Can someone help explain to me what an anterior cortical defect in the knee?

Divot. The cortex is the outer surface of the bone. An anterior cortical defect of the knee suggests that there is a small divot or defect in the outer front surface of your knee.
Yes. "anterior" means it's in the front of your knee (as opposed to "posterior" which is in the back). "cortical" means its in the hard outer layer of the knee (as opposed to "cancellous" which is the spongy bone that makes up the marrow). "defect" means that a peice is missing. "anterior cortical defect" means there is a piece or chunk missing from the bone in the front of the knee.