It is known that micro-organisms in irregular spaces beside the main canals and specially in lateral and accessory canals survive the limited effect of chemomechanical instrumentation.  There are overwhelming  amount of evidence in endodontic literature in support of the short-term use of CaOH (a week or longer).  It effectively reduces or eliminates bacterial infection from the root canal system.  Therefore, it is always beneficial to use an inter-appointment medicament such as calcium hydroxide, specially in non-vital or re-treatment cases.

CaOH is usually made into a paste by mixing with saline.  Inside the root canal system, CaOH dissociates into Ca++ and OH- ions resulting in an increased pH (up to 12.5) and an alkaline environment.  No micro-organisms survive at this pH and when they come into direct contact with CaOH.

The beneficial effects of CaOH include:

1. Prolonged and broad antibacterial action
2. Anti-inflammatory action (for example, stopping the exudative stage of inflammation)
3. Helping the local immune system
4. Enhancing the necrotic tissue dissolution when using in conjunction with NaOCl
5. Promoting bone formation and repair (complete repair is expected in 2-3 months)
6. Preventing re-infection inside the canal system by virtue of its physical presence

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My favourite preparation of calcium hydroxide is UltraCal® XS.  UltraCal® XS is an aqueous, 12.5 pH, syringeable, radiopaque, calcium hydroxide paste.  Flexible NaviTips are used to deliver CaOH inside the canals.  NaviTips are not side-vented needles.  So, care should be taken while placing CaOH suing this delivery method.  It should be delivered short of the apex (use the rubber stopper as a guide) while slowly withdrawing tips.