My bottom knuckle on my thumb hurts when i bend my thumb?

? trigger finger: Pain when bending the thumb is usually do to a slight swelling of the tendon that bends the thumb. This tendon must slide through a tunnel at the base of the thumb and the swollen tendon does not slide easily and can get stuck. This can be treated with a splint the limits motion, anti-inflammatory meds, or more commonly a cortisone injection. Surgery to release the tunnel is occasionally needed.