Can’t Straighten Knee: Why Knee Extension Lag Matters

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Most of us take for granted that we can fully straighten our knee. However, sometimes patients can’t do this and when that occurs, it’s a very big deal that usually causes other problems. Let me explain.

can't straighten knee

Standing Takes Little Energy

When the average person with normal knees stands, he or she can lock out the knees by slightly hyper-extending the joint. In that position, the thigh (quadriceps) muscles shut off and it takes very little energy to stand. This makes sense, as we were designed to stand for long periods without eating up precious calories.

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What Happens When You Can’t Extend Your Knee?

knee won't extend

When you can’t fully extend the knee, we doctors call that an “extension lag”. We also usually measure that in degrees. So 10 degrees of extension lag means that the knee is bent 10 degrees in flexion and won’t extend any further.

If you look at the images to the left, when standing with full knee extension, both the hamstrings muscles in the back and the quadriceps muscles in the front are, for the most part, shut off. However, when the knee won’t extend fully, the hamstrings are short and the quadriceps muscle is active. What does that cause?

Try standing right now and bending your knees slightly. What do you notice? First, not many of us can stand this way for long, as our thigh muscles will begin to burn. Why? They are actively working to support your body, just like in the picture to the right above. Do that for any length of time and the quadriceps tendon, knee cap cartilage, and patellar tendon will begin to get too much wear and tear. Do it for months or years and those tendons will get torn up and the back knee cap will lose cartilage.

What Are The Causes?

Knee extension lag can be caused by:

  • Scarring of the joint capsule after knee arthroscopic surgery (aka arthrofibrosis)
  • Chronic tightness in the hamstring muscle
  • Bone spurs
  • Torn and locking meniscus tissue
  • A posterior cruciate ligament that is scarred or too tight
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A Patient Example from this Week

I had a patient visit our licensed Grand Cayman clinic from the middle east whose one knee wouldn’t straighten fully. As I showed above, this was overloading and tearing up her quadriceps and patellar tendon. However, in her case, she reported that a massage therapist she knows could work her hamstrings and get the knee straight. I observed this as well, as when she was lying face up, her knee couldn’t straighten. However, when lying face down, the knee had no issue straightening. Hence, the tightness in her hamstrings limiting knee extension is positional, meaning that the hamstrings aren’t scarred (a contracture), but just “freaking out” in one position. We are in the process of looking at all of the nerves that drive those hamstrings.

What Are the Treatments?

For the above patient, it’s figuring out what’s causing the hamstrings to go into over-drive. In another patient with arthrofibrosis, it would be breaking up the scarring caused by knee surgery. Or it may be removing bone spurs in yet another patient. The point being, the focus needs to be on fixing the cause.

In addition, the quadriceps and patellar tendon are often fried and damaged. Hence, these tendons need to be treated. We often do this with platelet-rich plasma or if the damage is more severe, we use stem cell injections.

The upshot? Not being able to extend your knee is a big deal. Hence, figure out the cause and get it fixed as soon as possible!

This blog post provides general information to help the reader better understand regenerative medicine, musculoskeletal health, and related subjects. All content provided in this blog, website, or any linked materials, including text, graphics, images, patient profiles, outcomes, and information, are not intended and should not be considered or used as a substitute for medical advice, diagnosis, or treatment. Please always consult with a professional and certified healthcare provider to discuss if a treatment is right for you.

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18 thoughts on “Can’t Straighten Knee: Why Knee Extension Lag Matters

  1. Linda

    2 yrs I have not been able to straighten out my knees/ legs. Fluid extracted tested neg for everything. Dr said it was normal fluid of knee joint. Thanks for posting this info.

  2. Vanessa

    Hi doctor centeno how did you break up the scarring on patient with arthrofibrosis. My knee doesn’t fully extend after orthoscopy.

    1. Chris Centeno, MD Post author

      Yes, that’s possible to do, I would book a telemedicine visit so we can see if you’re a candidate.

  3. Laurie Ellis

    I had a partial knee replacement last year and was doing well until I got an infection and my surgeon ended up doing a washout surgery. Since then it has been impossible to get to full extension. I was stuck at 40 degrees and it never went down.
    I found a new surgeon and this year 2020, he did a revision total replacement. My surgeon was able to get me fully extended then but now I am having the same issue, I went through months of physical therapy until last month when we decided I needed a manipulation and once again my surgeon got me to zero degrees extension and 135 flexion and sent me back to PT the next day for 3 days in a row. Well, once again my muscle are fighting everything.

