What Causes Swelling Around The Knee Cap After Accidentally Twisting The Knee?

Hi, I was playing soccer yesterday, and I ran forward, went to turn and my foot got stuck and twisted my knee and I heard a big crack. Was difficult to walk on it straight after, about half an hour after it was impossible to walk on. Now i can put pressure on the leg with no pain but cant fully extend it or close it. There is a little bit of swelling around the knee cap. I just want to know what you may think, and if you think i need an operation. Thanks