Sudden pain in one of the knees is usually the result of overusing the knee or suddenly injuring it.
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.
Common causes of knee pain
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.
You can prevent future knee pain by:
- always warming up before exercising and stretching to cool down after exercise
- increasing your activity levels slowly over time
You can also try low-impact exercises, such as swimming, to improve your health and fitness without harming your knee joint.
Anterior knee pain syndrome
Knee pain felt at the front of the knee, around the kneecap, is called anterior knee pain syndrome. The cause is not understood, but it is usually made worse by sitting for prolonged periods or by climbing stairs.
Damage to the meniscus
Sitting between the upper and lower leg bones at the knee joint are rubbery pads of tissue called meniscus. These cushion the bones, acting as shock absorbers.
The meniscus can become worn as you get older, and are commonly the reason for knee pain in middle-aged people.
A meniscus can also be torn after suddenly twisting the knee joint, resulting in pain, swelling and, occasionally, locking of the knee. An operation is sometimes needed to repair the torn pad of tissue.
In older people, repeated attacks of knee pain are likely to be a sudden worsening of osteoarthritis, the most common type of arthritis. 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.
You should see a doctor if you think the cause of your knee pain is osteoarthritis.
Less common causes of knee pain
Overusing or injuring the tendon that connects the kneecap to the shin bone can cause patellar tendonitis (inflammation of the tendon). This condition is sometimes called “jumper’s knee”, as it can be brought on by jumping activities such as basketball or volleyball. The area may be swollen, red and warm.
Repetitive movement of the knee or kneeling for long periods can cause a build-up of fluid over the knee joint, known as bursitis or “housemaid’s knee”.
Housemaid’s knee tends to affect people with certain jobs that involve kneeling (such as carpet layers), or sports players (such as footballers).
Torn ligament or tendon
Knee pain may be caused by torn ligaments or tendons. Ligaments are tough bands of tissue that connect the bones at the knee joint; tendons connect the muscles to the bone. You can tear these tissues during running sports such as rugby or football.
Injured tendons or knee ligaments at the side of the knee may cause pain even when the knee is at rest, which may get worse when you bend the knee or put weight on it. There may also be warmth and swelling around the knee.
If you feel that your knee is also unstable or keeps “giving way”, you may have torn the anterior cruciate ligament (one of the main knee ligaments). This probably resulted from a sudden change in direction or a twisting movement, and you may have heard a pop when it happened. You should see a doctor if this happens, and you may be referred to an orthopaedic specialist for advice and treatment.
Bleeding into the 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 see a doctor immediately to have your knee treated if you have a very swollen knee.
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.
If the symptoms do not improve with time, you may need to see a doctor.
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 a doctor 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 pseudogout (see above). You should see a doctor urgently 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 a doctor
You should see a doctor 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
The doctor 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 an arthroscopy – a form of keyhole surgery that is used to look inside a joint and repair any damage that has occurred.