Graded thoracolumbar spinal injuries: development of multidirectional instability

Abstract

IntroductionInjuries of the thoracolumbar spine are serious disabling injuries. The treatment choice and clinical outcome are significantly dependent upon the determination of the stable/unstable status of the injured spine of the patient. There is controversy concerning the understanding and even the definition of clinical instability. However, mechanical derangement, in addition to clinical aspects such as pain and neurological deficit, is considered as one of the major determinants [21]. The mechanical injury can vary from a purely ligamentous injury to fractures of the endplate, vertebral body, and/or posterior elements. From the functional point of view, changes in the mechanical characteristics of the spine, documented in a three-dimenAbstract 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.

Biomechanical stability of five stand-alone anterior lumbar interbody fusion constructs

“…Increases in NZ have been considered as indicative of injury to osteoligamentous tissue [8,28,29]. This concept applied to a cage construct may provide a quantitative assessment of construct laxity and suggest possible osteoligamentous injuries/ruptures.…”

“…Muscle contraction, which is expected to reduce NZ [51], was absent in the cadaveric model. Clinically, an increase in NZ may depict failure of an FSU to sustain appropriate alignment while supporting physiological loads, thus indicating potential directions of segmental instability [8,28,29] that may lead to permanent deformity.…”

Biomechanical evaluation of the New Zealand white rabbit lumbar spine: a physiologic characterization

“…This was accomplished with a weight suspended from above with a pulley that was free to follow the motion of the headpiece below. This method of testing has been established and previously published for human specimens [11,12,20]. The specimens were kept moist with normal saline throughout testing.…”

Single and incremental trauma models: a biomechanical assessment of spinal instability

“…Raw data from the flexibility tests were analyzed to determine neutral zone and range of motion parameters in three planes of motion: flexion/extension, axial rotation, and lateral bending [11]. The positive and negative rotations in each plane were summed.…”