What is the primary defect in spondylolysis and spondylolisthesis?

The primary defect in spondylolysis and spondylolisthesis is associated with the pars interarticularis, which is a region of the vertebra located between the superior and inferior articular processes. In spondylolysis, there is a defect or fracture in the pars interarticularis, often due to stress or repetitive loading, which can occur in athletes. This defect may lead to instability in the spine.

In spondylolisthesis, this defect can progress to the point where one vertebral body slips forward on the vertebral body below it, leading to a misalignment of the spine. While this condition indeed results in a vertebra slipping, the underlying issue is the weakness or defect in the pars interarticularis that allows this slippage to occur.

The other options represent different pathological conditions that are not the primary defect involved in these specific disorders. Vertebral body slipping, while a consequence of spondylolisthesis, does not identify the root cause of the condition. Spinal canal narrowing may occur due to various spinal defects or conditions but is not the primary defect in spondylolysis or spondylolisthesis. Similarly, facet joint degeneration may be an associated finding