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A herniated disc typically occurs when the nucleus pulposus, the inner gel-like core of the intervertebral disc, protrudes through the annulus fibrosus, the tougher outer layer. The most common direction for this protrusion is posterior lateral.

This is primarily due to the anatomical structure of the lumbar spine, where the posterior longitudinal ligament is relatively weaker, allowing for more frequent herniations in the posterior lateral direction. Additionally, the herniation usually occurs lateral to the midline, specifically caudal to the disc level, as the nerve roots exit the spinal canal and can be compressed by the bulging disc material.

In contrast, the other directions listed—anterolateral, anteromedial, and posterior medial—are less common for disc herniation. Anterolateral and anteromedial herniations are relatively rare due to the strength of the anterior longitudinal ligament, which provides more support and protection to the front of the vertebral bodies. Posterior medial herniations, while possible, are also much less common as they may lead to more direct compression of the spinal cord or cauda equina, which is less prevalent in clinical scenarios.

Understanding this distribution is key for diagnosis