Lumbar Degenerative Disc Disease

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Internal disc disruption is a separate concept to disc degeneration. While there are many similar processes and features, internal disc disruption is a response to injury that occurs either through single or repetitive compressive loads. Degenerative disc disease does not correlate strongly with back pain, unlike internal disc disruption.

Degenerative changes are an expression of metabolic stress, not a disease. There are no known mechanisms whereby degenerative changes can be painful. There are lots of different triggers with a final common pathway. Degenerative joint disease is a disturbing label that patients associate with a poor prognosis. Genetic factors predispose to degenerative changes, but age is the strongest correlate. Specific metabolic causes are rare, and limited to diabetes mellitus and ochronosis. Impaired nutrition is promoted but the evidence is lacking. Smoking is a weak correlate. Low grade infection has been explored but the evidence is not conclusive.

In the lumbar spine degenerative changes are chemical in nature, expressed as changes in proteoglycan content and hydration. This is seen on MRI with changes in signal intensity.

Chondrocytes maintain the balance between synthesis and degradation of the extracellular matrix. Synthesis is promoted by growth factors. Degradation is achieved by the activation of various enzymes such as the MMP family, whose expression is controlled by various cytokines (see Fibrous Connective Tissues, and Synovial Joints). Osteophytes are simply adaptive modelling, where there is an attempt to increase surface area to reduce load. This can be be a normal joint with excessive load, or joint where the capacity to bear loads is compromised by degradation of the matrix.

With aging, chondrocytes might be subject to innate senescence. They may have genetic abnormalities that affect matrix quality. Normal cells may become impaired with the accumulation of toxins and mechanical stresses. Zygapophysial joints have not been explicitly studied with regards to ageing changes, but come under the umbrella of synovial joints - i.e. the changes are likely a combination of genetics and abnormal biomechanics. For lumbar disc degeneration the evidence is more explicit from twin studies. Environmental influences play a role, but larger proportions are explains by genetic factors.