Your knee twists, you hear a pop, and now you have knee swelling and pain when trying to walk or with bending your knee.  Perhaps you were struck by another player and have bruising, but no swelling.  Are these severe knee injuries?  When should you consider seeing a Sports Medicine Doctor following a knee injury?  Let’s go over 5 things that trained professionals look for to determine if your knee injury is serious. 

serious knee injury with popping

As an active athlete or competitor, you know that severe knee injuries are common. Both contact and non-contact knee injuries can be severe. This post will cover five signs to look for to help determine if you might have a severe knee injury.  In general, the most common and worrisome finding is immediate, significant swelling.

Even though most of the injuries I see in the office turn out to be mild,  it is essential for you to know what the warning signs are to look for a potentially severe knee injury.

Five signs of a potentially serious knee injury:

1. Your Knee Is Swollen

swollen knee serious knee injurySwelling in the knee immediately or shortly after an injury is a common sign that indicates you may have sustained a serious knee injury.  In many cases, the swelling is due to blood.  The bleeding will stop, but the swelling will remain. Blood in the joint is called a hemarthrosis.  Blood in the knee joint can cause a lot of pain due to inflammation. If we suspect that you have blood in your knee joint, that often means that you sustained a severe injury.  The bleeding is usually due to something inside the knee tearing.  

Swollen knee sign of serious knee injury

 Common causes of swelling after a severe knee injury include:

  1. A tear of the ACL
  2. Patella or kneecap dislocation
  3. A meniscus tear: specifically a Bucket-Handle Tear.  
  4. An MCL tear
  5. Injury to the articular cartilage.  

Over 70% of athletes with a swollen knee who felt a pop while running and pivoting will have an ACL tear or a patella dislocation.  Patella (kneecap) dislocations are more common than you think.  All of you dread an ACL tear.  But most of you are surprised when we see you in the office and tell you that your kneecap dislocated.  Most dislocated kneecaps will reduce spontaneously on the field. That means that the kneecap will go back into place on its own.  

Patellar dislocations may hurt just as much as an ACL tear. Because patella dislocations are such a common sports injury we wrote this post to go into far more detail about them.  

With any of the above issues, most of you will find it very difficult to walk without severe knee pain.  Crutches, icing, and elevation are beneficial in these cases. These knee joint injuries need to be evaluated for an X-ray.  Fractures are rare, but they do occur.  Very few knee injuries will require a brace unless you want to use it for comfort.   The most important reason for seeing a sports medicine physician after an injury that causes swelling is to look for these most common severe injuries.  

Many of these severe knee joint injuries should be evaluated sooner rather than later.  The initial treatment will be a reliable examination to arrive at a diagnosis, which will be followed with an MRI to confirm the diagnosis.  Depending on what we think you injured, we may consider rest, physical therapy, or surgery.  

2. Your Knee Is “Locked” And You Can’t Straighten It

If you had a knee injury and you are not able to fully straighten the leg,  you might have a “locked knee.”  A locked knee is simply a knee that cannot fully straighten.  There are different reasons a locked knee might occur. In some patients, swelling and inflammation can prevent you from fully straightening the knee.  In others, a meniscus tear has flipped into the middle of the joint and is causing mechanical locking.  You cannot straighten it with the meniscus in that position. 

Locked knee sign of a serious knee injury

Many athletes who have a locked knee will also find it very painful to bend the knee too.  They may feel sharp knee pain when bending which can occur when these bucket handle meniscus tears move.  

The most common cause of a locked knee is a unique meniscus tear called a bucket handle tear.  A bucket handle tear is considered a serious knee injury and will require surgery to fix or repair the tear.  The reason these tears are serious is that a large piece of the meniscus tears flips over and becomes stuck in the middle of the knee joint. You need that meniscus to protect the knee. The vast majority of bucket-handle tears can be repaired.  So the sooner we start the treatment process, the better the outcome might be after a meniscus repair. 

flipped over bucket handle meniscus tear stuck in the knee

Although we discuss bucket handle tears here. Not all locked knees will be found to have a bucket handle tear.  In older athletes, a flap tear, or different types of meniscus tears typically occur. These types of tears are also essential to identify early on.  Most bucket handle tears are repairable.  The torn bucket handle should not be removed from the knee, if at all possible.  These tears, which cause a locked knee, are often very large.  If the piece is removed and not repaired, then you have a significant chance of developing osteoarthritis.  These bucket handle tears do not require emergency surgery, but they are urgent, and you should see a sports medicine doctor if you feel like something is preventing you from straightening your knee all the way.

