Menu

Knee Anatomy, Function and Common Problems

The knee joint is a synovial joint which connects the femur (thigh bone), the longest bone in the body, to the tibia (shin bone).  There are two main joints in the knee: 1) the tibiofemoral joint where the tibia meet the femur 2) the patellofemoral joint where the kneecap (or patella) meets the femur. These two joints work together to form a modified hinge joint that allows the knee to bend and straighten but also to rotate slightly from side to side.

The knee joint is the largest joint in our body. It is vulnerable to injury as it bears an enormous amount of pressure while providing flexible movement. When we walk, the load on our knees is equal to 1.5 times our body weight. When climbing stairs it is equal to 3-4 times our body weight.  When we squat, the load on our knees increases to about 8 times our body weight!

Anatomical Terms

Anatomical terms allow us to describe the body and body motions more precisely. Instead of a doctor simply saying that “the patient’s knee hurts”,  he or she can say that “the patient’s knee hurts anterolaterally” to specify where exactly in the knee you are having pain.  Identifying specific areas of pain helps to guide the next steps in treatment or work-up. Below are some anatomic terms doctors use to describe location (as applied to the knee):

  • Anterior — if facing the knee, this is the front of the knee
  • Posterior — if facing the knee, this is the back of the knee. If used to describe the patella (knee cap), then it would refer to the side of the patella closest to the femur.
  • Medial — the side of the knee that is closest to the other knee, if you put your knees together, the medial sides of each knee would touch
  • Lateral — the side of the knee that is farthest from the other knee (opposite of the medial side)
  • Abduction — move away from the body (raising the leg away from midline i.e. towards the side)
  • Adduction — move toward the body (lowering the leg toward midline i.e. from the side)
  • Proximal — located nearest to the point of attachment or reference, or center of the body
    • example: the knee is proximal to the ankle
  • Distal — located farthest from the point of attachment or reference, or center of the body
    • example: the ankle is distal to the knee
  • Inferior — located beneath, under or below
  • Superior – located above

Structures often have their anatomical reference as part of their name, particularly if there are other similar structures close by. For instance, there are two menisci (or meniscus, singular) in the knee. As such, they are named the medial meniscus and lateral meniscus. Therefore, the medial meniscus would refer to the meniscus on the inside of the knee (i.e. closest to the other knee).

Structures of the Knee

Bones of the knee joint

Again, the knee joint is a hinge type joint. The part of the door that keeps it secured to the wall and allows it to open and close is called a hinge. The majority of the movement allowed by the knee is the same type of motion allowed by a door hinge. It additionally allows for a small amount of rotational movement.

If you think of the knee in layers, the deepest layer is bone and ligaments, then ligaments of the joint capsule, then muscles on top. Various nerves and blood vessels supply the muscles and bones of the knee.

Bones of the Knee

There are four bones around the knee: the thigh bone (femur), the shin bone (tibia), knee cap (patella), and the fibula (see image to the left):

  • Femur (thigh bone) – the longest bone in the body;  The round knobs at the end of the bone (near the knee) are called condyles. Within the knee joint, the end of the femur is covered in hyaline (or articular) cartilage.
  • Tibia (shin bone) –  runs from the knee to the ankle. The top of the tibia is made up of two plateaus (or flat surfaces) which are covered in articular cartilage (within the knee joint). Attached here are two C-shaped shock-absorbing cartilages called menisci. A knuckle-like protuberance on the front (or anterior aspect) of the knee is called the tibial tubercle. The patellar ligament (or tendon) attaches here (see below).
  • Patella (kneecap) – a semi-flat, triangular bone that is able to move as the knee bends. It’s main function is to increase the force generated by the quadriceps muscle (which straightens or extends the knee). For instance, if you break (or fracture) the patella, the quadriceps may not be able to effectively pull on the tibia and you may not be able to straighten your knee. This is one of the main reasons why patellar fractures often need to be fixed. The patella also protects the knee joint from trauma. The patella glides within the groove formed between the two femoral condyles called the patellofemoral groove.
  • Fibula— a long, thin bone in the lower leg on the lateral side which runs along side the tibia from the knee to the ankle. While about 80-90% of weight is carried by the tibia, the fibula does help to carry some weight as well. Importantly, it serves as an attachment for muscles like the biceps femoris (one of the hamstring muscles), lateral collateral ligament (see below), and also helps to form the ankle joint.

