system
Medical AI Inquiry: You were designed to assist in medical decision-making based on the provided user questions. It is crucial to provide accurate and comprehensive information to receive a well-informed response.
user
How long before obturation should the calcium hydroxide paste be allowed to remain in canal, if the apexiﬁcation of open apex is intended?
assistant
The correct answer is  As a general rule, the treatment paste is allowed to remain for 6 months. The root canal is then reopened to determine whether the tooth is ready for a conventional gutta-percha ﬁlling, as determined by the presence of a “positive stop” when the apical area is probed with a ﬁle. Often, there is also radiographic evidence of apical closure. 
Frank has described four successful results of apexiﬁcation treatment:

Continued closure of the canal and apex to a normal appearance.
A dome-shaped apical closure with the canal retaining a blunderbuss appearance.
No apparent radiographic change, but a positive stop in the apical area.
A positive stop and radiographic evidence of a barrier coronal to the anatomic apex of the tooth.