Understanding why your tibial slope can increase your odds of ACL injury

Posted on October 15, 2020

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(Last Updated On: October 15, 2020)

When it comes to predisposition for tearing the anterior cruciate ligament, we tend to focus on women. “They’re weaker, they have larger Q angles, is it something with their hormones?” We’ve found some truth in these hypotheses, though we’ve looked at the hormone factor and didn’t find anything. Men can only blame that on so much.

Predisposition goes deeper though. I covered meniscal wedge angles previously, let’s look at the tibial slope next.

Tibial slope x ray

Credit: Measurement of tibial slope angle after medial opening wedge high tibial osteotomy: case series

Let’s get our bones oriented first:

Tibial slope x ray with bones

We draw a line perpendicular to the tibia:

Tibial slope x ray with bones with tibial line

Then we draw a line perpendicular to that:

Tibial slope x ray with bones with tibial line with 90 degree line

The little box is a geometry sign for 90 degree angle.

Then we draw a line down the “tibial slope”:

Tibial slope x ray with bones with tibial line with 90 degree line with slope line

Giving us our tibial slope angle:

Tibial slope x ray with bones with tibial line with 90 degree line with slope line with angle

Now we remember back to what our ACL does:

Animations made from this video: https://www.youtube.com/watch?v=JWI_Qghqclw

Animations made from this video: https://www.youtube.com/watch?v=JWI_Qghqclw

It prevents the tibia from moving too far forward relative to the femur. Too much and the ACL tears:

ACL injury animation

Going back to our slope. Flat slope:

Flat tibial slope

Now let’s think about the femur being a ball on this slope (credit to the veterinarian world for this analogy):

Flat slope with ball

And our ACL is a rope attached to this ball.

Flat slope with ball and rope

As we change the slope then, the ball is going to roll back, pulling on the rope:

Tibial Slope and femur ball ACL GIF

If taking care of your ACL is a priority, we can imagine this isn’t a structure you want! In fact, dogs have a greater slope to begin with:

Credit: https://www.youtube.com/watch?v=WI9Jk6uL76U

Note the knee has been flipped horizontally compared to the pictures above. Credit: https://www.youtube.com/watch?v=WI9Jk6uL76U

What they’ll do with dogs is rather than reconstruct the ACL, they reorient the slope, so the knee isn’t as dependent on the ACL:

Tibial slope osteotomy before and after

Steep slope on the left gets cut (red dashed line) to become a flat slope on the right.

Dogs tend to tear reconstructions, so they aren’t done. Instead, surgeons make the knee less reliant on the ligament. Slope osteotomies aren’t really done in humans, certainly not anywhere near the scale reconstructions are done. (From what a surgeon has told me, “it’s a much harder surgery.”)

In a variety of ways some of us are not built for certain activities like others. If you search tibial slope, most of what you’ll see is the relationship between the slope and risk of ACL injury, as well as meniscus tears.

This gives some rationale for why some -few, but they’re out there (Adrian Peterson likely an example)- come back from ACL surgery like it’s nothing, while others never come anywhere near making it back. Some have a structure more built to take care of the ACL; some have one making the ACL more vulnerable. (How does Teddy Bridgewater dislocate his knee and tear his ACL by only dropping back to throw a football!? “Freak thing” = biomechanics.) Slope positioning is going to be one factor in this. If you have a big slope, that’s going to make it tougher to not stretch the new ACL out. (Though it makes it easier to not stretch the PCL!)

Unfortunately, we cannot overcome everything by working harder (which, actually, also has genetic components) and having a positive mindset. You can’t Oprah your way to changing your slope angle. This is not negative speak; it’s reality. A reality worth appreciating when you’re someone like the person who’s had two ACL tears, both non-contact, both seemingly inexplicable “My knee just gave out,” and you’re only 18. Chances are you’re not built to play X, Y, or Z. Despite how much you may love it.

Survival of the fittest is beautifully brutal, and endless in its shapes and forms.

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