I've had my TENS all summer and fall and still find it very helpful.
After a few minutes of TENS on my back I have to turn the intensity knob up. After another 20 min to half hour I turn it up again. There's probably something about the nerves getting used to the shock at a low level. I can get a couple of hours with the TENS and then relief stays for a long time after I take it off. But that's the back. I haven't tried it on post-op pain. It's not supposed to be put across or directly on the spine. I don't think it would work on bone pain anyway.
TENS didn't work for my pre-op bursitis and tendonitis and bone spur pain in my shoulder. I haven't tried it post-op for that pain, but I will today. I'm having muscle pain in biceps, back of shoulder and neck after babysitting grandkids last night. Since I get relief from back muscle spasm pain, maybe the TENS will help for these muscles too. I don't expect anything for the bone surgery part of the pain, and the bursa was removed.
I wouldn't accept a new unit until the old one wears out. They keep sending me the square pads for the belt unit, but I don't like that. I like the separate round pads that I can put wherever I want them. I can carry those and the little unit around in a sandwich-size plastic bag in my purse and put the unit on any time. I got it out after my endoscopy yesterday because the nurses wouldn't let me take my regular pain meds, even though I hadn't had throat spray and was allowed to sit there drinking coffee. It was nice for the long ride home.