Frymoyer has described a classification system for degenerative segmental instabilities that is an expansion of an earlier classification system .It is based on radiographic findings, as well as a history of previous spine surgery . The underlying concept is that segmental instability is best diagnosed when the deformity progresses over time. Indeed, the classification was based principally on such progressive deformities, rather than abnormal motions and translations identified on a single dynamic flexion-extension radiograph. A primary instability is one that cannot be attributed to any previous interventions (i.e., surgery, chemonucleolysis). Secondary instabilities are those that were created or exacerbated by an intervention.
Type I: Axial Rotational Instability
Axial rotational instability has a rotational malalignment as a major part of the deformity. Because cadaveric studies have shown that degenerative spondylolisthesis can have combinations of rotatory deformity, lateral bend, and translation , Frymoyer suggests that this and type II defects might be the same. Clinically, patients complain of recurrent, episodic low back pain provoked by twisting motions. Radiographically, this is manifest by malalignment of the spinous processes as well as pedicle rotation. Myelograms have been reported to show a pedicle-to-pedicle defect .
Type II: Translational Instability
Translational instability is synonymous with degenerative spondylolisthesis. Forward displacement of the cephalad vertebrae is the predominant deformity, although some rotational component can exist. Patients may complain of recurrent episodes of back pain along with an extensor lag. Classic radiographic signs described by Knutsson and Macnab, such as traction osteophytes and vacuum discs, are associated with this type of deformity. Women are affected five to six times more often than men, and the instability usually occurs after the age of 40 years. It occurs three times more often in African American women than in white women and usually involves the L4-5 interspace. Facet joint angulation has been implicated as a cause of the deformity.
Translational instability is synonymous with degenerative spondylolisthesis. Forward displacement of the cephalad vertebrae is the predominant deformity, although some rotational component can exist. Patients may complain of recurrent episodes of back pain along with an extensor lag. Classic radiographic signs described by Knutsson and Macnab, such as traction osteophytes and vacuum discs, are associated with this type of deformity. Women are affected five to six times more often than men, and the instability usually occurs after the age of 40 years. It occurs three times more often in African American women than in white women and usually involves the L4-5 interspace. Facet joint angulation has been implicated as a cause of the deformity.
Type III: Retrolisthetic Instability
Retrolisthetic instability is manifest by posterior displacement of the cephalad vertebra, posterior disc space collapse, and facet subluxation. The symptoms, which may include low back pain and nerve root signs, are most prominent in extension. Radiographically, the retrolisthesis increases with extension as well.
Type IV: Degenerative Scoliosis
The curve seen in this degenerative scoliosis is usually less than 40 degrees and is associated with central and lateral recess stenosis. The combination of the curve with degenerative disease of the disc and facets reduces canal size, and thus can be associated with central and lateral stenosis. Symptoms usually consist of low back pain with or without neurogenic claudication and/or radiculopathy.Patients with stenosis due to degenerative scoliosis, however, generally do not get relief from sitting .
The curve seen in this degenerative scoliosis is usually less than 40 degrees and is associated with central and lateral recess stenosis. The combination of the curve with degenerative disease of the disc and facets reduces canal size, and thus can be associated with central and lateral stenosis. Symptoms usually consist of low back pain with or without neurogenic claudication and/or radiculopathy.Patients with stenosis due to degenerative scoliosis, however, generally do not get relief from sitting .
Type V: Internal Disc Disruption
Various authors have tried to study the relationship between instability and disc disruption but have not found a definitive link. Soini et al. found no association between disc degeneration (as assumed by discography) and abnormal angular movement. It is not clear at this time what the relationship is between disc disruption and clinical instability.
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