Why Do We Have Knee Caps Anyway?

Ever stop to wonder what your knee cap (patella) is there for? It’s somewhat of strange thing to think about–and there’s generally not much reason to until something goes wrong with it, like dislocation or fracture. But despite being a bit of an oddity, its presence is certainly not incidental. Given the remarkable design of the body, you can rest assured the patella’s there for a good reason.

In fact, it actually serves a very important function–it makes your quadriceps muscles (on the front of the thigh) more effective.  The tendon of the quadriceps muscle crosses over the knee joint, attaching on the leg just under the front of the knee. When the muscle contracts, it rotates the knee into extension by pulling up on the lower leg. Because the patella lies right between the tendon and the knee, it positions the tendon farther away from the point of rotation than it would otherwise be.  Just that little bit of increased distance imparts a major mechanical advantage.  It’s simple physics–the farther away from the center of rotation, the less force needed to rotate the object. (It’s the same reason you’d prefer to use a long lever to jack up a piece of heavy furniture rather than a short lever….being farther away means you can lift the same weight with less effort). In reality, the knee cap makes the quadriceps muscle approximately 30% more powerful than without it!

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21 Comments

  1. Hi!
    I have just been diagnosed with a tear in my medial patellar tendon and one other one that I can’t remember. In addition to the tears I have the medial 1/3 of my patella is fractured. Due to the injury I am more susceptible to a patellar dislocation. I am being advised that I will probably have to have a reconstructive surgery to fix my tendons. Is that a normal way of fixing this kind of injury?

    Thank you!

    • Since I don’t know more of the details and haven’t had the benefit of evaluating you, I have to defer to your doctors on this one, Corey. Thanks for writing, sorry I can’t be of more assistance but don’t want to steer you wrong due to incomplete info.

  2. I love anatomy, and would love to get your take on why dont we have one on our elbows if we have the same type of joint?

    • Hi Jose,

      Well it’s always harder to know for certain why something does NOT exist, but my sense is that we don’t have an “elbow cap” because the elbow doesn’t have the same power requirements as the knee. It is largely not a weight bearing joint and thus there’s little need for the muscles of the elbow to generate the magnitude of force required of the quadriceps (for walking, running, stabilizing, etc). Also, the quadriceps, unlike the triceps, work primarily eccentrically (on “the negative”) for daily, routine functions, and I suspect that plays a role as well. Interesting question…If anyone has a better or even an alternative answer, please share!

  3. I have had microfracture surgury on my left knee. I also have a bone spur on under side of my knee pad. Doctors said they will not shave it off because it could cause other complications. My knee pops and clicks all the time. The doc said it is the spur. Why would they not remove it if they know what it is. It is very prominent on 3 d mri. What are what next steps. Have had 2 docs say the same thing. Really want a third opinion.

    • Hi Matt,

      I can’t comment on your specific situation, but as a general rule, spurs usually do not need to be removed. And popping/clicking unto itself is not necessarily a problem that needs treating. Also keep in mind that any treatment/intervention for any condition–whether it’s a surgery or just taking a medicine–has pros and cons to it, and the decision of whether to do a treatment depends on considering both. You only want to do something if the benefit clearly outweighs the risks and/or the negatives. Have you asked your surgeons why they recommend not removing it? Odds are that they have a well-reasoned answer.

  4. I have 20 year old knee replacements. The kneecap on my right knee has become dislodged and is basically ‘floating’ around in my knee. The orthopedic surgeon I saw recently said he could make a small incision and remove the kneecap. My other replacement parts are still intact. My question – can I function without a kneecap? I guess I am right now since it has broken loose but I’ve never heard of this.

    • Hi Kathy,

      When they do a knee replacement, it usually includes resurfacing the kneecap as well. To do that, a smooth metal piece is fixed onto the underside of the kneecap. If something breaks loose, it is generally that plate not the bone itself, in which case, one would still have a kneecap.

  5. I have seen 4 different orthopedic Drs. for terrible knee pain. they have all said it bone on bone. no cartilage, because of heart and kidney problems I cannot have surgery. I cannot walk. I am able to push my wheelchair with my feet but cannot stand to walk. I am getting therapy and the Dr says he can get me to walking.
    I am 78 and had severe knee pain since 40’s. my question is no one has told me but in manipulating my knees the other day my son noted that I have no kneecaps. i assume that is why I cannot straighten my knees currently I am walking in a pool a few days a week, i can walk there but not get my knees straight, left about 50 degrees, rite 30 degrees. is there ting that might help me?

