Mechanical Instability on Lumbar Spine at the Adjacent and Subjacent Segment after Rigid Fixation

To investigate the effect of PLIF and after rigid fixation on adjacent segment and subjacent mechanical instability moreover under loading adjacent and subjacent segment mechanical behavior. Eleven fresh-frozen lamb lumbar spines (Sacrum-T12) were loaded, in axial, flexion-extension, right-left bending directions. All specimens were tested intact and after implantation with posterior pedicle screws and rod fixation. All specimens were fixed in a biomechanical test frame. The biomechanical tests were performed by using the axial compression testing machine. The displacements between S1 and L5 of markers were measured superior-inferior displacement and anterior-posterior displacement with two non-contact CCD cameras. The axial compression was applied to each specimen with testing machine was used to generate an increasing moment up to 8400 Nmm in flexion and extension right left bending and axial compression. The load was applied to each specimen consecutively before and after intact spine and implanted rigid fixation. In all direction after rigid fixation of spine adjacent and subjacent segment (S1-L5, L5-L4) vertical (superiorinferior) displacements were lower but anterior-posterior displacements were higher. Displacement values were evaluated and compared statistically. When we checked the statistical data, there were numerical differences and there was significant change between the intact spine and the spine implanted pedicular screws and rods. Biomechanical alterations likely play a primary role in causing adjacent and subjacent segment disease. After rigid fixation adjacent and subjacent mobile segment (anterior-posterior) displacement of the lumbar spine was significantly greater than intact spine. Rigid fixation of lumbar spine alters adjacent and subjacent segment level ROM. Therefore abnormal stress and hyper mobility on adjacent and subjacent segment may cause spinal instability, which may develop facet joint degeneration and low back pain.