Knee pain that turns out to be a proximal tibiofibular joint cyst is, indeed, fairly rare. The word proximal tells us the problem is at the knee rather than down by the ankle (since both the tibia and fibula — the bones in the lower leg — connect in both places).
A cyst is defined as an abnormal closed, bladderlike sac (membrane) containing fluid, semifluid, or semisolid matter. The fluid usually comes from a channel (pathway or connection) between the synovial fluid inside the joint and the cyst. This isn’t always the case with cysts at the proximal tibiofibular joint. The etiology (cause or reason for their formation) is not always clear. Several theories exist as to the reason why cysts of the proximal tibiofibular joint develop.
The natural history (what happens over time) for this type of cyst is a gradual increase in size over time. Left untreated, these cysts can start to put pressure on the soft tissue and nerve structures in that area. Pain that used to go away with rest becomes constant pain. Compression of the nerve and muscles can also lead to numbness, weakness, and even a condition known as foot drop (patient can no longer lift the foot properly when walking). In the worst case scenario, the cyst may hemorrhage adding insult to injury.
Recommended treatment is surgical removal of the cyst and any connection to the joint. This is best done before nerve involvement develops. Some physicians have used aspiration (withdrawing fluid from the cyst with a needle) with steroid injection. But studies show this approach has a high recurrence rate. Even with surgical removal of the cyst and its connections, there is still a high rate of recurrence. Up to one-third of all patients with this type of cyst experience another cyst forming later.
Fortunately, a tibiofibular joint cyst is fairly benign if diagnosed and treated early on (before nerve damage). Recurrence is disappointing but still treatable. Getting a second opinion is always a good idea for something as rare as a tibiofibular joint cyst. Making sure you have an accurate diagnosis is essential in planning the best treatment approach. If your diagnosis is confirmed by your outside expert, you will likely be advised to pursue surgical excision right away. Follow your physician’s advice for the best outcomes.