Large lump on back of knee – orthopedics forum – ehealthforum electricity quotes by benjamin franklin


These are benign cysts filled with synovial (joint) fluid, essentially a ganglion cyst. There is a bursal sac in the back of the knee, just under the gastrocnemius tendon, which communicates with the inside of the knee joint. However, the stalk of the bursa acts like a one way valve, thus, the synovial fluid in the knee can get into the sac, but not back out, so it can fill up.

In kids, these usually go away on their own, without any treatment, though it may take a while. In the majority of cases, they do not cause the child any problems at all. A small proportion of kids will say that they have some dull aching occasionally associated with them. But, if the child has not given up any usual activities, then it is probably not affecting him/her to a significant degree.

Most of the time, these masses have a very characteristic appearance, and really do not need any further evaluation. But, if there is any concern, then an ultrasound of the mass can be obtained. In very rare cases, an MRI can be done. Again, the cyst will have a very characteristic appearance on these studies.

If there is still concern about what the mass is, then an aspiration of the cyst can be done. However, in small children, who do not tolerate being stuck very well, they would have to be sedated. This is because of the main blood vessel and nerve in the back of the knee are very close to the cyst. Any sudden movement while the needle was in place could cause injury to the neurovascular bundle. Aspiration of the cyst will produce the characteristic “apple jelly” like fluid, which confirms the diagnosis.

The cyst itself rarely needs to be surgically removed. Mainly because almost 100% of these will go away on their own. And, in kids, if the mass is excised, it has a tendency to just come back, till it decides on its own that it is time to go away.

Baker’s cysts in children are different critters from Baker’s cysts in adults. In children, these are almost always independent cysts, not associated with anything else. But, in adults, the formation of a Baker’s cyst is often associated with some type of internal derangement of the knee. Many adults with Baker’s cysts have torn meniscal cartilages, degeneration of the articular cartilage, or have an inflammatory arthropathy (such as rheumatoid arthritis).

However, there are very rare cases in children of different causes of masses in the back of the knee. There are other benign conditions, such as an osteochondroma, lipoma, fibroma, etc; each of which has a characteristic appearance. And, unfortunately, in extremely rare cases, there are malignant tumors which can show up as masses around the back of the knee. Most of the time, malignant tumors are quite painful, with the patient giving up favorite activities due to the pain.

So, you should probably have your son checked by an orthopedic surgeon, just to be sure. This will put your mind to ease, that if it is anything that needs to be treated, you will have had it checked out. But, again, the vast majority of these (given your almost classic description of the “golf ball”) turn out to be a Baker’s cyst.