Influence of spinal disc translational stiffness on the lumbar spinal loads, ligament forces and trunk muscle forces during upper body inclination.

Inverse dynamic musculoskeletal human body models are commonly used to predict the spinal loads and trunk muscle forces. These models include rigid body segments, mechanical joints, active and passive structural components such as muscles, tendons and ligaments. Several studies used simple definition of lumbar spinal discs idealized as spherical joints with infinite translational stiffness. The aim of the current sensitivity study was to investigate the influence of disc translational stiffness (shear and compressive stiffness) on the joint kinematics and forces in intervertebral discs (L1-L5), trunk muscles and ligaments for an intermediately flexed position (55°). Based on in vitro data, a range of disc shear stiffness (100-200N/mm) and compressive stiffness (1900-2700N/mm) was considered in the model using the technique of force dependent kinematics (FDK). Range of variation in spinal loads, trunk muscle forces and ligaments forces were calculated (with & without load in hands) and compared with the results of reference model (RM) having infinite translational stiffness. The discs' centers of rotation (CoR) were computed for L3-L4 and L4-L5 motion segments. Between RM and FDK models, maximum differences in compressive forces were 7% (L1-L2 & L2-L3), 8% (L3-L4) and 6% (L4-L5) whereas in shear forces 35% (L1-L2), 47% (L2-L3), 45% (L3-L4) and more than 100% in L4-L5. Maximum differences in the sum of global and local muscle forces were approximately 10%, whereas in ligament forces were 27% (supraspinous), 40% (interspinous), 56% (intertransverse), 58% (lig. flavum) and 100% (lig. posterior). The CoRs were predicted posteriorly, below (L3-L4) and in the disc (L4-L5). FDK model predicted lower spinal loads, ligament forces and varied distribution of global and local muscle forces. Consideration of translational stiffnesses influenced the model results and showed increased differences with lower stiffness values.