Filum terminale

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Structure of the spinal cord seen from a dorsal view.

The filum terminale is a thin filament of pial tissue that extends caudally from the apex of the conus medullaris (tapered end of the spinal cord) to the dorsal surface of the coccyx.

It measures about 20 cm in length and consist of two segments:

• The filum terminale internum, also known as the pial segment, refers to the proximal segment of the filum terminale enclosed within the lumbar cistern region of the spinal dural sac. It measures approximately 15 cm in length and is surrounded by cerebrospinal fluid and the spinal nerve roots of the cauda equina. This segment extends to the level of the second sacral vertebra, marking the distal termination of the spinal dural sac. At this point, the filum terminale pierces the arachnoid and dura mater, transitioning into the filum terminale externum, or the dural part.
• The filum terminale, also known as the dural segment or coccygeal ligament, is the shorter distal segment of the filum terminale. It merges with the dura mater and traverses the sacral hiatus to insert on the posterior surface of the vertebral bodies of the coccyx. There, it blends with the periosteum, creating a seamless connection.

The primary function of the filum terminale is to tether the spinal cord to the dorsum of the coccyx, providing stability and preventing excessive movement within the spinal column.