I am a frequent poster to letsRun.com and I just thought I'd share my story here in case or someone has this injury and happens to Google or do a LetsRun.com search for ganglion cysts adjacent to the Anterior Cruciate Ligament (the ACL).
The story in short: I've had trouble bending my knee on my runs for the past 10 months. I am a pretty serious runner - I was running about 80mpw before this problem and went to about 15 doctors in USA - some orthos and some physiatrists - and had 3 MRIs on it - because of this. One of the first times it became problematic was when I tried to go for a 13 miler with the last 4miles at half marathon pace and I had to cut the workout short as I struggled to flex the knee back. I jogged my way home limping slightly. I saw doctors but they didn't know what was wrong. The first doctor I saw (a local yocal ortho) just told me it was tendinopathy and that I needed to back off. I continued to run on and off through it. Months later, the knee was so bad that on an easy 10miler I ended up unable to bend my knee more than about 20 degrees without excruciating pain and having to just walk home 4-5 miles into the run. Since I was already miles from my house this really sucked. I wasn't getting through to the doctors that this kind of thing just doesn't happen to a serious distance runner without some kind of explanation. Despite 3 MRIs (each of which showed a ganglion cyst of the ACL), out of the 15 doctors I saw, only 2 of them (one of them suggested to me by someone on LetsRun.com and the other a top50 in the USA knee surgeon) came to a conclusion that the ganglion was the problem.
It was especially frustrating because when I visited most doctors, the main thing that would happen to me is that I would explain to them that I have an inability to flex the knee on distance runs and I'd be on the examiner's table, and the doctor would then do a standard tests checking my ligaments (such as Lachman test), not find anything, basically tell me things are fine with my knee, and send me on my way.
One doctor immediately pointed out the problem is the cyst. When I went to any 2nd get opinions on this though, this was often what I was told:
Doctors: "Those are almost always asymptomatic."
Radiologists: "Yes we do make note of them in our reports, but they are incidental findings."
The cyst has increased in size over the past 10 months. 1st MRI it was 1cm, 2nd it was 1.6cm, 3rd it was significantly larger.
I found a couple cases in the med literature that show these can cause problems in some cases, but most of them are from outside the US.
The bottom line is this - do not assume that if you have a ganglion cyst on your MRI that it is asymptomatic. This may be the case but not always. Doctors need to be aware that they can sometimes be symptomatic because (as I understand it) these things can change size and place during athletic activity/movement and press on things, causing pain and other problems.
Quote from http://smj.sma.org.sg/5011/5011me1.pdf
"Ganglion cysts of the cruciate ligaments are usually asymptomatic, and are often hard to diagnose clinically due to the lack of specific symptoms and signs. Symptomatic patients may present with pain aggravated with flexion or athletic activity, increased pain during posture change, locking, clicking or popping sensations, and a decreased range of motion.(1,8-13) "