Why is it so hard to straighten your knee after ACL surgery?

Posted on December 18, 2015

(Last Updated On: March 27, 2016)

Something ACL patients, really any knee surgery patient, struggles with the most is regaining full knee extension. Out of surgery, many lay their legs straight, and one knee looks like this, if not worse.

Leg extension one knee bent

Next thing you know they’re walking like this,

Bent knee walking GIF

We’re talking not only the ability to straighten your leg, but the ability to hyperextend your knee. If straight is zero degrees, then hyperextension would be another 7 degrees, give or take. Rather than your ankle and knee being lined up, your knee can go a little behind your ankle.

Standing knee hyperextension GIF

(You do not want to work on your knee range of motion while standing though!)

SMR Gastroc knee hypertension

I’ve had the amazing fortune of spraining my knee, having reconstructive ACL surgery, and having a partial meniscectomy. I have, for the most part, seen the gamut of knee injuries, personally and professionally.

Excluding extenuating circumstances, like complication from a knee replacement, by far the hardest time to regain knee extension range of motion is after ACL surgery. It was something I needed to work on every hour of every day (not hyperbole), for the first 6 weeks after surgery. It took until about the three month mark where I didn’t need to worry about it multiple times a day. By worry about it, I mean if it wasn’t done at these time intervals, I’d lose the range of motion, and it could take many minutes of stretching to regain it.

Many experience the same thing. Although, some regain full range of motion like it’s nothing. Beyond the readily apparent, like pain, inflammation, why is this so hard? Why do some have no problem at all? Is there anything unique to ACL surgery making it tougher than other injuries?

What does the anterior cruciate ligament do again?

femur tibia meniscus

The ACL goes from the back of the femur to the front of the tibia.

acl knee straight and knee bent anterior cruciate ligament cadaver

The ACL subsequently prevents the tibia from going in front of the femur. It pulls the tibia back to the femur.

To test if an ACL is torn, you’ll see an orthopedist gauge how far they can pull the tibia forward relative to the femur.

If it’s really lax, there’s probably a tear. (Why you don’t want a non-orthopedist testing your ACL.)

What happens to the ACL during knee flexion and extension?

I am simplifying this some. If you want more details, such as breaking the ACL into anteromedial and posterolateral bundles, check the Wheeles discussion. That level of specificity isn’t needed here.

As the knee bends, the ACL is tautened as the tibia moves forward relative to the femur / the femur moves backward relative to the tibia.

ACL through knee flexion GIF

acl knee straight and knee bent

This is one reason there are often, though not always, flexion range of motion restrictions immediately after ACL reconstruction. You go bending the knee too much, you might stretch out the graft.

When coming into knee extension, particularly some hyperextension, we can again see the femur moving back relative to the tibia. Really watch the last bit of this animation:

ACL through knee extension GIF

GIFs made from this video.

The more obvious: When you get a new ACL, the graft is tautened when put in your body. Your range of motion can be based on how tight the graft is. A really tight graft and it may be harder to work on flexion and or extension range of motion

The less obvious: I recently went in into an operating room and watched an ACL reconstruction. There are a couple moments where you knew the surgeon was tightening the graft. He would let out a huge grunt, like he was lifting heavy weights. As he jokingly said, “When you grunt, that’s how you know it’s tight!” The idea is this thing is really, really put in there.

-> In dogs, a veterinarian surgeon has told me they’ve given up on reconstructing the ACL. Due to the dog’s anatomy, their physical strength, and how hard it is to manage a dog post-op, they too often rip or stretch the graft. This another reason in humans we don’t (or at least shouldn’t!) go running like crazy a month post-op, or only do things based on pain. We need to give the graft (typically a tendon) time to set in the bones, as well as time to transform (into a ligament).

Not only that, but during the final phase of surgically inserting and tightening the graft, the tibia is pushed backwards relative to the femur.

