Due to research, the knee joint is the second joint in Brazilian Jiu-Jitsu to be injured, right after elbow joint. The reason may lie in a fact that heel hooks are forbidden in IBJJF and North American Brazilian Jiu-Jitsu Federation events.
Heel Hook is probably the most dangerous submission and that’s exactly the reason why it’s sanctioned in many jiu-jitsu competitions. If not released on time heel hook can do some severe damage to your legs.
Although heel hook is applied at the ankle the real damage comes from the rotation of the tibia on the femur that produces structural knee damage. Once the ankle is locked and the rotation starts the fibia will rotate on an almost fixed femur. Since knee joints range of motion is limited to flexion and extension there’s little to no room for any kind of rotation. It’s only about a few degrees available of internal rotation before damage will occur.
In a video below we can see a catastrophic leg break. According to a sound that the snap produced both fibia and tibula are broken and it’s not easy to tell what else broke in the knee.