Spacers can also get worn down, which is more likely to happen than for them to crack. If you're lucky, you can just have the spacer replaced - but that is still open surgery.
I did experience having a spacer crack and break, suddenly, after eleven years in a PKR. I had no previous warning, no signs of loosening, and I hadn't been running. I was just walking fast.
It was extremely painful and instantly crippling, because one part of the spacer shot out of my knee joint to the front and another part shot our backwards. I couldn't walk at all.
I had to travel home on crutches or in a wheelchair, from Canada to New Zealand. I had been on holiday in Canada.
My PKR had to be revised to a TKR. One piece of spacer was retrieved, but the other part is still buried deep in tissues at the back of my knee. I was on crutches for a total of four months, thanks to that spacer breaking.
I wouldn't wish that on anyone.