weidman didn't use a knee spike on the leg check that anderson broke his leg throwing. i'll have to go back and see earlier leg checks. he might of used it then, but not on the leg check that the injury happened on.
if you go back and look at the check that weidman used, it was entirely a defensive leg check. he brought his leg up and blocked anderson's kick with his chin and not knee.
i've been training in muay thai for seven years. i've used my knee to check kicks and to properly do so, you move your knee down and into the chin of the person throwing the kick. i did it in my most recent MMA fight when my opponent landed a few very hard leg kicks on me. after that, he didn't throw very many more low leg kicks.
it did look like weidman did work very hard on checking anderson's leg kicks. he did a great job of it. anderson's break was entirely a bad luck injury. maybe 1 in 100,000 if that.
weidman said that he had training partners breaking their legs in training due to checking kicks. i don't buy it. it's very common to get a bone bruise or hematoma on your shin from someone checking your kick, but breaks are extremely rare.
also, checking a leg kick with your knee is not dirty. it's a legitimate way to check leg kicks and is an advanced muay thai move that is used effectively by muay thai fighters. it's part of the game.