Mechanical response of the lumbar spine to seated postural loads

Study Design. In vitro force and deformation measurements formed the basis for a determinate, quasistatic analysis of principal forces in the seated lumbar spine. Objectives. To explore the relationship between seated postures and the mechanical response in component tissues of lumbar intervertebral joints. Summary of Background Data. Despite the high prevalence of low back pain syndrome, the precise mechanisms relating specific mechanical loads to spinal degeneration are not well understood. Simultaneous, time-dependent measurement of anterior column forces and articular facet forces has not been presented previously. Consequently, a determinate analysis of principal component forces has not been possible. Methods. Twelve lumbar spines (L1-S1) were subjected to constant loading conditions while in flexed and extended seated postures. Time-dependent forces were measured in the anterior column at the L4 and L5 superior endplates and in the four facets of the L3-L4 and L4-L5 motion segments. A quasi-static analysis of sagittal plane forces was used to compute the remaining principal joint forces, including ligament, disc shear, and facet impingement forces. Results. Component forces changed under static loading in both postures. There were significant differences between the mechanical responses of the two postures. Although the vertical creep displacement was greater in the extended seated posture (3.22 mm versus 2.11 mm), the escalation of forces was more severe in the flexed posture. Conclusions. The results suggest a mechanism of force balancing in lordotic postures under static loads, whereas flexed postures produce large increases to the tensile forces in the region of the posterior anulus.