3. Your Knee Feels Unstable, or You Felt a Pop

ACL serious knee injury
Torn ACL

Most ACL tears and patella dislocations occur from a twisting, non-contact injury.  A typical story is that you were turning or twisting hard, and you felt a pop.  As I mentioned earlier, most patella dislocations will reduce or go back into their usual place on their own.  But if your patella remains dislocated the knee will look strange. 

knee instability is a sign of as serious knee injury

If you felt or heard a loud pop as you twisted or turned to avoid another player, then you may have torn your ACL. Other causes of popping include a patella or kneecap dislocation.   If you felt or heard a loud pop in your knee, then there is a strong chance that you have a severe knee injury.  Most ACL injuries and patella dislocations are non-contact injuries.  A running back turning to head upfield.   A striker moving laterally to avoid the defense.  These are familiar stories when we see high school and college athletes who have torn their ACL.  

This post dives further into the immediate management of suspected ACL injuries.

Once again, another prevalent severe knee injury following a loud pop is a patella dislocation.  They are more common than most people think.  Everyone has heard of an ACL tear, but most are not aware that the patella or kneecap can dislocate.  Many patella dislocations will spontaneously reduce or go back into place.  That means that the patella was only dislocated for a second or two.  Patients with a patella dislocation often require an MRI to see if you injured the cartilage on the patella when it dislocated.  Surgery to repair the patella ligaments is usually not necessary for a first dislocation.  This post provides more information about patella dislocations.  


4. You Have Significant Weakness Trying To Straighten Your Knee

Severe weakness when trying to straighten the knee, even a few days after the injury, could mean that you suffered a severe tendon injury.   Common causes of weakness include patella dislocations, patella tendon tears, and quadriceps tendon tears.  Patella tendon and quadriceps tendon tears are not common in youth or collegiate sports, but we will see a few of these every year.   

knee weakness is a sign of a serious knee injury

In older weekend warriors who are wondering why their knee is weak after a serious injury, patella tendon and quadriceps tendon injuries are far more common. If you are over 35 and felt a loud pop in your knee while pushing off during tennis, or basketball then you need to consider that you have seriously injured your patella or quadriceps tendon(s).

After an injury to one of these large, critically important tendons, you will find that the knee will feel unstable.  You will think that the knee is unable to support your weight without giving way. You should be promptly evaluated by a Sports Medicine physician to determine the type of injury you had.  Both quadriceps and patella tendon injuries require surgery to repair these large essential tendons.

We discuss the management of patella tendon injuries in this post. The management of quadriceps tendon tears is very similar. 

Patella dislocations occur mostly in younger athletes.  Most patella dislocations happen when the knee is bent, the athlete is twisting, and then they are struck on the inner side of the knee. The patella will usually snap back into place, but the damage is done.  Any suspected patellar dislocation should be evaluated by a sports medicine physician since patella dislocations can cause injuries to the cartilage or the ligaments which hold the patella in place.

5. You Have Significant Difficulty Walking

Of course, many knee injuries make it painful and hard to walk.  For people with severe knee injuries, it is usually very hard if not impossible to walk.  You will often need to be helped off the field and require crutches.  Anyone who is placed on crutches should be considered to have a serious knee injury until an Orthopedic Surgeon evaluates you.  In many of these cases, an urgent X-ray is useful to rule out a fracture if the athlete cannot put any weight on the knee.

Take-Home Message:  What to look for if you are worried that you might have a serious knee injury.  

  1. Your knee is swollen
  2. You felt a pop
  3. Your knee is locked and will not straighten 
  4. You have a significant weakness when trying to straighten the knee. 
  5. You have persistent, significant difficulty walking or putting weight on your leg.  

