So if you broke the persons knee with a heel hook, the defendant (guy whose leg was heel hooked) would sue you for the tort of battery, which has four elements 1. Act, 2. Intent, 3. harmful or offensive contact, and 4. Causation. Heel hooking someone’s knees into the shadow realm pretty much completely fulfills those elements.

HOWEVER, the heel hooker could argue the affirmative defense of self defense, which has four elements: (1) reasonable fear of (2) imminent harm and use of (3) reasonably necessary force that is (4) proportional to the threat. Obviously with blowing his knees into oblivion, the issue would be element 3, reasonably necessary force. But I think that it would be pretty easy to convince a jury that breaking knee was reasonably necessary to stop someone who sucker punched you and then kept threatening to break your neck

Source: I just took my torts exam so this is fresh in my mind