    I got to 108 bending and my therapists are able to get me to negative 1 degrees on extension but it won’t stay there. The next session it bounces back to 10 degrees or more and my therapists are killing me and I walk out of PT in tears every session.

    I have machines from Ermi, inc that I use daily for both extension and bending but the extension does not want to stay down. Please help!

  4. Vanessa

    Hi doctor Centeno I am a candidate for stem cells procedure for arthritis on my knee .My knee doesn’t fully extend due to arthroscopic surgery 10 years ago due to scar tissue or arthrofibrosis Do you also do this kind of procedure . I’m 49 years old looking to get better.Didn’t know it could be scar tissue why a was not able to fully extend my knee . You info helps a lot of people America loves you.😊

  5. Vanessa

    Hi doctor centeno how is the procedure to break up scar tissue on a knee that doesn’t fully extend , and did you do that kind of procedure? Thanks.

    1. Chris Centeno, MD Post author

      Yes, we have done those types of procedures. That’s called hydro dilatation of the joint with manipulation under local anesthesia.

  6. Vanessa

    For me is a great deal straightened my knee again . Thank you so much Dr.centeno im looking forward to do it at your location in Colorado very soon.

  7. Debra Rosario

    I have MS & because of multiple back surgeries & pain when walking or standing over 6 min, my back has started to hunch over. I’m 59 yrs old & have always exercised & stretched. A few years ago, my personal trainer noticed that my legs/knees wouldn’t straighten. Despite 2 yrs of PT for this, it’s worse than ever & very noticeable & I feel it’s making me hunch over even more. I’ve had open surgery on my R knee & arthroscopic surgeries on both knees. I also broke my L knee a year ago so it’s bent even more than the right. Even during this pandemic, I’ve continued the PT stretches for my knees & to loosen my hamstrings along with using treadmill, recumbent bike & pool exercising almost daily but my knee extension lag continues to worsen. Could this be related to my MS? My legs don’t hurt at all so I’ve been blown off by all my doctors. My body is really looking deformed. Any advice would be greatly appreciated.

    1. Chris Centeno, MD Post author

      Either that or your surgeries. This is a common side effect of knee surgery.

  8. Vanessa

    Hi Doctor Centeno. How is the scar tissue detect? What type of exam is used to detect scar tissue?

    1. Chris Centeno, MD Post author

      A physical exam with ultrasound.

  9. Purvaja

    Hi dr,

    My knee won’t straighten and also hurts in the gastrocnemius area for months now i limp while walking. No one here has been able to give proper diagnosis or treatment even after multiple MRIs.
    Do you have a colleague in Canada you refer me to?

    1. Chris Centeno, MD Post author

      Sure, consider seeing David Smith. Send me an email at [email protected] and I’ll connect you.

  10. Linda Johnson

    I had Regenexx stem cell treatment on my left knee here in Atlanta office, will be a year now in December. I also had issues behind my knee, told it was Baker’s cyst, which they tried to remove but could not totally. My pain is better on the internal side of it but I’m still not able to fully extend my knee. It’s starting to cause issues with my right leg muscles now, making it hard to walk. I’m really getting frustrated and not sure just what to do now to try to fix my knee and leg issues. Was getting therapy before covid-19 kicked in, but all that has stopped now. Any thoughts on what direction I should go now to try to remedy these issues? Thanks.

    1. Chris Centeno, MD Post author

      The big question would be if there are bone spurs blocking the joint from extending. I would follow back up with your Regenexx doctor on that issue.

  11. Lillian

    Hi..I had a partial left knee replacement in 2015 and saw my knee doctor 3 times in 18 months following the replacement as I started having issues with my right knee (previously had a scope in 2005). I had 0 issues with my right knee until I had my replacement. I kept telling the doctor I could not extend my leg, and that my knee is slightly bent at all times when I walk, thus I am not distributing my weight evenly. I was told it was the best it will be and finally after the third visit I was sent for an MRI to find out my right knee was close to replacement status but that a knee debridement would be beneficial…18 months later and now I have right hip issues. Can my issues be stemming from not being able to extend my leg and walk without a limp, and what do you recommend? Thank you.

    1. Chris Centeno, MD Post author

      Yes, your hip issues could be caused by not being able to extend the knee.

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