Knee injuries are common across all sports.  Most knee injuries are mild, and the athlete can expect to return to play relatively soon.  Identifying the serious or severe knee injuries and acting quickly can make all the difference when it comes to getting you back in the game and minimizing your risk of further damage.

Disclaimer:  this information is for your education and should not be considered medical advice regarding diagnosis or treatment recommendations. Some links on this page may be affiliate links. Read the full disclaimer.

Related Posts

About the author:

Howard J. Luks, MD

Howard J. Luks, MD

A Board Certified Orthopedic Surgeon in Hawthorne, NY. Dr. Howard Luks specializes in the treatment of the shoulder, knee, elbow, and ankle. He has a very "social" patient centric approach and believes that the more you understand about your issue, the more informed your decisions will be. Ultimately your treatments and his recommendations will be based on proper communications, proper understanding, and shared decision-making principles – all geared to improve your quality of life.

59 comments on “Is My Knee Injury Serious? 5 Signs to Look For.

  • This is some great information, and I appreciate your point that swelling can indicate a serious knee injury. My daughter hurt herself doing gymnastics, and her knee is starting to swell up quite a bit. We’ll definitely look into visiting a professional to get it checked and to see what needs to be done to treat it. Thanks for the great post!

  • I was at soccer practice and I went to defend a shot and he had kicked my knee. ( I’ve hyper-extended) my knee before). But I felt like my knee had bent backwards. I can walk fine but I can’t completely straighten my knee while walking. Should I see a doctor or should I just ice and wait a few days?

    • That’s a painful injury. It may take a week or more to be able to straighten it… if it lasts longer than that have the knee checked out.
      Good luck

  • Hello, I am concerned about my injury, i Much appreciate the help. So, I was running down a stairs and i may have jumped off from four to fives step before hitting the pavement. I landed on my Right foot and ended falling afterwards. I didn’t noticed I hurt my knee with the leg I landed on until i got up and tried to use the right leg to walk and that’s when I fell immediately. I knew something was wrong then so I got carried by the shoulders.

    • Continuing…..
      I can’t lift my right leg. The only way I can lift it is by holding my right back thigh up which is tense and as well as my back leg. It is also swollen Around my knee. And the first day I couldn’t bend my knees or move my leg as much because of the swelling not until the second day but still not completely bend. I don’t feel much pain than I thought I was going to have.vshould I wait a few more days for result? Or how will I know when to get back on my Feets.

      • With that story and those complaints you need to be seen by an Orthopedic surgeon soon.

  • I was pulling myself up on the backend of our truck and felt a painful pull in my knee. It hurts to walk and I can’t straighten my knee. It’s swollen some and feels like it’s going to give out when walking. After getting up from the couch/ chair it’s quite stiff. I am wondering what the problem could be.

    • Kristi …. Anytime a knee injury causes the inability to straighten it we are concerned about a possible meniscus tear. Best to be evaluated if this has persisted for a short while (week).

  • We were on vacation and went to the NHRA race in TX. I was going up stadium stairs and my knee popped (not loud) and it hurt. Luckily I only had one step left. The knee was sore while sitting there but not to painful. I could move around and stand up. If I bent it, it would hurt but not bad. We got up to leave and started back down the steps and that was very painful. I had to put one foot down and leave the other leg “stiff” so I could get down the stairs. If I kept that leg straight it was better since I wasn’t bending it. When we got home-a week later it was better. I had a slight limp and that also went away. Today I was on my feet quite a bit and it’s sore again but I can walk on it. If it is swollen, its not much and I have no bruising and it’s not warm to the touch. When I bend my knee if feels “tight”. I hate to go to the Dr and he says that it will just take time for it to heal. I feel it isn’t serious because I can walk. What do you think? Thank you

    • HI Jenn…
      Obviously it’s difficult for me to say if I’m not able to examine you.
      In situations like this it is usually safe to wait a few weeks to see if your symptoms improve. If they do not, then you can talk to your doctor.
      Good Luck

  • I was weightlifting in my p.e. class a week ago, and my knee started to hurt after it had happend. I figured it was alright so the weeked following I was in a soccer game and a girl pushed me and my leg extended out and I fell on my right knee cap. The side of my knee on the outside hurts when I walk and on the bottom of my kneecap, it feels like someone is pushing needles through it. Do u think it could be serious?

