Understanding load-sharing in the spine during in-vivo conditions is critical for better spinal implant design and testing. Previous studies of load-sharing that considered detailed spinal geometry applied compressive Follower Load (FL), with or without moment, to simulate muscle forces. Other studies used musculoskeletal models included muscle forces, but model the discs by simple beams or spherical joints and omitted the articular facet joints. Therefore, it is imperative to develop a model that is able to predict the load-sharing and stress/strain distribution in passive structures while accounting for muscle forces. The current study developed and validated a computational tool that combines a musculoskeletal model (MSK) and a Finite Element (FE) model of lumbosacral spine to predict spinal loads and load-sharing in neutral standing and forward flexion postures. The model also investigated the effects of lumbo-pelvic rhythm and intra-abdominal pressure (IAP) during flexion on spinal load-sharing.

First, the MSK of the upper body available in AnyBody modeling environment was improved and validated. This model predicted muscle forces in the postures studied using inverse statics and considered the IAP variation due to the posture changes from upright to forward flexion. The muscle, gravitational forces and disc moment at the thoracolumbar junction T12-L1 were applied to the FE model as external loads to predict spinal load and load-sharing. The FE model was also validated using in-vivo data. Forward flexion was simulated in the MSK model by employing the spine rhythm measured in a previous in-vivo study. The FE model predicted intradiscal pressure (IDP) in the muscles, strains in the annular fibers, contact forces in the facet joints, and forces in the ligaments. The load-sharing of a spinal component at a given level is defined as the percentage of the total force/moment at that level resisted by that spinal component. The results revealed that spinal loads, which increased substantially from the upright to the flexed posture, were mainly supported by the discs in the upright posture, whereas the ligaments’ contribution in resisting shear and moment was more significant in the flexed posture.

Previous in-vivo studies suggest that the ratio of total lumbar rotation over pelvic rotation (lumbo-pelvic rhythm) during trunk sagittal movement is essential to evaluate spinal loads and discriminate between low back pain and asymptomatic population. Also, MSK models require the lumbo-pelvic rhythm to predict muscle forces, joint reaction forces and moment. This study also investigated the effects of three lumbo-pelvic rhythms defined based on in-vivo measurements on the spinal response during moderate forward flexion (60°). The developed tool was used to compute the disc force and moment, IDP, annular fibers strain, and load-sharing. The results revealed that a rhythm with high pelvic rotation and low lumbar flexion involves more global muscles and increases the role of the disc in resisting spinal loads, while its counterpart, with low pelvic rotation, recruits more local muscles and engages the ligaments to lower the disc loads. On the other hand, a normal rhythm that has balanced pelvic and lumbar rotations yields almost equal disc and ligament load-sharing and results in more balanced synergy between global and local muscles.

In addition, most of the MSK and FE models employed to study the spine behavior omit the IAP, a parameter that plays an important role in reducing the spine loading. Hence, the predictions of these models in terms of spinal loads are not realistic. The effects of IAP variation in forward flexion on spinal loads and load-sharing were also investigated using this novel tool. Two IAP settings (ON/OFF) were considered in the MSK model and the trunk muscle forces and reaction forces at the junction T12-L1 were compared. The effects of IAP on spinal loads and loadsharing were determined as well. The findings confirmed the unloading role of IAP, especially at large flexion angles. Inclusion of IAP reduced the global muscle forces, disc loads as well as IDP. The drop in disc loads was compensated by an increase in ligament forces. The annular fibers strain and IDP were more sensitive to IAP at upper levels of the spine.

The findings of this work are beneficial to clinical applications and disc implants design, and are expected to improve knowledge of spinal response in upright posture and forward flexion.