Knee Joint Anatomy, Function and Problems

Knee anatomy is about the structure of the knee – that is, the parts that makeup the knee. This article also tells you how a normal knee works and provides resources for problems of the knee joint or it’s parts including knee injuries.

Our knee is the most complicated and largest joint in our body. It’s also the most vulnerable because it bears enormous weight and pressure loads while providing flexible movement. When we walk, our knees support 1.5 times our body weight; climbing stairs is about 3-4 times our body weight and squatting about 8 times.

The knee joint is a synovial joint which connects the femur, our thigh bone and longest bone in the body, to the tibia, our shinbone and second longest bone. There are two joints in the knee—the tibiofemoral joint, which joins the tibia to the femur and the patellofemoral joint which joins the kneecap to 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 and from side to side.

The knee is part of a chain that includes the pelvis, hip, and upper leg above, and the lower leg, ankle and foot below. All of these work together and depend on each other for function and movement.

The knee joint bears most of the weight of the body. When we’re sitting, the tibia and femur barely touch; standing they lock together to form a stable unit. Let’s look at a normal knee joint to understand how the parts (anatomy) work together (function) and how knee problems can occur.

Anatomical terms

Anatomical terms allow us to describe the body clearly and precisely using planes, areas and lines. Instead of your doctor saying “his knee hurts” she can say “his knee hurts in the anterolateral region” and another doctor will know exactly what is meant. Below are some anatomic terms surgeons use as these terms apply 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, also used to describe the back of the kneecap, that is the side of the kneecap that is next to the femur
  • Medial — the side of the knee that is closest to the other knee, if you put your knees together, the medial side of each knee would touch
  • Lateral — the side of the knee that is farthest from the other knee (opposite of the medial side)

Structures often have their anatomical reference as part of their name, such as the medial meniscus or anterior cruciate ligament. The medial meniscus would refer to the meniscus on the inside of the knee, the anterior crucial ligament would be on the anterior side (front) of the knee.

Structures of the Knee

Bones of the knee joint

The main parts of the knee joint are bones, ligaments, tendons, cartilages and a joint capsule, all of which are made of collagen. Collagen is a fibrous tissue present throughout our body. As we age, collagen breaks down.

The adult skeleton is mainly made of bone and a little cartilage in places. Bone and cartilage are both connective tissues, with specialized cells called chondrocytes embedded in a gel-like matrix of collagen and elastin fibers. Cartilage can be hyaline, fibrocartilage and elastic and differ based on the proportions of collagen and elastin. Cartilage is a stiff but flexible tissue that is good with weight bearing which is why it is found in our joints. Cartilage has almost no blood vessels and is very bad at repairing itself. Bone is full of blood vessels and is very good at self repair. It is the high water content that makes cartilage flexible.

Bones of the Knee

The bones give strength, stability and flexibility in the knee. Four bones make up the knee (see above image):

  • Tibia —commonly called the shin bone, runs from the knee to the ankle. The top of the tibia is made of two plateaus and a knuckle-like protuberance called the tibial tubercle. Attached to the top of the tibia on each side of the tibial plateau are two crescent-shaped shock-absorbing cartilages called menisci which help stabilize the knee.
  • Patella—the kneecap is a flat, triangular bone; the patella moves when the leg moves. It’s function is to relieve friction between the bones and muscles when the knee is bent or straightened and to protect the knee joint. The kneecap glides along the bottom front surface of the femur between two protuberances called femoral condyles. These condyles form a groove called the patellofemoral groove.
  • Femur—commonly called the thigh bone; it’s the largest, longest and strongest bone in the body. The round knobs at the end of the bone are called condyles.
  • Fibula—long, thin bone in the lower leg on the lateral side, and runs along side the tibia from the knee to the ankle.

Ligaments of the knee

Ligaments in the knee

The knee works similarly to a rounded surface sitting atop a flat surface. The function of ligaments is to attach bones to bones and give strength and stability to the knee as the knee has very little stability. Ligaments are strong, tough bands that are not particularly flexible. Once stretched, they tend to stay stretched and if stretched too far, they snap.

  • Medial Collateral Ligament (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 (fibular collateral ligament) – attaches the lateral side of the femur to the lateral side of the fibula and limits sideways motion of your knee.
  • Anterior cruciate ligament – attaches the tibia and the femur in the center of your knee; it’s located deep inside the knee and in front of the posterior cruciate ligament. It limits rotation and forward motion of the tibia.
  • Posterior cruciate ligament – is the strongest ligament and attaches the tibia and the femur; it’s also deep inside the knee behind the anterior cruciate ligament. It limits the backwards motion of the knee.
  • Patellar ligament – attaches the kneecap to the tibia

The pair of collateral ligaments keep 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. There is also a patellar ligament that attaches the kneecap to the tibia and aids in stability. A belt of fascia called the iliotibial band runs along the outside of the leg from the hip down to the knee and helps limit the lateral movement of the knee.

