Compared to the shoulder and hip joints in what way is the knee poorly protected and thus especially vulnerable to injuries?
The knee is a weight-bearing, hinge type joint with one plane of motion.
The hip and shoulder are ball-and-socket joints and have a greater range of motion.
Since the knee can't move freely in all directions, pressure or shock at an angle in which it can't move can damage it far more easily then a shoulder or hip, which might be able to rotate away from the force.
Miami Heat guard Dwyane Wade's knee injury is keeping him from competing in the 2012 London Olympics.
The NBA star announced that he was withdrawing his name from the Olympic basketball team after doctors advised him that he required immediate surgery on his injured knee.