TRAUMATIC INSTABILITY OF THE LUMBAR SPINE - A DYNAMIC IN-VITRO STUDY OF FLEXION - DISTRACTION INJURY

Study Design, This in vitro study determined the effect on the lumbar spine of a dynamic flexion-distraction loading simulating a lap seatbe lt injury. The proportion by which the forces and the moments contribu ted to the injury of the lumbar spinal segment in such a situation was analyzed. The remaining stability of the injured lumbar motion segmen t was determined together with the threshold for lumbar spine instabil ity in such an injury. Objectives. Based on the experimental results i n this study, radiographic guidelines for instability criteria in lumb ar and thoracolumbar dislocations in the sagittal plane without concom itant compression fracture of the middle column were proposed. Summary of Background Data, A number of checklists and guidelines were sugges ted for the diagnosis of spinal instability after trauma, but no concl usive system was established. Those systems were mostly based on exper iments performed on spinal segments after sequential ablation of ligam ents and facet joints followed by static, unidirectional physiologic l oading. We believed that there was a need for more profound knowledge of spinal injury and for instability criteria of lumbar spinal injurie s based on more realistic experimental data simulating the clinical si tuation. In our injury model, we decided to study the biomechanic outc ome of a flexion-distraction injury similar to seatbelt type injury se en in frontal motor vehicle collisions. Methods. Twenty lumbar functio nal spinal units were first loaded statically with a physiologic flexi on-shear load to determine angulations and displacements under noninju rous conditions. Dynamic flexion-shear loading to injury with two diff erent load pulses was then applied. Static physiologic load was then a gain applied to determine any permanent residual deformation. Results, The viscoelastic effect of loading rate on translatory and angular di splacements and the values for translatory and angulation displacement s at first sign of injury (yield) and at failure were determined, Conc lusions. Radiographic guidelines for instability criteria in lumbar an d thoracolumbar fracture-dislocations without concomitant posterior ve rtebral body compression are proposed: 1. Instability exists if there is a kyphosis of the lumbar motion segment greater than or equal to 12 degrees (impending instability) or greater than or equal to 19 degree s (total instability) on lateral radiographs. 2. Relative increase in interspinous process distance greater than or equal to 20 mm (impendin g instability), greater than or equal to 33 mm; (total instability) on anteroposterior radiographs.