Tooth 18 and the wisdom tooth behind it
A lower wisdom tooth stuck at an angle against tooth 18 is linked with decay on the back surface of tooth 18. In a study of 327 lower wisdom teeth that were removed, 25.4% of the second molars in front of them had decay there. It was more common when the wisdom tooth lay on its side and touched the second molar low down, where the crown meets the root.
A CBCT study from China found decay on the back of 52% of these second molars, most often when the wisdom tooth leaned forward at 43° to 73°.
The authors of the first study suggest that removing lower wisdom teeth in these positions may be advisable to protect the second molar.
Hard to reach. Being so far back, its chewing surface and back side are easy to miss when brushing.
The same tooth on the right side, tooth 31, has its own section on cracks and unusual canals.
Where is tooth 18?
Tooth 18 is in your lower jaw on your left 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 left side.
In a mirror, tooth 18 shows on the left side of your reflection. Someone looking into your mouth sees it on their right.
In front of it (toward the middle) is tooth 19, the lower left first molar. Behind it is tooth 17, the lower left wisdom tooth.
Its mirror twin on the right side is tooth 31. The tooth in the same spot in the upper jaw is tooth 15, the upper left second molar.
Dental charts show the patient's right side on the left, so tooth 18 sits on the right half of the bottom row. The numbers then drop to the lower jaw: lower left teeth run from 17 (the wisdom tooth) to 24 (the front tooth).
When does tooth 18 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 18 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 18 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 18 in FDI and Palmer notation
| System | Tooth 18 is written | What it means |
|---|---|---|
| Universal (ADA, used in the US) | 18 | Adult teeth are numbered 1–32 around the mouth, starting at the upper right wisdom tooth. |
| FDI / ISO 3950 (international) | 37 | First digit 3 = lower left quarter; second digit 7 = seventh tooth from the middle. Said "three-seven". |
| Palmer | ┌7 (often typed LL7) | The ┌ bracket marks the lower left quarter; 7 = seventh tooth from the middle. |
"18" in FDI is a different tooth. Read as an FDI code, 18 means the upper right wisdom tooth (tooth 1 in Universal). Charts and X-ray reports from outside the US often use FDI, so check which system is in use.
"37" can only be FDI. Universal numbers stop at 32, so a tooth 37 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 LL) makes it this one.
All adult teeth in the lower left quarter
From the front of the mouth to the back:
| Universal | FDI | Palmer | Tooth |
|---|---|---|---|
| 24 | 31 | ┌1 (LL1) | Central incisor |
| 23 | 32 | ┌2 (LL2) | Lateral incisor |
| 22 | 33 | ┌3 (LL3) | Canine |
| 21 | 34 | ┌4 (LL4) | First premolar |
| 20 | 35 | ┌5 (LL5) | Second premolar |
| 19 | 36 | ┌6 (LL6) | First molar |
| 18 | 37 | ┌7 (LL7) | Second molar (this tooth) |
| 17 | 38 | ┌8 (LL8) | Wisdom tooth |
The Tooth Number Chart & Converter converts any tooth between the three systems.
Tools for tooth 18
- 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
What tooth is number 18?
What is FDI tooth 37 in the US system?
At what age does tooth 18 come in?
Can a wisdom tooth damage tooth 18?
Sources
- American Dental Association (MouthHealthy). Permanent Tooth Development (eruption chart). 2012. Accessed October 26, 2026.
- American Academy of Pediatric Dentistry. Dental Growth and Development (chronology after Logan & Kronfeld 1933, adapted 2003). Accessed October 26, 2026.
- World Health Organization. Oral health surveys: basic methods, 5th edition (tooth notation after ISO 3950). 2013. Accessed October 26, 2026.
- Nelson SJ. Elsevier. Wheeler's Dental Anatomy, Physiology and Occlusion, 11th edition. 2019.
- American Dental Association. ADA Dental Claim Form (tooth designation systems). Accessed October 26, 2026.
- Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Types of Teeth and their Functions. Accessed October 26, 2026.
- Dentalcare.com (continuing education course CE500). An Overview of Dental Anatomy: Permanent Dentition. Accessed October 26, 2026.
- Cleveland Clinic. Teeth: Anatomy, Types, Function & Care. last updated January 26, 2023. Accessed October 26, 2026.
- Centers for Disease Control and Prevention. Dental Sealant Facts. May 15, 2024. Accessed October 26, 2026.
- U.S. FDA label via DailyMed (Dentsply). XYLOCAINE Dental (lidocaine HCl and epinephrine) injection. updated August 13, 2026. Accessed October 26, 2026.
- Marques J et al. Journal of Clinical and Experimental Dentistry 9(6):e794–e798. Impacted lower third molars and distal caries in the mandibular second molar. Is prophylactic removal of lower third molars justified?. 2017. Accessed October 26, 2026.
- Kang F, Huang C, Sah MK, Jiang B. Journal of Oral and Maxillofacial Surgery 74(4):684–692. Effect of Eruption Status of the Mandibular Third Molar on Distal Caries in the Adjacent Second Molar. 2016. Accessed October 26, 2026.
- 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 by the Dental Health Time editorial team, against the sources above. How we review.