Which structure forms the posterior boundary of the spinal canal and can hypertrophy to contribute to canal narrowing?

• A. Ligamentum flavum
• B. Posterior longitudinal ligament
• C. Interspinous ligament
• D. Ligamentum nuchae

Answer: (A)

The ligamentum flavum is the structure that forms part of the posterior boundary of the spinal canal and can thicken with age or degenerative change. It runs between the laminae of adjacent vertebrae and lies right along the back wall of the canal. When it hypertrophies or buckles inward, it reduces the space available for the dural sac and nerve roots, contributing to canal stenosis and symptoms like neurogenic claudication. The posterior longitudinal ligament stays along the front (anterior) aspect of the canal, so its thickening narrows the canal from the front rather than the back. The interspinous ligament lies between spinous processes and mainly limits flexion, not the primary posterior boundary. The ligamentum nuchae is a cervical structure; it does not form the posterior boundary of the spinal canal throughout the spine.