Pathophysiology and pathoanatomy of degenerative disorders of the adult spine

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What factors play a role in the development of degenerative disorders of the spine?

The prevalence and progression of spinal column degenerative changes are associated with increasing age, but remain asymptomatic in many individuals. Mechanical, traumatic, nutritional, biochemical, and genetic factors interact and contribute to the development of spinal degeneration. The relative importance of these factors varies among individuals and remains incompletely understood. Evidence suggests that spinal degenerative conditions have important genetic influences.

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What mechanical factors have been associated with disc degeneration?

Disc degeneration has traditionally been linked to mechanical factors such as excessive or repetitive loading resulting in structural injury and subsequent development of axial pain symptoms. Factors traditionally associated with the occurrence of disc degeneration according to this injury model include age, occupation, male gender, cigarette smoking, and exposure to vehicular vibration. An observation cited to support a mechanical basis for disc degeneration is the development of degenerative changes adjacent to previous spinal fusions.

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How does genetics influence the development of disc degeneration?

Findings from multiple studies including the Twin Spine Study have shown a substantial genetic influence on intervertebral disc degeneration and the presence of pathoanatomic findings including Modic endplate changes, Schmorl nodes, endplate defects, lumbar spinal stenosis, and back pain. (1) The traditional environmental and mechanical factors associated with disc degeneration in an injury model had only a modest influence on disc degeneration and back pain in more recent studies. Gene loci associated with the development of disc degeneration include type 1 collagen genes (COL1A1) , type IX collagen genes (COL9A1, COL9A2) , vitamin D receptor genes (Taq1 and Fok1) , interleukin-1 and interleukin-6 genes, and metalloproteinase-3 genes. Current thinking is that genetic factors, with or without the presence of other risk factors such as spine injury or aging, lead to the development of intervertebral disc degeneration in specific populations. (5)

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Can psychosocial factors influence a patient’s perception of axial pain associated with degenerative spinal disorders?

Yes. A variety of factors have been shown to influence a patient’s perception of axial pain associated with degenerative spinal disorders. When pathologic processes stimulate pain-sensitive structures in the lumbar spine and pelvis, neural signals are transmitted through the dorsal root ganglion to the spinal cord and ultimately to the brain for processing. Perception of these stimuli may be modulated by a variety of factors along the pathway of signal transmission. Psychologic and social factors have been demonstrated to influence this process. Such factors include chronic pain illness, depression, somatization (expression of emotional and psychologic symptoms in physical terms), and secondary gain (e.g., worker’s compensation claims, litigation claims following motor vehicle accidents). (3) One of the most challenging aspects in the evaluation and treatment of patients with degenerative spinal disorders is our poor understanding of why similar appearing degenerative changes may be asymptomatic in one patient but cause severe pain and impaired function in other individuals. (7)

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Is it accurate to classify degenerative spinal disorders as diseases?

Strictly speaking, degenerative changes involving the intervertebral discs and zygapophyseal (facet) joints are not a disease in the traditional sense, but rather the natural consequences of mechanical stresses applied to the spine throughout life. Although disc and facet joint degeneration are universal in the aging spine, these changes are inconsistently and only occasionally associated with pain and functional limitation. There is concern that labeling patients with a diagnosis such as “degenerative disc disease” may have a negative impact on certain patients who may perceive their diagnosis as a progressive, irreversible disease that may not improve over time. Caution is recommended in interpreting the clinical relevance of spinal degeneration features noted on imaging studies due to the high prevalence of imaging findings of spinal degeneration in asymptomatic subjects. (2) Data show that the prevalence of degenerative findings observed on spinal imaging studies increases with age and exceeds 90% in individuals >50 years of age who are evaluated with magnetic resonance imaging (MRI). It is critical that imaging findings of spinal degeneration are interpreted in the context of the individual patient’s clinical condition.

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