Why can ACL surgery increase arthritis risk?

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Alright, so the title of this is a little misleading. After all, whether you get ACL surgery or not doesn’t impact your risk of arthritis. I wrote about this in depth in,

–Reconstructive ACL surgery: Is it beneficial?

Here’s a more recent study backing that up,

–Does Anterior Cruciate Ligament Reconstruction Improve Functional and Radiographic Outcomes Over Nonoperative Management 5 Years After Injury?

However, the rationale for why the risk of arthritis goes up whether you have surgery or not can change.

If you don’t get surgery, your knee is more mobile than it otherwise would be. Hence, the giving way sensations many who tear their anterior cruciate ligament feel. Where the knee all of a sudden seemingly gives out underneath you.

When the ACL is torn, the knee is no longer as stable as it was. This excessive flexibility can now make the knee more likely to become arthritic. One element of arthritis is excessive bone growth. You can make an argument the body throws down extra bone at areas that move too much. In other words, “arthritis is a normal response to an abnormal stimulus.”

-> And Flexibility- when more is less

But if you get surgery, then this rationale goes out the window, right? Since the surgery gets rid of that hypermobility?

Not necessarily. Just because you get surgery doesn’t mean the surgery is completely successful. Your new ACL might not be as stable as your old one, where the above rationale still holds.

There is more. What if the new ACL is stable? What if the knee isn’t giving way or moving too much? How could arthritis come about then?

Let’s talk about this,

–Anterior Cruciate Ligament Reconstruction Affects Tibiofemoral Joint Congruency During Dynamic Functional Movement

First, let’s get oriented.

Femur is on top of the knee; tibia is below; ACL in between:

In ACL surgery, we take the old out for the new. We put a hole in the front of the tibia; the back and side of the femur, and we tighten the new ACL in those holes:

What this study found (edited for clarity),

“Lateral compartment congruency in the ACL reconstructed knee was greater than in the non-operated knee. From 6 to 24 months after surgery, dynamic congruency decreased in the medial [inside] compartment and increased in the lateral compartment in the reconstructed knee. In the lateral compartment, side to side differences in joint congruency was related to contact location and femur global curvature, and in the medial compartment, side to side joint differences in joint congruency was related to contact area.”

Let’s get oriented more,

The study found on the lateral side, for the knee which had surgery, the femur and tibia touched one another in a different location than the knee which didn’t have surgery.

For the medial side, on the knee which had surgery, the study found the top and bottom of the knee touched one another differently in terms of how much space.

That’s a bit overkill for our purposes here. All we really need to digest is the knee doesn’t compress the same anymore.

We thus have rationale for why ACL surgery can increase arthritis rates. If you’re 20 years old and tear your ACL, your knee has had 20 years of acclimating to a certain kind of loading. But, after surgery, the knee now doesn’t compress the way it used to. (Especially if you don’t have proper rehab) that’s grounds for irritating the knee. For instance, if now one part of the knee, which hasn’t been used to loading, is having to deal with 10,000 steps per day, that’s a lot of work for the cartilage to acclimate to, and cartilage adapts very, very slowly. When cartilage can’t adapt, we often find arthritis -less cartilage / more bone- isn’t far behind.

It’s similar to if you’ve never ran before, then at 20 years old you try to run a marathon. You are going to jack your knees up, because they aren’t used to the loading.

Furthermore, our body tends to like even distribution of load. We don’t like to run or lift with one side of our body doing more than the other.

-> Again, if you grow up this way, you’re more likely to get away with it i.e. how you start off in youth matters. But trying to make a change like this as an adult is much more precarious.

-> The above is even more reason why ACL patients don’t feel their best til around two years. The graft might be securely held, yet the knee can feel weird for years. This is another reason to not rush the process.

Why does this happen?

Look back at how the new graft is put in place,

For reasons we’ve gone over, one of the top priorities when installing this graft is securing it. Orthopedists put A LOT of strength into doing so. They really pull it tight.

But think about the force of the graft. It may compress the lateral side more than the medial,

Opposite our ACL, we have the PCL:

You can easily imagine if those aren’t in perfect equilibrium relative to where they were before we tore our ACL, our knee has been changed. You can easily imagine how hard it is for a surgeon to insure that’s happened. To get the tension just right. The strength of the graft just right. The angle of the graft just right. The hardware to hold things in place for months just right. Our biology to adapt just right.

Long story short, it’s quite conceivable the surgery is not going to precisely replicate what the natural-in-your-body-for-decades ACL did. We’ve only talked about fairly static loading. You have to also imagine the ability of the ACL to respond to much more dynamic activity, like cutting back and forth. What if the new ACL doesn’t respond quite as quickly? Then again you’re changing how the knee is loaded.

That doesn’t mean you’re guaranteed an arthritic, bum knee. Our body, our knee, can adapt. But that takes time. We have to give the literally new knee time to adapt to its new loading. But also keep in mind you may never feel quite the same ever again.