Tooth 21: Decoding the Secrets of Your Lower Left First Premolar

Tooth number 21, which is, according to the universal tooth numbering system, the 1st premolar of the lower left quadrant, is often represented as 4 in the FDI World Dental Federation notation. The lower left quadrant consists of:

  1. 3 molars
  2. 2 premolars (Bicupsid)
  3. 1 Canine (Cupsid)
  4. 2 Incisors (Lateral & Central)

The quadrant starts from tooth 17, which is the 3rd molar of the lower left quadrant. After completing the 3 molars, comes the turn of the bicupsid, aka premolars, where tooth 21 comes as the 2nd premolar. After the canine, the lateral and central incisors appear as teeth 23 and 24.

tooth 21

We are coming to the location of tooth 21 to assure you that according to the universal tooth numbering system, it lies in the lower left quadrant and comes in between tooth 20 and tooth 22, which are the first bicupsid and the only canine in the quadrant. All the quadrants, which are four in number, comprise the same set of eight teeth in all.

What is the Function of Teeth 21 and How is it Different from Adjacent Teeth?

Premolars, especially tooth number 21, play an important role in the early stages of chewing and grinding food before it goes to the molars for processing. Tooth 21’s cusps and occlusal surface contribute to its ability to break down food particles. All this chewing and grinding helps with digestion. 

Tooth 21 may have a different number of cusps or root structure than the adjacent canine and second premolar. Tooth 21 may exhibit unique traits, such as particular fissure patterns or size variances in comparison to surrounding teeth.

Premolars, including tooth 21, have played significant roles in archaeological studies, providing insights into the diet and lifestyle of ancient populations.

Do all Teeth Have same Anatomy? 

  • Enamel: Tooth number 21 has a highly solidified, transparent outer covering that protects it from external forces and acids.
  • Dentin: A yellowish tissue that lies beneath the enamel and makes up the majority of the tooth’s structure. It is not as durable as enamel, but it does offer support.
  • Pulp: The pulp chamber and canals contain nerves and blood vessels that play an important role in tooth sensation and feeding.
  • Cementum: This solid tissue covers the tooth roots and helps to secure the tooth to the jawbone.
tooth number 21 anatomy

Root and Crown Being Part of Tooth 21 Anatomy:

Crown Contains:

  • The occlusal surface of tooth number 21 is designed to facilitate chewing and grinding. Its unique anatomy aids in the breakdown of food particles.
  • Cusps represent elevated figures on the occlusal surface that improve the effectiveness of chewing.
  • Pits and fissures on the occlusal surface can collect germs, making them more susceptible to decay. Cleaning these regions on a regular basis is essential for preventing cavities.

Roots of Tooth Number 21:

  • Premolars typically have one or two roots. The roots secure the tooth in the jawbone.
  • The root canal system includes nerves and blood vessels that run from the pulp chamber to the tip of each root. This complicated system necessitates extreme care during dental operations.

The Issues You get With Tooth Number 21

Premolars, including tooth number 21, are prone to cavities, particularly in pits and fissures. Regular dental check-ups and preventive measures, such as dental sealants, can help lower the risk. Because of its position and function, tooth 21 is prone to fractures and infections that can compromise the pulp and root canal systems.

cavities in premolars

How Denstist Treat the Issues of Tooth Number 21?

Dental Fillings

Dental fillings are frequently used to cure cavities caused by decay. They seek to rebuild the tooth’s structure while preventing additional demise.


  • The dentist usually uses a local anesthetic to numb the tooth and surrounding areas.
  • Decay removal involves using a dental drill or laser to remove the decayed section of the tooth, leaving a clean cavity.
  • The prepared cavity is formed to hold the filling material.
  • Fillings can be made from a variety of materials, including amalgam (silver-colored), composite resin (tooth-colored), gold, or ceramics
  • A specific light is used to cure and harden materials, such as composite resin. The dentist then forms and polishes it to achieve a natural appearance.

Root Canal Treatment

Root canal therapy is required when the pulp of a tooth becomes infected or inflamed as a result of decay, trauma, or other causes. The treatment is designed to remove the infected pulp while saving the tooth.


  • The dentist makes a hole in the tooth crown to reach the pulp chamber.
  • Pulp removal involves removing infected or inflammatory pulp from the pulp chamber and root canals with a specialist device.
  • The tooth’s inside is completely cleansed, and the root canals are shaped to allow for easy filling.
  • Root canals are cleansed and formed before being filled with gutta-percha, a biocompatible substance that seals the space.
  • A temporary or permanent filling is placed in the access opening, and a crown may be indicated for further support and security.

Tooth Extraction

Tooth 21 extraction is necessary when a tooth is badly damaged, decaying, or cannot be saved by typical dental operations.


  • The difficulty of the extraction determines whether a local or general anesthetic is used.
  • The dentist uses specialized devices to carefully free the tooth from its socket.
  • Once the tooth has been sufficiently loosened, it is carefully extracted from the socket with forceps.
  • The dentist may clean the extraction site and, in some situations, stitch it to promote healing.
  • Patients are given post-extraction care recommendations, which include information on pain relief, swelling, and dental cleanliness.

Each of these procedures is intended to meet the patient’s individual demands and the condition of the tooth. Dentists prioritize keeping natural teeth wherever possible. And the treatment used is determined by factors such as the level of damage, the presence of infection, and the patient’s overall oral health.