The World Federation One Stop Fiqh

Ruling 182

If the palms or knees of someone who moves around on his hands and knees become impure, then to consider them as having become pure as a result of him moving around on them is problematic [i.e. based on obligatory precaution, they must not be considered as having become pure]. The same applies to the bottom of a walking stick, the bottom of an artificial leg, the shoe of a quadruped animal, the wheel of a car or cart, and similar things.