Ligaments of the knee

Ligaments in the knee

Ligaments are strong, tough bands that are not particularly flexible. The function of ligaments is to attach bones to bones and to help keep them stable. In the knee, they give stability and strength to the knee joint as the bones and cartilage of the knee have very little stability on their own.

  • Medial Collateral Ligament (or the tibial collateral ligament) – attaches the medial side of the femur to the medial side of the tibia and limits sideways motion of your knee.
  • Lateral Collateral Ligament (or the fibular collateral ligament) – attaches the lateral side of the femur to the lateral side of the fibula and also limits sideways motion of your knee.
  • Anterior cruciate ligament (ACL) – attaches the tibia and the femur. It’s located deep inside the knee and in front of the posterior cruciate ligament. It mainly serves to limit forward motion of the tibia relative to the femur. It also limits some rotation and sideways motion of the knee. The ACL can be torn with sudden pivoting motions of the knee.
  • Posterior cruciate ligament (PCL) – like the ACL, it attaches the tibia and the femur. It lies behind the anterior cruciate ligament. It mainly limits backward motion of the tibia relative to the femur. Like the ACL, it also limits some rotation and sideways motion of the knee. The PCL can be torn with a forceful landing on the shin.
  • Patellar ligament (or tendon) – attaches the kneecap to the tibia. It is less of ligament and actually a continuation of the quadriceps tendon.
  • Joint Capsule – a thick, fibrous structure that wraps around the knee joint. Inside the capsule is the synovial membrane which is lined by the synovium, a soft tissue structure that secretes synovial fluid, the lubricanr of the knee.

The pair of collateral ligaments keeps the knee from moving too far side-to-side. The cruciate ligaments crisscross each other in the center of the knee. They allow the tibia to “swing” back and forth under the femur without the tibia sliding too far forward or backward under the femur. Working together, the 4 ligaments are the most important in structures in controlling stability of the knee.

Cartilage of the knee

Cartilage of the Knee

There are many types of cartilage in our body, each with a slightly different function. For instance, the medial and lateral meniscus (discussed below) are made up of fibrocartilage which make them strong and rubbery and able to add additional stability to the knee.  On the other hand, like bones of most joints, the end of the femur and tibia and the undersurface of the patella are covered in hyaline cartilage. Hyaline (also known as articular) cartilage is both flexible and slippery. The flexibility helps it to act as a shock absorber. Articular cartilage is made even more slippery by an oily lubricant made within the joint, called synovial fluid. This allows the two bones to move smoothly on each other without pain. If this articular cartilage wears away, joint movement can become painful and limited (this is known as arthritis). Unfortunately, cartilage has almost no blood supply and is very bad at repairing itself.

  • Medial Meniscus

    The medial meniscus is a crescent shaped structure that exists on the inside of the knee. It is made of fibrocartilage. It acts as a shock absorber in the knee and adds stability to the knee joint.  It is attached to the tibia as well as to the joint capsule of the knee.

  • Lateral Meniscus

    The lateral meniscus sits on the lateral tibial plateau. It is a crescent shaped structure that is also made up of fibrocartilage. It acts as a shock absorber in the knee and adds stability to the knee joint.  It is attached to the joint capsule of the knee as well.  It is somewhat more mobile than the medial meniscus.

In a healthy knee, the rubbery menisci act as shock absorbers. They both sit on top of the tibia and help to spread the load of the femur over a larger surface area on the tibia. If the menisci are removed (because they are torn, etc.) the underlying articular cartilage sees a heavier load and is at risk of wearing down faster (i.e. development of osteoarthritis)

Additionally, together, the menisci create a shallow socket on the tibia that accommodates the end of the femur. This assists with knee stability.

Muscles Around the Knee

The muscles around the knee help to keep the knee stable, well aligned, and moving. There are two main muscle groups around the knee: the quadriceps and the hamstrings. The quadriceps are a collection of 4 muscles on the front of the thigh and are responsible for straightening the knee by bringing a bent knee to a straightened position. The hamstrings are a group of 3 muscles on the back of the thigh that provide the opposite motion by bending the knee from a straightened position.

The iliotibial band is a broad tendinous extension of the tensor fascia lata and gluteus maximus that also helps to stabilize the knee.

