There are many potential causes of back pain after a spinal fusion surgery. The spine is a highly dynamic mechanism, and the attempt to make one or more of its segments into a rigid unit may not yield perfect results.
Spinal fusion is done to eliminate painful motion from an unstable segment of the spine. During the procedure, a piece of bone graft or similar material is applied between two vertebrae, and the segment is secured with rods and pedicle screws. Ideally, the hardware used provides enough stability to allow the bone graft to slowly fuse to each vertebra, thereby joining them into one rigid segment.
A number of things can go wrong during and after surgery. First, the hardware may be implanted in such away that it interferes with nerves and muscles around the spine. Even though surgeons have extensive knowledge of spinal structures, the area is complex enough that even an adept surgeon may place a screw or rod in a position to cause pain. It is also possible that the fusion never takes place; this could be due to excess motion of the segment post-surgery. Hardware may break or migrate during the healing process, creating pain and increasing the risk of failed fusion.
It's important to be aware that pain after surgery could indicate a hardware problem. If pain occurs soon after surgery (after the initial post-operative pain subsides), it is possible that either the placement of hardware is the problem or that hardware has migrated. Sometimes after a fusion procedure, the rod moves out of place before the fusion sets up or scar tissue forms to secure it. In either case you may feel localized pain and tenderness when the skin over the rod is touched. You may also have signs of nerve interference (shooting pain, numbness, and/or weakness along a nerve path).
Over time, metal fatigue can cause the rods or screws to fracture. This can lead to nerve pain and muscle spasms, since the fragmented implant may be interfering with a muscle's movement.
Pain caused by hardware can only be resolved by removing the hardware in a surgical procedure.
Lonstein et. al. performed a study assessing the complications associated with pedicle screws used in 915 spinal fusions performed between 1984 and 1993. They found that late-onset pain required the removal of hardware from those who received 24.3% of the original procedures. 20% of those who had hardware removed were found to have pseudarthrosis, or failed fusion. In the other 80% of cases, then, it is possible that hardware was the main cause of pain.
The study above is rather old, but new studies into the complications associated with spinal fusion surgery are lacking. The rate of spinal fusion procedures has increased 250% since the mid-90's with the greatest increase in the Medicare population. This has raised concerns that surgeons are performing unnecessary surgeries for the kickbacks they receive from hardware manufacturers.
Though spinal fusion boasts of a 95% or greater success rate, it is important to remember that this rate doesn't take into account a number of complications that may result. If you're feeling pain after spinal fusion, request imaging tests to see if your hardware is out of place or fractured.