Crippled Limbs are caused by excessive damage to the left arm, right arm, left leg, right leg, and in Fallout 3: the torso, or head. In Fallout, Fallout 2 and Fallout Tactics, the eyes can get damaged as well.
Fallout, Fallout 2 and Fallout Tactics
In Fallout and Fallout 2, crippled limbs are a serious problem. Having arms crippled lowers hand-to-hand damage and accuracy with all weapons; with both arms crippled, weapons cannot be used at all. Crippled legs lower mobility, increasing the amount of Action Points required to move during battle. If either or both eyes are damaged, the accuracy with every weapon drops substantially. In order to heal crippled limbs a decent Doctor skill and a first aid kit or doctor's bag is required, or finding a doctor that can heal crippled limbs. In Fallout, the doctor in Vault 13 can do this.
The Ripper is a very effective weapon to use when attempting to cripple an enemy's limbs.
Fallout 3
In Fallout 3, the head and torso can be crippled as well as arms and legs, though the Crippled Eye is gone, there is a similar effect with a crippled head. When the head is crippled, a concussion is received, constantly blurring the vision and making sounds like ringing in one's ears making it difficult to see or hear what is happening; a -4 reduction to Perception is also taken. Having the torso crippled affects Endurance, increasing sensitivity to being hit and potentially disrupting actions. Having crippled arms has the same effect as in Fallout and Fallout 2, lower damage with melee and lower accuracy, and crippled legs lowers overall speed. Having any crippled limbs in Fallout 3 means either a stimpak can be used, a doctor be found, or even sleeping for about an hour. Eating food or drinking water does not regenerate limbs. After completing the optional method of completing "Contract Radiation Sickness" in Chapter 1 of the "Wasteland Survival Guide" Quest, Moira will remove the 600 rads of radiation from the Lone Wanderer and cause a mutation, granting a new perk; "Rad Regeneration", which allows regenerating crippled limbs while experiencing advanced radiation poisoning.
Fallout: New Vegas
In normal play, a doctor's bag or sleeping will heal all crippling. Stimpaks can be used to heal individual limbs. In Hardcore mode, stimpaks will not heal limbs and doctor's bags will work on one limb at a time or used generally to increase the health of all limbs slightly. Sleeping will only heal crippling if it is in an "owned" bed.
Appearances
Crippled limbs have appeared in Fallout, Fallout 2, Fallout 3, Fallout 4, Fallout Tactics, Fallout: New Vegas and the Van Buren tech demo.