Your Body

Understanding the Knee


Knee Pain

Sudden pain in one of the knees is usually the result of overusing the knee or suddenly injuring it. In many cases, you don’t need to see your GP however if you are not recovering make an appointment to see you doctor.

The knee joint is particularly vulnerable to damage and pain because it takes the full weight of your body and any extra force when you run or jump. This is why you are more susceptible to knee pain if you are overweight.

This page specifically covers the most common and more unusual causes of knee pain.

Pain in more than one joint may be caused by arthritis, which is covered separately.

Common causes of knee pain

Simple strain

If you think your pain is the result of having done more activity than you’re used to, you’ve probably just strained the knee. This means that the knee tissues have stretched, but are not permanently damaged.

Patellofemoral Pain Syndrome (PFPS)

The term Patellofemoral Pain Syndrome (PFPS) is commonly used by practitioners to describe pain affecting the anterior (front) aspect of the knee and more specifically the patella (kneecap) and the surrounding area.

The primary cause is poorly understood, but it is widely agreed amongst clinicians to be multifactorial in nature. A study by Witvrouw (2005) classified PFPS into two main categories, Mal-alignment and Muscular dysfunction.

PFPS is one of the most commonly assessed and treated knee conditions presenting to General practitioners and sports medicine clinics. According to a systematic review by Bizzini (2003) it accounts for approximately 25% to 40% of all identified knee injuries. Patellofemoral pain (PFP) is particularly prevalent in the young active population from ages 15 – 30 and has been highlighted by Boling (2010) as having a greater incident rate in females; who were 2.23 times more likely to develop PFPS compared to their male counterparts.


Damage to the menisci

The medial and lateral menisci are shock absorbers and force distributors located between the femur and the tibia. Consequently, menisci can tear due to traumatic injury or degenerative wear, and can compromise force distribution across the knee joint, increasing force concentration on the cartilage and other joint structures.

Damage to the meniscus due to rotational forces directed to a flexed knee (as may occur with twisting sports) is the usual underlying mechanism of injury.

Tears produce rough surfaces inside the knee, which cause catching, locking, buckling, pain, or a combination of these symptoms. Abnormal loading patterns and rough surfaces inside the knee, especially when coupled with return to sports, significantly increase the risk of developing arthritis if not already present.

Traumatic meniscal tears most commonly occur as said before, during twisting sports such as football, rugby, and basketball, but skiers, runners, and tennis players are also at risk. Older patients can tear their meniscus during normal activities of daily living, usually as a consequence of aging and degenerative wear of the knee joint.

The degree of vascular penetration (blood supply) into the periphery of the meniscus ranges from 10% to 25% of the meniscal width. Consequently, most areas of the meniscus cannot heal by themselves because they are not vascularized.

Additional risk factors include occupations that stress the joint (e.g., construction work and manual labor jobs that involve knee flexion while lifting heavy objects), malalignment of the knee joint, previous ligament injury (especially anterior cruciate ligament injury), and knee instability. Increased friction from various sporting turfs has been suggested as potential risks that may lead to a meniscal tear. Poor ground or weather conditions increase the likelihood of slips, falls, and improper landings, further increasing the risk of meniscal tears.


In older people, repeated attacks of knee pain are likely to be a sudden worsening of osteoarthritis, the most common type of arthritis in the UK. Osteoarthritis causes damage to the articular cartilage (protective surface of the knee bone) and mild swelling of the tissues in and around the joints.

A painful fluid-filled swelling may develop at the back of the knee as a result of osteoarthritis – this is known as a Baker’s cyst, or popliteal cyst.

Osteoarthritis can sometimes affect younger people, especially those who are overweight or have had serious injuries to the knee in the past.


Patella Tendonitis/Tendinopathy

What is Patella tendonitis

Patellar tendinitis is an injury that affects the tendon connecting your kneecap (patella) to your shinbone. The patellar tendon plays a pivotal role in the way you use your leg muscles. It helps your muscles extend your knee so that you can kick a ball, run uphill and jump up in the air.

Patellar tendinitis is most common in athletes whose sports involve frequent jumping — such as basketball and volleyball. For this reason, patellar tendinitis is commonly known as jumper’s knee. However, even people who don’t participate in jumping sports can experience patellar tendinitis. The Frequent stress placed on the patellar tendon results in tiny tears in the tendon, which your body attempts to repair. But as the tears in the tendon become more numerous, they cause pain from inflammation and a weakening of the tendon structure. When this tendon damage persists over more than a few weeks, it is called tendinopathy.


Contributing factors

A combination of factors may contribute to the development of patellar tendinitis, including:

Intensity and frequency of physical activity. Repeated jumping is most commonly associated with patellar tendinitis. Sudden increases in the intensity of physical activity or increases in frequency of impact activity also put added stress on the tendon.

