Patients offered a fusion increasingly ask whether a disc replacement would be preferable. The answer depends heavily on where in the spine the problem lies.
More and more patients troubled by back pain, having been offered a spinal fusion, ask me whether disc replacement might be the better option. It is a reasonable question, and the answer differs considerably between the neck and the lower back.
First, what spinal fusion does
Fusion is used to treat back pain caused by instability of the spine, where one bone has physically slipped forward on the one beneath it, and particularly where that slip threatens the nerves controlling movement and sensation in the legs.
The construction is straightforward: screws and two rods, together with a device between the two bones called a cage. This allows the segment to fuse together, stabilising it. It can be effective both for the back pain and for any nerve entrapment.
One worry people often raise is that it immobilises the segment. In practice, immobilising a single segment creates surprisingly little stiffness. You have many other segments capable of movement, and they compensate. We perform this procedure on some highly professional athletes.
A second indication is where back pain has been shown to arise from inflammatory changes in the bone around a disc, without instability. Here the alignment is well maintained but the bone shows inflammatory change. We do sometimes see this in people with no back pain at all, so the diagnosis must be made carefully. But where physiotherapy and other treatments have demonstrably failed, fusing the segment can help.
Fusion in the neck
The cervical spine presents a particular challenge. If a slipped disc is compressing the spinal cord or a nerve, reaching it from the back of the spine is difficult, because the spinal cord itself is in the way, unlike in the lumbar spine.
So we usually approach from the front of the neck and remove the disc, putting a cage in its place to allow the bones above and below to fuse.
The problem that led to disc replacement
Fusion is effective, but it has a recognised drawback.
When a segment is fused and its movement removed, the segments above and below must compensate. Each vertebra is joined to the next by the disc at the front and the facet joints at the back. What we have observed over the years is an acceleration of wear and tear at the adjacent levels.
This is what prompted the development of motion preservation surgery, and disc replacement in particular.
How disc replacement works, and where it works best
In the cervical spine, where this surgery is rather more established, the disc is removed and a device fitted that permits a degree of motion between the two bones. This can help prevent symptomatic problems at the levels above and below, and studies show reasonably good results.
There are caveats. The complication rate and the reoperation rate are both slightly higher, which is why careful patient selection is paramount. We must examine the facet joints at the back of the spine and be satisfied they are not significantly worn. If we preserve motion at joints that are already degenerate, we may simply create ongoing neck pain instead.
My own approach in the neck
Where the disease affects a single level, I usually fuse the segment. Losing motion at one level alone does not cause lasting stiffness in the neck, and with careful rehabilitation and exercise therapy we can do a good deal to prevent adjacent segment disease.
Where two levels need surgery, I may offer a disc replacement at one and fuse the other, so that some motion preservation is retained.
Why the lumbar spine is different
Results in the lower back have been less convincing.
The reason lies in the facet joints. Even when the disc is replaced and motion preserved, the facet joints at the back of the spine become considerably more painful, and this is worse in the lumbar region, where facet joint degeneration and enlargement are far more common. It is genuinely rare to find a badly worn disc accompanied by little or no facet joint wear.
There are, however, newer devices in clinical trials. One I find particularly interesting replaces the disc and removes the facet joint, with a construct performing the function of both. By removing the facet joint you eliminate facet joint pain, while also addressing the inflammation and degenerative change around the disc.
The wider point
It is worth stepping back. The usual indication for spinal surgery is to protect nerves: to relieve pain radiating down the legs from the lumbar spine, or down the arms from the neck, or to preserve function in the limbs.
For back pain or neck pain itself, there are very few indications where surgery helps.
The key takeaway
Where surgery is genuinely required, there are sound indications for disc replacement in the neck, though it depends on the individual patient, a careful review of their scans, and how their symptoms fit together.
In the lumbar spine the picture is less clear. I shall be watching the development of these newer devices with interest.
