Risks Factors for Adjacent Segment Degeneration After Spinal Fusion

Reason for Your Back Surgery

Depending on the diagnosis that leads to your back surgery, you may be at an increased risk for ASD.

Dr. John Toerge, an osteopathic physician, professor of medicine at Georgetown University, and Medical Director of the National Rehabilitation Hospital’s Musculoskeletal Institute, says that people who undergo a spinal fusion for degenerative disc disease are at an increased risk for ASD. Toerge says this is because degeneration has already started in the levels above and below the problem area, even though you may not have noticed symptoms. Generally, the surgeon does not fuse those adjacent levels, he adds.

Toerge says that patients with severe arthritis may also be at an increased risk for ASD. "These people have fewer mechanical elements that can reduce the risk," he explains. "With diminished residual capacity, patients with advanced arthritis have little room for error, and as such, are more prone to further degeneration in the spine."

Your Age

It is widely accepted that age plays an important role in the risk for ASD.

As we age, our spines tend to degenerate, which complicates the idea that back surgery causes ASD. In fact, a 1999 study on risk factors for ASD in the neck, conducted by Hilibrand and published in The Journal of Bone and Joint Surgery, found that preexisting degeneration as seen in films (such as MRIs and CT scans) was one of the biggest risks for ASD.​

"The natural history of degenerative changes in the spine is a compounding variable when determining the cause of ASD," says Dr. Frank P. Cammisa, Chief of Spinal Surgical Service at Hospital for Special Surgery in New York. "If these changes are already occurring in your spine, they may be present (or they may develop) in more than one level, with or without surgery."

Location of Your Surgery

Your spine has opposing curves, which help you balance as you move. These curves are divided into areas: Neck (cervical), upper and mid back (thoracic), low back (lumbar), and sacral area. If your surgery takes place where one curve transitions into the next—for example, where the thoracic becomes the lumbar (T12-L1)—your risk for ASD may be higher.

Toerge calls these transition areas "active motion segments." He says that fusions at active motion segments often present problems later. This is because, he says, such a fusion may result in increased load on the neighboring intervertebral joints, which in turn may increase the risk of ASD, as well as adjacent segment disease.

The Hilibrand study mentioned above found that risk for ASD varied according to the location of the fusion. The researchers identified the C5-C6 and C6-7 levels (these are the two lowest intervertebral joints in your neck) as posing the greatest risk of any area in the neck for degeneration not previously evident on films. These two motion segments, or levels, are very close to or at the active motion segments mentioned by Dr. Toerge.

Length of the Fusion

In general, your risk for ASD is higher when more levels are fused.

Dr. Cammisa says spinal problems necessitating a long fusion (multiple levels fused) pose more of a risk for ASD. Scoliosis is an example of this. Cammisa explains if you’re fused from T4-L4 (the range of motion segments, or intervertebral joints, that spans from the middle of your chest to just below your belly button) to correct​ scoliosis, it is likely that over the years you’ll develop ASD at T4-5 and L5-S1. (T4-5 and L5-S1 are the motion segments located directly above and below T4 and L4, respectively.)

A 2016 review and meta-analysis published in the journal Clinical Spine Surgery fusion length is ​the biggest factor associated with adjacent segment degeneration and disease. The authors suggest that limiting the number of levels fused may be a better strategy than changing how the fusion is done.

Posture Before and During Your Back Surgery

Your posture, as well as the alignment of your bones during the surgery, may affect your risk for ASD. If you have a kyphosis at the time of the fusion, you may later experience strain on your facet joints. This may lead to pain as well as the degenerative changes indicative of ASD. It may also lead to spinal arthritis at the facet joints.

Two postural misalignments associated with the development of degenerative spinal changes and ASD are related to one another. If your posture is such that your pelvis is tilted back (called pelvic retroversion) during the surgery, the muscles responsible for holding you upright may fatigue more easily afterward. Over time, this may lead to pain and degenerative changes in that area of your spine.

The angle of your sacrum during surgery makes a difference, too. Normally, the top of the sacrum tilts slightly forward (as does the pelvis, discussed above). If your sacrum happens to be vertical or near a vertical position during the surgery (which it may well be if your pelvis is tilted back), your risk for ASD may be increased.

And finally, do you have forward head posture? If so, and you’re having a spinal fusion, your risk for ASD may again be increased.

While some of these issues can and should be addressed by your surgeon at the time of the procedure, remember that you bring your posture with you to the operating table.

For many of us, posture is an accumulation of habits over time; for others, it is part of our structure. If your kyphosis, forward head, sacral angle, and/or pelvic tilt related posture issues are not built into your bones (and in some cases, even if they are), seeing a physical therapist for a home exercise program before you have the surgery may help you decrease some of your ASD ​risk.

"Carefully selected exercise to stabilize the risky areas can be very helpful for reducing your symptoms," Toerge adds.