[annapurna]

While it is possible to break a fusion, breaking a twenty-year old fusion would likely be a fairly traumatic event of sufficient significance that you wouldn't be questioning what happened. A simple flexion-extension xray can, should you still be worried, confirm that your fusion is still fused. It might miss a hairline crack, just like a x-ray could miss a hairline fracture, but it's not likely to miss anything else.

A discogram might be worth investigating the condition of L4-5 by itself to see how much, if any of your pain comes from the disk. Facet injections are a possible diagnostic tool but my memory suggests that they can be confusing to interpret when you have a potentially painful disk working in conjunction with damaged facets.

You might consider your combination of a microdisectomy and a foraminotomy together with some of the investigational stem cell techniques to see if you can restrengthen the disk after the microdisectomy and buy time.

I disagree with Jeff's comment about putting off surgery for the longest possible time. Far too often, that's interpreted to mean that nothing should be done until the last possible moment: when the patient isn't able to think coherently and is pressured to do something as fast as possible. I'd suggest that you plan now to have surgery. Figure out what you'd do and what would be the bet to address your current condition. Once you have it figured out, sit down and work through the risks and benefits and decide what kind of shape you'd need to be in to be willing to get the surgery. Don't actually get the surgery until you're happy with the decision and have reached the point where the risks and benefits are correctly balanced for you but don't put off the thinking until your time pressure limits your options.