion errors, was usually better than the image resolution; the repeatability of the results was generally better than 2 mm. Data show a consistent error due to STM above all in L4, L5 and S1 vertebrae (up to 24 mm). In order to show the STM effect in relation to the acquired posture, some relations have been also proposed and validated on a sample of 6 subjects.