I woke up two mornings ago and noticed a large lump on the back of my knee. It's about 1 and a half inches long and about 1 inch wide. I have just been watching it to see if it will go away, so far, it has not. I did a vigorous work out the night before I got the lump, I don't know if that matters. Anyone know what it could be?
It is most likely a Baker's cyst. There is a large bursa in the back of the knee around the head of the gastronemius muscle, which communicates with the knee joint cavity. If there is any swelling in the joint it can leak into the cyst. This causes the back of the knee to feel full, or even have a lump.
A Baker's cyst in of itself is benign, what you need to find out is what is going on in the knee to cause the swelling. You may not notice any swelling in the knee, per se, because it has leaked into the cyst.
There are, of course, other rare causes of a lump in the posterior knee. An aneursym would have a pulse in it. A tumor is usually very firm, slow growing, and painful. There are also some skin cysts, such as an epidermal inclusion cyst, that could cause a lump.
If you are concerned about it, you should see your physician. Good luck.
Without an examination it is not possible to tell you the exact cause. But, that said, the most common cause of discrete lumps in the popliteal fossa in children is that of a Baker's cyst.
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.