During a physical exam, the doctor will look for a soft mass in the back of the knee. If the cyst is small, comparing the affected knee to the normal knee can be helpful. There may be limitation in range of motion caused by pain or by the size of the cyst. In some cases there will be signs and symptoms of a meniscal tear.
Transillumination, or shining a light through the cyst, can demonstrate that the mass is fluid filled.
If the mass demonstrates any abnormal signs, like rapid growth, night pain, severe pain, or fever, a more involved work-up is indicated to rule out non-cystic tumors that can grow in the back of the knee.
X-rays will not show the cyst or a meniscal tear but will show other abnormalities that may be present including arthritis.
MRI can be helpful to visualize the cyst and to demonstrate any meniscal injury.
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