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Full Knee Hyperextension After ACL Reconstruction

acl hypermobility knee extension

The most common question we get after our ACL reconstruction rehabilitation course on MedBridge is why do you emphasize that you need to achieve full knee hyperextension after ACL reconstruction?

So I thought I would dive in deep into this topic.  And I think it comes with some natural questions or natural responses that I hear:

"So you're saying you want me to hyperextend the knee?" after they have it reconstructed. Isn't that one of the ways that we actually tear the ACL is by knee hyperextension?"

And yes, that is true, it is a mechanism of injury.  But that's not exactly maybe the spirit of it. The way we need to think about it is. . .  are we wanting full knee hyperextension or are we wanting symmetrical extension range of motion equal to the contralateral side?

Are we wanting full knee hyperextension or are we wanting symmetrical extension range of motion equal to the contralateral side?

  And clearly, the emphasis is symmetrical full knee extension range of motion.

And some people have hyperextension as full knee extension range of motion normally.  It's very common. But what I get pushback on or questions on is when I say the word hyperextension, because when we say hyperextension, that indicates a joint going beyond where it should go or its physiological norms. 

But the reason I use it is that it causes people to think, “Do I want zero degrees of knee extension or do I want what the other side has if the other side is normal?”. And I actually want what the other side has. 

Let's go through the research of why that is and a little bit of the history of it. This has been a concept that's really been around for a long time. So why does it matter?  

First of all, so that lack of range of motion is related to anterior knee pain. The research is pretty clear on that . . . that anterior knee pain and lack of range of motion are related to decreased quad function. Quad function is clearly related to power production.  And also there is some good research that it decreases a person's functional perception and their satisfaction with their surgery  . . . because it's related to the anterior knee pain loss of extension range of motion. And there's some research that points to there may be an increased risk when you lose range of motion both in flexion and extension.

So let's dive in a little deeper than that . . .This is actually a concept that comes from the early 80s, but a study from 1989 (Sachs et al 1989) . . . quad weakness, that is strength less than 80% of the normal side.

Some people are like, oh, my gosh, I would never have that. Well, actually, I still do see that. I have patients come to me with that still after six months or nine months . . . was present in 65% of patients and correlated positively with a flexion contracture or greater than five degrees of lacking knee extension range of motion, patellar irritability, and when they use patellar tendon grafts.

Okay, so I just set the stage. This has been a concept that really has been around for a long time. Shelbourne was the one who really popularized ACL reconstruction rehabilitation from an accelerated perspective. Rather than taking over a year, let's see how we can get this down.He said non-compliant patients tend to do better. Those that weren't casted or locked in a brace for too long tended to do better.

And so really, this big push to get return to sport even at four months . . . while I'm not advocating that . . . the pendulum from a research perspective and our knowledge perspective has swung the other way . . . that we should be waiting a full nine months at a minimum for full return to sport. 

But what we need to see here is what are the good things that came out of this ACL accelerated ACL reconstruction rehabilitation? And maybe time frame isn't one of the good things, but some of the concepts are. So a 1997 article (Shelbourne and Trumper 1997) said anterior knee pain after ACL reconstruction is not an inherent complication associated with patellar tendon harvesting. Increased incidence of anterior knee pain with a bone patellar tendon bone graft can be prevented by obtaining full knee hyperextension postoperatively.

Let's dive in a little bit more. This was a very recent study (Marques et al 2020)  looking at anterior knee pain predictive factors. So this was about 438 ACL reconstructions. Anterior knee pain was found in 6% of the patients. So you can see from that early or late 80s where everybody had anterior knee pain, at least we've gotten that down, that it's down to 6% of the patients with anterior knee pain. There are two independent factors. And so when you do a logistic regression, we're looking at factors that don't necessarily just relate to each other, that they are risk factors in and of themselves . . . bone patellar tendon bone graft and knee extension deficit. And in this study, knee extension deficit once again greater than five degrees of lacking symmetry.

And as we look at knee extension range of motion, what is that? Well, that's probably within the error of the measure, actually, or right at the peak of the error of the measure. So anterior knee pain is related to the bone patellar tendon bone graft as well as a knee extension deficit. 

Then let's look at one of Shelbourne's studies with 780 patients (Shelbourne et al 2012). I like this definition of normal range of motion. It was defined as within two degrees of the opposite knee including hyperextension and knee flexion being within five degrees. So they looked at with that definition of 780 patients at ten years, on average, post-surgery, what are the risk factors for having osteoarthritis?

