To be honest that would happen to anyone getting a shin kick blocked by the knee on a frontal orientation, turning my knee into a kick (directed at the lowest rib and below) is the first thing I have always done in a fight to do exactly what happened to silva simply because the lower part of someones leg is no match for the leg from the knee to the hip.
There is just so much more bone mass inline with the contact on the defender than there is on the attacker in that type of strike/block. When attacking you try to hit with the smallest possible contact area to deliver the most concentrated load, and when you defend and you can't evade completely or parry to change the angle of attack you try to absorb the strike with as much contact area as possible to relieve the amount of PSI felt on strike through distribution.
This was a by the book attack-defend scenario where physics had it's way. He lifted his leg and if you follow the direction of the incoming attack it met full force with the block that demonstrates back-up mass (all of the defender's body mechanics at the time of the block show him aligning knee, hip, chest basically creating a wall which he also rotates into the actual point of contact so add torque as a multiplier)Amongst fighters however, the move is actually a defensive attack.
I am actually surprised that this doesn't happen more often in MMA fights as a small kick to the legs in an MMA fight is like a jab in boxing and are thrown frequently...and to be honest when ever I see a kick to the legs like this when watching a fight I say the same thing to whoever I am watching with, usually something like... "if that was me I would lift my leg and rotate my knee into the guys lower leg and let him snap his own chit like a twig"...(skipping past the friendly man to man arguments over who would be whooping whose ass, and the pessimistic dbag in the room who thinks tv is the deciding factor and says...those guys are on tv fighting so they would whoop your ass.) ... like I said before, any martial artist knows that the knee check in that fight was less defense and more offense just by judging the mechanics(which are less computational and more reaction, muscle memory and repetition) and I have to think that with Silva breaking his opponents down by using his length to weaken the opponents legs (their base) that Weidman prepared for that in his training up to this fight...all the motion going into the block appears to say he practiced it
Honestly every-time I see a leg kick in MMA fights....I pretty much see what everyone saw in the video but in my head right before the kick lands or get's blocked...I am genuinely surprised that this type of break does not happen at least once a card.
In a real life situation a leg kick should be the end of the one receiving it...as in a official MMA type fight most go in with the mentality to win a fight, not defend their life (that's what a ref is for)...and if you were defending your life and were kicking at someones leg...you would kick straight forward (less roundhouse) and put the dudes kneecap through the back of his leg in order to render him useless as fast as possible.
Either way even if Silva calls it a career, he is still a monster for what he accomplished in his prime...he struck fear in those who opposed and only had a downhill slide when his spider tactics backfired and he ate that fateful punch while dancing, which was ironic because it was those very tactics that made him dangerous in the first place. 'Live by the sword and die by the sword' has more meaning when it is double edged I suppose.
He was certainly entertaining and while he is one of the greatest MMA fighters and one of my favorite all time fighters to watch (in the long run only one other person in mma has come close to him as far as tactics and style go), I would rather see him chill out and not fight anymore than do permanent damage, people say that the bone will be stronger than ever when healed but mentally, knowing that happened and having to step back in the octagon as a fighter and a showman in front of millions is a psych-out by itself and could lead to a horrible mistake which could cause a permanent injury. He's put himself at risk long enough now for peoples entertainment and I'd rather see his style and tactics on the big screen in an action movie. Come on, can't you see Silva as a Bond supervillain's Henchman giving bond the business or something like that?