I Have Been Experiencing Knee Pain For The Past Month

I have been experiencing knee pain for the past month or two. Saw the orthopaedist after my primary doctor ordered xrays, blood work and an MRI. According to the test there was nothing found as to a reason for the pain. The doctor gave me a shot of Marcaine and said it would take a couple of days to be pain free. What is this medicine and what does it do.