Once again, here is the original image:
Here is a magnified view of the abnormality, at the front of the knee just below the patella (kneecap):
The patellar tendon is torn. It should be a smooth black stripe, as in the normal image:
The patellar tendon is normally very strong. In fact, you can feel how stout your own patellar tendon is by placing your finger just below your kneecap while your knee is extended, then gently bending your knee back and forth a few degrees (i.e., 2-3 inches).
Hence, patellar tendon ruptures are fairly rare sports injuries. Here’s more information.
Some of the other guesses were reasonable. However, I only gave one image (out of over 100), so many of the other structures of the knee were not included. For instance, the cruciate ligaments were not fully included on this one image and they happenened to be intact. But one would have required seeing the full data set to know one way or another. There probably was also some hemorrhage in the skin and fat just anterior to (in front of) the patellar tendon tear.
If you enjoy these semi-regular radiology case presentations, please let me know. My practice is very busy, so it’s easy for me to find and post interesting case examples to NoodleFood.