Graded thoracolumbar spinal injuries: development of multidirectional instability.

Injuries of the thoracolumbar spine are serious, disabling, and costly to society. These injuries vary from mild ligament tears to severe bony fractures. Increased range of motion (ROM) and neutral zone (NZ) have been suggested as indicators of the resulting clinical instability. The purpose of the present study was to investigate the relative sensitivities and merits of the ROM and NZ in relation to spinal injuries of the thoracolumbar junction. A graded spinal trauma experiment was designed, in which the threshold of injury and injury progression were examined. Ten thoracolumbar human spine specimens (T11-L1) were traumatized using a high-speed incremental trauma model. The ROM and NZ, which indicate altered mechanical properties, were determined for three physiological motions: flexion/extension (FE), lateral bending (LB), and axial rotation (AR). The injury threshold was found to be 84 J (or 84 Nm) by examining both ROM and NZ for all motion types (P < 0.05), but the NZ was more sensitive. At the injury threshold, the NZ showed an overall average increase of 566% above that of the intact, while the equivalent increase in the ROM was only 94%. The NZ was also a more sensitive parameter documenting the progression of the injury beyond the injury threshold. After the maximum trauma of 137 J, the NZs for the three motions (FE, LB, and AR) increased by 700%, 1700%, and 3000% above their respective intact values. Corresponding increases in the ROM were much smaller: 115%, 184%, and 425% respectively. Direct extrapolation of the in vitro experimental findings to the clinical situation, as always, should be done with care. Our findings, however, suggest that the ROM, as measured from functional radiographs of a traumatized patient, may underestimate the true injury to the spinal column.