Time-dependent changes in the lumbar spineʹs resistancc to bending

Objective. To show how time-related factors might affect the risk of back injury. Design. Mechanical testing of cadaveric lumbar motion segments. Background. High bending stresses acting on the lumbar spine are associated with injuries to the intervertebral discs and ligaments. Since these soft tissues are viscoelastic, the bending stress (‘bending moment’) must depend on the speed of movement and the duration of loading, but this has not previously been quantified. Methods. Forty-five cadaveric lumbar segments, consisting of two vertebrae and the intervening disc and ligaments, were loaded in combined bending and compression in order to simulate movements and postures in living people. The relationship between flexion angle and bending moment was determined at different loading rates, and after sustained loading in bending and in compression. Results. Rapid flexion movements increased the peak bending moment by 10–15% compared to slow movements. On average, repeated flexion over a period of 5 min reduced the peak bending moment by 17%, and 5 min of sustained flexion reduced it by 42%. Two hours of compressive creep loading reduced the height of the intervertebral discs by 1.1 mm, increased the range of flexion by 12%, and reduced peak bending moment by 41%. Conclusions. The scale of these changes suggests that, in life, the risk of bending injury to the lumbar discs and ligaments will depend not only on the loads applied to the spine, but also on loading rate and loading history.