Abstract
Purpose of the study
To incorporate magnetic resonance (MR) image data in a finite element (FE) model to estimate intervertebral disc stress as a function of posture.
Background
Determining the stresses on the intervertebral discs is important for understanding disc degeneration and developing treatment strategies. The effect of different postures on disc stress has previously been investigated through disc pressure measurements and through computational modelling. Kinematic data derived from MR images and used in an FE model may provide a non-invasive way of assessing a wide range of subjects and postures.
Methods
Two-dimensional FE models of the lumbar spine were created for four subjects. Vertebral bodies were modelled as rigid bodies, the disc was modelled with an isotropic elastic annulus (E = 2.5 MPa, ν=0.4) and nucleus (E = 1 MPa, ν=0.45). The geometry was defined from MR image data obtained in the supine posture; vertebral body translation and rotation were determined from images acquired in standing and sitting.
Results
The principle stress distribution in standing and sitting differed between subjects. Stress peaks occurred in different discs (L4L5 v L5S1) and in different regions of the annulus (anterior v posterior). In three subjects the compressive stress at L4L5 was largest in sitting, for the fourth subject it was largest in standing; shear stress at L4L5 was highest in sitting for all four subjects.
Conclusion
Kinematic data from MR images provides a way of assessing the effect of postural change on disc stress; inter-subject differences in L4L5 compressive stress are consistent with disc pressure measurements.
