Multi-planar bending properties of lumbar intervertebral joints following cyclic bending

Objective. To assess the changes in the multi-planar bending properties of intervertebral joints following cyclic bending along different directions. Design. An in vitro biomechanical study using porcine lumbar motion segments. Background. Repeated bending has been suggested as part of the etiology of gradual prolapse of the intervertebral disc, but the multi-planar changes in bending properties following cyclic loading have not been examined in detail. Methods. Porcine lumbar motion segments were subject to 1500 cycles of bending along directions of 0° (flexion), 30°, 60°, or 90° (right lateral bending). The multi-planar bending moments and hysteresis energies were recorded before loading and after various cycle numbers. Results. Repeated bending at 30° and 60° resulted in greater decreases in mean bending moment and hysteresis energy than bending at 0° or 90°. No significant differences were seen between loading groups for the change in bending moment along the anterior testing directions, but significant differences were observed in the posterior and lateral testing directions, with bending at 30° causing a significantly greater decrease in bending moment in the postero-lateral directions. Conclusions. The change in mechanical properties of porcine intervertebral joints due to cyclic bending depend on the direction of loading and the direction in which the properties are measured. Loading at 30° provokes the most marked changes in bending moment and hysteresis energy.