I was specifically told to look into Biopace rings when I first started riding after my amputation (ca. 1990-91). I've got a
Van Ness rotationplasty, which means I can't bend my right leg much more than about 90 degrees (though there are ways to get around that). My right q(g)uads are also much less developed. I was told that the Biopace rings might help -- I was under the impression that it was to help with the rotation limitation of my right leg, but in retrospect it may have been to reduce strain on my left knee, too.
Haven't ridden with one for years, though. I'd be interested in experimenting with one. Hmmmm.