Instantaneous center of rotation in the lumbar spine and biomechanical effects of its alteration

The low back pain is the most common illness after the cold and 80% of world population was affected by it in some part of their lives. Besides the importance of it with respect to the health, effects of the low back pain with respect to the work force and economics cause considerable losses. Thus, understanding the complex motion of the spine and the changes that occur with various deficiencies bears great importance for surgical and preventive treatment.Instantaneous center of rotation is one of the most important characteristics to determine the quality of spinal motion. Even though researchers have investigated the use of instantaneous center of rotation as a diagnostic tool for so long, there is no comprehensive study that investigated the changes in the instantaneous center of rotation after disc degeneration and ligament failure.In this study, finite element model of the L4-5 spinal unit was constructed from computed tomography images by using reverse modeling. Material properties of various spinal structures were taken from the literature. The model was validated by comparing the range of motion results of the model and previous studies. Mild and moderate disc degeneration were simulated by changing the disc height and material properties of the constructed model. Change of instantaneous center of rotation, range of motion, facet loads and maximum annulus stresses were investigated after the simulated disc degeneration. In the next step, ligament failure was simulated by subsequently removing the ligaments from the model and change of instantaneous center of rotation, range of motion, facet loads and maximum annulus stresses were investigated after the simulated ligament failures. The results of these investigations were evaluated along with the previous related studies in the literature and discussed.In the result of the study, it has been found that isolated early-to-mid stage disc degeneration has a limited influence on the location of the instantaneous center of rotation. This result shows that changes in the spinal motion can reach to a clinically detectable level only after certain point in the degeneration process. In the result of the next investigation, it has been showed that the change in the quality of the motion of the spine (instantaneous center of rotation) adjunct to the change in the quantity (range of motion) could have a diagnostic value revealing more detailed information on the type of the injury, the state of the ligaments, and load transfer and sharing characteristics of the segment.