1 Introduction to posttraumatic stability as related to classification systems

Biomechanical testing

The potential for dynamic vertebral deformation was tested in a fresh cadaveric experiment, shown in Fig 5.2-1 , with vertical compressive or compression/flexion loads applied. Volumetric and pressure changes to the spinal canal could be measured with a malleable plastic tube and a saline pump system attached [6]. The results shown in Fig 5.2-2 indicated a significant difference between occlusion measured during impact, immediately postimpact, and by postinjury CT scanning. During impact, the maximum canal occlusion (decrease in the midsagittal diameter) was 71.5%. Immediately post impact (while still in the test frame), it averaged 46.7%, demonstrating the amount of initial elastic recovery of the spine, and after removal of specimens and CT scanning, the average canal occlusion had decreased to 21.5%. The decrease from transient to postimpact canal midsagittal diameter was 70%. Therefore, most of the canal occlusion had disappeared postimpact. These findings, from the perspective of laboratory testing, demonstrate why it may be difficult to relate post-traumatic measured canal compromise with the actual occlusion that had occurred during the traumatic event. It stands to reason that a similar observation could be made for sagittal angular deformation, with a transient major bending deformation partially reducing by “springing back”. In addition, a spontaneous traumatic deformity reduction may be expected with recumbent patient positioning. Further variables to consider in measuring sagittal angular deformation can be expected with patient body habitus, mobility of spine segments, and type of support structure, such as a rigid board versus a soft mattress. In this context determination of injury severity by measuring static post-traumatic images in a number of dimensions may well under appreciate the actual deformation that the spinal column underwent at the point of impact.

2 Treatment consideration

A spinal injury classification system should also be a guide to the surgeon as to how to best stabilize the injured spine mechanically to prevent further damage to the neurological structures. We hypothesized that different spinal fracture patterns result in different patterns of damage and that some of the load carrying structures (the facets for example) may remain intact after specific types of trauma. In a different set of in-vitro tests we set out to identify the residual mechanical stability related to the type of fracture produced in the thoracolumbar spine. Mechanical stability of the spine pre and postinjury was defined in terms of the neutral zone concept described by Panjabi [7]. The neutral zone is an area in which the spinal motion segment can be positioned without applying significant load, and in which the spine will remain after the load is released. Every normal joint has a range of motion in which it displaces without requiring the application of significant load. When the joint is injured, this range increases. The neutral zone provides an easy to visualize method for describing the remaining spinal joint stability. For this series of experiments, thoracolumbar spinal fractures encompassing compression injuries, flexion-distraction injuries, as well as burst fractures were created by impact. Once damaged, loads were applied to specimens in flexion, left lateral bending, extension, right lateral bending, and torsion and in combinations of these loads. For each measurement, the load on the specimen was applied and then released, and the position of the center of the vertebra measured. The measured positions described an area in the horizontal plane, which correlated to the particular type of fracture induced. The outer boundary of the area is the limit of the unstable region, since load is required to move in any direction outside the boundary of the neutral zone.

Fig 5.2-3a demonstrates the difference in neutral zones between an intact vertebral motion segment and the same one after compression fracture. The specimen maintained almost complete mechanical integrity in flexion/extension/lateral bending as indicated by how the intact and injured specimen neutral zone boundaries overlap. However, the torsional neutral zone was increased significantly. In contrast, as Fig 5.2-3b shows, a burst fracture has a much wider neutral zone boundary in flexion and right and left lateral bending.

Currently the concept of a “neutral zone” has become a well recognized biomechanical term utilized widely in research endeavors. As the graphs show it is possible to express spinal stability increasingly well in a graphic fashion in an in-vitro setting. At this point, we currently still lack a translational application which would allow a neutral zone diagram to be applied to any given patient’s fracture based on expected biomechanical performance of the injured segment in order to allow the treating physician to choose the most appropriate care from a variety of nonsurgical or operative options. However, these results do indicate generally how different fracture patterns affect remaining mechanical stability of the spine after injury.

3 Summary

In summary, the current attempts at identifying ‘spinal stability’ remain somewhat arbitrary subjective, and are largely empirically based. Under-appreciation of spinal injury severity based on reliance on static recumbent postinjury films remains a real danger in management aspects. Ultimately preservation of neurologic function and optimization of its recovery potential in case of injury with restoration of a pain-free and physiologically aligned spinal column remain the parameters used for identifying a structurally stable spine. Hopefully translational research will enable application of visual decision-making aids in the future to help in determining stability needs of patients.

4 References

1 Frymoyer Classification of Degenerative Segmental Instability

Frymoyer JW, Selby DK (1985) Segmental instability. Rationale for treatment. Spine; 10:280–286.

Related posts:

5.1 Spine Classifications and Severity Measures 5.3 Deformity severity measures: 5.3.2 Spine Classifications and Severity Measures 5.3 Deformity severity measures: 5.3.4 Deformity severity measures: spondylolisthesis 5.5 Osteoporosis severity measures 5.4 Degenerative measures: 5.4.1 Degenerative measures: cervical myelopathy 5.3 Deformity severity measures: 5.3.1 Spine Classifications and Severity Measures

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