Tendons in the Knee

Tendons are elastic tissues that technically part of the muscle and connect muscles to bones. Many of the tendons serve to stabilize the knee. There are two major tendons in the knee—the quadriceps and patellar. The quadriceps tendon connects the quadriceps muscles of the thigh to the kneecap and provides the power for straightening the knee. It also helps hold the patella in the patellofemoral groove in the femur. The patellar tendon connects the kneecap to the shinbone (tibia)—which means it’s really a ligament.

Cartilage of the knee

Cartilage of the knee

The ends of bones that touch other bones—a joint—are covered with articular cartilage. It’s gets its name “articular” because when bones move against each other they are said to “articulate.” Articular cartilage is a white, smooth, fibrous connective tissue that covers the ends of bones and protects the bones as the joint moves. It also allows the bones to move more freely against each other. The articular cartilages of the knee cover the ends of the femur, the top of the tibia and the back of the patella. In the middle of the knee are menisci—disc shaped cushions that act as shock absorbers.

  • medial meniscus

    The medial meniscus is made of fibrous, crescent shaped cartilage and attached to the tibia, on the inside of the knee

  • lateral meniscus

    This is made of fibrous, crescent shaped cartilage and attached to the tibia, on the outside of the knee

  • articular cartilage

    Found on the ends of all bones in any joint—in the knee joint it covers the ends of the femur and tibia and the back of the patella. The articular cartilage is kept slippery by synovial fluid (which looks like egg white) made by the synovial membrane (joint lining). Since the cartilage is smooth and slippery, the bones move against each other easily and without pain.

In a healthy knee, the rubbery meniscus cartilage absorbs shock and the side forces placed on the knee. Together, the menisci sit on top of the tibia and help spread the weight bearing force over a larger area. Because the menisci are shaped like a shallow socket to accommodate the end of the femur, they help the ligaments in making the knee stable. Because the menisci help spread out the weight bearing across the joint, they keep the articular cartilage from wearing away at friction points.

The weight bearing bones in our body are usually protected with articular cartilage, which is a thin, tough, flexible, slippery surface which is lubricated by synovial fluid. The synovial fluid is both viscous and sticky lubricant. Synovial fluid and articular cartilage are a very slippery combination—3 times more slippery than skating on ice, 4 to 10 times more slippery than a metal on plastic knee replacement. Synovial fluid is what allows us to flex our joints under great pressure without wear.

Muscles Around the Knee

Muscles of the knee

Muscles Around the Knee (anterior view)

The muscles in the leg keep the knee stable, well aligned and moving—the quadriceps (thigh) and hamstrings. There are two main muscle groups—the quadriceps and 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 straight position. The hamstrings is a group of 3 muscles on the back of the thigh and control the knee moving from a straight position to a bent position.

The Joint Capsule

The capsule is 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 that secretes synovial fluid when it gets inflamed and provides lubrication for the knee.


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 most significant bursa and is located on the front of the knee just under the skin. It protects the kneecap. In addition to bursae, there is a infra patellar fat pad that helps cushion the kneecap.


Plicae are folds in the synovium. Plicae rarely cause problems but sometimes they can get caught between the femur and kneecap and cause pain.

Knee Arteries and Veins

Knee arteries and veins

Knee arteries and veins

Knee Function

So now we have all the parts, let’s see how the knee moves (articulates)—which is how we walk, stoop, jump, etc.  The knee has limited movement and is designed to move like a hinge.

The Quadriceps Mechanism is made up of the patella (kneecap), patellar tendon, and the quadriceps muscles (thigh) on the front of the upper leg. The patella fits into the patellofemoral groove on the front of the femur and acts like a fulcrum to give the leg its power. The patella slides up an down the groove as the knee bends. When the quadriceps muscles contract they cause the knee to straighten. When they relax, the knee bends.

In addition the hamstring and calf muscles help flex and support the knee.

Problems in the Knee

The knee doesn’t have much protection from trauma or stress (pressure or force). In addition to wear and tear on the knee, sports injuries are the source of many knee problems.


