Pathologic processes and surgical manipulations of the spinal column may result in alterations of the biomechanical properties of the spine through increases or decreases in the range of motion or stability of the spine. A decrease in range of motion between two adjacent vertebrae subsequent to arthrodesis or ankylosis appears, clinically, to be well tolerated without significant alterations to the functions of the spine; however, a decrease in spinal column stability as a result of pathologic changes or surgical alterations can result in catastrophic spinal cord injury. In order to determine the effect of various surgical procedures and trauma on the spinal column, in vitro biomechanical studies may be employed using a servohydraulic testing apparatus and cadaver vertebral motion units. The T₁₃ - L₁ vertebral motion units of 48 mix breed dogs were dissected free of surrounding musculature and prepared for biomechanical testing by mounting with cross pins and polymethylmethacrylate. Specimens were surgically altered by facetectomy, lateral fenestration, diskectomy, and combinations of these procedures. Specimens were subjected to lateral bending at a rate of 2.5 cm per minute to failure in a swing arm bending jig designed to simulate four point bending. The slopes of bending moment vs. angular displacement curves were compared and significance determined by the method of least squares. A statistical difference (p < 0.05) was found between the stiffness of all diskectomy groups when compared to any other group. Unilateral and bilateral facetectomies, and fenestration induced a non-significant decrease in stiffness in comparison to control specimens. This data may be combined with that of previous testing of the canine thoracolumbar spine in flexion-extension and rotation to determine the clinical effects of surgical manipulations and trauma on spinal stability. These results suggest that fenestrations and facetectomies do not appear to increase the risk of injury to the canine thoracolumbar spinal cord during lateral bending in the in vitro model; however, thoracolumbar spinal fractures involving the vertebral body as represented by the diskectomy in vitro model may significantly destabilize the spine in lateral bending. / Master of Science