    • Hi Charles,

      I can’t speak to the cause of the difficulties you’re describing without actually evaluating you, but I’ve never heard of nor seen anyone not having kneecaps, and their absence would be glaringly obvious on basic knee x-rays which it sounds like you’ve had your share of over the years. So the fact that none of your orthopedic doctors told you that your kneecaps were missing makes me very doubtful that they are actually MIA. And believe it or not, they can also be harder to feel on exam than you would think. I would seek out a rehab specialist to evaluate and offer specific suggestions based on their findings and your situation. Good luck!

    • maybe get another opinion. I am 63 and just had a total knee replacement with Dr. Jeremy Reid of Reconstructive Orthopedics in New Jersey. They have several offices. I had my surgery on Nov. 21, 2016 in Virtua Hospital in Vorhees, NJ and it was an excellent experience and would go to him and there is I need my other knee done.I was up doing knee exercises and walking that nite. Came home Nov. 22 and walked the stairs without a walker or cane. Hardly used them.

  6. Hi everyone I was in a Cheerleading Sports incident 3 yrs ago. Long story short: I was a Varsity Cheerleader and my stunt position was as flyer. For those who don’t know that is the one thrown in the air. My coach decided to get one of the girls who had never rly performed in a stunt before to be my back spot(the one in the back of the stunt that is vital to catching the flyer). The 2 coaches did not get a mat to practice on. I had hit the ground a few times during practice. Only my legs were caught by the bases(girls on the side that also catch flyer). While being thrown in the air the stunt went off so my legs became twisted like with eachother because the way i was thrown was at what felt like 2 different intervals on both of my legs. I hit the hard ground and my teammates said it was like i seemed to have bounced from the ground as a teammate pushed the backspot who who was to scared to ever catch me out of the way and immediantly to my aid. Yes my head had hit the ground several times. No i was not checked for a concusion. I wasnt worried about my head as the pain i felt was in what seemed like my right leg. More specifically my knee. My mom took me to the hospital straight after practice as the pain had me in tears. I was given an x-ray. I read the you have to have 3 diffrerent scans on your knee to be for sure that there is no injury. One of them being an MRI scan. I was told that nothing was wrong and bones were fine. Ive been reading a lot lately about the knee cap and how a sudden twist or jerk can cause it to become out of place or dislocated. My knee is presently and has been sideways on my straight leg. I have not praticipated in sports since as the pain becomes unbearable. If I get involved in intermediate physical activty my knee becomes swollen and there fore my leg is difficult to move.I have lost the majority of my toned muscles as i had a toned body and am now back to my high metabolistic self with a petite apearence. I am tired of the body shaming. I want to be fit again. I just don’t know what kindve process I would have to go through with my knee. Im scared that I may never be able to do alot of physcial activity or develop some sort of terminal problem. I am a worry wart btw so idk if this is just me overthinking things.

    I do not know what exactly I should do.Are there any sort of at home exercises to strengthen my knee and condition myself????? The incident was yrs ago when I was in 7th grade 12 yrs old. I am now 16 and will be a Junior nxt yr. I loved playing sports but it had become impossible. I have gotten better as I take walks for miles and will soon try to start doing beginners yoga once again along with short exercises as long periods cause pain. I hope that these are the right things to do for the time being , but i would love some expert advice and guidance on things. Pls comment back. You have no idea how much I would appreciate it.

    I rly just want to mentally prepare myself for the process IK I will eventually have to go through and any advice to help my knee cap for the time being.

    If you want any details or more information feel free to ask.

    Deeply appreciate anything someone helpfully has to say. Thank you for taking the time to read&thank you for any words.

  7. I am looking at surgery for partial replacement of right knee. It is bone on bone and extremely painful. Just had 3rd cortisone injection and it is wearing off after less than a week. They surgoene said he will resurface the knee cap with surgical plastic and resurface the knee joint with titanium I believe. Is this a common procedure? He said pain relief should be 80-90%. Does this sound correct?

