Be the whip.Straighten your thumb somewhat (mine doesn't bend reliably anymore), and you'll find it starts to protrude just past the surface of the fist. That tip of the thumb is what's transmitting impact into the arthritic knuckle.
Besides that, I do have super-human punches. I hit like a steam engine. No, really. It's like getting punched by the Hulk. That's my story and I'm sticking to it...until someone makes me prove it, then I'll have to come up with a new one.