Less than 40% relief is generally considered a "failure". One thing to note is that the trial tends to produce more relief than the permanent implant, so besides the fact that insurance generally won't cover it if it's under the 40% threshold, it would likely not be worth the pain & risks of surgery for that level of relief. Also, the SCS will only lessen the pain at the specific level where it is attached, so it's not as if everything becomes much easier. In my case, I can use it for base of head/top of neck pain (where they meet) plus one line of shooting pain down my right arm and also if I turn it up high enough I can get my hands & feet. It is about "50-60% better" **in the areas it affects** when I turn it on (8 to 4/5, 9 to 4/5). I can hold a conversation & sit at a computer to type now, but it doesn't mean that now I can go walking or do a bunch of housework and stuff and not pay the consequences of doing that. The pain around my temples, jaw line, low back, etc. is still at 100% (8/9 is still 8/9) because the SCS doesn't touch those areas. That means that overall my pain scale rating is maybe improved one or two levels by the SCS (8 to 6/7 or 9 to 7). I agree that that is a *huge* improvement, but I just wanted to be clear that it is not like pain meds which act on all your pain. The SCS will only work on one specific area.
The reason there are thresholds for how much pain it needs to relieve is because the SCS is extraordinarily expensive. The device itself is $8-10K and then there's the surgery & follow-up on top of that. Generally altogether it runs $20-40K. In my case, the cost of major neurosurgery (removing part of my skull & top vertebrae) ran less than half of what the SCS implant cost. For better or worse, insurance companies are in the business of making money, so if they don't feel they can recover their "investment" in the SCS through fewer office visits and fewer minor procedures like injections or RF they do not pay for the implant. Beware about overstating how much relief you're getting, because then the evil insurance people start calling you & sending you angry letters about why are you still on so many meds/still seeing the PM every week after having the surgery. The insurance people suspected I was dishonest about level of relief during the trial b/c I was having a hard time healing after the procedure & needed extra visits/more meds for several months afterwards. My PM & PCP wrote off a lot of the bills I had over the first few months, plus they had to submit a ton of documentation going years back showing that I've always had very slow healing times plus they had to run all sorts of extra blood work to prove that my sed rate was super high & that I wasn't lying about the relief and we weren't making stuff up. So aggravating! >:| Eventually my docs did get some of their bills covered, but trust me when I say that it's hard enough when you're being honest about how much relief you're getting. To play games just to get the implant would not be worth it b/c they've got all these stupid actuary tables that show norms about if you have_____ relief from the trial you're supposed to have _____ fewer meds and/or _____ fewer office visits; if you don't fit the norms you will have major probs and may not be able to see your doc for a long time afterwards.
I know it's frustrating to not have something work out. It's a tough balance wanting to keep our insurance premiums lower versus wanting every possibly treatment available covered no matter the cost, but that's the reality of it. The pain pump is less expensive & should be easier to get approved. It's also centrally acting, so it would probably help the arthritis much more than the SCS can.