How does lumbar degenerative disc disease (DDD) cause radiculopathy?

Mechanisms of Radiculopathy in Lumbar Degenerative Disc Disease

Lumbar degenerative disc disease causes radiculopathy through mechanical nerve root compression resulting from disc space collapse, disc herniation, foraminal narrowing, and secondary bony changes including facet and uncovertebral joint hypertrophy. 1, 2

Primary Pathophysiological Mechanisms

Direct Mechanical Compression

• Disc herniation is the most common mechanism, with herniated disc material directly compressing the spinal nerve root as it exits through the neural foramen 1, 3
• More than 90% of symptomatic lumbar disc herniations occur at L4/L5 and L5/S1 levels, producing back and leg pain in typical lumbar nerve root distributions 3
• The herniated nucleus pulposus creates focal compression of the nerve root, leading to radicular signs and symptoms consistent with the specific spinal nerve under compression 1

Foraminal and Lateral Recess Stenosis

• Progressive disc space collapse from degenerative disc disease reduces the vertical height of the neural foramen, creating a narrower exit pathway for the nerve root 4, 5
• Facet joint hypertrophy and uncovertebral joint enlargement further narrow the neural foramen, compressing the exiting nerve root 1, 4
• Lateral recess stenosis occurs when degenerative changes compress nerve roots within the spinal canal before they exit through the foramen 4

Secondary Degenerative Changes

• Disc degeneration leads to segmental instability, which can cause dynamic compression of nerve roots with movement 1, 6
• Degenerative spondylosis develops as a consequence of disc disease, contributing additional compressive elements through osteophyte formation 7, 6
• In degenerative lumbar spinal stenosis with scoliosis, L3 and L4 roots are more compressed by foraminal or extraforaminal stenosis on the concave side, while L5 and S1 roots are affected by lateral recess stenosis on the convex side 4

Clinical Correlation

• The straight-leg raise test demonstrates 91% sensitivity (though only 26% specificity) for herniated disc causing radiculopathy 3
• Neurological deficits follow predictable patterns: knee strength and reflexes (L4), great toe and foot dorsiflexion strength (L5), foot plantarflexion and ankle reflexes (S1), and corresponding dermatomal sensory distributions 3
• Imaging findings must correlate with clinical symptoms to confirm that the anatomic compression is truly causing the radiculopathy, as degenerative changes are commonly seen in asymptomatic individuals over 30 years of age 1, 3

Important Clinical Pitfalls

• Enhancement of lumbar nerve roots on MRI in degenerative disc disease represents intravascular enhancement of radicular veins rather than clinically significant nerve root pathology, and should not be misinterpreted as evidence of symptomatic compression 8
• Over-reliance on imaging without clinical correlation leads to unnecessary surgical intervention, as spondylotic changes correlate poorly with the presence of symptoms 1, 3
• The natural history favors improvement within the first 4 weeks with conservative management in most patients, making premature surgical intervention inappropriate unless red flags are present 3