I'm not going to even try to help diagnose over the Internet... So first and foremost if it is bruised, swollen, painful, and/or doesn't move like normal then go to a doctor. Knuckles are not cartilage... they are bone and can be broken or fractured. If not treated, they can limit movement, cause infection, not heal properly, in some cases. Typically the pinky finger is more prone to breaking/fracture. But any of them can break.
Here is some good info: Broken Knuckles