Cracking your knuckles is totally benign and has never been shown to cause any significant damage to the joints or to the hand. All you are doing when you crackle your knuckles is moving the fluid inside the joint around rapidly, which causes 'cavitation', or the production of small air bubbles in the joint fluid that burst and produce the noise.
Therefore, if your finger stays numb, you will need to go see your primary care doctor
to help you figure out what is causing this. If you have recently jammed or sprained your finger, or otherwise injured it, this could be the cause. If you have, your doctor may want to perform some x-rays to make sure the bones of the hand are all intact.
Similarly, there are some nerve and over use injuries that can produce numbness
in the fingers. The most common of these is carpal tunnel syndrome
, which is associated with repetitive actions at the wrist, such as typing on a keyboard. Carpal tunnel syndrome causes numbness, pain, or weakness in the first three fingers of the hand (thumb, index, and middle fingers). It can be diagnosed or ruled out by your primary care doctor - please see him or her soon.