Spondylolisthesis: What radiographic finding defines the condition?

• A. Forward slipping of a vertebra on another due to pars interarticularis defect
• B. Lateral bending of the spine
• C. Disc herniation with nerve root compression
• D. Fracture of the pedicles

Answer: (A)

The key finding that defines spondylolisthesis on imaging is forward (anterior) displacement of one vertebral body relative to the vertebra below, best seen on the lateral X-ray. This forward slip occurs because of a defect in the pars interarticularis (spondylolysis) that allows the vertebra to translate anteriorly, most commonly at the L5–S1 level. On radiographs, this anterior translation is the hallmark feature, and it’s often graded by how much of the vertebral body has slipped.

Lateral bending of the spine describes scoliosis, not spondylolisthesis. Disc herniation with nerve root compression explains symptoms and different imaging findings (disc bulge and nerve impingement) rather than the visible forward slip. A fracture of the pedicles is a different injury pattern and does not define spondylolisthesis.