A certain proportion of people experience severe leg pain following a spinal fusion operation.
To understand why this happens, we need to take a step back and look at what you actually do when you do a spinal fusion.
In a spinal fusion, the disc is surgically removed. Usually, the way to access the disc is from the back.
To get to the disc, the surgeon needs to remove part of the bony arch that is at the back of the spine. This operation called a laminectomy. Once some of the arch at the back of the spine has been removed, this opens up space to access the disc.
The next step is to pull the nerve aside to get it out of the way of the operation. The surgeon puts traction on the nerve so that you can get to the disc itself. They then use special instruments and go into the disc, scrape out all the contents, and then scrape all the way around to remove as much of the disc wall as they can.
Then, the surgeon will insert a cage between the two vertebrae to recreate the space where the disc used to be. This is to keep a space for the nerves need to branch out from the spinal cord at each level.
This cage is then filled with crumbled pieces of bone intended to graft and grow to fill up the space.
Lastly, the surgeon takes two strong rods and attaches them into the vertebrae above and below with big screws. This is to hold everything in place while the bone grows.
At the end of all this, you have a metal cage packed with bone, two strong metal rods and four large screws. (This is just for a one-level fusion. There’ll be more hardware if you’re fusing more levels.)
Now, the question we’re looking at is: what can cause leg pain from a back operation?
As we’ve seen, a fusion is a complex and destructive operation in which lots of things can go wrong. We’re now going to look at each separate thing.
The first possibility is that the nerves themselves can be damaged. These are the nerves that run from your back down into your leg.
This damage can happen in a number of ways. The first way is that when the surgeon does the laminectomy, they’re using a sharp instrument to cut through bone. If they make a mistake here, this instrument could damage the nerve.
The second is when the surgeon does the most basic thing, which is to pull the nerves apart and traction on them, they could traction too much and damage the nerve by stretching it.
Thirdly, when the surgeon goes in to destroy the disc, any mistake during this process could damage the nerves.
Lastly, damage can happen when they put the screws in. If the screws are not drilled into exactly the right place, they can damage the nerves. I’ve even seen fusions where the surgeon has stuck a screw right into the joint. This didn’t end up with a good post-surgery result.
Any of these four errors can cause nerve damage, giving you pain running down your leg. This type of pain is called neurogenic or neuropathic pain.
It’s a pain like sciatica: a sharp stabbing pain that runs in a thin band down your leg. It can have associated numbness, tingling and weakness in your leg.
So nerve damage during the operation is the first cause of severe leg pain after a spinal fusion.
The second cause is when something goes wrong after the operation, and the bone graft doesn’t take.
The bone fragments scattered in the spacer cage are intended to grow in a bony graft to stabilize the fusion. This process can take up to a year. But if the bone graft doesn’t take, you end up with just the metal cage, the rods and screws holding the vertebrae apart.
This is simply not adequate to cope with the strains we put our backs through during a normal day. The main strength of a fused joint over time comes from the bone growing.
In fact, as we get older, our spines can create a natural fusion. Over years of use, boney growths emerge in our vertebrae to stabilise the joints as our discs gradually wear out.
Originally, these boney growths were seen as a problem, but now it’s clear that at least some of the time they are our body creating a natural fusion. Many people with these boney growths do not experience back pain.
However, if the bone graft doesn’t grow after a fusion operation, then the metal spacer starts to force its way into the bone of the vertebrae above and below. This results in what’s called a pseudoarthrosis, or false joint.
Because the spacer is able to move, and it’s harder than bone, in time, it pushes into the vertebrae. This further damages the joint and means it may make abnormal movements, which can stress the nerves around the joint.
This would usually give you pain in your back. However, there is a very common condition called somatic referred pain.
Soma is Greek for body. Somatic referred pain isn’t caused by a pinched nerve. It’s pain that you feel in a larger distribution than you should because of how pain messages are transferred to the brain and interpreted.
Here’s how it works. The nerves from the damaged joint pick up pain messages then join into the spinal cord. This is a trunk line of information going from your body up to your brain.
Other nerves joining at the same place also send branches down your leg. When the messages from your damaged joint join into the trunk line of your spinal cord, the message that reaches your brain is a general one which contains mixed zones.
The message would certainly have back pain, but due to somatic referral, it may also include buttock pain and leg pain, as all these areas are fed by nerves in the same vertebral segment.
The pain spreads because of a medical term: convergence. Convergence means that information coming from the joint or the disc isn’t as precise as messages coming from a highly sensitive organ like your skin. Instead, joint pain can be a vague, deep, achy pain, which becomes mixed with other messages in the spinal cord.
When these messages reach your brain, your brain interprets them as a mixture of pain that often includes terrible leg and buttock pain with the back pain.
The real cause is coming from the failed fusion, but because of somatic referral, the perceived area of pain is much larger.
In summary, these are all these things that can go wrong in a fusion operation. These are also all the reasons why people can end up with not only back pain but also leg pain following the operation.
The problem with a fusion operation is that there’s no going back from it.
Once you’ve fused a joint and done a diskectomy (removal of the disc,) that’s it.
If the surgery fails, all you can do is try to do the fusion again. This means the surgeon goes in and scrapes more bone from somewhere in the body to fill the spacer cage.
It’s a very difficult thing to fix, especially when you get a pseudoarthrosis. The chances of a second bone graft taking are not that good once the area is already inflamed.
If the surgery does fail, people often end up with worse pain than before. These are some of the people I’ve seen in my chronic pain clinic. The tragedy is that while some of them did need a fusion, many of them may not have needed the operation at all.
Once it’s done, though, they’re stuck with it. This is why I see a spinal fusion as a last resort, and only to be undertaken when all other paths have been tried.
If you are experiencing severe leg pain after a fusion operation, it’s important to determine the cause.
If your pain is sharp, shooting, with numbness, weakness, or tingling, then it’s more likely coming from a damaged nerve.
If the pain you’re experiencing is a deep aching pain, then somatic referral is the more likely cause, and you’ll need to have the joint more closely examined.
While it’s certainly difficult to recover after a failed back surgery, it’s not impossible. Firstly, you need to examine the physical causes of the pain.
And then you need to look at an extremely important factor most health professionals ignore: your pain system.
If you have been in severe pain for longer than 3 months, it’s likely your pain system has become sensitised, and this can have a big effect on your successful recovery
Here’s a case study of a lady who had pain following spinal fusion, and was able to dramatically reduce it – without surgery or drugs.
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