Biomechanical response of a lumbar intervertebral disc to manual lifting activities: a poroelastic finite element model study.

STUDY DESIGN

Determination of damage to a lumbar disc caused by lifting using a poroelastic finite element model study.

OBJECTIVE

Compare the biomechanical response of a lumbar disc under 8 different lifting conditions and identify the loading conditions that produce the greatest deformations and highest stresses in various tissue components of the disc.

SUMMARY OF BACKGROUND DATA

Lifting has been associated epidemiologically with back injuries and back pain complaints. Forces high enough to cause mechanical damage to the disc have been predicted using analytical methods.

METHODS

Using kinematic data from series of lifts obtained in our laboratories and an electromyography (EMG)-driven muscle optimization model, representative force patterns for each of 8 lifts were obtained and used as input into a poroelastic finite element model. Disc tissue displacements and stresses were determined for all the 8 lift conditions. RESULTS.: Lifting the box from a maximally lateral flexed posture up to waist level (Task 5) produced the largest translational and rotational motions of all studied. This lift activity also produced maximum von Mises stresses in all disc components: annulus, nucleus, and endplates. The largest facet joint forces were also observed during lift Task 5.

CONCLUSION

Asymmetric lifting involving lateral bending of the trunk produced large motions that might cause localized disc tissue injury. Stresses larger than the failure strength of the corresponding disc tissues were experienced during asymmetric lifting. Lifting that involves lateral bending of the trunk was the most hazardous type of loading with regard to damage to the disc.