Hi there, welcome to HCM, thanks for your question Synovial cyst s are fluid filled soft tissue structure s which are formed due to degeneration in the joints between bones of your back (known as facet joints between L4 and L5.I. e 4 th and 5 th spinal bone in your situation ). These cysts get filled like a balloon filled with water when you stand or walk and get deflated when you rest or lie down due to decompression like a deflated baloon. Since they lie in close proximity to the nerves in your spine, they are associated with pain radiating down the leg, simolar to sciatica pain seen with disc herniation During early stages when the cysts are small, the pain can respond to steroid injection s which help in reducing the inflammation associated with compression of the neves.But once the cuses reach a certain size the mechanical compression of nerves become significant and atop respondin to injections.At this stage surgery is indicated. Surgical options include only decompression or decompression with fusion with screws.Whether to use screws and fuse or not will depend on assessment for instability of the bones at the involved level by X rays/CT scan or MRI.However most commonly most of the synovial cysts in spine respond cert well to decompression surgery wit encouraging results. Hope this answers your question. All the best with best regards Dr.SBK