https://promedspine.com/nerve-spinal-cord-compression/
[...] Thecal Sac Compression – The spinal cord and nerves are also protected by a membrane filled with cerebral spinal fluid known as the thecal sac or dural sac. Flattening or compression of the thecal sac will occur in other conditions that cause pressure or pinching of the spinal cord, such as stenosis. [...]

https://www.spine-health.com/conditions/spinal-stenosis/majo...
Narrowing of the central canal (called central stenosis) in the lumbar (low back) area can compress the thecal sac. This narrowing can cause pressure on the nerve bundle at the end of the spinal cord (cauda equina), or the individual nerves within the thecal sac.

thecal sac indentation

https://www.practo.com/consult/mild-thecal-sac-indentation/t

This is the thecal sac or dural sac. See also:

https://www.proz.com/kudoz/english-to-greek/medical-general/...


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Note added at 1 day 16 hrs (2019-05-16 10:04:06 GMT)
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Thecal sac indentation
This results in symptoms of canal stenosis. In our study 25 patients had mild and one patient had moderate degree of thecal sac deformation. No thecal sac indentation was found in rest of the patients.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3872655/