Knee joint effusion is quite common after any injury to knee joint, osteoarthritis, rheumatoid arthritis, meniscal and ligaments injury. There is collection of fluid within joint, physical therapy quite beneficial in your case. 1st need ultrasonic treatment so that inflammation reduce and healing become fast.
Then complete role of exercises that includes stretching and strengthening exercises.
Next Steps
Do regular icing for reduction of effusion, at least twice a day