A biomechanical study of an interspinous stabilization spinal implant (Coflex) was carried out using eight human lumbar L4/L5 motion segments. Each motion segment was tested in compression, then flexion/extension, then lateral bending, and then axial rotation at five conditions: 1) intact; 2) partial destabilization (by cutting the supraspinous and interspinous ligaments, the ligamentum flavum, the facet capsules, and 50% of the inferior bony facet bilaterally); 3) stabilization with the Coflex device; 4) complete destabilization with total laminectomy; and 5) stabilization with pedicle screws and rods. The most important result is that the motion segment after destabilization and insertion of the Coflex device does not allow significantly more or less motion than the intact specimen in either flexion/extension or axial rotation. Thus the Coflex offers nonrigid fixation and can return a partially destabilized specimen back to the intact condition in terms of motion in flexion/extension and axial rotation.