Thank you for your reply.
I am sorry for not getting back to you sooner. I was already logged off when you replied.
The most likely cause of knee instability is an injury or tear to one of the ligaments which holds the knee together. If you are a frequent runner, you could have damaged a ligament due to constant use of the knee (we often see this type of injury in runners due to use of the joint in a repeated fashion).
You will need to see an orthopedic doctor and have MRI of the knee done (x-rays will only show bones and soft tissue, MRI will look at ligaments and muscles). Many ligament injuries are treated with ice, anti-inflammatory medications (Motrin, Advil, Aleve) and rest. Sometimes physical therapy may be needed and if the tear is severe, you might need arthroscopic surgery to repair it.
Until you see your doctor, I would suggest using a knee brace (to improve stability) and to ice the affected knee. Also I would suggest not running on it until further examination is done (to avoid further damage).
I hope you find this information helpful. Please feel free to follow-up with any other questions you might have.
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Best wishes and good luck,