Spine instability and segmental hypermobility biomechanics: A call for the definition and standard use of terms

Abstract

Abstract 1.(1) Confusion can be avoided if standard terms are used in discussions of spine and hypermobility. Thus, an acceptable vocabulary would include the terms: of the (cervical, thoracic, or lumbar) spine; segmental hypermobility; flexibility or stiffness; load (force, moment); displacement (rotation, translation); progressive deformity; neural deficits; flexion-extension or traction-compression radiographs. 2.(2) The definitions of spine and segment should be dissociated from any causes or consequences of or hypermobility. 3.(3) Loads and motions, not just motions, are used to define stiffness, flexibility and instability. 4.(4) Avoid the use of misleading terms. For example, alternative terms for slip and dynamic have been suggested. 5.(5) The most important terms are: 6.(a) Spine instability: The term spinal instability should be used to describe very specific mechanical behavior. A structure is unstable only if a small additional load causes a large, perhaps catastrophic, displacement. Spine mechanical occurs very rarely. When it does occur it is most often associated with trauma, severe osteoarthrosis, tumor, infection or iatrogenic causes. It should not be used to describe larger-than-normal motions, nor should it be used to describe symptomatology. Spine means that spine stiffness is so low for the anticipated range of daily loads that sudden failure in which large and catastrophic displacements occur is imminent. This usually only occurs in trauma and a limited number of special circumstances. If a spine will not exhibit catastrophic displacements under loadings incurred in daily activity, it does not exhibit signs of mechanical instability. Spine can only be quantified when spine loads and displacements are known, at least approximately. 7.(b) Segmental hypermobility: Segmental hypermobility should be used to describe the larger-than-normal intervertebral motions measured radiographically by computed tomography, or by MR, that can be induced by standardized provocation tests. The object of standardized tests, such as those discussed in this article, is to standardize the loads, at least approximately, and so make the results comparable to those of other studies. Again, segmental should not be used to describe causes, consequences, or symptoms.