A Class 3 lesion progresses to a Class 4 lesion with the involvement of the main apical foramina or possibly a large lateral canal. It is sometimes difficult to decide whether the lesion is primarily endodontic with secondary periodontal involvement (Class 2), or primarily periodontal with secondary endodontic involvement (Class 4), particularly in the late stages. If there is any doubt, the necrotic pulp should be removed; any improvement in the periodontal disease suggests that the classification was in fact of a Class 2 lesion.