Why can you not jump after total knee replacement?

Jump??: First of all, who told you that you can't? Probably not a good idea to do that in early post-operative or rehabilitation period, but if the surgery goes well and there are no post-operative issues, there is no reason why you can't jump. Of course prior to doing that you would have to build the strength back up in your quads and hamstrings.

Not Advisable: Every stress put across a total knee replacement subtracts from the longevity of the replacement. Smart exercise as discussed with your orthopedist and your physical therapist is advised.

Hard on the parts: While it is possible to jump on a replaced knee, it is thought that these types of activities can cause wear and earlier failure of the device. Further, a bad fall could cause a fracture in the bone around the implant, which can require surgery and possibly long recovery. Finally, loosening of the components would also typically require more surgery. Always best to err on the side of caution.

Wear and loosening: This can cause the plastic bearing to wear down faster from the impacts of the metal part into the plastic, or it can cause the implant to loosen itself from the bone by breaking the bond between the cement and the implant or the cement and the bone.