After I had my hip replaced, I noticed that my back stopped aching. Is it just a coincidence or is there some connection?

Actually, there is a connection. About 30 years ago, the hip-spine syndrome was reported by two orthopedic surgeons. No further studies have been done on the topic until recently.

A group of surgeons in Tel-Aviv, Israel studied 25 patients having hip replacements. They observed that low back pain (LBP) in this group was almost as bad as their hip pain. After the surgery, hip pain and function improved. But so did the LBP.

The group reported the most improvement in LBP by three months after the hip replacement. They didn't gain any further change in their back symptoms. But at the end of two years, the changes were still maintained.

Patients reported being able to stand, balance, and walk better as a result of the hip replacement. The authors concluded that total hip replacement does have a positive effect on spinal posture and symptoms.