Adjacent Segment Disease

The spine is a column of vertebrae separated by discs and small joints that share movement and load between them. When a fusion is performed, two or more of these vertebrae are deliberately joined into a single solid block so that they no longer move relative to one another. The fused segment stops moving; the segments immediately above and below it keep moving and keep carrying load, and it is these neighbouring, or adjacent, segments that the term refers to.

The entire field turns on a single distinction. Adjacent segment degeneration means changes seen on imaging at the level next to a fusion — narrowing of the disc, the formation of bony spurs, a small slip, thickening of the small joints, or changes in the signal of the disc on an MRI. These are structural, radiographic findings, extremely common as people age and as a fusion matures, and in a large proportion of cases they cause no symptoms whatsoever.

Adjacent segment disease means something different and more specific: the appearance of new symptoms — recurrent back or neck pain, pain radiating into a limb, numbness or weakness, or the heavy-legged ache of canal narrowing — that correspond to those changes at the adjacent level. Disease, in this technical sense, requires both the imaging change and a matching clinical problem.

Reported frequency varies widely, in part because of this distinction. Radiographic adjacent-level degeneration is found in around a quarter to a third of people after lumbar fusion, with some series higher and others lower. Symptomatic adjacent segment disease leading to further surgery is much less common: figures around 2–3% per year are quoted in long-term studies, with cumulative rates of 16–24% at five to ten years in some series and lower in others. In the cervical spine, a frequently quoted long-term study found symptomatic disease in around 25% of people at ten years.

Two ideas compete to explain it. The first is altered mechanics: a fused level no longer absorbs movement and load, transferring both to the levels above and below, which over years may degenerate more rapidly than they would have done. The second is the natural history of degeneration: people requiring a fusion have already shown a tendency to disc and joint wear, and the adjacent levels were often already on a degenerative trajectory before surgery. Most authors regard the truth as a combination of the two, with the relative contribution varying from person to person.

Factors reported to raise the risk include age, pre-existing degeneration at the neighbouring level, longer fusions, fusion ending at vulnerable transitions (cervico-thoracic, thoraco-lumbar, lumbo-sacral), and poor spinal balance. Restoring good sagittal alignment at the original operation is regarded as one of the most effective protective measures.

Symptoms are those of any degenerative condition at the affected level: new back or neck pain, pain radiating into the arm or leg in a new dermatomal pattern, numbness or weakness, or the claudication-type heaviness of canal narrowing. The clue that points to adjacent segment disease is the pattern of symptoms, which fits the level next to the fusion rather than the fused level itself, and the timing — typically appearing years after the original operation rather than immediately.

An important and separate entity at the top of a long deformity fusion is proximal junctional kyphosis, an increase in the forward angle at the junction just above the top of the construct. The more serious form, proximal junctional failure, involves actual breakdown of the junction — fracture, hardware pull-out or a slip — often with pain and sometimes nerve symptoms, and frequently requires revision surgery.

Assessment combines the pattern of symptoms with imaging. Standing X-rays and full-length alignment films show the geometry of the spine and the relationship of the fusion to the levels above and below. MRI shows degeneration, disc signal change, canal and foramen narrowing at the adjacent level. CT is used where bone detail is needed. The central question is whether the imaging change corresponds to the clinical problem.

It is also necessary to exclude other causes: a problem at the original operated level (residual or recurrent stenosis, pseudarthrosis of the fusion), instability that is masquerading as adjacent level disease, and unrelated causes of pain. The label adjacent segment disease should be reserved for the situation in which the symptoms and the imaging genuinely match.

The evidence base is largely observational and consensus-based rather than built on large randomised trials. The following describes what the literature reports, not guidance for any individual.

Conservative care is the usual starting point for most people, including pain relief, activity modification, physiotherapy, attention to weight and bone health, and in selected cases targeted injections. Many people settle with these measures.

Surgery is generally reserved for symptoms that persist despite conservative care or for progressing nerve symptoms. It may involve decompression alone at a stable adjacent level, or extending the fusion to include the affected level, with attention to restoring spinal balance. Approaches from the front or side may reduce the burden of revision in selected cases.

Disc replacement is sometimes discussed as a way to reduce adjacent segment disease compared with fusion. The evidence is mixed: some trials report fewer further operations with disc replacement than with fusion, but a high-quality randomised trial in the neck found that an artificial disc did not prevent adjacent segment disease. Motion preservation may reduce some changes in selected patients but does not reliably abolish the problem.

A final, important point: extending a fusion solves the problem at one level but creates a new neighbouring level, which can itself become symptomatic over time. This is one reason such decisions are made carefully and individually, weighing benefit against the prospect of moving the problem along the spine.

For the much larger group with mere adjacent segment degeneration on imaging and no symptoms, the outlook is generally good and the situation is one of monitoring rather than treatment. For those with true adjacent segment disease, conservative care settles many; for those requiring surgery, outcomes are generally good when the symptomatic level is correctly identified and proportionately addressed, with the recognised proviso that the problem may recur at a new neighbouring level over the longer term.

This page is general medical education and is not a substitute for individual advice. New or worsening weakness or numbness, saddle numbness, or any disturbance of bladder or bowel control requires urgent assessment.

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This information is for general patient education and is not a substitute for an individual consultation. Always seek personalised advice from a qualified spinal specialist.