    • Hi Madelyn …
      Perhaps it is time for you to see a doctor so they can examine your knee.

  • The other night I was at a softball game and as I was rounding second my leg got caught in the dirt. When this happened my knee twisted. I felt a pop and there was a burning sensation for about 30 seconds. Since then I have been having sharp pains on the left side of my knee. Yesterday while I was getting ready for work I noticed that the left side of my right knee was swollen. And to make things a little more complicated I have been feeling clicking in that knee as well. I have had three surgeries on that knee for meniscus tears. Does this sound like that could be the case yet again. In total I have 6 knee surgeries all together. 3 on each knee and I am not looking forward to another one. I am scheduled to see my Orthopedic surgeon on Monday to have it looked at.

    • HI Mark…
      Twisting injuries with resulting pain on the inside of the knee can represent an injury to the MCL, or the meniscus. With any twisting injury accompanied by a pop, sadly an ACL injury is also possible. Good luck with your exam on Monday. The exam itself should tell you whether or not the ACL or MCL is injured.

      • Thank you for the reply. I sent you a picture I took the other day of the swollenness. Today I am having a little more pain then yesterday and the swelling hasn’t changed. But I am optimistic lol. Have a great weekend.

  • One night I was longboarding and I went downhill going fairly quick I saw a lot of big cracks and holes in path ahead of so I hopped of my board, bad idea, and using my right leg to catch myself keep in mind that all that momentum went to my right leg primarily my knee. As for my knee, I can bend it and squat without problem no pain whatsoever until I try to fully straighten or when I add weight when standing while my leg is straightened. I feel the pain slightly below the knee cap. It swell up a little bit on on side of the knee cap but not so much that it is noticeable at first glance.

    • After an injury, swelling is the number one predictor of a serious injury. Most people with a swollen knee after an injury should see a doctor. The loss of the ability to straighten your knee could be due to the swelling, or perhaps due to something being torn deep inside the knee. Good Luck!

  • I was dancing at home and I went to turn my body turned but my right knee didn’t it popped alright it hurt so bad but it’s not dislocated cause I can still walk on it but it’s a struggle to go up stairs cause it hurts kinda I also have a dance audition in school in 6 days and I need it to get better…

    • I’m sorry to hear that Jessica. I would suggest that you see a Sports Medicine doc for a good examination.

  • My knee has been messed up for a while but it was just a slight injury. And it still pops. Then a couple days ago it hurt more. This morning I couldn’t even move my leg because it hurt so bad and i had my leg bent and I tried to Streighten it and made it pop so I could walk. And finally it popped and it hurt so bad. Is my knee broken or sprained

    • It’s hard to say… you should see a Sports Medicine doc to help you through the diagnostic process. Many things can cause these symptoms. They include meniscus tears, arthritis, cartilage issues.

  • I tripped in my house and fell super hard ( picture being body slammed face down. Hit my left knee the hardest and then bounced to the right side from hip to ankle(63 years old) wood floor under carpet.
    Went to ER had X-rays no breaks seen. It’s been 3 days and I still have a knife sharp
    Burning pain and bruising in the left knee cap. I came down on that knew so hard o nearly passed out. Should I see someone again? The pain is so bad I can barely walk. Can’t bend the knee.

  • Hello,

    I have had this knee problem randomly since I was 26 years old, I am 28 now. What happens is I’ll be doing something like playing spike ball in the sand and it feels like the outer side of my right knee dislocates or something because it feels out of place, and it won’t let me put pressure on it with out hurting and it won’t straighten out all the way. It will be like that for a few seconds and I’ll use my injured knee and do a kicking motion and it pops (hurts and makes loud noise) what feels like back into place and I can straighten my leg out and everything goes back to normal, but it accompanied with pain and a little swelling. I have had this happen about 10-15 times over the last 3 years. Any idea what could be going on?

    • HI Jordan, It’s hard to say from where I sit… but common causes of these symptoms include a tear of the lateral meniscus, and a possible injury to the proximal tibia/fibula joint. That joint can be unstable after certain injuries.

