Scientists aren't sure the answer to this question. Perhaps there's a little of both. Pain lasting more than six months is called chronic pain. Chronic low back pain is complex and poorly understood.
It's clear that the changes take place in how chronic pain is transmitted. Surgery may not change the pain pathway. In fact it could make things worse instead of better. Since the cause of back pain is often unknown, the best treatment is also unclear.
A recent study in New Zealand reported that improvement--but not cure--is a common response to surgery to fuse the spine. Patients didn't have great functional improvement, but their pain levels and drug use were less. Many were able to return to work. Most were glad they had the operation.