Gravity center estimation for evaluation of standing whole body compensation using virtual barycentremetry based on biplanar slot-scanning stereoradiography - validation by simultaneous force plate measurement

Background Whole body standing alignment (WBSA) in terms of biomechanics can be evaluated accurately only by referring the gravity line (GL) which lies on the gravity center (GC). Here, we introduce a method for estimating GL and simultaneous WBSA measurement using the EOS® imaging system and report on the reproducibility and reliability of the method. Methods A 3-dimensional (3D) avatar to estimate GC was created following three steps: 3D reconstruction of the bone based on EOS images; deformation into a generic morphotype (MakeHuman statistical model) before density integration with 3D rasterization of the full body into 1-mm3 voxels (the content of each voxel is considered homogeneous); computation of the density of all the voxels provides the center of mass, which can be projected onto the floor as the GC of the full body, providing the GL in relation to the WBSA. The repeatability, reproducibility, and accuracy of the estimated GC and body weight of the avatar were compared with clinical estimation using a force plate in healthy volunteers and patients with degenerative and deformative diseases. Results Statistical analyses of the data revealed that the repeatability and reproducibility of the estimation was high with intra-rater and inter-rater intraclass correlation coefficient. ≥0.999. The coordinate values of the GC and body weight estimation did not differ significantly between the avatar and force plate measurements, demonstrating the high accuracy of the method. Conclusion This new method of estimating GC and WBSA is reliable and accurate. Application of this method could allow clinicians to quickly and qualitatively evaluate WBSA with GL with various spinal malalignment pathologies.

Analysis on influence of static calibration on the axle-group weigh-in-motion system accuracy

The axle-group weigh-in-motion system has two functions: static weighing and dynamic weighing. According to the weighing model, the accuracy of dynamic weighing is affected by the static performance. This paper analyses the size of various factors affecting the static performance, such as sensor tilt installation, platform deformation, platform tilt installation, and these errors will lead to sensor swing, bearing head tilt, gravity line of action and sensor axis direction is not consistent, thus affecting the static weighing accuracy. However static calibration is the best way to reduce or even eliminate the above errors. The dynamic truck scale of different manufacturers with or without static calibration is used in the test process. The results show that the dynamic performance index can meet the requirements only after the static calibration is used.

What is the best lateral radiograph positioning technique for assessment of sagittal balance: A biomechanical study on influence of different arm positions

Purpose: To evaluate the influence of different arm postures from the physiological standing position using force plate analysis of the gravity line. Methods: Forty healthy volunteered university students were enrolled. Each subject assumed different standing positions including standing with arms resting on the side (control), with fist over the clavicle (clavicular position), with active shoulder flexion in 30°, 60° and 90° with elbows extended (active flexion A), with hand rest on a bar with a static support (passive flexion P), and with hand rest on a bar with a drip stand (passive flexion D). The offset of the gravity line from the heel was measured by force plate analysis. The offset of the gravity line in different arm positions was compared with the control using paired t-test. Results: The mean anterior offset of the gravity line in control position is 39.80% of the foot length. All testing positions showed anterior shift of the gravity line compared with the control position from 0.51% to 7.50%. There were statistically significant changes of the gravity line from the control position in all ( p < 0.05), except in the clavicular position ( p = 0.249). Conclusion: All testing positions cause anterior shifting of the center of gravity from the physiological standing position. Clavicular position is the best comparable posture to the physiological standing position in taking a lateral radiograph. We recommend using the clavicular position as the standard testing position in the assessment of the sagittal profile.