Complex sentence: Periodontal abscesses are less common than apical abscesses but still frequent, the key difference being that the pulp of the tooth tends to be alive, and will respond normally to pulp tests.
Split into simpler sentences: Periodontal abscesses are less common than apical abscesses, but are still frequent. The key difference between the two is that the pulp of the tooth tends to be alive, and will respond normally to pulp tests.