The Filum Terminale: A Cadaver Study of Anatomy, Histology, and Elastic Properties

Abstract

The filum terminale is a fibrous band, consisting of the filum terminale internum (FTI), connecting the conus medullaris (CM) with the dural sac (DS), and the filum terminale externum (FTE), connecting the DS with the coccyx. Despite its importance in tethered cord syndrome, published anatomic and physiologic data on the filum terminale remain scarce. We describe 1) the dimensions and position of the FTI and FTE; 2) the histology of the FTI-DS-FTE transition zone; and 3) the extensibility and elastic properties of the FTI and the CM. Anatomic measurements were performed on 10 fresh and 10 embalmed human cadavers. Four other fresh cadavers were used for strain and elasticity measurements. The mean FTI and FTE lengths were 158.75 and 69.33 mm, respectively. From cranially to caudally, the FTI diameter decreased from 1.93 to 0.69 mm. The most frequent vertebral level of the CM-FTI and the FTI-DS-FTE junction were L1 and S2, respectively. FTE length correlates with body length (r= 0.54; P= 0.014) and with FTI-DS-FTE junction vertebral level (ρ=-0.76; P < 0.001). Histologically, the FTI fuses with DS fibers and continues as FTE. The FTI and the CM show an exponential loaded weight-strain relationship, with the FTI showing higher strain than the CM and almost perfect elastic properties. The CM strain is increased when the dentate ligaments are cut. The FTI is an overturned oblate cone-shaped structure, showing bigger strain under weight loading compared with the CM, thereby protecting the CM from traction, together with the dentate ligaments.