What Causes Sharp Burning Pain In Knee?

I am having sharp, burning pain in my left knee when I am on my knees. This is the only time it hurts, though recently the pain lingers when I get up. It is starting to throb some. Again the pain is triggered when I am put my knee on any surface. No injury that I am aware of. I can walk fine and have no discomfort or pain from activity, just when I kneel feels like someone is shoving a knife in my left knee.

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