Tendons in the Knee

Tendons are elastic tissues made up of collagen. They are the continuations of muscles and allow them to connect to bones. There are numerous tendons around the knee that also help to stabilize the knee. They are associated with muscles discussed in the section above (see above).  One of the most important tendons is the quadriceps tendon. This lies on the front of the knee and connects the quadriceps muscles of the thigh to the tibia via the patella and patellar ligament (or tendon). It provides the power necessary to straighten the knee.

Bursae

There are up to 13 bursa of various sizes in and around the knee. These fluid filled sacs cushion the joint and reduce friction between muscles, bones, tendons and ligaments. There are bursa located underneath the tendons and ligaments on both the lateral and medial sides of the knee. The prepatellar bursa is one of the larger bursae of the knee and is located on the front of the patella (hence pre-patellar) just under the skin. It protects the patella. Sometimes, whether due to direct trauma or even infection, it can become irritated, swollen, and painful.  This is known as prepatellar bursitis.

The pes bursa is another important bursa that overlies some of the hamstring tendons which attach to the medial side of the tibia.  It too can sometimes become irritated, causing pes bursitis, which can be painful.

Plicae

Plicae are folds in the synovium within the knee joint itself. Plicae rarely cause problems but sometimes can get caught between the femur and patella and cause pain.

Knee Arteries and Veins

Probably the most important thing to know about the blood supply to the knee is that it is very abundant. There are many collateral vessels (basically extra vessels) that give blood supply to the structures of the knee.

Knee arteries and veins

Knee arteries and veins

Problems in the Knee

As the knee has many structures associated with it, there are many problems that can occur around the knee.  In addition to wear and tear type issues of the knee, sports injuries are the source of many knee problems.

Symptoms

Knee symptoms can be quite variable. Pain can be dull, sharp, constant or an on-and-off type pain. As there are many structures of the knee that are prone to injury, depending on the underlying problem, the location of pain can be variable as well.

With certain types of injuries (ligamentous injuries), knee range of motion may actually increase as the knee becomes unstable.  On the other hand, with arthritis of the knee, range of motion can decrease.

Depending on the injury, you may experience “mechanical symptoms”. Mechanical symptoms are ones that affect the normal function of the knee.  Catching or locking of the knee, whether painful or painless, are examples of mechanical symptoms (see below). Additionally, you may hear grinding or popping (crepitus).

  • Swelling
  • One of the most common symptoms associated with knee problems is local swelling. The accumulation of too much synovial fluid (synovial effusion) is usually due to irritation or inflammation of structures within the joint.  Bleeding into the joint (called a hemarthrosis) can also cause a joint to swell. Swelling immediately following an injury is usually from bleeding. More delayed swelling or on-and-off swelling are usually from excess synovial fluid production from an irritated knee.  The best initial home therapy for swelling is R.I.C.E. therapy.
  • Chronic swelling can start to limit full range of motion. This may eventually lead to muscle atrophy (or wasting) from non-use of the muscles around the knee.
  • Locking (or Catching)

    Locking or catching of the knee usually occurs when there is a loose body or a torn meniscus in the knee. The loose body can be as small as a grain of sand or as big as a quarter. It is usually a piece of cartilage that has been chipped off of the end of the femur or tibia or a piece of torn meniscus that has become free.  As it floats around the knee, it can suddenly limit normal motion and be associated with significant pain. A torn flap of meniscus (that is still attached) can do the same thing. If the symptoms are bad enough, knee arthroscopy may be needed to address the issue.

  • Giving Way

    Sometimes, depending on the underlying problem, your knee may occasionally feel unstable and you may feel like you’ve momentarily lost control of the muscles around the knee. This may cause you to stumble or even fall. There are many reasons why this can occur, including injuries to ligaments. Sudden sharp pains in the knee from a loose body or a torn meniscus can also cause your knee to reflexively feel weak and give way.

  • Snaps, Crackles and Pops

    Popping or crackling noises – medically termed crepitus – coming from the knee without any associated pain are oftentimes normal. However, If you have pain, swelling or loss of knee function, you should seek the opinion of an orthopedic surgeon. The most common causes of crepitus are osteoarthritis and a condition called —chondromalacia patella—where the cartilage under the patella starts to wear down. These conditions result in rough surfaces within the knee that rub on each other and cause noise (i.e. crepitus).

