Elephants technically do not have knees despite the fact that they have joints in positions that people assume are "knee joints." The joint in the middle of each leg is actually considered a "wrist," according to Kruger National Park.
The reasons animal biologists consider the elephant joints to be wrists is because elephants don't have separate hand and foot bones as humans do. An elephant's legs provide functionality similar to what humans get from the combination of their arms and legs. Elephants reach out to push or grasp things with their feet, similar to how humans would with wrists and hands.