Collocations for knuckle - noun

bare
bleeding, bloodied, bruised, swollen
white
She clenched the phone till her knuckles were white.

brass

crack
He rubbed his hands together, cracking his knuckles as he tried to control his anger.

bruise, graze, scrape

bone (usually knucklebone in AmE)

rap sb on the knuckles, rap sb over the knuckles (both figurative)
She was rapped over the knuckles by her boss for criticizing the company in the press.