Knee symptoms come in many varieties. Pain can be dull, sharp, constant or off-and-on. Pain can also be mild to agonizing. The range of motion in the knee can be too much or too little. You may hear grinding or popping, the muscles may feel weak or the knee can lock. Some knee problems only need rest and ice, others need physical therapy (knee rehab exercises) or even surgery.

  • Swelling

    One of the most common symptoms is local swelling. There are two types of swelling. One is caused by the knee producing too much synovial fluid and the other is caused by bleeding into the joint (hemarthrosis). Swelling within the first hour of an injury is usually from bleeding. Swelling from 2-24 hours is more likely to be from the joint producing large amounts of synovial fluid trying to lubricate an abnormality inside the knee.  The best home treatment for swelling is R.I.C.E. therapy. Chronic swelling can distend the knee, prohibit full range of motion and the muscles can atrophy from non-use. Also, if the cause of the swelling is blood, the blood can be destructive to the joint.

  • Locking (or Catching)

    Locking or catching is when something is keeping the knee from fully straightening out. This is usually a loose body in the knee. The loose body can be as small as a grain of sand or as big as a quarter. The best treatment is removal of the loose body by arthroscopy. Another type of locking is when the knee hurts so bad that you just won’t use it. The best treatment here is rest and maybe some ice; swelling is not usually present.

  • Giving Way

    If your kneecap slips out of is groove for an instant, it causes your thigh muscles to loose control causing the feeling of instability—that is, you don’t feel like  your knee is stable, won’t support your weight—and you usually try to grab hold of something for support. Giving way can also be caused by weak leg muscles or an old ligament injury.

  • Snaps, Crackles and Pops

    Noises – medically termed crepitus – coming from  your knee without pain are likely nothing to worry about. Sometimes the noise is caused by loose bodies that just float around and are not causing pain or injury to the knee. However, If you have pain, swelling or loss of knee function, you should see an orthopedist. The most common cause—chondromalacia patella—is caused by an injury. Another common cause is a dislocating kneecap—that is, a kneecap that keeps slipping out of its groove. Pops without trauma (injury) are not worrisome, pops with trauma can mean ligament tears. Crackling, grinding or grating (crepitus) means there is a roughness to the bone surfaces and likely from degenerative disease or wear-and-tear arthritis (osteoarthritis).

  • Pain and Tenderness

    Where and how bad the pain is will help find the underlying cause. It also helps to know what caused it and what makes it hurt. Pain that gets worse with activity is often tendinitis or stress fractures. Pain and tenderness accompanied by swelling can be more serious such as a tear or sprain. Some pain can be caused by muscles spasms associated with trauma.

Pathological Conditions and Syndromes in the Knee

  • Osteochondritis Dissecans
  • Osteoarthritis (Degenerative Arthritis) – Caused by aging and wear and tear of cartilage, symptoms may include knee pain, stiffness, and swelling.
  • Infectious Arthritis
  • Chondromalacia Patellae – Pain from irritation of the cartilage on the underside of the kneecap; a common cause of knee pain in young people.
  • Gout – A form of arthritis caused by buildup of uric acid crystals in a joint. Sometimes the knees may be affected causing severe pain and swelling.
  • Plica Syndrome
  • Rheumatoid Arthritis – An autoimmune condition that can cause arthritis in any joint, including the knees. If untreated, rheumatoid arthritis can cause permanent joint damage.
  • chondromalacia
  • osteoarthritis

Traumatic Knee Injuries

Repetitive Knee Injuries

  • Patellofemoral Syndrome (Runner’s Knee)
  • Tendonitis
  • Bursitis (Housemaid’s Knee)
  • Illiotibial Band Syndrome
  • Osgood-Schlatter Disease

Types of Knee Surgery

In Closing

The knees are some of the most used joints of the body. Susceptible to wear and tear, these joints deserve your utmost care and concern, especially in the event of injury, trauma, and disease. If you suspect that you might be suffering from a knee problem, be sure to check with your doctor to get a definitive diagnosis and appropriate treatment.


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

  2. 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

  3. 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 ?

  4. 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….

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

  6. 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.

  7. 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.

  8. 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

  9. Helen Thomas says

    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

  10. 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

  11. 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.

  12. jitendra kumar says

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

  13. 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

  14. 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

  15. 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.

  16. shivakumar says

    my left leg bending problem, 6 years ago surgery knee

  17. 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?

  18. 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

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

  20. 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


  21. 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

  22. 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.

  23. 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….

  24. 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.

  25. 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.

  26. 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

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

  28. 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.

  29. 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.

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