Abnormal intervertebral motion is considered one of the primary causes in precipitating symptoms that may lead to low back pain. This abnormal motion may be the result of injury, cumulative fatigue damage, or the surgical procedures themselves undertaken to relieve the patient of current low back pain. Lumbar fusion is a procedure frequently employed in the surgical treatment of low back pain. The main emphasis of this proposal is to evaluate various internal fixation devices in terms of their biomechanical responses. Their performances, we hypothesize, are functions of (a) the extent of injury inflicted on the spine prior to their use, and (b) the load history (fatigue) to which the spinal structures get exposed. Both of these factors lead to a decrease in spinal stability. The present in vitro study proposes to investigate these hypotheses, using whole lumbar spine specimens, by comparing the load-deformation behavior of the normal/fatigued, injured, and stabilized specimens. The effect of fatigue in producing instability will be differentiated by testing specimens in two different groups. In Group A, specimens will be tested as received while specimens in Group B will be subjected to cyclic torsional loads prior to injury and stabilization. The injuries to be inflicted at L4-5 and/or L5-S1 (single and multilevel) will mimic discectomy and/or spinal decompression. The injured specimens are reduced with a Luque D-loop or a plate-screw system. The three-dimensional motion (load-displacement) behavior of each vertebra of the lumbar spine segment (L1-S1) will be obtained through the Selspot II System and a servo-controlled biaxial MTS used to administer fatigue loads. The points of insertion of ligaments (morphology) are to be determined by means of a specially designed digitizer (morphometer). The principles of rigid body kinematics are utilized to combine the load-displacement and morphological data to yield in situ ligamentous strains. The purpose of this proposal is to gain knowledge and a better understanding of the relationship between fatigue, injury, degeneration and stabilization of the spine and its motion pattern. The proposal will also provide, for the first time, a comparative evaluation of various spinal fixation devices.
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