In addition to the primary reasons for failed back surgery syndrome (FBSS) discussed on the prior page, there are several reasons why a spinal fusion might fail to alleviate a patient’s back pain after surgery, including fusion and implant failures, and transfer lesions.
Failure of Solid Fusion After Spine Surgery
When the fusion is for back pain and/or spinal instability, there is a correlation (although weak) between obtaining a solid fusion and having a better result of the spine surgery. If a solid fusion is not obtained through the spine surgery, but the hardware is intact and there is still good stability to the spine, the patient may still achieve effective back pain relief with the spine surgery. In many cases, achieving spinal stability alone is more important than obtaining a solid fusion from the spine surgery.
On postoperative imaging studies it is often very difficult to tell if a patient’s spine has fused, and it can be even harder to determine if a further fusion surgery is necessary. In general, it takes at least three months to get a solid fusion, and it can take up to a year after the spine surgery. For this reason, most surgeons will not consider further spine surgery if the healing time has been less than one year. Only in cases where there has been breakage of the hardware and there is obvious failure of the spinal construct would back surgery be considered sooner.
Implant Failure in Spine Surgery
An instrumented fusion can fail if there is not enough support to hold the spine while it is fusing. Therefore, spinal hardware (e.g. pedicle screws) may be used as an internal splint to hold the spine while it fuses after spine surgery. However, like any other metal it can fatigue and break (sort of like when one bends a paper clip repeatedly). In very unstable spines, it is therefore a race between the spine fusing (and the patient’s bone then providing support for the spine), and the metal failing.
Metal failure (also called hardware failure, implant failure), especially early in the postoperative course after back surgery, is an indicator of continued gross spinal instability. The larger a patient is and the more segments that are fused, the higher the likelihood of implant failure. Implant failure following spine surgery should be very uncommon in normal sized individuals with a one level fusion.
Transfer Lesion to Another Level after a Spine Fusion
A patient may experience recurrent pain many years after a spine fusion surgery. This can happen because the level above or below a segment that has been successfully fused can break down and become a pain generator.
- This degeneration is most likely to happen after a two-level fusion (e.g. a fusion for L4-L5 and L5-S1 levels) and in a young patient (in the 30-50 year old age range).
- It is much less likely to happen if only the L5-S1 level is fused, as this segment typically does not have much motion and fusing this level does not change the mechanics in the spine all that much.
- Most of the motion in the spine is at the L4-L5 level, and to a lesser extent at L3-L4. When the L4-L5 level is included in the spine fusion it transfers a lot of stress to L3-L4. This does not present as much of a problem for elderly patients, since they tend to not be as active nor do they have the fusion for as many years.
- Transfer lesions are far more common in degenerative osteoarthritis conditions ( e.g. degenerative spondylolisthesis) and far less common in disc degeneration problems (e.g. degenerative disc disease or isthmic spondylolisthesis).