A less invasive form of fusion avoids the cage altogether. Here is who it suits, what the evidence shows, and why a perfect post-operative X-ray is not the goal.
If you have been told you may need lumbar fusion surgery, it is natural to worry about how extensive the operation is, particularly if you are older or have other medical conditions that raise the risks.
There is a less invasive version of the procedure that is worth understanding, along with the circumstances in which I would choose it.
What fusion usually involves
Fusion joins two vertebrae together, typically where the segment is unstable and nerves are being compressed as a result.
The operation uses screws placed into the bones of the spine, connected by rods to lock them together, realign them, and assist in decompressing the nerves. Usually it also involves placing a device between the bones called a cage, which is the medium through which the bones fuse.
The alternative: posterolateral fusion
A minimal access posterolateral fusion does not use a cage at all. This reduces both the invasiveness of the operation and the time spent in theatre.
Take a common scenario: a spondylolisthesis, where one bone has slipped forward relative to the one beneath, and perhaps another has slipped backwards relative to the one below that, producing stenosis where the nerves in the central canal are compressed. This causes back pain and pain radiating down the legs, particularly on standing and walking.
Here we place only screws and rods at the back of the spine, through very small skin incisions and using image guidance to position the screws accurately. The rods realign the segments. The disc is not touched and no cage is placed.
Realigning the bones itself generates some decompression of the nerves. Through a further small incision, working under the microscope, we can remove any thickened tissue compressing the nerves and see directly that they are free.
So why is a cage usually used?
Because it works better for fusion. Placing a cage in the disc space between the two vertebral bodies provides far more surface area of bone for the fusion to take. Studies of post-operative X-rays across different fusion types confirm that the interbody cage achieves a better fusion.
The difficulty is what it costs to get there. Reaching that disc space means removing the facet joint and performing a good deal of dissection near the nerves, which adds considerable operative time.
Conversely, the drawback of posterolateral fusion is that achieving fusion between the bones at the back is harder. To help it along, we use a drill to create a channel between the bones posteriorly and lay down bone graft, allowing them to fuse over time.
What the evidence shows
A 2026 study compared patients undergoing lumbar fusion with and without a cage.
Those who had a cage inserted had a slightly higher complication rate in the perioperative period. But the overall outcomes between the two groups, in terms of pain and general quality of life, were the same.
My own protocol
If you are relatively young, or fit and healthy without significant medical comorbidities raising your surgical risk, I carry out a standard lumbar interbody fusion, still through a minimally invasive approach. Placing the cage gives better spinal alignment and maintains the disc height between the two bones, which makes for a longer-lasting construct.
If you are in your seventies or eighties, where surgical risks do increase somewhat, I do not rule the cage out entirely. I discuss matters with my anaesthetist and any other physicians involved in your care. But I will often consider the posterolateral fusion instead: screws into the bones, realignment, and decompression. It saves time and frequently achieves a good result.
In medicine we treat the patient, not the scan. The goal is not a picture-perfect X-ray afterwards. It is a good outcome, with improvement in your symptoms and your quality of life.
A promising idea, still being researched
I want to end with a hypothesis currently under investigation.
If we place only screws, omit the interbody cage, and allow fusion to occur at the back of the spine over laid-down bone graft, the resulting construct may behave differently. Bone has a degree of elasticity; it is slightly bendable, and so permits a small amount of motion between the bones.
The theory is that this tiny amount of motion may help prevent wear and tear developing in the adjacent segments above and below.
This is still being studied, but it is a promising concept. If we can improve posterolateral fusion, we may eventually be able to dispense with the interbody cage altogether.