So they are abnormal knee flexion at early and final follow-up, lacking knee flexion range of motion, abnormal knee extension at final follow-up. Remember, we're defining flexion lacking five degrees, extension lacking two degrees, including hyperextension, partial medial menisectomy and articular cartilage damage.

We're not surprised if we damage the cartilage or the meniscus to such a degree that probably are going to set us up for increased osteoarthritis or the impact or the degree or severity of that injury is probably at least more. But what I find really interesting, the odds of having osteoarthritis were two times more for patients with abnormal knee range of motion. And it's the same or similar odds as if you had a partial menasectomy or articular cartilage damage. Things that we would clearly correlate or tie to the development of knee osteoarthritis. This is saying range of motion, not having full hyperextension and full knee flexion range of motion increases a person's rate of osteoarthritis.

That's huge. That's a big deal, and that's really important. The other thing is anterior knee pain is related to quadriceps weakness independent of the type of graft used (Marques et al 2020). So when we look at bone patellar tendon bone versus hamstring or other cadaver graft, that type of thing, we want to look at quad strength. When we look at quad strength, anterior knee pain is related to that. If quad strength is decreased in those who lack range of motion, we can clearly see why it's a risk factor.

The other thing is just to kind of step back looking at knee flexion range of motion, and this is a study with Kate Webster (Webster and Feller 2019), that they looked at patients with a flexion deficit of five degrees had over two times the odds of sustaining graft rupture. The reason I put this out here is that's kind of partly predicting retear rates, right? So lacking full flexion range of motion. So I know we're asking about hyperextension, but I want to emphasize this full range of motion is essential. Full range of motion is essential for normal proprioception. It's essential for normal quad function, hamstring function, hip function. All of that is important, and the basis of it is full range of motion. So one of the things that we looked at, you've probably heard me talk on this, full knee flexion range of motion is being able to get the heels, the ischial tuberosities to the heels. I like this position shown below, it really helps me identify that.

If you see the picture with the person, there's that little bit of space on the picture on the right between the ischial tuberosity and the left heel, that's not okay. We should be restoring that fully. Now our goal is not to restore that fully at six weeks. It does take months, but they're not done with my care until they can do this. We do this very slowly. We don't go aggressively at it, but we need to restore that full ischial tuberosity to heel range of motion.

Now, of course with everything, there are considerations.

  1. People who are hypermobile Our mind immediately goes to, well, what about that person who is hypermobile? That person who has more than ten degrees of knee hyperextension or has congenital hypermobility on their Beighton scale really has a lot of factors. Well, if they have more than ten degrees of knee hyperextension, I'm probably getting them to that five or ten degree range. But I tell you what, one thing to really aggravate a person who has a hypermobile knee is to have a 15-20 degree difference between their left and right knees. Now again, we find that person, people who are hypermobile, their knee range of motion tends to come back once we get it to about that five degrees of hyperextension. I don't really worry about it much more than that. 
  2. Hamstring grafts Hamstring grafts have a higher rate of failure and in general, increased laxity. So with a hamstring graft, because they don't have that harvest site mobility that a bone patellar tendon bone graft has, hamstring grafts, I will tend to get full knee extension easily. I want to make sure that with a heel prop, they can get that knee, back of the knee fully down with a good quadricep contraction. But I'm not necessarily going for a lot of range of motion beyond that initially, because those grafts, in my opinion, and I think the research bears this out, tend to stretch out a little bit more than bone patellar tendon bone. And they also don't have the anterior knee pain, which a lot of times occurs with bone patellar tendon bone grafts, which inhibits knee extension range of motion. So one of the things that I like the way this study says it is,

    "The combination of extension mechanism weakness and graft stiffness will significantly limit extension when a patellar tendon graft is used compared to a hamstring graft." 

    So the research kind of bears this out as well.

  3. Quad tendon graft. Again, I'm going to put that in the same category along with the BEAR procedure. I'm going to put those in the same category as hamstring graft that I want to get full extension a little bit beyond zero comfortably. But I'm not going to force that because I think that bone patellar tendon bone is the one that really loses their knee extension range of motion.

Bottom line, it's important to achieve full knee hyperextension because it decreases anterior knee pain, it increases quad function. Therefore, when you increase that quad function, you increase power production, functional status and satisfaction. And I don't know that knee hyperextension has not been identified as a risk factor, but surely lacking knee flexion range of motion has. And it makes a lot of sense that if you have anterior knee pain, quad function deficit, why wouldn't that be a risk factor for retear or certainly other injury? And this is particularly important with bone patellar tendon bone grafts.

If you have any other questions, questions on this, additional questions about ACL reconstruction rehab, I love answering these questions, so feel free to reach out to me at or there's my email address as well.


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