According to Chad Buchanan, the surgery went as planned and there were no surprises:
I agree that it's a concern that he's losing more cartilage, but given that neither of us have a medical degree, it's pretty hard to know whether this is a real long range problem or not. It may be that Greg's cartilage is made of "chalk", as you suggested, IIRC. It may also be that there are other reasons that we don't understand that account for the damage. I'm certainly not optimistic that Greg will have a long career, but I'm going to wait and see what happens rather than reaching a knee-jerk conclusion.