If your symptoms are interfering with your quality of life, you should see an orthopedic surgeon to have your knee evaluated. He or she will perform a history (ask you questions about your symptoms and how they started) and a physical exam.  Imaging studies then may be ordered to help figure out what is causing your symptoms.  If you have had imaging studies performed elsewhere, it is always best to bring the actual CD of the study (i.e. not just the report) to your surgeon so that they can actually look at it in person. This will speed up the time it takes to figure out how to best treat your problem. 

Pathological Conditions and Syndromes in the Knee

Types of Knee Surgery

 

Note that the information in this article is purely informative and should never be used in place of the advice of professionals.

Comments

  1. colette cudworth says

    my name is colette, I had a tkr 2 years ago and last year began having cramps on the side of the knee and in the back. After surgery, I couldn’t straighten it and experienced pain that was incredible. I was on dilaudid for four months and finally begged my dr. to do a manipulation. I still can’t straighten it, and wear pain patches every night. does anyone know why this is happening? I am up walking out the cramps up to 4 times a night. Could it have anything to do with the tendons? thank you for any info

  2. Jennifer says

    17 years ago I was diagnosed with osteoarthritis in my left knee. Last year was in a lot of pain and went to my GP who sent me for physiotherapy. The pain was still there so I went to see a chiropractor who believed it is my inside leg muscles and I had acupuncture. It is still very painful and my GP dishes out tramadol like sweets. I have iron deficiency anaemia and had an iron infusion yesterday and my leg feels much better. One of my blood tests showed I had a vitamin d deficiency last year. Could these conditions be affecting the pain?

  3. Martin Amell says

    I’m very interested in the process or phenomenon of popping my knees. So far I’ve been able to identify 3, perhaps 4 different ways I’m able to pop them, the most dramatic is by pressing my feet together, relaxing the muscle in and around my knees and BOOM! A loud pop which also temporarily changes the location of the bones or at least it seems so.
    There’s also the squat pop or while laying down what my wife calls kicking myself in the ass. There’s a few feet pressing pops I can achieve and I can also achieve a standing twist pop. None of them have been painful and I’ve been doing this for many years now. I’ll be 60 in July so it’s nothing new to me, but more and more facinating as I get older.
    Can you elaborate on any of the popping I find so satisfying?

  4. will a buised bone behind the knee cap area right itself and how long does it take

  5. My name is Farukh I operated tibial plateau fracture on 27 Dec,18 .Fracture is completely healed up and iam doing proper exercise to mobilize my stiff muscle.All of the muscle are now working except knee muscle ,which still tight .in the consequences I can’t bend down my knee,please advise

  6. Marian harney says

    I was walking something like bones cracked at back of my knee I couldn’t put my heal to ground for 2days still v tendor and stiff what could it b ?

  7. I fell on both knees on a concrete driveway and have had trouble standing up straight and walking naturally causing my back to ache. I have no pain or trouble sitting upright in a chair. Should I try to exercise to strengthen my glutes. Would that improve my walking.??? I have no pain otherwise …..I am willing to try any kind of exercise to gain my mobility. Thanks for your help….

  8. Can one donate ligaments from his leg to another needy patient.

  9. Cindy Schmidler says

    Thank you Laura. We updated the article to add the term “catching” which I agree is appropriate. We left the word “locking” in the article because people continue to search using that word.

  10. Because your site doesn’t allow editing after submission, please delete my previous entry and use the following instead:
    I wish the medical community would stop referring to the knee as “locking” when something is keeping the knee from fully straightening because technically the knee is being prevented from locking out. It sounds ridiculous to have a knee that is locked from locking out. I prefer to describe this as catching. Sometimes it catches only briefly and other times it takes a little time for it to work the obstruction out of the way, but using the term locking seems like a poor choice for the sake of good communication.

