Effects of nucleus pulposus location on spinal loads and joint centers of rotation and reaction during forward flexion: A combined finite element and Musculoskeletal study

The mechanical environment of the intervertebral disc and spinal loads are intimately associated with low back pain (LBP) caused by mechanical load. The cause and effect relationship between postures and pain has been therapeutically investigated and widely used to guide patient care. Shift of the nucleus pulposus (NP) inside the intervertebral disc when the spine changes posture in the sagittal plane, known also as NP migration, has been observed and quantified in in-vivo studies. However, the effects of this phenomenon on biomechanics of the disc and the surrounding spinal structures has not been investigated. This study aimed to quantify the effects of the NP location in a moderate flexion posture on spinal load and load-sharing using a Finite Element (FE) model of the L4-L5 functional spinal unit (FSU) driven by muscle forces, reaction forces, ligament forces and disc moment predicted by a Musculoskeletal (MSK) model of the trunk. The results showed that intradiscal pressure (IDP) and compressive force are sensitive to the nucleus location while the effects on the center of rotation (CoR), center of reaction (CTR) and moment rotation curves were negligible. Also, our findings revealed that FE models should consider the effects of NP location during bending to predict more realistic results as the nucleus displacement caused by disc bulge predicted by these models is much smaller than the real shift observed in in-vivo. In addition, this study confirmed that position of the rigid joint in MSK models that fix this latter to the CoR, must be modelled carefully for more accurate muscle forces and spinal loads prediction.