2 Metaphors for knuckles

Jack was swallowing spasmodically; he moistened his lips; he grasped the balcony railing so tight that his knuckles were white knobs on the bronze back of his hand.

The fingers which had been playing with the boa had crumpled the end of it into a ball, which they were gripping so tightly that the knuckles were little white spots set in a blood-red background.

2 Metaphors for  knuckles