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You probably have ligament injury
. Ideally MRI
should also be done to see what structures in the knee have been damaged. Since Doctor has given you plaster and pain killers, do follow the advice. Do not use that leg. It may take a while to get fully alright as the ligament may take 4-6 weeks to heal. Better to wait and watch as I cannot examine you from here. Maybe a partial ligament tear which will heal soon. Are you having pain on the outer part of knee.