Tight leg muscles. Reduced flexibility in your thigh muscles (quadriceps) and your hamstrings, which run up the back of your thighs, could increase the strain on your patellar tendon.

Muscular imbalance. If some muscles in your legs are much stronger than others, the stronger muscles could pull harder on your patellar tendon. This uneven pull could cause tendinitis.


Pain is the first symptom of patellar tendinitis. The pain usually is located in the section of your patellar tendon between your kneecap (patella) and the area where the tendon attaches to your shinbone (tibia).

The pain in your knee may:

Initially be present only as you begin physical activity or just after an intense workout.  Progress to the point that it interferes with your sports performance.  Eventually interfere with daily tasks such as climbing stairs or getting up from a chair.


Housemaid’s knee

Prepatellar bursitis (also known as beat knee, carpet layer’s knee, coal miner’s knee, housemaid’s knee rug cutter’s knee, or nun’s knee) is an inflammation of the prepatellar bursa at the front of the knee. It is marked by swelling at the knee, which can be tender to the touch but which does not restrict the knee’s range of motion. It is most commonly caused by trauma to the knee, either by a single acute instance or by chronic trauma over time. As such, prepatellar bursitis commonly occurs among individuals whose professions require frequent kneeling.

A definitive diagnosis of the condition can usually be made once a clinical history and physical examination have been obtained, though determining whether or not the bursitis is septic is not as straightforward.

What is a Bursa

A bursa (plural bursae) is a small fluid-filled sac lined by synovial membrane with an inner capillary layer of viscous fluid (similar in consistency to that of a raw egg white). It provides a cushion between bones and tendons and/or muscles around a joint. This helps to reduce friction between the bones and allows free movement. Bursae are filled with synovial fluid and are found around most major joints of the body.

Signs and Symptoms

The primary symptom of prepatellar bursitis is the swelling of the area around the kneecap. It generally does not produce a significant amount of pain unless pressure is applied directly to the swelling.[4] The area of swelling may be red (erythema), warm to the touch, or surrounded by cellulitis (localized or diffuse inflammation of connective tissue), particularly if the area has become infected. In such cases, the bursitis is often accompanied by fever. Unlike arthritis, prepatellar bursitis generally does not affect the range of motion of the knee, though it may cause some discomfort when the knee is completely flexed.


Prepatellar bursitis is caused by either a single instance of acute trauma to the knee, or repeated minor trauma to the knee. The trauma can cause extravasation (leaking of fluid) of nearby fluids into the bursa, which stimulates an inflammatory response.  Other possible causes include goutsarcoidosisCREST syndromediabetes mellitusalcohol abuseuremia, and chronic obstructive pulmonary disease.  Some cases are idiopathic (arising spontaneously), though these may be caused by trauma that the patient does not remember


A diagnosis of prepatellar bursitis can be made based on a physical examination and the presence of risk factors in the person’s medical history


Torn ligament or tendon

There are also four ligaments around the knee joint. A ligament is a tough strip of connective tissue that joins one bone to another bone around a joint. The knee joint ligaments help to stabilise and support the knee when it is moved into different positions.

Each ligament has a different job to do: (see video link)

The anterior cruciate ligament (ACL) is one of the ligaments inside the knee joint. It runs diagonally connecting the anterior (front) of the tibia to the posterior (back) of the femur. This ligament helps to stabilise the knee joint by controlling backward and forward movements of the knee. It stops the tibia bone from moving forwards in front of the femur.

The posterior cruciate ligament (PCL) is the other ligament inside the knee joint. It also runs diagonally across the knee connecting the posterior (back) of the tibia to the anterior (front) of the femur. The ACL and PCL cross each other inside the knee joint and some people call them the cross ligaments. The PCL helps to control the forward and backward movements of the knee.

The medial collateral ligament (MCL) is one of the ligaments on the outside of the knee joint. It runs between the femur and the tibia on the inner side of the knee. It helps to protect and stabilise the knee joint against any blows or forces that may be directed on to the outer side of the knee. It helps to limit the amount that the knee moves from side to side

The lateral collateral ligament (LCL) is the other main ligament on the outside the knee joint. It runs between the femur and the fibula on the outer side of the knee. It helps to protect and stabilise the knee joint against any blows or forces that may be directed on to the inner side of the knee. This ligament also helps to limit the amount the knee moves from side to side.


What is a knee ligament injury?

The ligaments around the knee are strong. However, sometimes they can become injured. They may be sprained (stretched), or sometimes ruptured (torn). A ligament rupture can be partial (just some of the fibres that make up the ligament are torn) or complete (the ligament is torn through completely). The majority of knee ligament injuries are sprains and not tears and they tend to settle down quickly.


Anterior cruciate ligament injury

ACL injury most often occurs during sports such as football, basketball, skiing and tennis. The injury often happens if you land on your leg and then quickly pivot or twist your knee in the opposite direction.

