A 33-year-old man is brought to the emergency department after being involved in a major motor vehicle accident. He is unable to move his legs and complains of severe pain in his mid to lower back. On physical examination, he is found to have exquisite tenderness over some of the bony prominence of his lower back, but no gross physical deformity can be appreciated. On neurologic examination, flaccid paralysis of both lower extremities and complete anesthesia to all sensory modalities below approximately the L3 dermatome are noted. Catheterization of his bladder yields approximately 700 mL of urine. Plain radiographs of the spine reveal compression fracture in the body of L3 with greater than 50% of loss in its height. A computed tomography (CT) scan through this area reveals a burst fracture of the body of L3. There are large fragments of bone driven dorsally with an 80% canal compromise. What is the cause of weakness?