Surgical fusion of the spine for degenerative disease is becoming a popular way to treat this problem. And that’s because surgeons now have at their disposal better ways to perform the surgery and improved hardware such as pedicle screws and locking plates to hold the bones together. Even so, there is a major concern about the number of failed spinal fusions requiring revision (a second) surgery.
In this review article, orthopedic spine surgeons from George Washington University Medical School bring us up-to-date on the problem of pseudarthrosis after spinal fusion. Pseudarthrosis means false joint and refers to movement that occurs at the fused site. It can occur without symptoms so the patient doesn’t even know he or she has it. Or it can cause back and leg (or arm) pain, depending on whether the fusion is at the cervical (neck) or lumbar (low back) level.
There can be other causes of failed spinal fusion such as the hardware coming loose or infection and poor wound healing, but pseudarthrosis accounts for almost one-fourth of all revision fusion surgeries. To help us understand why this happens, the authors present the many possible risk factors, and then walk us through the diagnosis and follow-up treatment.
So, who’s at risk for pseudarthrosis? Smokers and patients who do not follow the guidelines for movement restriction during the post-operative period are at the greatest risk of failed fusion. But anyone who has reduced blood supply or metabolic disorders such as heart disease or diabetes can also experience delayed wound healing or infections that can leave patients with lower fusion rates.
How does the physician diagnose pseudarthrosis? It can be discovered in the patient who doesn’t have any real symptoms when dynamic imaging studies are done. Dynamic means the X-rays are taken as the patient is moving. But this method isn’t very reliable and wouldn’t be done routinely after surgery if the patient wasn’t having any problems.
When it comes to diagnostic imaging, there just isn’t a good way to tell if the fusion failed. When reading dynamic radiographs, the radiologist knows that just because there isn’t any obvious motion doesn’t mean the fusion is complete. And just how much motion constitutes a failed fusion remains fuzzy. There’s a lot of debate about what is and what isn’t a solid fusion. Some experts think there’s a difference in springiness between a fusion with and without hardware to hold it together during the healing phase.
Thin-slice CT scans have been used to assess the fusion site. But the results don’t really add anything more than what is seen on the X-rays. The one exception to this is in the case of locked pseudoarthrosis. Thin-cut CT scans help show this problem more clearly than dynamic radiographs. Locked pseudoarthrosis describes a situation in which the top and bottom of the cage inserted between the two vertebrae has fused solid but the middle (inside the cage) has not filled in with bone and solidified.
MRIs can be a bit iffy in patients with hardware in place because the implants cause artifacts (unexplained shadows and altered densities). Those changes interfere in judging whether or not the fusion is completed. There has been some question about the use of ultrasound and bone scans to help diagnose pseudarthrosis. Not enough study has been done to clear up any questions about these modalities. When imaging studies do not aid in the diagnosis, the surgeon can rely on a follow-up surgical procedure to confirm any diagnostic suspicions. Only patients with painful, disabling symptoms would undergo a second (diagnostic) procedure.
Once the diagnosis has been made, what is the treatment for pseudarthrosis? What can the surgeon do for this problem? Some of the decision depends on how the first fusion was done and the location (neck or low back). If there are no symptoms, then it might warrant a wait-and-see approach. But for the patient with painful symptoms, if no graft was used in the first procedure, the surgeon may choose to take bone from a donor bank or from the patient and place it around the fusion site. There are advantages and disadvantages for each choice.
Allograft (donor bone from a bank) is dead and doesn’t produce new bone. It just gives a scaffold or place for the patient’s body to fill in with bone produced during the healing phase. That’s the down side. The up side is that there are no problems at the donor site with pain, infection, or poor wound healing. Autograft (bone taken from the patient) are still alive and capable of producing more bone cells. That’s a benefit as the body fills in the fusion site faster. But then there’s the risk of donor site morbidity (problems) as described.
If bone graft was used (and failed) in the first surgery, then metal plating or a device called a cage may be used. Plating anteriorly (from the front of the neck) is usually advised when there are multiple levels being fused. Cages have been made of titanium for the most part. But newer implants made of plastic or porous tantalum are being tried in hopes that fusion rates can be improved with better bone in-growth.
For both the cervical and lumbar spine, human bone morphogenetic protein (rhBMP-2) has been tried with mixed results. Its use is still in the experimental phase. There hasn’t been much published yet on results of this technique. And research is ongoing in an attempt to find bone graft substitutes. Products commercially available that have been approved for use are showing some improved results in early studies but there are reported side effects with cervical fusion (e.g., neck swelling, difficulty swallowing) that have raised some concern.
Finally, how well does it work to try a different approach when treating failed fusions? Studies show that fusion is almost always possible but symptoms don’t always change as a result. Risk factors, psychologic issues, and type of graft enter into the mix and affect the outcomes. Risk factors for revision spinal fusion are similar to first-fusion procedures but also include worker’s compensation status, active lawsuits, use of narcotics before surgery, and neurologic problems present before surgery.
There isn’t a one-size-fits-all method of recovery after a failed first fusion. Each patient is evaluated on an individual basis. Surgeon preferences and experience can also make a difference in choosing type of revision surgery performed. The bottom line is: if the revision surgery is successful, the outcome is better than if the person remained unfused.