Impact of Personalized Mass Distribution on the Spinal Load Predictions of Rigid and Flexible Thorax Models during Forward Flexion

Accurate modeling of musculoskeletal (MSK) dynamics tailored to individual patients is crucial for evaluating adult spinal deformities, especially in the thoracic region. This research explores the impact of personalized upper-body mass distribution on predictions of spinal loads using both rigid and flexible thorax models in neutral standing and forward flexion postures. Two MSK models were employed: a rigid thorax model with thoracic spinal discs treated as rigid joints, and a flexible thorax model with thoracic discs modeled as spherical joints allowing three degrees of freedom. These models incorporated constant percentage base (CPB) and body shape-based (BSB) mass distributions, resulting in four configurations: Rigid-CPB, Rigid-BSB, Flexible-CPB, and Flexible-BSB. In neutral standing, the root means square error (RMSE) averaged 16.96% BW in the anteroposterior direction and 24.79% BW in the proximodistal direction between the Rigid-CPB and Flexible-BSB models. These errors increased during forward flexion to 43.72% BW and 84.86% BW, respectively. In the mediolateral direction, RMSE was 6.69% BW during forward flexion and 0.02% BW during neutral standing. The normalized RMSE (nRMSE) averaged below 8.5% in both postures. This study underscores the effectiveness of the Flexible-BSB model in simulating intricate spinal deformities, offering valuable biomechanical insights. It emphasizes the critical role of personalized mass distribution and thoracic flexibility in improving the precision of MSK simulations for clinical applications.