Pain in Back of Knee When Straightening Leg

Pain behind the knee (posterior pain)

Expert reviewer, Mr Damian McClelland, Trauma and Orthopaedic Consultant, and Clinical Director for Musculoskeletal Services at Bupa
Next review due June 2025

Pain behind the knee is also called posterior knee pain. The word 'posterior' means behind. As well as pain, you may have some swelling. This may be just at the back of your knee, or it can go up into your calf. The swelling may be bad enough to stop you bending your leg properly.

Types of pain behind the knee

There are a number of health conditions that may result in pain behind your knee. Two common conditions that cause it are a:

• posterior cruciate ligament injury
• popliteal cyst, also called Baker's cyst

A posterior cruciate ligament injury can happen if you overstretch or tear this ligament, which runs across your knee from your thigh to your shin bone. It often results from a heavy blow to the front of your knee while it's bent. This can happen if you hit your knee on the dashboard during a car accident, or over-straighten your leg and bend your knee backwards. Doctors call this hyperextension.

A cyst is a collection of fluid or material inside a thin layer of tissue. A popliteal cyst is a cyst in the shallow pit at the back of your knee. It's often linked to other conditions that affect the knee, such as osteoarthritis, rheumatoid arthritis, or cartilage injuries. If you injure your knee, it can cause a collection of fluid to develop within your knee. Sometimes you can feel this in the depression at the back of your knee.

Osteoarthritis of the knee is another common cause of knee pain. The smooth, shiny cartilage that lines your knee joint becomes worn and rough. This causes pain and damages your knee over time. It mostly affects people over 50. The older you are, the more likely you are to get it.

Causes of pain behind the knee

There are a number of causes of pain behind the knee, which include:

• your knee joint wearing down as you get older – for example, you may have osteoarthritis or rheumatoid arthritis
• a knee injury (ligament injury or torn cartilage)
• a benign or cancerous growth (this is very rare)
• an infection (septic arthritis)

Symptoms of pain behind the knee

Symptoms of pain behind the knee will vary, depending on the cause. You may have swelling or pain when you move your knee a certain way.

If you have a posterior cruciate ligament (PCL) injury, you'll probably have injured other parts of your knee too. But if you've only injured your PCL, you may just feel a bit of discomfort. You may feel pain behind your knee, especially when you kneel. If your symptoms don't improve, it may be uncomfortable going down an incline, for example walking or running downhill, or going down stairs.

A popliteal cyst causes swelling and sometimes pain at the back of your knee, which you'll probably notice. If you have a very large swelling, it may stop you from fully straightening your leg. The swelling may come and go, and get worse or better over time. If your cyst bursts, you may hear a pop and feel warmth spread down your calf. It may start to look red or bruised anywhere from the back of your knee down to your ankle and the top of your foot.

If you have a swollen, tender calf, it's very important to see a doctor. The swelling can also be caused by a clot in your leg (deep vein thrombosis), which you will need urgent treatment for.

Osteoarthritis usually causes pain when you bear weight on it, and gets better when you rest. Your knee may be stiff and you might not be able to move it as well first thing in the morning, or after you sit for a while. This usually eases once you start moving around. You may also have some swelling over your knee.

Diagnosis of pain behind the knee

Your doctor will examine your knee and ask about your medical history and symptoms, including:

• the type of pain you have, when it started and whether it comes and goes
• how active you are
• any activity, accident or injury that could have caused it

If they suspect a posterior cruciate ligament injury, they may refer you to have an X-ray or a magnetic resonance imaging (MRI) scan. If you have signs of a popliteal cyst, your doctor may suggest you have an ultrasound scan.

Self-help for pain behind the knee

If you injure your knee, you should follow the POLICE and HARM procedures.

• POLICE stands for protect, optimal loading, ice, compression and elevation. These are things you should do to help your symptoms.
• HARM stands for heat, alcohol, running and massage. These are things that you should avoid for a bit so you don't damage your knee further.

If you can't put weight on your leg, you may need crutches.

Popliteal cysts often get better on their own and you may not need any treatment. But it's a good idea to see a doctor if you have pain behind the knee. It may be a sign of something more urgent (such as a blood clot in your leg). If you have a posterior cruciate ligament injury, you can develop complications later if you don't get medical help for it.

You should see a doctor if:

• you cannot put weight on your leg
• you have severe pain, even when you don't put any weight on it
• your knee buckles, clicks, or locks
• your knee looks deformed or misshapen
• your knee is hot, red or very swollen or you have a fever
• you have pain, swelling, numbness, tingling, or a bluish discoloration in your calf

Treatment of pain behind the knee

Treatment for pain behind your knee will depend on what condition is causing the pain.

For information on how to treat pain behind the knee, please see the relevant page for each type of knee condition. For example, treatment for:

• posterior cruciate ligament injury
• osteoarthritis

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Pain in Back of Knee When Straightening Leg

Source: https://www.bupa.co.uk/health-information/knee-clinic/explore-knee-pain/pain-behind-knee

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