    • I was riding a 4 wheeler and my son wrecked his 3 wheeler. I stoped and got off mine ran to him full speed and I feel in a hole I got up and tried to run again and I couldn’t go I had to crawl to him and my knee instantly swelled and I have bruising. I feel little zings below the knee that drive me crazy. Any opinions would be great..

      • It’s hard to say… I’ve seen many different types of injuries when folks step into deep holes while running. We’ve seen many ligament injuries as well as some fractures. An x-ray and a good exam will help clarify what’s going on.

  • I got up out of my chair last night and my knee clicked, there’s no swelling but there’s a lump on the lower corner of my knee. When I walk it hurts, but I can straighten my leg out but when I bend my leg it clicks. Do you have any idea what has happened?

    • There a list of possibilities… some more common than others. An x-ray and a good exam should help clarify what this is. Tears of the lateral meniscus on the outside of the knee will produce a cyst (fluid-filled sack). Neither are dangerous, but those cysts can ache.
      A meniscus that has flipped into a certain position can cause a lump that is tender to touch too.
      An arthritic knee will have lumps that we may not have noted before the pain started. Now that pain has drawn your attention to that region you are aware of it.

      Those are the most common reasons… see your doctor and hopefully they can clarify it further for you

  • Fiancé suffered a bad knee injury at work that resulted in significant swelling, pain, and crutches for a day or two. It’s been 4 days and the swelling has gone down, he’s able to bend it pretty decently, and just a little soreness. Should he keep his follow up appt and still be seen by a specialist or could this mean nothing too serious is wrong since it’s getting better?

    • We like to examine knees that developed significant swelling and stiffness after an injury. That’s our way of determining that there was no significant damage, especially to the ligaments.

  • I was at the park yesterday and went to the playground to save a child that was up high and couldn’t get down by the time I got up there the child was coming down I had to slide down a pole landed fine I thought but heard 3 loud cracks now I can’t walk sit or even cough got crutches but even the simplest move I’m in escrusiating pain .

    • Sounds like you need an X-ray and a good exam by a knee specialist.
      Good luck !

  • Like 1 month ago I was on a step stool I almost fell but I caught myself and landed on my left leg
    My knee is a little swollen and it hurts when I squat or walk down the stairs or bend it also it hurts when I try to straighten it
    I had an X-ray but it was normal
    Do I have a sprain/ or tear ?

    • Hi Michelle,
      That’s hard to answer. It is possible a meniscus tear exists. It is also possible that it is a strain and will resolve over the next month or so. If the pain persists much longer, or certainly if the pain worsens consider seeing a knee specialist.

  • Yesterday I was running and my knee gave out on me and I fell. I went to the emergency room and they took xrays and said it look fine, it’s just sprained. I can’t walk to much on it and it’s really stiff it feels like I need to pop it but it won’t. I have to work Wednesday which only leave me six days to heal I don’t know if it will heal in enough time

    • I don’t know either :-(. If the knee is swollen and you can’t straighten it all the way you should see a knee doc (Sports Medicine Doc) for an examination.
      Good Luck

  • I was playing soccer and went to defend a ball. My leg was planted and she continued to run through and hit my knee. I heard a loud pop and then tried to straighten my knee, I heard another pop following this. I had immediate swelling, and I am unable to walk on it/put much weight on it. I have torn my other ACL and meniscus on my other leg, it had three consecutive pops when it happened and it stayed in a bent position. This one I can straighten a little but not much. The pops were not the same, could it be the same thing?

    • It’s possible Emily. It sounds like you need to see a knee doctor before heading back onto a soccer field. You need a good examination and possibly an imaging study such as an MRI.

      • I felt my leg caught stuck in the turf. I felt my knee pop. I could not extend my leg fully straight. Can’t put body weight cause it hurts it’s not swollen. Had exray and their is no fracture but very pain full to extend

  • I felt my leg caught stuck in the turf. I felt my knee pop. I could not extend my leg fully straight. Can’t put body weight cause it hurts it’s swollen. Had exray and their is no fracture but very pain full to extend. When I try to put my foot in the ground it hurts and can’t fully extend my knee.

