I assume you have had recurring disc herniations at L5-S1 and have had three surgeries. Yet another herniation has occurred that is compressing the nerve root. The typical surgical planning that occurs after the third herniation is a TLIF fusion performed on this level but there are exceptions to the rule. This TLIF fusion prevents recurring herniations and further damage to the nerve root.
The TLIF ablates the disc space, and allows plenty of room to decompress the nerve root which is most likely scarred down after three surgeries. The fusion of L5-S1 will probably not be noticeable to you in function after healing as there is most likely no disc material left in the disc space and the motion of this space is currently negligible.
Dr. Corenman