This study determined the biomechanical response of the in vitro lumbar motion segment (functional spinal unit, FSU) under a dynamic flexion-shear load. Two test series were conducted using 10 specimen in each, with two different load pulses. These are: (1) a moderate pulse (mean acceleration 2.5 g, duration 150 ms); and (2) a severe pulse (mean acceleration 8 g, duration 250 ms). The results showed that the moderate load pulse caused initial flexion-distraction injuries (so-called lap seat belt injuries) at a mean bending moment of 113 Nm, and a mean shear force of 346 N. The maximum flexion angulation attained during the loading sequence was 14 degrees. The severe load pulse caused evident signs of failure or total rupture of the segments at a mean bending moment of 151 Nm, and a mean shear force of 481 N. The flexion angulation just before failure was 19 degrees. A statistically significant correlation 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 vertebrae. Comparisons were made with previous established thresholds for static flexion-shear loading. The results indicated that thresholds for initial and ultimate flexion-distraction injury respectively are in the same range for static and transient loading conditions.
Abstract