A METHOD FOR STUDYING THE BIOMECHANICAL LOAD RESPONSE OF THE (IN-VITRO) LUMBAR SPINE UNDER DYNAMIC FLEXION SHEAR LOADS

A method was developed to study the biomechanical response of the lumb ar motion segment (Functional Spinal Unit, FSU) under a dynamic (trans ient) load in flexion. In order to inflict flexion-distraction types o f injuries (lap seat-belt injuries) different load pulses were transfe rred to the specimen by means of a padded pendulum. The load response of the specimen was measured with a force and moment transducer. The f lexion angulation and displacements were determined by means of high-s peed photography. Two series of tests were made with ten specimens in each and with two different load pulses: one moderate load pulse (peak acceleration 5 g, rise time 30 ms, duration 150 ms) and one severe lo ad pulse (peak acceleration 12 g, rise time 15 ms, duration 250 ms). T he results showed that the moderate load pulse caused residual permane nt deformations at a mean bending moment of 140 Nm and a mean shear fo rce of 430 N at a mean flexion angulation of 14-degrees. The severe lo ad pulse caused evident signs of failure of the segments at a mean ben ding moment of 185 Nm and a mean shear force of 600 N at a mean flexio n angulation of 19-degrees. Significant correlations were found betwee n the load response and the size of the specimen, as well as between t he load response and the bone mineral content (BMC) in the two adjacen t vertebrae. Comparisons with lumbar spine response to static flexion- shear loading indicated that the specimens could withstand higher bend ing moments before injury occurred during dynamic loading, but the def ormations at injury tended to be smaller for dynamic loading.