| Thecal sac, dural sac | |
|---|---|
 A section of the spinal cord with the dura opened to show the interior of the thecal sac. | |
 The spinal canal in cross-section; the outer layer of the thecal sac, the dura, is colored green and the subarachnoid space is blue. | |
| Anatomical terminology |
Thethecal sac ordural sac is themembranoussheath (theca) or tube ofdura mater that surrounds thespinal cord and thecauda equina. The thecal sac contains thecerebrospinal fluid which provides nutrients and buoyancy to the spinal cord.[1] From theskull the tube adheres to bone at theforamen magnum and extends down to the secondsacral vertebra where it tapers to cover over thefilum terminale. Along most of thespinal canal it is separated from the inner surface by theepidural space.[2] The sac has projections that follow thespinal nerves along their paths out of thevertebral canal which become the dural root sheaths.[3]
Thelumbar cistern is part of thesubarachnoid space. It is the space within the thecal sac which extends from below the end of the spinal cord (theconus medularis), typically at the level of the first to second lumbar vertebrae down to tapering of the dura at the level of the second sacral vertebra. The dura is pierced with a needle during alumbar puncture (spinal tap). Forepidural anesthesia an anesthetic agent is injected into the space just outside the thecal sac and diffuses through the dura to the nerve roots where they exit the thecal sac.[4][5] Forspinal anaesthesia in general, an injection can be givenintrathecally into the subarachnoid space, or into thespinal canal. Thisroute of administration may also be used for the delivery ofdrugs which will evade theblood–brain barrier.[6]
Disruption of the dural sac may occur as a complication of a medical procedure, or as a consequence of trauma causing acerebrospinal fluid leak, or spontaneously resulting in aspontaneous cerebrospinal fluid leak.[7]
If the spinal cord is not free to move within the thecal sac due to abnormal tissue attachments, especially during growth,tethered spinal cord syndrome may occur.[8]
In a split cord malformation, some portion of the spinal cord is divided into parallel halves. The thecal sac may be divided and surround each half with a spike of cartilage or bone dividing the halves (Type I), or both halves may be present within the same sac where the dura is bound to a band of fibrous tissue (Type II).[8]