Dynamic load response of the in vitro lumbar spine in flexion

Tue biomechanical response of the in vitro lumbar motion segment (functional spinal unit, FSU) under a dynamic (transient) flexion-shear load was detennined. Tue load was transferred to the specimen by a padded pendulum and simulated a flexion-distraction injury, a so called lap seat-belt injury. Tue load response of the specimen was measured with a force and moment transducer, and the motions were detennined with high speed photography. Two series of tests were made with 10 specimens in each, with two different load pulses: one moderate pulse (mean acceleration 2.5 g, duration 150 ms) and one severe pulse (mean acceleration 8 g, durarion 250 ms). Tue resuJts showed that the moderate load pulse caused initial flexion-distraction injuries at a mean bending moment of 1 13 Nm and a mean shear force of 346 N. Tue maximum flexion angulation attained during the loading sequence was 14 •. Tue severe load pulse caused evident signs of failure or total rupture of the segments at a mean bending moment of 1 5 1 Nm and a mean shear force of 481 N. Tue flexion angulation just before failure was 19·. A statistically significant correlation (r>0.7, p<0.05) was found between the load response and the height of the segment, the load response and the lateral disc diameter, and the load response and the bone mineral content (BMC) in the venebrae. Comparisons were made with previous established thresholds for st.atic flcxion-shear loading. Tue results indicated that thresholds for initial and ultimate flexion-distraction injury respectively are in the same range for static and transient loading conditions.