Sensitivity of Thoracolumbar Spine Musculoskeletal Model Loading in Neutral Standing and Forward Flexion Static Postures to Thoracic Disc Stiffness

This study aimed to quantify the sensitivity of a thoracolumbar musculoskeletal model with a flexible thoracic spine and articulated ribcage to disc flexural stiffness variation inherent from in-vitro cadaveric data. The model was personalized to a normal weight subject, whose upper body segmental masses and centers of mass were computed using a body-shape-based approach. Joint flexural stiffness curves were defined based on in-vitro flexion-extension moment-rotation data from several specimens taken at various thoracic levels, with stiffness variation reaching 1.2 Nm/deg. Neutral standing and forward flexion postures were simulated using in-vivo measured spinal rhythm. The finding revealed negligeable sensitivity of joint reaction forces, less than 2.5% Body Weight (BW), to flexural stiffness in neutral standing posture. The sensitivity became more pronounced, especially at mid-level thoracic joints, with deviations reaching up to 14% BW and 26% BW for antero-posterior shear and compressive forces, respectively, in 60-degree forward flexion. Very low Root Mean Square Error (RMSE) and normalized RMSE values, calculated using intradiscal pressure based compressive forces, indicated no effects of flexural disc stiffness variation on model prediction validation. The findings underscored the importance of cautious consideration when utilizing flexural stiffness from a single cadaveric specimen.