Filum Terminale

The filum terminale is a slender fibrous structure that extends inferiorly from the tip of the conus medullaris and helps anchor the lower end of the spinal cord within the vertebral canal. It begins as a continuation of the pia mater, descends among the nerve roots of the cauda equina, passes through the lower end of the dural sac, and ultimately attaches to the coccyx.

The filum terminale is divided into two parts according to its relationship with the spinal meninges: the filum terminale internum, which lies within the dural sac, and the filum terminale externum, which extends from the termination of the dural sac to the coccyx. The external part is also known as the coccygeal ligament.

Although extremely thin compared with the spinal cord and nerve roots surrounding it, the filum terminale is an important landmark in the anatomy of the lower vertebral canal. Its course illustrates the relationship between the conus medullaris, cauda equina, lumbar cistern, dural sac, and coccyx.

Location and Extent

The filum terminale begins at the inferior tip of the conus medullaris, the tapered terminal portion of the spinal cord. In adults, the conus usually ends near the level of the L1-L2 intervertebral disc, although its exact vertebral level varies between individuals.^[1][2]

From the conus medullaris, the filum descends through the lumbar cistern within the bundle of lumbar, sacral, and coccygeal nerve roots forming the cauda equina. It reaches the inferior end of the dural sac at approximately the S2 vertebral level.

At S2, the relationship of the filum to the meninges changes. The portion above this level is the filum terminale internum. After passing through the end of the dural sac, the filum acquires a covering from the dura and continues inferiorly as the filum terminale externum before attaching to the coccyx.^[1][3]

Structure

The filum terminale is composed predominantly of connective tissue and represents a continuation of the spinal pia mater from the conus medullaris. It contains collagenous and elastic fibers and becomes progressively associated with meningeal coverings as it descends.

The two portions are distinguished by both their location and their coverings:

This division corresponds closely with the termination of the dural sac. The internal portion lies within cerebrospinal fluid, while the external portion continues below the dural sac as a fibrous anchoring structure.

Filum Terminale Internum

The filum terminale internum, also called the intradural portion of the filum terminale, begins at the apex of the conus medullaris and extends inferiorly for approximately 15 cm to the lower end of the dural sac at about S2.

Because it lies within the dural sac, this portion is surrounded by cerebrospinal fluid in the subarachnoid space. The descending nerve roots of the cauda equina surround it as they travel toward their respective intervertebral foramina.

The filum terminale internum is much thinner than the surrounding nerve roots and can be distinguished anatomically by its midline course and different tissue composition. Small vessels may accompany it along its descent.

Filum Terminale Externum

At approximately the S2 vertebral level, the dural sac ends and the filum terminale becomes invested by dura mater. From this point inferiorly, it is called the filum terminale externum.

The external portion descends through the sacral canal and ultimately attaches to the posterior surface of the coccyx. Because it forms a fibrous connection between the lower end of the dural sac and the coccyx, it is also called the coccygeal ligament.

This distal attachment helps secure the dural sac and spinal cord longitudinally within the vertebral canal while still allowing the degree of movement required during normal motion of the vertebral column.

Relations

The relationships of the filum terminale change along its course because the internal portion lies within the subarachnoid space while the external portion lies below the dural sac.

Relationship to the Conus Medullaris

The filum terminale begins directly from the inferior end of the conus medullaris. The conus represents the terminal tapering of the spinal cord, whereas the filum continuing from it is predominantly fibrous rather than a continuation of the functional spinal cord.

Relationship to the Cauda Equina

Within the lumbar cistern, the filum terminale internum descends among the nerve roots of the cauda equina. These structures should not be confused. The cauda equina consists of lumbar, sacral, and coccygeal nerve roots, while the filum terminale is a non-neural anchoring structure derived principally from pia mater.

The nerve roots diverge toward their respective exit foramina as they descend, whereas the filum maintains a predominantly central course toward the lower end of the dural sac.

Relationship to the Lumbar Cistern

The lumbar cistern is the enlarged portion of the subarachnoid space extending inferiorly from the conus medullaris to approximately S2. It contains cerebrospinal fluid, the cauda equina, and the filum terminale internum.

This arrangement is clinically important because the lumbar cistern can be accessed below the level of the spinal cord for procedures such as lumbar puncture. The needle enters a region containing mobile nerve roots and the filum rather than the spinal cord itself.

