An avulsed permanent tooth has been dry for more than 60 minutes. After removing the necrotic periodontal ligament, the root is soaked in sodium fluoride solution for 20 minutes. What is the purpose?

Correct answer: D. To slow replacement resorption and delay ankylosis

• A. New ligament cannot reattach to a root stripped of vital periodontal ligament — bone deposits directly onto the root instead, which is precisely what ankylosis means.
• B. Sterilisation of the canal is achieved by extraoral root canal treatment, which is performed separately before replantation in these cases.
• C. Once the cells are dead nothing revives them. Fluoride does not restore viability.
• D. CORRECT. Beyond 60 minutes dry, the periodontal ligament cells are dead and replacement resorption with ankylosis is INEVITABLE. The therapeutic goal changes from saving the ligament to BUYING TIME. Soaking the root in 2% sodium fluoride — classically 2.4% acidulated phosphate fluoride at pH 5.5 for 20 minutes — converts root surface hydroxyapatite to the more resorption-resistant fluorapatite, slowing osteoclastic replacement so the tooth can serve as a space maintainer through the growth years.

Note: Extraoral dry time is the single strongest predictor of outcome. Under 60 minutes in a suitable medium, aim to preserve the ligament. Over 60 minutes dry, accept ankylosis and manage it — the tooth is a temporary space maintainer, and infraocclusion in a growing patient will eventually need decoronation.