well everybody broke the Leg on the same place
Warning: long post ahead!
That particular area is not really meant to be slammed into a hard object (shin, elbow, knee would be a hard object), and therefore it is suseptable breaking.
There was a show, either on Natl' Geographic or one of the sports stations that was covering athletes, and seemingly "inhuman" feats, and scientifically analyzing how these athletes are able to perform such feats.
They did a great segment on martial artists who do competitive breaking where they break bricks, cinderblock slabs, ice, wood, and so forth with various different parts of their body. Anyhow, these giant dudes were slamming into all different types of hard objects, and it was pretty cool. What was cooler, though, was when they hooked them up to sensors attached to load cells to get a figure of how much force per square inch they were generating. They did this specifically with a forearm break, where the expert breaks through a series of cinder blocks with the forearm.
Anyway, They estimates how much force he generated on the forearm, and I forgot the exact figure. But the explanation was that a "normal" untrained person's forearms would break under that kind of force. However, due to training, the expert breakers bodies were able to adapt with training, build more calcium and stronger bones and tissue in those areas to make them more resilient to damaging themselves. This, added with appropriate technique allows them to break objects with parts of their body where ordinary people would break themselves.
I assume that it is the same with shin or instep roundhouse kicks. If you are training and conditioning, you are less likely to break that bone because your body will adjust. Technique plays a role as well. This is why "shin conditioning" in Thai boxing was an important part of training. Now-a-days in MMA, not much attention is payed to this type of conditioning and technical awareness, so it leaves athletes more suseptable to this type of injury. If you add genetics or medication or steroids that might weaken bones, some athletes will be more suseptible then others.
There are 2 lessons we can take away from this. 1 is that if I am teaching self-defense to people who aren't martial artists, and who are not compulsive enough to spend time doing body conditioning every day, then I am not including Thai kicks and such into the program. There are other techniques that are effective that will leave them less open to injury; and the fact is if you crack or break something by delivering a strike in self-defense, you could be worse off then when you started.
The second lesson is that if you are an athlete training for fights, put the time into adding body conditioning into your program. You don't have to be esoteric about it. Just be sure your can hit hard-density heavy bags with your techniques before you get into the ring.
C.