No. Their joints work in the same way as any other quadruped. But in a horse, say, the 'knee' in the middle of the back leg is actually the ankle, which is why it seems to bend the 'wrong' way - the knee (the joint between femur and tibia) is higher up the leg. (my annotations below)
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http://www.newrider.com/Library/Misc_Tips/skeleton.jpg
Elephants have the joint between femur and tibia lower down, as their weight is more directly over the leg than a horse. Therefore the knee is where you expect it, in human terms, and the ankle is lower and much less flexible.
View attachment 2593
http://www.talkorigins.org/pictures/holden/struct-fig2a.gif
All mammals have two humeri, two radii, two femora and two tibiae, except some cetaceans (dolphins and whales) who have only vestigial or missing hindlimbs. Two femora and two tibiae means two knees, maximum.
Montage, no, not a biology teacher, an archaeologist specialising in animal bones.