Not just the knuckles. In MMA and fighting, hitting with the bottom part of the fist is also quite effective.
Actually, as an Ohio State Highway Patrol Academy trained recruit,I learned that making the fist and using the back part of the hand, (Swung sideways)is the hardest part of the body (sorry Laz). I’ve used that method, didn’t get hurt and broke a lot of people down in one punch. I also learned that a head butt, walking down the bar on a riotous bar, will calm crap down real fast. If done correctly, people don’t like blood. Geez, I’m glad I’m retired.