Relationship to the Dural Sac

The spinal dura and arachnoid continue inferior to the conus medullaris to form the dural sac, which terminates at approximately S2. The filum terminale internum lies within this sac. At its lower end, the dura invests the filum and contributes to the formation of the filum terminale externum.

Development

The anatomy of the filum terminale is closely related to development of the caudal spinal cord and the unequal growth of the spinal cord and vertebral column.

Early in development, the spinal cord extends much farther caudally relative to the vertebral column than it does in the adult. As development proceeds, the vertebral column grows more rapidly than the spinal cord. Consequently, the conus medullaris comes to occupy a progressively higher vertebral level, while the lower spinal nerve roots lengthen as they descend toward their exit foramina.

This differential growth produces the characteristic adult arrangement of the conus medullaris, cauda equina, and filum terminale. The filum represents a remnant associated with the regressed caudal portion of the developing spinal cord and maintains the inferior connection of the spinal cord and meninges within the vertebral canal.^[1]

Function

The principal mechanical role of the filum terminale is to provide longitudinal stabilization of the spinal cord and dural sac. By connecting the conus medullaris and dural sac to the coccyx, it limits excessive superior and inferior displacement of these structures within the vertebral canal.

This inferior anchoring works together with other stabilizing relationships of the spinal cord. Laterally, the denticulate ligaments extend from the pia mater toward the dura and help stabilize the cord within the dural sac. Inferiorly, the filum terminale provides a longitudinal attachment.

The filum remains sufficiently compliant to accommodate normal changes in length and position that occur with movements of the vertebral column. Its function therefore involves stabilization rather than rigid fixation.

Distinction from Nearby Structures

Several structures occupy the lower vertebral canal and can be confused when first learning the anatomy. Their relationships are easier to understand by considering their origins and composition.

The filum terminale is therefore neither a spinal nerve nor an extension of the cauda equina. Its central course through the nerve roots reflects its origin from the inferior end of the spinal cord rather than from a spinal nerve segment.

Anatomical Variations

The length, thickness, and tissue composition of the filum terminale vary between individuals. Variation in the vertebral level of the conus medullaris also changes the length of the intradural portion.

The filum may contain variable amounts of fibrous, elastic, vascular, and adipose tissue. Small amounts of fat within the filum can be seen on MRI in some individuals without necessarily producing neurological abnormalities.

More substantial fatty infiltration or abnormal thickening can be associated with congenital abnormalities affecting the distal spinal cord. The significance of these findings depends on their relationship to the position and mobility of the conus medullaris and associated neural structures.

Clinical Significance

The filum terminale is particularly important clinically because abnormalities of its length, thickness, composition, or attachment can interfere with the normal mobility of the distal spinal cord.

Tethered Cord Syndrome

Tethered cord syndrome refers to neurological dysfunction associated with abnormal fixation and traction on the spinal cord. An abnormally thick, shortened, or otherwise restrictive filum terminale can contribute to tethering of the conus medullaris.

As the vertebral column grows or moves, an abnormally fixed distal cord may be exposed to traction. Tethering can occur in association with several forms of spinal dysraphism and other congenital abnormalities, although the underlying anatomy varies between patients.

Thickened and Fatty Filum

The filum terminale can occasionally be abnormally thick or contain prominent adipose tissue. A fatty filum is readily visible on MRI because fat has a characteristic imaging signal.

The presence of fat alone does not define the functional effect of the abnormality. Imaging findings are interpreted together with the position of the conus medullaris, other associated abnormalities, and the neurological anatomy of the distal spinal cord.

Surgical Anatomy

The distinction between the filum terminale and the surrounding nerve roots is critical during procedures involving release of a tethered spinal cord. The filum lies among the roots of the cauda equina, so accurate identification is necessary before it is divided.

Its tissue appearance, midline course, vascular pattern, and relationship with the conus can assist identification, with neurophysiological techniques also used during appropriate procedures to distinguish neural structures from the filum.

Imaging Anatomy

MRI can demonstrate the conus medullaris, cauda equina, filum terminale, dural sac, and surrounding cerebrospinal fluid. The level of the conus and the appearance of the filum are important features when evaluating congenital abnormalities of the distal spinal cord.

The filum terminale forms the final longitudinal attachment of the spinal cord and its coverings. Its course from the conus medullaris through the cauda equina and dural sac to the coccyx provides a useful anatomical framework for understanding the organization of the lower vertebral canal.