Baker’s Cyst is a fluid filled sac that develops behind the knee.  The fluid originates from the synovial joint of the knee.  Often the cyst will arise after trauma to the knee and may go unnoticed for some time.  Patients with Baker’s Cyst often have swelling and pain behind the knee.  The cyst will become more prominent when the knee is  flexed.  Patients with Rheumatoid Arthritis are more prone to the development of Baker’s Cyst.  These cysts can rupture spontaneously or during squatting.  MRI scans and Ultrasound of the knee are useful to confirm the diagnosis.  Treatment of Baker’s Cyst begins with conservative modalities such as rest, alteration in activites, ice, physical therapy and medications such as non-steroidal anti-inflammatories.  Your doctor may try to drain fluid from the cyst or inject the cyst with a steroid.  Occasionally, surgery may be necessary to remove the cyst or treat any underlying cause such as a meniscal tear of the knee which may be causing the problem.