The biomechanical responses of 48 lumbar motion segments (consisting of two adjacent vertebrae, the intervertebral disc and all intervening ligaments) exposed to loads similar to those in frontal car accidents were determined by means of a new method of applying dynamic (transient) flexion-shear loads. The applied loads caused flexion-distraction injuries in the specimens, so called lap seat-belt injuries. Good repeatability and high accuracy were achieved with the dynamic load application method.

The peak values of the applied load pulses in the present experiments varied between 5-12 g, with a rise time between 5-30 ms and a duration between 150-250 ms. The specimens could withstand loads up to 225 Nm and 720 N in flexion before obvious fractures occurred. Signs of injury in the bony structures were observed at lower load levels. The energy absorption of the specimens varied between 15-35 J. The tensile force affecting the posterior structures were 3-5 kN and the deformation rate of the posterior ligaments during the loading sequence varied between 0.5-2.6 m/s.

The results showed that the magnitude of the applied load pul.se and the loading rate determined the degree and severity of spinal injury. The duration of the load pulse did not affect the load and injury response. The specimens could withstand higher loads and absorb more energy when the loading rate was increased, but the deformations at injury were smaller when the loading rate was high. There is thus an indication of viscoelastic behaviour in the specimens. The biological parameters bone mineral content, anterior-posterior length and height of the specimen showed high correlations with the dynamic load response of the lumbar spine.