Knee replacements are tough. The metal components are much stronger than natural bone and the high density platic spacers are also very resilient.
Unfortunately, if you apply a force to your bone that would have broken it before your knee replacement, it will probably break it afterwards. The main difference now is that many of the methods used to fix fractures around the knee can't be used as the knee replacement gets in the way.
This raises an interesting question that was discussed at the recent Australian Orthopaedic Association conference. Knee replacements increase function and decrease pain, allowing people to return to activities they had previously ceased. So should you return to high risk sports such as snow skiing just because you can?
It has been noted that the number of sports that surgeons are happy for their patients to return to has been increasing but there has been no evidence to back this decision. It has been hypothesized that some surgeons are promoting the return to more extreme sports as a form of advertizing. Given that there are known difficulties for traumatic injury treatment following knee replacement, and the unknown effect of some sports on the life of a replacement, it is a fair question to ask.
Of course one of the main benefits of knee replacement surgery is allowing people to return to activities that had been denied them. Everyone should make an informed decision on the benefits and risks of any activity but when the risks are unknown a higher degree of scepticism should be employed. The goal of a knee replacement should be remembered - to allow everyday activities without pain. As younger people investigate the option of a total knee replacement, this issue is likely to receive more attention.
We may well be able to quantify the risk of certain activities in the future, but until then it is "buyer beware".