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Update on Adjacent Level Disease After Cervical Spine Fusion

Posted on: 08/21/2008
There's an ongoing debate about adjacent segment disease (ASD) after fusion of the cervical spine. ASD refers to the degenerative changes seen in the disc and vertebra at the next spinal level. The changes may not cause painful symptoms but they are visible on X-ray. If asymptomatic, the condition is referred to as adjacent level degeneration. Once symptoms develop, the label changes to ASD.

It's clear that after fusion, the next vertebral level starts to degenerate, too. The question is: does this happen as a result of changes in force and stress from the fusion? Or would this happen anyway because of the natural progression of degenerative disease?

The answer to these questions is important because disc replacements for the cervical spine are now approved for use by the Food and Drug Administration (FDA). The goal of these implants is to preserve motion, reduce stress, and decrease the chances of developing ASD. But maybe these efforts aren't needed. Maybe ASD is going to occur no matter what course is taken.

The authors of this review looked at studies on ASD published from 2006 to 2007. They reported the incidence and outcomes of ASD after fusion. And they offer some conclusions on this topic.

Studies report between 25 and 92 per cent of patients with cervical spine fusion develop ASD. That means that at least one in four (and maybe more) are affected by ASD after fusion. But not all patients with ASD require further surgery. And the rates of ASD after fusion aren't higher than after combined discectomy and fusion or after discectomy alone.

As time goes by, the rate of ASD increases after surgery. This usually happens at an average of seven years. The cause may be altered stress at the disc spaces above and below the fusion level. With a loss of motion at the fused site, increased stress is placed on the next available mobile unit.

What's the source of this stress? That remains the subject of many studies. Loss of motion at the fused site may transfer load to the adjacent level. Disc replacements may help prevent this from happening by preserving motion at the diseased segment. But some scientists point out that ASD occurs without fusion. This has been observed in patients who have decompressive surgery. In this procedure, bone is removed from around a spinal nerve to take pressure off it and reduce neck pain.

Another possible cause of the stress may be the location of metal plates used to hold the segments in place until fusion takes place. Plate position placed too close to the next disc may create problems.

Researchers have also looked at the possibility of cervical kyphosis creating a problem at the next level after fusion. Kyphosis is a forward curve of the vertebral bones. The position puts increased stress on the anterior (front) portion of the vertebrae. So far, it looks like patients who had kyphosis before the fusion surgery tend to stay stable. It's the patients who develop new kyphosis after fusion that also go on to develop ASD.

But while some studies conclude that using a disc replacement will prevent ASD, others show mixed results. In some studies, there was no difference in rates of ASD two years after fusion versus after disc replacement. Experts point out that there are various factors that might influence study results. For example, disc replacement doesn't change the condition of the nearby facet (spinal) joints.

Bone spurs and hypertrophy (thickening) of the articular joint cartilage of the facet joints may alter the biomechanics of the joint. The end-result could also be ASD. And in studies comparing ASD after fusion versus after disc replacement, the presence of pseudoarthrosis may alter the results. Pseudoarthrosis means there's enough motion at the fused segment to create a false joint.

Right now, after looking at all the data, it appears as though ASD occurs despite newer treatment methods and surgical techniques. It may just be that a disc under stress (for any reason) is going to respond to stress over time with degeneration at the next level. Discs above or below the fused level that are stiff before surgery tend to degenerate faster after surgery.

The answer to what causes ASD will be discovered over time. Long-term studies of artificial disc replacements will show if motion saving implants are the answer. If ASD is really a multifactorial problem, then further steps will be needed to prevent ASD.

Ravi K. Ponnappan, and Alan S. Hilibrand. Cervical Spine. Adjacent Segment Disease of the Cervical Spine: Fact or Fiction. In Current Orthopaedic Practice. July/August 2008. Vol. 19. No. 4. Pp. 420-424.

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