  • Pull the tibia forward relative to the femur => ACL tautened
  • Push the tibia back relative to the femur => ACL slackened

posterior drawer test

Meaning the graft is tautened in a position the ACL is usually slackened. Tighten graft -> Push tibia back relative to femur to slacken graft -> Then tighten graft again. The new ACL is put in tighter than a normal ACL would be.

This was being done by two grown men at the same time. One guy was working with the femur and tibia, the other guy was working with the tibia some, but also finishing up on the graft.

-> I got more of an understanding why orthopedic surgeons plan to retire by 63 years old. It’s a physical job.

Two men putting the leg into position, specialized materials apparently stronger than anything in the body, with one guy using damn near all the strength he has. That’s how this thing was put into place. This is thing is put in tightly.

Beyond wanting the graft secure, the idea here is despite all this, over time, the graft will likely still loosen up some. By placing the graft in a position where the ACL is actually a bit shortened, as it loosens up some, it will then be in a normal resting position. It’s a fail-safe; attention to detail at the highest level.

But going back to our knee extension range of motion, what do we know happens as we approach some hyperextension?

ACL through knee extension GIF

The ACL stretches a little bit. Our knee has just been manipulated with two men using super strong materials to hold their manipulation, which prevent this anterior tibial translation. No wonder it’s hard to get knee extension range of motion back!

Fear not?

We often worry about bending the knee too much too soon, but we don’t worry about straightening it too much, too soon.

By regaining knee hyperextension range of motion, we’re talking only 7 degrees or so, relative to neutral. The graft is placed in a position where it’s expected it can loosen up to this point. Furthermore, the force on the ACL is quite small in hyperextension. Maybe 10% of what the ACL can handle. (Initially, something like a quadrupled hamstring graft is stronger than a standard ACL. So they often have an even better ratio in this regard.)

In knee flexion, we’re talking up to 120 degrees, relative to neutral, and the ACL is the primary restraint to anterior translation of the tibia from 30 to 90 degrees. Accounting for 80% of the work. Furthermore, anterior tibial translation -stretching the ACL- is greatest between 20 and 45 degrees of knee flexion.

There is a significant discrepancy in ACL stress, and range of motion, between going to full extension and full flexion. This is one reason some will end up locked in a brace for 10-14 days, before being allowed to bend the knee. The decision is to eliminate all bending to minimize ACL stress. But that bit of stress in full extension? Not worth worrying about, and regaining full extension is much more important than full flexion. The cost / benefit of stretching the graft versus regaining full extension is clearly in favor of full extension, while the cost / benefit of stretching the graft versus regaining full flexion is in favor of being careful with knee flexion. Particularly active flexion, where passive isn’t as much of a concern.

-> For 8-12 weeks is a good ballpark. Patellar graft is believed to be set by ~8 weeks; hamstring graft by ~12 weeks.

–> Takes ~18 months for them to transform to a ligament though!

-> If you’ve had a meniscal repair, common with ACL pathology, you’ll want to be even more careful with knee flexion, as flexion mobilizes the meniscus.

That said, this doesn’t happen to everyone. Even the surgeon I watched, when I asked him about locking people straight, he said it was mainly to let the leg calm down more than anything else. It seems some feel good enough about the graft, or the materials they’re using, they don’t worry about range of motion after surgery. (Passive range of motion. Everyone worries about active, and how intense the active is.) Although, you’re unlikely to see anyone allowed to bend past 90 degrees for at least a couple weeks. The swelling probably won’t let you anyways.

I’m not sure if every surgical team does things this way, where they place the graft in a position of being extra taut. I unfortunately didn’t think to ask this question at the time, although I’m not sure they would have known. My impression is this isn’t always done, and this is why some people may not have as hard of a time regaining range of motion immediately post-op. However, their graft may not be as well set as someone else. It’s quite possible having trouble regaining knee hyperextension after ACL surgery is a sign the surgical team did a good job, and that if you regain things too easily, things may not have been as well set as they could have been.

With the body, it remains whenever you gain something, you lose something.

For a plan dedicated to making the initial ACL recovery as smooth as possible, go here.

For a plan focusing solely on being able to straighten your knee again, go here.


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