This study used a "third thumb
," which is neither the right size nor position (it should be smaller and higher to mirror the flesh thumb), and should have been controlled by hand or arm motion rather than foot, but which worked anyway.
Amusingly, they didn't mention -- and likely are not aware of -- the very old, static prosthetic thumb. It's commonly called a shucking peg
after its most famous use, but can be used in many other tasks. There are many styles, but they break into two basic forms: a protrusion that works like an extra thumb, or a hook that's more like an extra fingernail. They can be used to hold, puncture, tear, or open things.
Humans use a lot
of prosthetic devices that aren't really thought of that way. Clothes are prosthetic fur, sometimes literally. Shoes give tender feet the protection of hard hooves, and gloves make up for missing fingerpads. A hat provides the sun shade that some other species get from a tail. A purse, briefcase, backpack, etc. is like the marsupial pouch or folds of skin that some species use to carry everything from babies to food. Jewelry is a mate attraction device, like the fanciful colors or large horns of some species, and it takes effort to obtain which is why it's sometimes used as a gauge of productivity and thus fitness.