Adjacent Segment Fusion

When a spinal segment is fused, it stops moving. The levels immediately above and below now have to do a little more of the work of bending and carrying load. Over time, the level next to a fusion may show wear on its X-rays and scans — this is called adjacent segment degeneration. In some people that wear becomes symptomatic, producing new back pain, or leg pain and numbness from nerve compression at the neighbouring level — and that is adjacent segment disease. The distinction matters: many people show some degeneration on imaging without any trouble from it, whereas a smaller number develop genuine, symptomatic disease that may need treatment. It is one of the most important long-term considerations after any lumbar fusion.

The honest answer is that it is common enough to take very seriously. Pooled analyses report radiographic adjacent segment degeneration at a rate of roughly six percent per year and symptomatic adjacent segment disease at roughly two percent per year, with symptomatic disease clinically affecting somewhere in the region of five to thirty percent of people over time, and reoperation rates commonly cited around one in ten to one in six. The classic long-term study by Ghiselli and colleagues estimated that the chance of a neighbouring level becoming symptomatic and needing surgery was about sixteen and a half percent at five years and thirty-six percent at ten years. Longer-term series of circumferential fusions have reported reoperation for adjacent levels rising towards a quarter at ten years and higher still by fifteen. These are not reasons to avoid fusion when it is needed — but they are powerful reasons to do it in a way that protects the neighbours.

There is a genuine and long-running debate about cause, and the truth involves both sides. Part of adjacent segment disease is simply natural history: the same wear-and-tear process that damaged the level I fused is often present, quietly, in the neighbours too, and it would have progressed to some extent regardless of any surgery. But part of it is mechanical, and driven by the fusion. When a rigid, fused segment sits in the spine, the biomechanical studies show that the levels next door experience increased movement, higher pressure inside the disc, and greater load — they are, in effect, made to work harder. On top of that, the surgery itself can add insult: stripping and damaging the muscles and ligaments, disrupting the posterior tension band, and — a specific and avoidable problem — violating the facet joint of the level above when placing the topmost screws. So adjacent segment disease is best understood as a natural degenerative tendency that a fusion can accelerate, especially if the fusion is done in a way that overloads or disturbs the neighbours.

If I had to name the one modifiable factor that matters most, it would be sagittal alignment, and this is the crux of my whole approach. A lumbar spine that has been fused flat — under-corrected, with too little lordosis, and a mismatch between the curve of the lower back and the shape of the pelvis — forces the segments above to work abnormally hard to keep the body upright. The evidence here is striking: a mismatch between pelvic incidence and lumbar lordosis has been shown to predispose strongly to adjacent segment disease, with a markedly higher revision risk when the malalignment persists, and inadequate restoration of segmental lordosis, particularly in the lower lumbar spine, has been linked to higher rates of adjacent segment disease and revision within just two years. In plain terms: if I fuse someone flat, I hand the bill to the level above. That is precisely why I treat every case as a deformity case, why I obsess about restoring lordosis and matching it to the pelvis, and why I regard good alignment as the best protection against adjacent segment disease that I can offer.

Alignment is the biggest lever I control, but it is not the only factor. The literature identifies several others: the number of levels fused, with longer fusions of three or more levels carrying more risk; older age; a higher body weight; osteoporosis, or weak bone; degeneration already present in the adjacent level before surgery; and facet joint violation by the topmost screws. Some of these — age, existing degeneration, bone quality — I cannot change, but I can account for them in planning. Others — how many levels I fuse, whether I violate the facet above, and, most of all, the alignment I leave behind — are within my control, and are exactly where careful technique pays off years later.

Prevention runs through everything I have described. First and foremost, I restore lordosis and global sagittal balance, planning the shape of the spine around the person's pelvic morphology so the neighbours are not overloaded. I fuse as few levels as the problem genuinely requires, no more. I take care to protect the facet joint of the level above when placing screws, to avoid tipping a healthy neighbour into trouble. And in selected cases there are motion-preserving or so-called topping-off strategies — dynamic devices at the top of a construct designed to create a gentler transition between the rigid fusion and the mobile spine above — though the evidence for these remains mixed and they are not a substitute for getting the alignment right. The through-line is simple: the way to protect the neighbours is to leave the spine balanced and to disturb it as little as possible.

When adjacent segment disease does become symptomatic and does not settle with non-surgical measures, the treatment is directed by what is causing the symptoms. If the trouble is nerve compression at the neighbouring level, that level is decompressed to free the nerves. Because the neighbour has usually become unstable or is contributing to the imbalance, the decompression is generally combined with extending the fusion to include that level — and the evidence shows that decompression with extension of the instrumented fusion gives durable improvement in pain, disability and quality of life at two years. Crucially, when I extend a fusion for adjacent segment disease, I do not simply bolt on another level; I take the opportunity to restore the global sagittal balance of the whole spine, because if I extend the fusion without correcting the underlying malalignment, I will simply pass the same problem up to the next level. Less invasive anterior or lateral interbody approaches can sometimes achieve this with less disruption. The principle, once again, is that the durable answer is a well-aligned spine.

For most people, adjacent segment disease develops gradually, years after the original surgery, and is monitored over time; not everyone with degeneration on a scan will need anything done. Where surgery is required, recovery follows that of a lumbar decompression and fusion — a hospital stay of several days and a graded return to activity over the following weeks and months while the bone knits and, ideally, within a spine whose alignment has been restored. The long-term follow-up of any fusion is, in large part, a watch on the neighbouring levels, which is why alignment and careful technique at the first operation matter so much.

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