So i rapidly started GMing a 4th ed campaign this week. This was an injury-dismemberment idea i came up for it (though we aren't using it, because my players have been playing Dark Heresy for years and their sick of getting limbs blown off, which i think is a fair reason not to want a dismemberment system).
What i have tried to do is to piggy back as much as is possible on to the healing surges and death save system already in place in 4th ed. This is what i came up with:
When you are dropped to 0 or less HP, in each of your turns you roll a d20 and consult the table below. On the first roll on the table after you are dropped to 0 you can get a result that is lower than 1 and risk dismemberment or injury. Otherwise when you are dropped to zero HP and remain unconscious, any of the subsequent rolls have a minimum roll value of 1 (as you roll this each turn). If you are healed to full by another player or creature before your first roll on the table, you make a roll on it anyway to see if you lost an arm or some such, and can ignore the effects that aren't dismemberment or injury.
For each time you have already been dropped to 0 or less HP in a day you apply a negative of -1 to this table, and therefore repeated unconsciousness is how you end up risking dismemberment. This means if you have already been dropped to 0 HP or less twice in one day and are rolling on this table for your third bout of being a 0 or lower HP, you are at a -2 penalty, so you are more likely to bleed out, and actually risking injury.
On a natural 20 you always regain consciousness and use a healing surge, but if you have suffered an injury then this effect persists once you are up an fighting again.
A character dies when they reach their bloodied value in a negative HP. This represents dying of injuries and bloodloss, or catastrophic injury like decapitation.
-5 or lower Loss of limb. Roll d20 – 1-5 Left Leg, 6-10 Right Leg, 11-15 Left Arm, 16-20 right arm. You evidently cannot use that limb anymore for checks or attacks and suffer appropriate penalties permanently. Some effects are summarised here. Loss of leg results in halving of movement speed and dexterity scores and imposes a -5 circumstance penalty on athletics and acrobatics checks and the like. Loss of arm results in an inability to wield weapons or implements with that arm anymore (obvious) and a -5 circumstance penalty on certain checks (disable device, search etc). You are bleeding out unless tourniquet is applied (DC15 heal check) and when you lose 1 hp in a round from rolling on this table you lose D3 HP instead of 1.
-4-3 Serious injury. When awake and for 2d4 weeks afterwards (unless magically healed) you grant combat advantage to enemies attacking you as your injury hampers your movement, using secondwind removes this penalty for the duration of an encounter. You temporarily lose a healing surge whilst injured. You are bleeding out unless tourniquet is applied (DC15 heal check) and when you lose 1 hp in a round from rolling on this table you lose d2 HP instead of 1.
-2-0 Light Injury. Whence awake and for 2d4 days afterwards (unless magically healed) you grant combat advantage to enemies attacking you as your injury hampers your movement etc. Using secondwind removes this penalty for the duration of the encounter.
0-10 Lose 1 hp.
11-19 No change