What Causes Swelling And Inability To Straighten Leg After Arthroscopic Surgery?

Had arthroscopic surgery 8 weeks ago for torn meniscus; still can't entirely straighten my knee/leg and have some swelling; surgeon sent me to do physical therapy 2 weeks ago; still doing exercises but little improvement. 3 weeks after surgery the dr. removed 50cc's of fluid the swelling was so bad. Do I need to have an MRI to figure out why my knee is like this? Could the dr. have damaged something else during surgery?