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patients with massive midline disc protrusion and patients with bilateral disc herniation and patients involved in strenuous labor or sports activities and in obese patients. It is argued that these patients are more susceptible to degenerative disc changes, thus decreasing the functional outcome. When discussing instability, they state that spinal instability is a difficult concept to define. Instability, as seen on radiography, is often clinically asymptomatic and an entity that is clinically apparent may be radiographically subtle.
To paraphrase the work of Panjabi and co-workers, spinal instability is the inability of the spine to prevent initial or additional damage to the neuronal elements incapacitating deformities or pain from structural changes. In the lumbar spine the loss of normal integrity of the functional spine unit may lead to segmental instability.
Frymoyer states that segmental instability is defined as loss of spinal motion stiffness such that force application to the motion segment produces greater displacement than would be seen in the normal structure resulting in a painful condition, the potential for progressive deformity and neurological structures at risk. A loss of normal integrity may lead to degenerative discogenic pain. The pathology of discogenic pain and degenerative instability has been described eloquently by Kirkaldy-Willis and Farf an, 1982, who postulated that minimal changes in segmental stability might lead to major dysfunction. Three sequential phases of degenerative processes were theorized: 1) dysfunction, 2) instability, and 3) restabilization. Nuclear degeneration, annular tears and facet hypertrophy characterize the dysfunctional phase. The phase of instability reveals laxity of the posterior facets and ligaments, disc space reduction and increased motion. Instability leads to an attempt at restabilization. This is exemplified by osteophyte formation, facet hypertrophy and dissection of the disc with an increase of intradiscal collagen. The end-stage may be foraminal and central spinal stenosis. This cascade involves all of the spinal columns, thus precipitating the rationale for the 360° of circumferential fusion.
Haid and Dickman define degenerative disc disease of segmental instability as clinical signs and symptoms consistent with mechanical low back pain. Exacerbations are produced by activity and relieved with rest. Intermittent radicular findings correlating with that level are not mandated, but are strongly suggested. Radiographic findings are consistent with disc degeneration and include decreased disc signal on T2 weighted MR images, subchondral changes on MR imaging, disc space collapse and traction spurs. Degenerative disc degeneration may or may not be associated with abnormal olisthesis, superior facet nerve root impingement or lateral recess stenosis. It is not exclusive of other pathological entities.
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