  • Hi. I twisted my knee 2 weeks ago from playing basketball. I was turning to my right and my right knee was planted on floor and heard 3 clicks/pops when it happened. I didnt fall down on floor but my knee felt super weak. I had both my hands to help me from falling to the floor. Week and a half later. I twisted it again easily and swelled up again. Whenever i bend it, there is a cyst like figure on the right side of my knee cap. And after a few days later pains started showing up from around the knee. Then after that, sharp pains hit from knee down to nearly my ankle. And knee up to my buttocks.

    • You should see an Orthopedist who specializes in knees. A good exam will be key to figuring out what happened.

    • Did you find out what was wrong?? Similar case just happened to me whilst playing soccer 2 weeks back and I’m still in recovery. Haven’t had it examined but I’m thinking of checking it out.

  • Hello,

    I was delivering a washer and dryer to one of my clients and as I was carrying the dryer with my partner, my knee hyper extended in a 8-9 inch hole in the ground that was covered by leaves. I felt my knee cap shoot straight up and felt like a pop. As of right now, there isn’t any swelling but it is stiff and hard for me to bend my knee while walking. Should I be concerned about a major knee injury?

    • Hi Gregory … Hyperextension injuries can cause ligament injuries… PCL or ACL. Sometimes it’s just a stretch or a sprain, and sometimes it’s a tear of the ligament. It is worth your time to have a knee doc take a look and examine your knee.
      Good Luck

  • So in pe 3 days ago we were playing four corner football indoors and I tripped over the ball and hit my knee really hard on a really hard floor, I tried to get up normally but I wasn’t able to bend my knee, since then my knee’s been swollen and ongoing pain, and even if I try to bend my knee it hurts to much. I tried asking my perants if we could see a doctor but they said it’ll go by the next day, and it’s still painful and swollen. What should I do?

  • I’m 12 years old. And I’m a dancer. I do pointe ballet. And last week we were doing splits and a girl tripped and landed on my knee. And it hyperextended my knee. And now i cant really walk on it or straighten. Its swollen and bruised. I heard a pop when it happened and it hurts in the back of my knee and my knee cap area. What do you think happened?

    • HI…
      As knee specialists, we will always want to examine a knee with that type of injury. We would want to make sure that all of your ligaments in the knee are ok before you return to dancing.

  • 3 weeks ago, I was jogging. No issues whatsoever and my knee began to hurt. I finished my workout by walking. The next morning there is some noticable swelling int he front below the kneecap but the swelling isn’t anything like what I see in the online photographs. My knee was very tender to walk on for a few days but it’s been 3 weeks and it does not hurt to walk at all. Going up stairs feels like there’s some pressure but no pain. The only time it really hurts is if I twist or get out of my car using that leg, which is in an over extended position. Wondering if I can ease back into jogging. The swelling is still present. It hasn’t gotten worse or better.

    • Many sports docs would suggest that you should try it out and see what happens… if you are unable to return to running then you can see a knee doc.
      FWIW, it is not unusual for the knee to stay “cranky” for 8-12 weeks. So improvement after just 3 weeks is likely a good sign.

  • I tore my ACL in December and had surgery mid January. So far so good in recovery so far but last night I was walking down the stairs and almost fell but I caught myself. When I walk my knee hurts below the knee cap but I can straighten and bend without pain. I can’t tell if it’s swollen because it’s still quite bigger from the surgery. My orthopedic appointments have been canceled due to coronavirus. Do you think I’ve caused more damage to my knee?

    • It’s hard to say Susan. Ultimately you will need to be examined when your doctor’s office reopens. Nost of the time, these little issues do not cause a new tear. Good luck.

  • I would go to a doctor but covid 19 is spreading and i dont want to get infeced over my stupid leg.

  • I was kneeling then I stood up pressing g uward with my left let and suddenly my left leg made this super loud pop and I fell down and my leg wont straghten or bend on its own. I have to grab from the back of my thigh and lift it. It can also not bear any weight.
    Should I go to the dr or ER?

Sorry, comments are closed.