One of the things to do is ice the knee before and after using the CPM (continuous passive motion) machine or having a physical therapist work with on post op knees. This is what most people are told to do:

Take paper cups and fill with water. Place in freezer. When needed, take a totally frozen paper cup and peel around the cup edge exposing the ice surface. While holding the paper cup, rub the ice around the knee area. You can either put the rest of the frozen ice back in the freezer to use later or toss away.

[1] Icyball therapy is more convenient is reusable, sterilizable and more effective.  This tool can be used on many parts of the body that need icing.