If there's such a thing as nerve root tethering that is neither visible on MRI or to the eye, then I very much doubt that anyone could prove that there was such a thing. What I'm saying is that it's extremely unlikely to the point of being a - No.
Other nerve problems, yes it could be true sciatica, sciatica proper or piriformis syndrome - they are all one and the same thing. This is a lot more likely to cause pain from the buttock down the thigh and leg, but not up as far as the lumbar area, but it's not impossible.
Short of actually of examining you properly, then there isn't much more that I can say that I haven't already said on this Thread - http://www.healthypages.net/forum/tm.asp?m=87060 and others in this Forum.
In light of the new information, then the most likely cause is a facet 'joint lock' with referred pain which is not the same as nerve root irritation. Put simplistically when you have nerve impluses firing off from a certain level of the spinal cord they can spill over into nerves for other areas, so the brain perceives that you have pain in that other area, where there may not actually be a problem.
Also, what happens when the pain has been there for a long time is that some will actually grow more nerve endings in the local area and dedicate more area of brain to that area. This means that the area becomes over sensitised and prone to reacting more severely than in the next person.
Add to that - your repeated surgery will caused scar tissue, possibly some degree of nerve injury and cortizone, as it speeds tissue repair, then they tend to be weaker. But if present this would only likely to be in a small area around the point of injection.
Any recovery that you make is going to be largely down to you!
Whether that means;-
* Hooking up with the right practitioner, osteopath or whoever, seeing the treatment and advice given or
* Or you saying to yourself - Well, no one can fix this for me and I'm damned if I'm gonna take this lying down.
I can't say, but essentially all healing is done by the person themselves - no one else can fix it for you, but there are those out there that can probably do a lot to help.
I also can not speak for the practitioners that you have already seen - good, bad or otherwise. Sometimes the muscle spasm is so severe that it's not always possible to release a 'locked facet' joint. When they are this severe, then you usually find that the person is very stressed, anxious or something is keeping them physiologically tense.
In such cases I tend to needle the area with western acupuncture for 5-10 mins prior to even attempting to release the offending joint. Acupuncture needles are excellent at reducing muscle spasm in most people and actually releasing the joint in turn causes a reflex relaxation of the local muscles, although many need further work on the soft tissues after this to break the pattern and it locking up again as soon as they leave or the following day.
If it's driving you up the wall, then getting stressed about it will only make it ten times worse. If this is the case, then learning to meditate or relax is a good place to start.
Maybe you need to accept that it may never go a away. But if you believe that it'll never go away, then it probably won't. There's unlikely to be any 'quick fix' or 'magic bullet' for this. It's probably going to take time and effort on your part.
Good luck - Andrew.