The thecal sac is a protective membrane that encloses the spinal cord and cerebrospinal fluid. Thecal sac indentation occurs when external factors, such as a herniated disc or spinal stenosis, exert pressure on the sac, causing it to become compressed. Indentation means there is some compression of the thecal sac itself, but it does not appear to affect the nerves. Other terms for thecal sac indentation include effacement of the thecal sac, encroachment of the thecal sac, and compression of the thecal sac.

The severity of the condition depends on several factors, including the underlying cause and the degree of indentation. In many cases, mild to moderate thecal sac indentation may not lead to significant symptoms or long-term complications. In fact, if the thecal sac itself is being compressed but the spinal cord and nerves are spared, there may be no symptoms at all. While thecal sac indentation can cause concern, it is not always a serious condition. Mild to moderate cases can often be managed with conservative treatments, while severe cases may require surgery.