  11. seriously anyone with issues go and see a professional, this is just a basic guide and this is definitely not the place to try and get a diagnosis for any pain you may be having. Contrary to what some of you may think the internet is not the place to find the answer to everything or to be asking people what they think your problem is. We are all so incredibly different and what one person may have may not be what you have. IF IN DOUBT GO AND SEE A MEDICAL PROFESSIONAL and push them until you get an answer better yet ask more than one if your not happy with the answer

  12. Helen Thomas says

    Hi,
    I will be 50 in 3 months and had a cartlidge repair 3 weeks ago now as suffering for about 4 years but no tablet would shift the pain and i have been on tramadol now for at least 7 years but were getting to the stage even they wouldnt do that. I am still in pain everyday after my operation I can barely walk and haven’t slept for more than 2 hours properly since I’ve had it done.the surgeon haven’t really explained exactly what he has done yet in surgery,but i have never listened really.But before you decide you have

  13. Esther bechy says

    Hi… Am Esther… Am a student of college of education ijaniki… Am currently experiencing a kind of shock from the beginning of my waist down to the toe of my left hand side.. Can hardly bend… As if d muscle there is weak or damaged…. Pls wat do I do

  14. Shirley Sturgis says

    The information was very helpful. Now I know more about the knee and have more understanding of what could be happening. Thank you. My pain is above the right knee. Cannot sit with knee bent for a long time, especially riding in back seat of a car.

  15. jitendra kumar says

    Hi
    My have fixed on point but the side wise having disturbed little bittle . so what should I do for treatment .

  16. Hi, actually i have pain back side of my right knee. I am bsically a fitness instructor, coz of this pain, i can not fold my leg properly in my yoga classes. I feel back of my knee there is a balloon filled with water. What is this, plz explain with pictures
    Regards

  17. Roy Beddington says

    I had bi-lateral knee replacement surgery in October 2015. I spent two weeks in a rehab hospital, and upon release could walk unassisted up and down stairs, get in and out of my car and go on short walks, 1 -2 kms, at a stretch. For four months everything went well, no pain which had been caused by severe oesteo- arthritis. I went to the pool to swim and walked to the gym to work on strengthening my quad muscles which had pretty well atrophied during my arthritis. Then one day while walking in the mall my left knee swelled up severe pain ensued and it was all I could do to hobble back to the car. Four days of bed rest and I was okay. But this condition has attacked both knees on separate occasions coming at rest or after moderate exercise. Several trips to my surgeon have been un- helpful. His x-rays show no problems with the prostheses so he has absolved himself of my misery. My physio people also are at a loss to explain what is happening, having never seen anything like this before. My GP and Chiropractor agree it could be Bursitis, but the surgeon and physio folks disagree. Something is causing this very painful and frustrating situation and the medical fraternity have been of little help. Prognosis anybody….Please

  18. Israel Lohm says

    I’m Israel Lohm of Papua New Guinea, upstairs iomop. I’m so amazed to learn how the knee joint works by the muscles and tendons and components working the knee joint. Especially the very great potential of the medial and lateral meniscus to perform their function as shock absorbents. Wow! A countless gratitude to the intelligent designer.

  19. shivakumar says

    my left leg bending problem, 6 years ago surgery knee

  20. the outside of my leg at the knee really aches at times. It comes and goes meaning it will ache a week or three, then just goes away. I ice it, take Mortin and elevate in the evening. I am overweight which I’m sure doesn’t help, but I’m working on that. Is this an injury or wear and tear and should I go see my doctor?

  21. seye star says

    My name is seye from Nigeria, I had an operation on my right leg over 8 years ago when my patellar was shifting periodically for about 10 years. After the successful operation there was no pain or any discomfort….. but recently I was hit by a reversing car while I was standing and that resulted to surgical operation on my right knee for ruptured tendon. The operation was done without any knee replacement rather the tendon was repaired. I have POP on my leg for 6 weeks now but some of the doctors said 6 weeks it’s okay while some said I should make it 3months. It’s a teaching hospital, I want to know which one is the best. The previous one lasted 6 weeks and I don’t why this one should be different. Kindly reply thanks

  22. What causes the leg just above the knee and to the outside of the leg to burn. It burns really bad walking or standing.

