Spinal Instability and Spinal Fusion Surgery Clinical Presentation

Updated: Mar 15, 2022
  • Author: Peyman Pakzaban, MD; Chief Editor: Brian H Kopell, MD  more...
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History and Physical Examination

The clinical manifestations of spinal stability fall into three categories, as stated in the definition of instability (see Definition of Spinal Instability):

  • Neurologic deficit due to cord, cauda equina, or nerve root compression
  • Pain
  • Incapacitating deformity

Definition of Spinal Instability

In the widely quoted work by White and Panjabi, [1] spinal stability was defined as the ability of the spine under physiologic loads to limit patterns of displacement so as not to damage or irritate the spinal cord and nerve roots and, in addition, so as to prevent incapacitating deformity or pain due to structural changes. Conversely, instability was defined as referring to excessive displacement of the spine that would result in neurologic deficit, deformity, or pain.

Instability can be acute (eg, spine fractures and dislocations) or chronic (eg, spondylolisthesis). Acute instability has been further subcategorized as overt versus limited, whereas chronic instability has been subdivided to include glacial instability (progressive deformity) and instability associated with dysfunctional motion segment. [38]

A simpler conceptual approach would be to think of instability as overt, anticipated, or covert.

Overt instability

Overt instability refers to excessive motion that is readily documented by radiographic studies and results in pain, deformity, or neurologic deficit. Those spine fractures, dislocations, tumors, and infectious processes that significantly disrupt one or more spinal motion segments produce acute overt instability. (See the image below.)

Bilateral jumped facet syndrome is example of over Bilateral jumped facet syndrome is example of overt spinal instability due to trauma. Note grossly abnormal displacement of C5 relative to C6 with neck flexion.

Spondylolisthesis with abnormal dynamic displacement, documented on flexion-extension radiographs, is an example of chronic overt instability. In addition, any spinal deformity (kyphosis, hyperlordosis, scoliosis, or spondylolisthesis) that progresses with time as documented by serial radiographs (ie, Benzel glacial instability) falls in the category of chronic overt instability.

Overt instability generally requires stabilization, either by external means (bracing) or by internal means (fusion). [39]

Anticipated instability

Anticipated instability (see the image below) refers to instability that would be produced by a surgical procedure that is required for proper decompression of neural elements or resection of an offending lesion. 

Example of anticipated instability. (A) Large mass Example of anticipated instability. (A) Large mass affecting right C3-4 facet joint and lateral masses in patient with severe right-side neck and shoulder pain; (B, C) complete resection of tumor and simultaneous C3-4 anterior fusion to circumvent anticipated iatrogenic stability produced by radical resection of facet and lateral masses.

For instance, corpectomy or total facetectomy would constitute indications for fusion at the time of the original operation. A comprehensive anterior cervical diskectomy (with complete resection of the posterior longitudinal ligament and portions of both uncovertebral joints performed for adequate neural decompression) may also be considered in this category, in that it disrupts two of Denis' three spinal columns.

Covert instability

Covert instability is a more elusive concept, referring to circumstances in which excessive motion cannot be grossly demonstrated but is presumed to exist on the basis of the combination of clinical and radiographic findings. Fixed spondylolisthesis (without movement on flexion-extension radiographs) in the setting of progressively worsening back pain or radicular symptoms is a good example of covert instability (see the image below). Pseudarthrosis with intact instrumentation also falls in this category.

Spinal stenosis with fixed degenerative spondyloli Spinal stenosis with fixed degenerative spondylolisthesis in elderly patient is common example of covert instability. Acceptable surgical treatment options include decompression alone vs decompression with fusion.

Controversy arises when the concept of covert instability is applied to degenerative diseases of the spine. In this context, the concept of microinstability is sometimes evoked to justify fusion for a wider range of conditions, including recurrent disk herniation, disk degeneration with diskogenic pain, painful facet arthropathy, spinal stenosis, and failed back syndrome without overt instability.