Modélisation de la colonne vertébrale thoraco-lombaire humaine en position neutre debout: Distribution des charges et analyses de stabilité

The human spine is constantly exposed to combined loads and displacements, static and dynamic, for short and long durations according to the type of activities and exercises carried out.

The kinematics redundancy in biomechanical models of complex joints, such as the human spine, has presented an obstacle in estimating the muscle forces as well as joint reaction loads. Accurate determination of load distribution among passive and active components of the human trunk in various recreational and occupational physical activities is of prime importance for the determination of optimal postures and exercises, design of implants, and effective prevention, evaluation and treatment of spinal disorders.

The stability requirement of the passive-active system under external loads and postures is also important and needs to be considered. The passive ligamentous thoracolumbar and lumbar spines are known to exhibit large displacements or hypermobility (i.e., instability for an imperfect system such as the spine) under compression loads <100 N. Bearing in mind that such compression forces are only a small fraction of those carried by the spine in recreational and occupational activities; the question thus arises as to how then the system is stabilized in vivo?

A previously developed sagittally symmetric T1-S1 beam-rigid body model based on kinematics, while accounting for nonlinear ligamentous properties and trunk musculature, combined with optimization, solved the redundant active-passive system by a novel kinematics-based approach that used both the posture and gravity/external loads as input data (Pop, 2001). This model is made of six deformable beams to represent T12-S1 discs and seven rigid elements to represent T1-T12 (as a single body) and lumbosacral vertebrae (L1 to S1). A sagittaly symmetric muscle architecture with 46 local and 10 global muscles was used. The local muscles attach the pelvis to lumbar vertebrae (except the iliopsoas (IP) that originates from proximal femur), and the global muscles attach the pelvis to the thoracic cage. The computational model was performed to investigate the muscle activity, internal loads, and system stability margin of the human spine in neutral standing postures under gravity ± loads carried either in front of the body or on sides.

The system stability is examined using both linear buckling and nonlinear analyses assuming various muscle stiffness values. The former is performed using the updated geometry and stressed conditions of the spine at the final configuration to evaluate the stability margins as a function of the muscle stiffness coefficient q. Nonlinear analyses are performed for different q values thus identifying the critical q value above which a convergent solution in a force-controlled loading environment exists; i.e. the structure remains stable. These analyses of such an imperfect system are more accurate and reliable than the linear stability analyses often performed on the undeformed and unstressed system. (Abstract shortened by UMI.)