Pain cases generally depend on testimony. They generally do not involve a situation where the claimant is provably unable to do a certain task, but involve a limitation that pain imposes on them. That's not necessarily provable by an Xray or even by MRI or CT scans. Even so, without medical proof of the underlying problem that is causing the pain, they a nearly impossible to prove.
I tried a case earlier this year where the man had had a two level fusion; fusions usually are pretty succesful in the neck. This man, though, had a poor recovery which was documented by MRI. He was denied at the first two stages and he had been declined by another experienced SSDI/SSI lawyer. I decided that his pain complaints were legitimate and tried the case. The judge agreed.
I also got disability a few years ago for a woman who passed many gallstones (seen on Xray) per week and who was on morphine for the severe pain. She was a young mother who couldn't do much for herself or her family because she spent most of her time crying due to the pain. The judge agreed that that pain was disabling.