According to a recent study from the Mayo Clinic in Rochester, Minnesota intraoperative acetabular fractures are rare. Out of 7,121 total hips done, only 21 patients had such a fracture. This is about a 0.4 per cent rate.
The Mayo study was able to identify the possible cause for this problem. They looked at the cup design used most often that fractured during the procedure. It looks like the elliptical shape and flare of the edges caused the most problem. Usually the shape of the opening for the cup didn't match the shape of the cup.
Uncemented cups tend to fracture more often than cemented cups. This may be because more force is needed to set the cup up against the bone. If a fracture does happen, the surgeon checks to see if the hip is still stable. If it is, then bone chips are used to reinforce the fracture site. If it's not, then the cup is taken out and a new one put in.
Your grandfather may have to wait a little longer to put his full weight on the leg. Otherwise, his rehab will continue as planned. He should be up and about fully in about six to eight weeks.