Women are more likely to injure their ACL than men although it is not certain why this is. About half of people with an ACL injury also have injury to their meniscus or another ligament in the same knee.

Posterior cruciate ligament injury

Injury to this ligament is not as common as an ACL injury. This is because the PCL is wider and stronger than the ACL. There are a number of ways that the PCL can become injured. For example, it may be injured during a car accident if the front of your bent knee hits the dashboard. It may also be injured from falling on to your bent knee. Your PCL can also be injured if your knee is hit from the front whilst your leg is stretched out in front of you with your foot on the ground – for example, during a game of football.

At first, some people with a PCL injury may not have much in the way of symptoms (see below). It may take a while for you to realise that there is a problem. For example, you may later notice pain that comes on when going up and down stairs or when starting a run; or, your knee may feel unstable when walking on uneven ground.

Medial collateral ligament injury

Injuries to the MCL can happen in almost any sport and can affect people of all age groups. They often happen when your leg is stretched out in front of you and the outer side of your knee is knocked at the same time – for example, during a rugby or football tackle.

Lateral collateral ligament injury

Injury to the LCL is less common than injury to the MCL. This is because your other leg usually protects against injury to the inner side of your knee. (It is usually a direct blow to the inner side of your knee that causes an LCL injury.) But, this ligament injury can sometimes happen if one leg is stretched out in front of you and doesn’t have the other leg for protection – for example, during a rugby or football tackle.

How is a knee ligament injury diagnosed?

If you feel that you may have a knee ligament injury, you should see a doctor/physiotherapist. Your doctor/physio will usually start by asking you questions about the injury, what happened and where you are feeling any pain. They may then examine your knee to look for signs of swelling of your knee joint. They may also move your leg into different positions to test your knee ligaments. From this examination, they may be able to get an idea of the possible injury that you have. If your doctor is concerned that you may have a more serious knee ligament injury, they may refer you for further tests such as an ultrasound scan or an MRI scan. Such tests should be able to show up any tears or rupture of your ligaments


Bleeding into the knee joint

An injury that causes significant damage to the knee joint may cause bleeding into the joint spaces, known as haemarthrosis. This can happen if a cruciate ligament is torn or if there is a fracture to one of the bones of the knee.

Signs of haemarthrosis are swelling of the knee, warmth, stiffness and bruising. You should go to hospital immediately to have your knee treated if you have a very swollen knee.

Osgood-Schlatter’s disease

Swelling and tenderness over the bony bump just below the kneecap is known as Osgood-Schlatter’s disease.

This is a common cause of knee pain and swelling in teenagers, particularly teenage boys who sprain or overuse their thigh muscles when playing football or other sports.

Mild cases usually settle with rest and taking anti-inflammatory medication. Severe cases may need referral to a specialist.


Gout and pseudogout

If the knee joint is also hot and red, the cause is likely to be gout or pseudogout, which are types of arthritis.

Gout is caused by a build-up of uric acid in the body. Uric acid is a waste product that is produced during the process of metabolism (when the body breaks down food to use as energy). Usually, uric acid is excreted by the kidneys.

People whose kidneys do not excrete uric acid properly, or those who produce too much, can have high levels of uric acid in their blood. If the level becomes very high, crystals form in the joints. The crystals cause the joints to become inflamed and painful.

Gout will cause severe pain in the knee, limit movement of the joint and may cause a slight fever.

Usually, gout affects the joint of the big toe first, before it affects the knee joint or any other joint.

Pseudogout is a similar condition to gout in that crystals of calcium are deposited in and around the joint. But unlike gout, pseudogout can affect the knee joint first.

You should see your GP if you think the cause of your knee pain is gout or pseudogout.


Septic arthritis (infected knee)

Septic arthritis is a serious condition that causes a very painful, hot, swollen knee. You may also have a fever and any movement of the knee will be very painful.

It can be mistaken for gout or pseudo gout (see above). You should see your GP urgently, or go to accident and emergency (A&E) if you suspect you have septic arthritis.

Septic arthritis is treated with antibiotics and surgery is often needed to clean out the infection.

When to see your GP

You should see your GP if:

  • you cannot put weight on your knee at all
  • you have severe pain even when you’re not putting weight on it
  • your knee locks or painfully clicks (painless clicking is OK)
  • your knee keeps giving way
  • your knee looks deformed
  • you have fever, redness or heat around the knee, or it is very swollen
  • you have pain, swelling, numbness or tingling of the calf beneath your affected knee the pain is still severe after three days of caring for your knee at home

Your GP will do a careful examination of the knee and take your medical history.

Investigations may include blood tests, an X-ray (if a fracture is suspected) or an MRI scan. Treatment may involve physiotherapy, painkillers and sometimes anarthroscopy – a form of keyhole surgery that is used to look inside a joint and repair any damage that has occurred.