Do you mean "slipped tendon"? I ask because it looks like slipped tendon (perosis) to me.I think the tendon is slipped too.
If this is slipped tendon it has nothing to do with being a Pekin duck, it is a condition that ducks, peafowl, turkey, and chickens get from eating diet lacking in certain vitamins, minerals, amino acids, or from injury.
Here is a thread about slipped tendon:
https://www.backyardchickens.com/threads/slipped-tendon-perosis-in-ducks.1213225/#post-19336951
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In this case, the vet said it was issues with the tendons, and based on the dozen or so I have seen, it looks like slipped tendon to me, which, as I said above, can be cause by many things.

