Which statement correctly differentiates isthmic spondylolisthesis from degenerative spondylolisthesis on imaging?

• A. Isthmic spondylolisthesis results from pars interarticularis defect (stress fracture) often in younger patients; degenerative spondylolisthesis results from facet joint and disc degeneration, common in older patients.
• B. Isthmic spondylolisthesis results from disc degeneration in older patients; degenerative spondylolisthesis results from pars defects in younger.
• C. Both conditions arise equally in all ages with identical imaging features.
• D. It is the same as lateral recess stenosis.

Answer: (A)

Differentiating these two on imaging hinges on the underlying structural defect and the typical age group. Isthmic spondylolisthesis comes from a defect in the pars interarticularis, often a stress fracture, which allows forward slippage and is classically seen in younger patients. On imaging, this pars defect is the telling feature and is often highlighted on oblique views showing the characteristic pars fracture (the Scotty dog collar sign). In contrast, degenerative spondylolisthesis results from aging-related changes in the facet joints and intervertebral discs, leading to slippage without a pars defect and is more common in older individuals. Imaging shows facet joint arthropathy and disc height loss with osteophytes, and the pars interarticularis defect is absent. This combination of pars defect in younger patients versus degenerative facet/disc changes in older patients makes the statement the best fit. The other ideas either reverse the cause and age patterns, claim identical imaging for both, or confuse the condition with another entity like lateral recess stenosis.