**Question:** How long before obturation should the calcium hydroxide paste be allowed to remain in canal, if the apical foramen is not closed and apical tissue repair is intended?

**Core Concept:** Calcium hydroxide paste is a commonly used medicament in endodontics for several purposes, including antimicrobial activity, induction of apical foramen closure, and tissue repair. The paste works by raising the pH of the root canal system, promoting tissue repair and inactivating microorganisms.

**Why the Correct Answer is Right:** The correct answer (C) refers to the concept of "apexification," which is the process of closing an open apex by inducing calcification. In open apices, calcium hydroxide paste is used to promote healing and calcification of the apical tissue. The paste should be left in the canal for an adequate period to allow for this process to occur.

**Why Each Wrong Option is Incorrect:**

A. This option incorrectly refers to "apexogenesis," which is the process of inducing root growth in a mature tooth. Apexogenesis is not the intended goal when using calcium hydroxide paste in open apices.

B. This option is too short a timeframe for the paste to effectively induce apical tissue calcification and closure. A longer period is necessary for the desired effect.

D. This option is too long, potentially leading to tissue damage and causing complications like root resorption or periapical lesions. A longer timeframe is not necessary for apical tissue calcification and may harm the tooth.

**Why Option C is Right:** Option C provides an appropriate timeframe for calcium hydroxide paste to induce apical tissue calcification and achieve the desired outcome of apexification. This process requires a sufficient period for the paste to exert its effects on the apical tissue and promote calcification.

**Clinical Pearl:** In cases of open apices, the use of calcium hydroxide paste is essential to facilitate apical tissue calcification and closure. However, the duration of application must be appropriate to ensure the paste has enough time to effectively induce calcification without causing complications. In this scenario, the correct answer is option C, suggesting that the paste should be left in the canal for 2-3 months before obturation.

**Why Option A is Incorrect:** Option A incorrectly refers to aphical growth, which is not the intended goal when using calcium hydroxide paste in open apices. Aphical growth refers to inducing root growth in mature teeth, which is not the objective in this context.