Why a root canal may need two visits?

Although infection after a root canal rarely happens, it can occur if a tooth is not cleaned carefully. Therefore, in more complex teeth with difficult-to-clean root systems, your dentist may need more than one session to treat your infected tooth.
During the first visit, your dentist will remove the infected and damaged tissue and place a temporary antibacterial solution over your tooth. The second appointment involves more cleaning and sterilization of all the areas of the inside of your tooth. After making sure the root canals are infection-free, the dentist fills the canals with rubber-like material, seals the tooth, and restores it with a permanent filling or a crown.
A root canal, also referred to as endodontic treatment, is a dental procedure which aims to treat or remove the damaged and diseased soft tissue from within an infected tooth.
The soft tissue inside your tooth is called pulp—a living tissue consisting of blood vessels, nerves, and connective tissues. Every tooth has a pulp that is located in the pulp chamber extending to the canal roots of a tooth. The pulp feeds the tooth and keeps it and the roots healthy. It is the connection path to the gums and bones.
When an infection or inflammation begins to compromise the pulp, it will affect the entire pulp tissue. If not treated with a root canal, the infection can spread down to the roots and cause the formation of a pocket of pus at the tooth’s root, known as a dental abscess. This is a very serious infection and can spread to other teeth and other parts of the body.
Common reasons that cause infection in the pulp include:
Tooth decay or deep cavities that has damaged enamel extensively, allowing bacteria to penetrate into the pulp
Dental injuries such as cracked or chipped tooth
Having repeated dental work done on the same tooth
Damage to the cementum—the layer covering the roots— due to decay and untreated exposed tooth root

What does the root canal procedure involve?Root canals are routine dental procedures that can be performed by a dentist in your regular dental appointment. More commonly and especially with complex cases, your procedure may be referred to an endodontist. They are dentists who specialize in dental pulp and have additional training with pain diagnosis and difficult-to-do root canals that can save your natural teeth.