Regarding the topic; personally i think it's retarded to use any knuckles to punch someone with, i've remained injury free using elbows, forearms and open palms. There is not much difference between two knuckles and three, both have a good chance of getting fucked up if you hit the wrong target or at the wrong angle, which can and will happen in a high stress scenario. Use whatever your coach teaches you to use.
The only fight I've been in as an adult I jacked my hand up pretty bad after a left hook caught the guy right on a tooth. It still hurts when I bend that finger all the way.
All this talk about knuckles is great but in my experience it all goes to hell in a real fight. For example the guy I fought had his hands in front of his face so I threw a left hook, thing is I was still at long range so my arm was somewhat extended and I ended up hitting where the metacarpal and the phalange meet instead of on the knuckle.
If I somehow ever get in a fight again I will do my best to remember not to use a closed fist, I work in IT and it sucks trying to get by with one hand.
A couple of older guys i know who did a bit of prizefighting mentioned that they'd often hit each other in the body alot more than you'd expect.. for the sake of their hands
I'm sitting here staring at my hand and I can't really figure out what is wrong with it that makes hitting with the first three knuckles strange. I basically just line up on my middle knuckle. On me it is clearly the largest/most conditioned. My hands aren't particularly flat or anything.
My ex-boss, as trained boxer did not differentiate between glove and bare knuckle, other than doing last 3 knuckle push-ups to condition said fists.
Knuckle push-ups was advice I gave after he approached me once he knew I was from Karate and fought/sparred bare handed.
He obviously took my advice since the next two street fights he got into didn't involve bandages but band-aid for skin cuts.
Not to mention all the pictures posted by whathappened are 1 ali posing for a picture and 2 live fighting. niether picture proves anything. Ali was posing not punching, and in a fight it's likely at least a few punches will land without using the intended striking surface. One isolated picture is not proof of anything. In summary the pictures are not only irrelevant to the OP they can be considered irrelevant to this whole topic.
My posting of the boxers is relevant based on my experience with them. They (the ones I know) punch with the same set of knuckles with or without gloves.
I've met at least two who train for the street, their last 3 knuckles and fingers are calloused and of higher bone density than the index or thumb.
Wait I just realized someone was trying to say that the picture of Ali "proves" that HE punched with the last three knuckles. lol
You can't recognize any bone remodeling in that picture? Hint: It isn't on the last three knuckles.