Static and Dynamic Load Response of the Lumbar Spine in Flexion

Doctoral thesis, 1992

The aims of this study were to develop experimental methods for static and dynamic loading of spinal segments in vitro that would simulate flexion-distraction injuries (lap seat-belt injuries) and increase knowledge about the biomechanical response and the mechanisms of injury of the lumbar spine under static and dynamic (transient) loads in flexion. The experiments were carried out on the lumbar functional spinal unit (FSU) consisting of two adjacent vertebrae, the intervertebral disc and all intervening ligaments. The results will be used for the development of future preventive measures and as input data to establish injury criteria for the lumbar spine. The ultimate strength of the lumbar spine during static flexion-shear loading was determined on 16 lumbar FSUs by means of a new method of applying static combined loads. The specimens could resist a mean (S.D.) bending moment of 156 (11) Nm combined with a mean (S.D.) shear force of 620 (53) N before complete disruption occurred. The mean (S.D.) tensile force acting on the posterior structures was 2.8 (0.2) kN. The flexion angulation just before failure was 20o and the anterior horizontal displacement was 9 mm. The bone mineral content in the vertebrae appeared to be a reliable predictor of the ultimate flexural strength of the lumbar FSU. The threshold values for the loads and the deformations during static flexion-shear loading was determined on 10 lumbar FSUs. Before the first sign of a permanent deformation of the osteo-ligamentous components, the specimens could resist a mean (S.D.) maximum bending moment of 121 (10) Nm combined with a mean (S.D.) maximum shear force of 486 (38) N. The flexion angulation was 16o and the anterior horizontal displacement 7 mm. The absorbed energy at the initiation of trauma was 10 J. The threshold for injury occurred at about 80% of the ultimate flexural strength of the lumbar FSU. The bone mineral content in the vertebrae appeared to be a reliable predictor of the structural properties of the specimen at the threshold of flexion-distraction injury. The biomechanical responses of 48 lumbar FSUs 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 peak values of the applied load pulses 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. The tensile force affecting the posterior structures were 3-5 kN. The results showed that the magnitude of the applied load pulse 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. 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 FSU. The methods developed for static and dynamic loading of spine segments showed good repeatability, were easy to handle and had high flexibility. Load response and deformations of the specimens could be measured with high accuracy. The different results obtained for lumbar spine response to static and dynamic flexion-shear loading showed that the specimens could withstand higher loads and absorb more energy before injury occurred during dynamic loading, but the deformations were smaller. There is thus an indication of viscoelastic behaviour in the specimens. The results of this study indicate that dynamic experiments must be performed when injury mechanisms which occur in real life accidents are to be studied.