In this paper, we prove a new result of digital topology which state that the digital fundamental group -- a notion previously introduced by Kong in [Kong89] -- is sufficient to characterize topology preservation within digital surfaces. This proof involves a new tool for proving theorems in this field: the Intersection Number which counts the number of real intersections between two surfels loops lying on a digital surface. The main property of the intersection number and the reason why it is useful is the following: the intersection number between two paths does not change after any continuous deformation applied to the paths.