Analysis of the Influence of Lumbosacral Fusion on the Adjacent Moving Segment

The term “adjacent segment disease – ASD” is the designation for clinical degeneration of the adjacent active segment to the fused segment. This disease occurs as the subsequent complication of spinal fusion, which could result in more surgery. The fusion spine segment significantly reduces movement which causes load enhancement in adjacent active segments and causes a change of mobility. The changes are probably the primary cause of the occurrence of ASD. The strongest degeneration occurs at the nearest point under or above the fusion. The purpose of this work was to analyze the loading of the adjacent disc to the fused segment for flexion and extension movement of the whole segment. The problem was comprehensively solved using complex experimental and computational modeling. Thanks to experimental measurement we recorded the response of the test specimen to flexion and extension movement. The test sample was composed of three vertebrae and two discs. The specimens were taken from a pig. Two states were modeled: healthy and damaged with a LEGACY implant. In healthy specimens the deformation mainly takes place in the disc. In the case of the fusion of two vertebrae with the same loading the deformation mainly take place on an adjacent disc. One way to determine the state of stress inside discs is by computational modeling. The strain-stress states were done by solving the computational model in the ANSYS system. Two variants were solved: healthy and damaged with a LEGACY implant. The model was loaded by angular displacement according to flexion and extension movement. The most important findings are: the specimen with the fused segment has significantly higher stiffness compared to the healthy specimen. The disc above the fused segment treated with LEGACY is more loaded (higher stresses) in flexion than in the healthy specimen.