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The thecal sac is the outermost structure containing the lower end of the spinal cord, but at the level of the thecal sac the spinal cord is no longer a cord, but, rather, a group of nerves that resemble a 'horse's tail', so at this lower level of the back the spinal cord is often called the 'cauda equina'.
The deformity of your thecal sac is likely caused by a herniated disc, but some thecal sac deformities are congenital (there since birth).
A thecal sac deformity can mean many things, if it is associated with a herniated disc there is a chance that surgery will be recommended. Your physician may refer you to a neurologist or a neurosurgeon to consult and to make decisions about whether anything needs to be done in your particular situation.
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