Adamantium Skeleton/Fallout 4

Adamantium Skeleton is a perk in Fallout 3, Fallout: New Vegas and Fallout 4.

Fallout 3 and Fallout: New Vegas
Adamantium Skeleton simply takes any sustained limb damage and halves it, making it twice as difficult for you limbs to be crippled. This perk does not affect limb damage from falling, nor does it prevent any scripted situations that cripple limbs, such as falling in the Mothership Zeta trash compactor.

Fallout 4
With the first rank of this perk, damage done to limbs is reduced by 30%. The second rank raises this reduction to 60%, and the third rank negates all limb damage entirely.

Behind the scenes
This perk pays homage to Marvel Comics character Wolverine, who possesses an adamantium-laced skeleton.