  23. reet monga says

    my husband is 38 years since 15 days he is having pain at the side of the knee ayurveda Dr says that lock has opened between the two bone near knee no
    w I want to know that what is the remedies for this

    pain

  24. I am a soccer player I have lateral collateral ligament injury how can I recover it very fast…I want to play again I cant leave to play soccer

  25. Natasha Peterson says

    I am 22 years old and have had knee pain for half of my life. I have recieved so many different diagnoses that I honestly have n have idea what is causing my pain. First it was growing pains, later it was patellar tendonitis, and my most recent diagnosis was patellofemoral syndrome. Every doctor suggested the same treatments which were Tylenol, rest, and physical therapy none of which worked. Physical therapy made other joints in my body hurt, I even had a physical therapist refuse to treat me because my body reacted so badly. I have had xrays MRIs tested for every type of arthritis and given knee braces. The only time I have not felt knee pain in the last 11 years is when I was pregnant. I have gone to local doctors and have driven hundreds of miles to go find out what is wrong with me and no such luck. If you have any ideas I would love to hear them, as l long as physical therapy is not my only solution.

  26. john o'neil says

    Some three weeks ago I was papering a wall, but half way through doing it, I felt my right leg stiffening up at the knee, and it became painful , at the moment I have substantial pain coming from the knee. Painkillers dont seem to help, and ive not had a decent nights sleep since.xray shows slight osteo arthritis..but obviously its not that, as pain is constant ….help….

  27. plz. give suggestion on my MRI of left knee report is submitting here.
    – Excessive lateral patellar shift with no obvious pateller facet cartilage thinning or patellofemoral bone bruise/contusion.
    – Fairly large pockets of heterogeneous fluid signal intensity collection in the medial and lateral patellofemoral recesses extending superiority to suprepatellar bursa and inferiorly to the joint space with medial and lateral patellar retinaculum tears.
    – Mild thickening of the anterior cruciate ligament with altered heterogeneous signal intensity and maintained alighnment- Mild interstitial tear is likely.
    – Edema/thickening involving the medial and lateral colateral legaments-Grade II injury.
    – Edema involving the Hoffas pad of fat.

  28. I’m Mary ann asor from Philippines. I have severe problem in my knee. It just started when I twisted my left knee and felt pain and irritation when walking and even when im stable. I have not consulted a doctor. Instead I just always rotate my legs to find comfort but it had just become worst.Until I felt my knee becomes loosed and that I felt difficulty it my veins had loosed. Now I dont know what had happened.I felt difficulty in breathing. Even my veins in my back has been affected. I felt my veins in my back become twisted also. I felt very uncomfortable and difficulty in breathing.And even my mind is already affected.I have become slow in thinking. I thought because my blood circulation was blocked. I’m so worried and afraid that i think of suicide because im afraid. Pls. help me..pls. reply. Thank you.

  29. DONALD YOUNG says

    I have a knee injury in my left leg .The pain and swelling is right where the laderal is . Pain is described as none stop ackeing the whole leg musles and staping pain where the ladral is . To describe better where the injury is it’s the front left side below the kneecap on my left knee .I must have twisted my knee when I was pushed as I was getting off a chair .The pain is none stop and I have no money or insurance please what can I do to get rid of this problem. Thank you

  30. My name is Munther I face a problem in my knee when I play football it twisted I do not know what happened

  31. flex belt coupon says

    I had patellar tendonitos once and it was more than just a pain in my knee if you know what I mean. I have to say it lasted close to 9 months and I couldnt perform any running at the time. Basically, If I were to try to extend my leg upwards, once at a 90 degree angle like a sitting position, I would get an excrutiating pain in the knee. Almost like somone digging a knife into it. Its an injury I hope no one gets.

  32. Ann Allegrini says

    Was about to have knee reconstruction. After six months of visits to the Rothman Center in South NJ I was told it cannot be done. I need a muscle. I tore my patello-femoral ligament. I have had synvisc shots and six weeks of PT at this point and was told I was doing well in PT.

    I have Nail Patella Sydrome and a previous reconstruction surgery done 41 years ago had excellent results..but I took a bad fall while out walking as I had done every morning. NPS patients do not do well with partial knee replacements and cannot have a total knee replacement. Reconstruction was discussed and I agreed to it. The other day one of the two surgeons who would be performing the surgery said I need a muscle. He didn’t define it but said “he doesn’t make muscles”. The surgery will not take place. Isn’t there muscle grafting when all that is holding off a surgery is a muscle; I had been doing well in physical therapy; I don’t know why all of a sudden a muscle is the reason. I need to find out if there is a orthopedic surgeon who can determine if a muscle graft would fix my patello-femoral ligament and put my 10 millimenter kneecap (Yes I know it is small) back where it belongs. Thank you…glad I found this on the web.

Speak Your Mind

*