Filum terminale externum

The lower portion of the filum terminale, a pia mater extension, is referred to as the filum terminale externum or the coccygeal ligament. This structure descends from the S2 vertebral level and attaches to the back of the first segment of the coccyx.

In adults, the spinal cord terminates at the L1 vertebral level, but the protective coverings of the spinal cord extend beyond this point. One of these coverings, the pia mater, continues from the tip of the conus medullaris to the bony coccyx. This extension is known as the filum terminale. It's important to note that the arachnoid membrane, along with the subarachnoid space beneath it, terminates at the S2 level. However, the filum terminale passes through this space to reach the coccyx. Consequently, the filum terminale is divided into two parts: the upper part, known as the filum terminale internum, which extends from the tip of the conus medullaris to the S2 level, and the lower part, known as the filum terminale externum, which extends from S2 to the coccyx, beyond the subarachnoid space.