Spinal injury with finger numbness. how does that happen?

Nerve damage: Sensation in the body is governed by nerve cells which run from the spine all the way to the tips of the fingers. That is why damage to the spine can affect sensation even in a finger tip. In general, the further away from the central nervous system (spine and brain) an injury occurs, the easier it is for it to heal. A spinal disc, pressing on a nerve, can be removed to help in some cases.

Pinched nerve: It sounds as though the nerve that leads from your spine to your finger has been compressed or bruised in some way. Check with your doctor and let them know about your symptoms. These may resolve on their own with time and nsaids, or may be something more severe.