You do not need to whiten your back teeth: whitening only has to reach the teeth that show in your widest natural smile, which for most people means the upper front teeth back to the premolars and the lower front teeth, while the molars rarely show. In a video study of 100 adults, the second premolar was the last upper tooth visible in 57 per cent of posed smiles and 74 per cent of natural, unposed smiles. That is why whitening trays and in-chair whitening are usually planned around the teeth your smile displays, not the whole mouth.
Written by Dr James Tran, BDent (University of Sydney), MICOI, GradDip (Oral Implants), general dentist at Lumi Dental in Melrose Park.
The short version
- Whiten what shows. For most people that is six to ten upper teeth and the lower front teeth.
- Smile studies report the first premolar is almost always visible and the second premolar often is. Molars show in few smiles.
- Back teeth tend to look darker than front teeth even after whitening, mostly because they sit in shadow at the corners of the mouth; our article on buccal corridors and narrow smiles explains that dark space.
- Custom trays usually cover the visible teeth, often to the second premolar or first molar.
- Crowns and fillings do not change colour with whitening, so visible ones need planning first.
- A wide smile in daylight, photographed at arm's length, tells you where your whitening needs to stop.
The social six, the smile eight and the smile ten
The "social six" are the upper central incisors, lateral incisors and canines: the teeth you show when you talk. The "smile eight" adds the first premolars, just behind the canines. The "smile ten" adds the second premolars. Behind those sit the molars, which do most of the chewing and are mostly hidden by the cheeks.
Research supports these groupings. In a study of 120 adults aged 20 to 40, most displayed their first premolars when smiling, and around a third showed both premolars. A study of 188 smile images concluded that the upper first premolars are almost always displayed, sometimes with teeth further back, and lay raters scored smiles showing teeth to the second premolar as the most attractive. In my experience a few people with broad arches show a sliver of the first molar. Very few show anything further back.
| Tooth group | Typically visible in a broad smile? | Usually included in whitening? | Notes |
|---|---|---|---|
| Upper incisors (4) | Yes, in almost everyone | Yes | Noticed first |
| Upper canines (2) | Yes | Yes | Often start a shade darker |
| Upper first premolars (2) | Yes, in most people | Yes | Part of the "smile eight" |
| Upper second premolars (2) | Often | Usually | Often the end point of the tray |
| Upper first molars (2) | Sometimes, at the very edge | Sometimes | If your smile shows them, or for tray grip |
| Upper second and third molars | Rarely | Rarely | Hidden by the cheeks |
| Lower incisors and canines (6) | Often when talking or laughing | Yes, if the lower arch is treated | Often look darker |
| Lower premolars and molars | Less often | Premolars sometimes, molars rarely | Mainly for tray retention |
The single rule: whiten every tooth your widest smile shows
If a tooth appears in your broadest natural smile, it should be in the whitening plan. If it never appears, there is usually no reason to treat it. For most people that lands at the second premolar on the upper arch and the canine or first premolar on the lower arch. A wide smile may need the first molar; a narrow one may stop at the first premolar.
If the trays stop one tooth short, the last visible tooth stays at its old shade and can look like a dark step at the corner of the smile. Going far beyond the smile adds peroxide to teeth nobody sees, and more treated teeth can mean more sensitivity.
Why back teeth often look darker anyway
Patients I see often point to a premolar or canine that looks yellower than the front teeth. Most of the reasons are about light rather than stain.
Shadow at the corners of the mouth
The space between the outer surface of the back teeth and the inside of the cheek is called the buccal corridor. Less light reaches teeth in that space, so they read as darker on camera and in person.
Thicker teeth and more dentine
Canines and premolars are bulkier than incisors, with more dentine under the enamel. Dentine is naturally more yellow, so these teeth often start a shade or two darker. Our article on why canines look darker than front teeth covers this.
Fillings and crowns
Back teeth are more likely to carry older fillings and crowns. teeth.org.au, the Australian Dental Association's patient site, notes that whitening gel lightens enamel only and will not change the colour of porcelain crowns, veneers or white fillings. A review of tray whitening in the British Dental Journal adds that exposed roots and existing restorations will not bleach well, if at all, and that patients should be shown every restoration visible in their smile before they begin.
