Possible causes for pain in the back of the knee include:

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- Osteoarthritis: This is a wear-and-tear condition that wears down the cartilage and meniscus of the knee over time. Patients with this typically develop the pain over time, and are in their late 50's to 60's. Patients report feeling pain, soreness and tightness behind the knee. They frequently feel this at night, and it is worse with more walking.

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- Meniscus tears: The meniscus is a shock-absorbing cushion in the knee. Tears of the meniscus can cause patients to feel severe pain at the back of the knee. Frequently, patients are in their 40's to 50's, and many patients start having this pain after some event, for example a period of intense exercise or going on a holiday.

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- Baker’s cyst: A Baker’s cyst is a collection of fluid that builds up at the back of the knee. Baker's cysts may cause discomfort and swelling at the back of the knee, or many times they may not cause problems at all.

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- Muscle strain/injury: This would be more clearly related to an episode of exercise. The muscles at the back of the knee include the hamstrings and the gastrocnemius. Muscle strains would typically get better on their own within a few days, and if such pain persists, a medical evaluation is needed. 

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- PCL tear: The PCL is a ligament behind the knee. Again, the patient would usually have had a significant injury such as a sporting injury or road traffic accident.

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- Other causes: Various other conditions can cause pain at the back of the knee, including blood clots in the veins and serious causes such as a tumour. The important thing is not to self-manage for beyond a short period of time and seek medical attention early.