A young adult with isthmic spondylolisthesis at L5-S1 has radiculopathy. Unlike an L5-S1 disc prolapse, which compresses the traversing root, the root compressed in isthmic spondylolisthesis at this level is:

• A The L4 root, compressed in the neural foramen
• B The traversing S1 root, compressed in the lateral recess
• C The exiting L5 root, compressed by fibrocartilaginous callus at the pars defect and the pedicle ✓
• D The L5 root, compressed centrally by the bulging annulus

Explanation

In isthmic spondylolisthesis the posterior part of the neural arch stays behind while the vertebral body slides forward, so the L5 nerve root, which exits under the L5 pedicle across the slipped segment, gets pinched between the fibrocartilaginous callus of the pars defect and the pedicle. Hence the exiting L5 root is affected, whereas an L5-S1 disc prolapse compresses the traversing S1 root. Degenerative spondylolisthesis, by contrast, narrows the canal and compresses the traversing root at L4-L5.

Reference: Essential Orthopaedics (Maheshwari), 5th ed.

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Written and medically reviewed by the StethoPrep medical team.