Your screen is a "packet" with many layers of tech, haha easy explained, the crack might be tiny, but if it is deep enough it will damage the digitizer or lcd, or just simply the whole thing, but as you refer for, is the touch defective. it is kinda self explaining i would think this answer?
I am sorry if this is a bad answer or really bad written but it is the best i could write in english, haha sorry.
Here is a little picture of how touch screen works:
[image|998720]