Hi there,
I am sorry to hear that your symptoms are getting worse.
Do you have any other symptoms such as do you ever feel that your knee is slightly unsteady/unstable or not able to support you?
Does it ever lock into a position?
You say it is the back of the knee, is it more one side than the other or is it the middle?
I would be interested in hearing the answers to these questions and the results of the MRI scan.