I have noticed a bump on the outside part of my knee cap when my knee is in flexion. No surgery on knee. Sprained my ACL two years ago and an ultrasound on the knee two years ago shows cartilage missing under kneecap. I do notice swelling underneath the kneecap as well and a sharp pain similar to runners knee.
Dear patient What is your age? If you are young age possibility are 1. Discoid meniscus 2.  Meniscus cyst 3. Pigmented VN synovitis. If you are elderly knee effusion with osteophytes can be diagnosis. I would like to advise MRI of the involved knee as it helps in confirming one diagnosis and rules out other possibilities.  Visit radiology center nearby you and get it done.  You need to consult expert arthroscopic surgeon with report. All the best.