You might be "reading into" the protectiveness quotient too much. Riding with sturdy, thick-soled, heavy boots that are 10" in height (or preferably greater...) are far more protective than shoes, sneakers, ankle boots, or flip-flops. I wouldn't worry too much about how a boot will or will not transfer energy from the ankle to the knee. The simple fact-of-the-matter is that the heaviest, most-protective gear you can put on your feet will make you happiest in the unfortunate event of a tumble.
~ Blue Jays ~