There are many different reasons this can happen. It's not always clear why it happens to any one person. Low bone mass in post-menopausal women may be a factor. Tobacco use in any patient can delay or prevent healing. Sometimes it just takes longer to heal compared to the average patient and fusion occurs much later.

Occasionally, the screw "backs out" or loosens too soon. Without this instrumentation to hold the spine stable during healing, motion can occur at the fused segment.

You may not have to do anything. Your surgeon is the best one to advise you on this. Some patients who don't have any symptoms adopt a "wait-and-see" approach. Dynamic (moving spine) X-rays can be taken to help with this decision. If there is an apparent loss of fusion on regular X-rays but no motion on dynamic films, then no additional treatment is required.

In these cases, your body may have formed a fibrous union. A thick band of scar or connective tissue may have formed instead of bone. Unless you develop more pain or new pain, no additional surgery is needed. You may see a physical therapist for a short rehab program to help prevent future problems.