Tooth Number 31: Lower Right Second Molar

Cracked tooth 31

In a study of 175 cracked teeth, the lower second molar was the tooth most often cracked (25.1%). Most patients were 50 or older, and teeth with no filling or only a small one cracked more often.

When a cracked tooth had a root canal, 90% were still in place 2 years later in that study, but teeth with deep gum pockets (over 6 mm) did worse.

C-shaped canals. Lower second molars sometimes have one C-shaped canal instead of separate ones. A worldwide CBCT study of 3,600 of these teeth found it in 13.9%, rising to 44% in China, with no difference between the left and right sides.

In a survey of 3,350 endodontic patients, the lower second molar was the third most often treated tooth (12.0%).

The same tooth on the left side, tooth 18, has its own section on decay linked with a stuck wisdom tooth.

Where is tooth 31?

Tooth 31 is in your lower jaw on your right side. It is the seventh tooth from the middle: start at the gap between the two bottom front teeth and count 7 teeth toward the back on your right side.

In a mirror, tooth 31 shows on the right side of your reflection. Someone looking into your mouth sees it on their left.

In front of it (toward the middle) is tooth 30, the lower right first molar. Behind it is tooth 32, the lower right wisdom tooth.

Its mirror twin on the left side is tooth 18. The tooth in the same spot in the upper jaw is tooth 2, the upper right second molar.

Dental charts show the patient's right side on the left, so tooth 31 sits on the left half of the bottom row. Lower right teeth finish the circle, from 25 (the front tooth) to 32 (the wisdom tooth).

When does tooth 31 come in?

The lower second molars usually come in at 11–13 years according to the ADA chart. The AAPD chart gives 12–14 years.

In the ADA chart, they usually come in after the lower second premolars (11–12 years) and before the upper second molars (12–13 years). In the AAPD's 13-step order, from the first molars (step 1) to the wisdom teeth (step 13), they are step 12.

Under the gum, according to the AAPD chart, it starts to harden (calcify) at about 30–36 months, has a finished crown by 7–8 years and has a finished root by about 14–16 years, a few years after it comes into the mouth.

What does the lower second molar do?

Molars do the heavy grinding. Cleveland Clinic notes that about 90% of chewing happens on the molars. The lower second molar is a bit smaller than the first molar and usually has four cusps.

It is also called the "12-year molar". Second molars are named for the age they usually arrive, about 12 years.

Adult molars don't replace any baby tooth. They come in behind the baby teeth as the jaw grows.

How many roots does tooth 31 have?

2 roots (front and back), usually closer together than the first molar's.

Common problems with lower second molars

• Sealants. These thin coatings are painted on the chewing surfaces of back teeth. The CDC says they prevent 80% of cavities over 2 years in the back teeth, where 9 in 10 cavities occur.
• Treatment. Small cavities are fixed with a filling. If decay or an injury reaches the nerve, root canal treatment can save the tooth, often followed by a crown.

If tooth 31 hurts, feels loose, or the gum around it swells, the Dental Emergency Checker can help you decide how soon to see a dentist. Swelling that spreads to the eye or neck, or makes it hard to breathe or swallow, is an emergency: call 911. These are typical problems for this kind of tooth, not a diagnosis.

Tooth 31 in FDI and Palmer notation

"31" in FDI is a different tooth. Read as an FDI code, 31 means the lower left central incisor (tooth 24 in Universal). Charts and X-ray reports from outside the US often use FDI, so check which system is in use.

"47" can only be FDI. Universal numbers stop at 32, so a tooth 47 on any chart is this tooth.

Palmer needs the bracket. Every quarter of the mouth has a tooth 7 in Palmer notation; only the ┐ (or the letters LR) makes it this one.

All adult teeth in the lower right quarter

From the front of the mouth to the back:

The Tooth Number Chart & Converter converts any tooth between the three systems.

Tools for tooth 31

• Adult Teeth Predictor: likely dates for your child's adult teeth.
• Dental Emergency Checker: how soon to see a dentist if it hurts or breaks.
• Dental Numbness Calculator: lower molars usually get a nerve block, which lasts longer (about 3–3½ hours with lidocaine, vs about 2½ hours for a shot by the tooth).
• Dental Insurance Calculator: what your plan may pay for a crown or root canal.
• How Long Does Dental Work Last?: how long fillings, crowns, veneers and implants usually last, from long-term studies.
• Dental Treatment Plan Decoder: plain English for treatment-plan shorthand like "#30 MOD comp".

Frequently asked questions

Sources

1. American Dental Association (MouthHealthy). Permanent Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
2. American Academy of Pediatric Dentistry. Dental Growth and Development (chronology after Logan & Kronfeld 1933, adapted 2003). Accessed October 26, 2026.
3. World Health Organization. Oral health surveys: basic methods, 5th edition (tooth notation after ISO 3950). 2013. Accessed October 26, 2026.
4. Nelson SJ. Elsevier. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition. 2019.
5. American Dental Association. ADA Dental Claim Form (tooth designation systems). Accessed October 26, 2026.
6. Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Types of Teeth and their Functions. Accessed October 26, 2026.
7. Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Permanent Dentition. Accessed October 26, 2026.
8. Cleveland Clinic. Teeth: Anatomy, Types, Function & Care. last updated January 26, 2023. Accessed October 26, 2026.
9. Centers for Disease Control and Prevention. Dental Sealant Facts. May 15, 2024. Accessed October 26, 2026.
10. U.S. FDA label via DailyMed (Dentsply). XYLOCAINE Dental (lidocaine HCl and epinephrine) injection. updated August 13, 2026. Accessed October 26, 2026.
11. Kang SH, Kim BS, Kim Y. Journal of Endodontics 42(4):557–562. Cracked Teeth: Distribution, Characteristics, and Survival after Root Canal Treatment. 2016. Accessed October 26, 2026.
12. von Zuben M et al. Journal of Endodontics 43(9):1442–1447. Worldwide Prevalence of Mandibular Second Molar C-Shaped Morphologies Evaluated by Cone-Beam Computed Tomography. 2017. Accessed October 26, 2026.
13. Wayman BE, Patten JA, Dazey SE. Journal of Endodontics 20(8):399–401. Relative frequency of teeth needing endodontic treatment in 3350 consecutive endodontic patients. 1994. Accessed October 26, 2026.
14. Almadhoon HW, Abuiriban RW, Almassri H, Al-Hamed FS. Journal of Evidence-Based Dental Practice 22(2):101712. Efficacy of 4% articaine buccal infiltration versus inferior alveolar nerve block for mandibular molars with symptomatic irreversible pulpitis: a systematic review and meta-analysis. 2022. Accessed October 26, 2026.

Last reviewed October 26, 2026 by the Dental Health Time editorial team, against the sources above. How we review.