    • Hi Mike,

      The procedure you describe–resurfacing the inside of the joint with titanium and plastic–IS what is meant by a knee replacement. When it involves resurfacing of only one or two areas in the knee–e.g. the medial compartment–as opposed to all surfaces within the joint, it is considered a partial knee replacement as opposed to a total knee replacement. Both kinds of replacements are very common and in the hands of a skilled surgeon, extremely successful. And the reason is very simple: the issue in such patients is that they have severe arthritis causing intolerable pain, and what a joint replacement does is literally cut out the arthritis. In other words, the whole point of a joint replacement is to completely get rid of the thing causing the pain. In fact, the only time(s) I’ve found that a joint replacement doesn’t result in massive relief is if 1) the arthritis wasn’t actually the cause of the pain, i.e. the diagnosis was wrong (just because someone has proven arthritis doesn’t mean that that arthritis is the cause of their joint pain), 2) the surgery isn’t done by a skilled surgeon so the replacement is not properly installed so to speak (there’s an art to putting in a joint replacement well), 3) there are complications from the surgery such as joint infection, which could open up a whole other can of worms (of course, all surgeries carry some risks), or 4) proper post-operative rehab was not completed (do NOT believe a surgeon who tells you you won’t need rehab–rehab is your absolute best chance for the best outcome! There’re are several reasons why surgeons, even well meaning and skilled surgeons, sometimes tell their patients they won’t need to do rehab after the surgery, but they could not be more wrong on that. Don’t be fooled into thinking that just because you can walk on the replaced knee you’re good to go…there is so much more to recovering full function and avoiding future problems. Even doing rehab before the surgery can help make the recovery from surgery easier and less painful.
      So patients should always try to confirm that arthritis truly is the cause of the pain (the doctor shouldn’t be making the diagnosis just based on an x-ray image), find a skilled surgeon (e.g. they do a large volume of those surgeries, public ratings look good, someone you know had him/her do the surgery and had good outcome, etc), and do formal physical therapy afterwards.
      All that being said, there are other treatments to try short of surgery. Certainly steroid injections, but also hylaronic acid injections which sometimes work in people who didn’t have relief with steroids. Physical rehab can help tremendously by strengthening the muscles to better support and protect the knee. Water-based exercises are particularly good for painful arthritic joints. But ultimately, if the arthritis is truly severe, surgery is generally the only real option for significant relief.

      Good luck!

  8. I had both my knee caps,removed at age 16.due to dislocation and bone on bone.These days they wouldn’t dream of doing such surgery.My knees are full of arthritis,bug sets and overall a,lot worse,I’m,51 now.drs shudder when I tell them of the 8surgeries on them both.Seems I’ve just got to live with it.Anyone else had this done?no knee cap replacement,nothing,just took them out and that was that.

  9. hey ,thank you very much for this information ,now i get what was going on with my knee .recently have been working out my legs like crazy but now days my knees have been paining me mostly when i have been sitting for like an hour and after i walk…so now am wondering if i can just get kneecaps or check on the doctor..thank you

  10. I,ve had 3 knee surgery’s to repair my knee pain. The last was a full replacement.
    It still feels like there’s something catching under the side of my knee cap. MRI,s
    show nothing. The pain is incredible ! Any ideas ?

    • Walt,
      Your comment certainly makes me wonder what all those surgeries were supposed to be treating in the first place. Seems like you might be long on treatments but short on diagnoses. From your brief description, I would consider more of a functional problem than structural. In other words, not necessarily some THING that you can touch or see on MRI, but things around the kneecap just not working right and causing your kneecap to catch (or at least feel like it’s catching). If you had a full knee replacement, they not only directly looked at the undersurface of your kneecap, but they likely resurfaced it. They should’ve seen any local structural problem like a spur or loose debris if that were the issue. You need to go to a specialist who will thoroughly evaluate your knee–and that doesn’t mean just x-rays or MRIs. It means someone who will do an extensive hands-on EXAM, delving into the function of the muscles, nerves, ligaments and into the details of the pain more deeply. It’s not the easiest thing to find, but sports medicine physiatrists (PM&R doctors like me) are more geared towards that type of approach, so you can see if there’s one in your area. (aapmr.org is a good resource for that). Good luck. It sounds very solvable.

  11. this subject is so informative thx for explaining it so well you gave me more explaination that the silly cheap scate GP’s at the medical centre thankyou . now i know why i’m in so much pain .

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