Primary injury to the spinal cord has four morphologic types: impact plus persistent compression, impact alone with transient compression, distraction, and laceration or transection. Morphologic injury on a cord not only instantly injures or destroys resident cells, but also causes delayed damage and death to cells that survive the original trauma. The biological response to a spinal cord injury is divided into three
phases (Table I) that follow a distinct but somewhat overlapping temporal sequence: acute (seconds to minutes after the injury), secondary (minutes to weeks after the injury), and chronic (months to years after the injury).