What is effacement of the ventral thecal sac?

Basically, it means . There is slight or minimal compression externally placed onto the outer covering of the spinal cord which can be from a variety of conditions usually not leading to any neurologic findings or even symptoms.
Back pain. The thecal sac covers the spinal cord. It becomes thinned , or effaced, when pressure is applied to it from degenerative disk disease of the spine. In other words, the bulging disk squeezes the sac as it presses towards the spinal cord.