Why do gaps appear after receiving periodontal treatment? Periodontitis is caused by a buildup of bacteria, tartar formation, infection of the periodontal tissues, and the fact that the bone supporting the teeth has become more or less resorbed. In layman’s terms, the bones of the teeth have declined and the flesh should naturally recede downward. However, prior to periodontal treatment, the gums swell and tartar builds up, occupying the gaps and creating the illusion of “no gaps”. When the tartar is removed during treatment, the gum swelling is relieved or eliminated, the gum margin is restored to where it should be, and the gap appears.