Before revision: Location of septa in the sinus Underwood initially divided the maxillary sinus into three regions relating to zones of distinct tooth eruption activity: anterior (corresponding to the premolars), middle (corresponding to the first molar) and posterior (corresponding to the second molar) .
After revision: Location of septa in the sinus Underwood divided the maxillary sinus into three regions relating to zones of distinct tooth eruption activity: anterior (corresponding to the premolars), middle (corresponding to the first molar) and posterior (corresponding to the second molar) .