The filum terminale is a small thin filament of connective tissue that extends inferiorly from the apex of the conus medullaris to the sacrum.
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Gross anatomy
The filum terminale is continuous with the pia mater and is described as having two sections:
filum terminale internum: upper three quarters of the filum; covered by the spinal dura and arachnoid meninges; about 62.5% of filum terminale internum and the dural sac terminates at or below S1/S2 disc space while 11.3% terminates above S1 4.
filum terminale externum: lower quarter of the filum; fuses with the investing dura mater and continues inferiorly to attach to the dorsal coccyx; it contains all three layers of meninges, also known as coccygeal ligament 5.
Variant anatomy
fatty filum terminale: can be considered as a cause of tethered cord 4 or may be considered a rare (<0.5%) anatomical variant in asymptomatic patients 3