How far custom whitening trays usually go
There is no single rule for where take-home trays end; it is a clinical choice, and many stop at the second premolar or first molar. One UK whitening laboratory, after analysing a large number of its trays, moved to ending them at the first molars, reasoning that the molars are not really visible in a smile and that trays fit more loosely at the very back, letting gel escape and saliva dilute it.
The bite matters too. The British Dental Journal review describes trimming the last tooth off each side of a tray when it props the back teeth apart and stops the front teeth meeting. That keeps the bite comfortable. Our guide to how custom whitening trays work explains the rest of the design.
The lower front teeth usually show when people talk or laugh, so they belong in the plan, though some people start with the upper arch and compare. Our article on whitening only your top teeth covers that approach.
In-chair whitening and the back of the smile
In-chair whitening uses a stronger gel painted on by the dental team with the gums protected, so the clinician chooses exactly which teeth to treat. That is usually the teeth that show, often second premolar to second premolar on the upper arch plus the lower front teeth. Molars are rarely included: they are hard to reach and seldom seen. If take-home top-ups follow, the trays should cover the same teeth, or a premolar missed by the trays can fall behind.
When a back tooth has a crown or a large filling
A crown or large filling on a premolar is a common reason the end of a smile looks uneven after whitening: the natural teeth lighten and the restoration does not.
The usual approach is to whiten first, then wait at least two weeks before matching or replacing a visible restoration; the British Dental Journal review explains that the shade needs about two weeks to settle and that bonding placed straight away is weaker. A restoration behind the visible part of your smile can usually be left alone. Our article on whitening with veneers or crowns goes through the options.
A simple self-check before your consult
- Stand facing a window in daylight. Avoid warm indoor lighting, which makes teeth look more yellow.
- Hold your phone at arm's length and take a front photo while smiling as widely as you naturally would when laughing.
- Count the teeth you can see on the top and bottom, from the centre line outwards on each side.
- Note any tooth that looks a different colour, and any visible filling or crown.
Bring the photos to your appointment; a relaxed smile at home is often wider than a posed one in the chair.
How we approach this at Lumi Dental
Whitening at Lumi Dental starts with the dentist checking for decay, recession and worn edges, and noting every filling and crown inside your smile. The team takes clinical photographs to record which teeth show in a broad smile, and that sets the end point for treatment. Take-home trays are made from an in-house 3D intraoral scan, described on the our technology page, and are planned to cover the teeth your smile displays with enough extension for a secure fit. The whitening options are named Lumi Home, Lumi Refresh and Lumi Radiance, and the teeth whitening page explains how they differ.
Questions patients ask
Can you whiten molars?
Yes, molars respond to whitening gel like other teeth. Most people do not need it because molars rarely show, but if yours do, they can be included in the trays.
Why do my side teeth look darker than my front teeth?
Usually a mix of shadow at the corners of the mouth, thicker dentine in canines and premolars, and older fillings. Our article on why lower teeth look darker and whiten less explains a similar effect on the lower arch.
How many teeth show when you smile?
Most adults show six to ten upper teeth in a broad smile, from the canines back to the first or second premolars on each side. Many also show the lower front teeth, especially when talking or laughing.
Do whitening strips reach the back teeth?
Not always. Strips are a standard size rather than made to your teeth, so on a wide smile they may not cover the premolars, which can leave a darker band at the edges. Custom trays are planned around the teeth your own smile shows.
Will my crown on a back tooth need replacing after whitening?
Only if it shows in your smile and no longer matches. Crowns behind the visible area can usually stay. A cosmetic consult before you start can show which restorations fall inside your smile.
Thinking about whitening? Complimentary cosmetic consult in Melrose Park
If you are unsure how far back your whitening should go, the team at Lumi Dental can photograph your smile, check your fillings and crowns, and plan trays around the teeth that show. Book a complimentary cosmetic consult at Melrose Central in Melrose Park, or read more about teeth whitening at Lumi Dental. Patients visit from Ryde, Parramatta, Ermington and Rydalmere, with on-site parking.
Sources
- An orthodontic analysis of the smile dynamics with videography (PMC)
- Kolte A and colleagues: tooth and gingival display in maxillary anterior teeth and premolars (PubMed)
- British Dental Journal: tray bleaching review on examination, tray design and restorations
- Australian Dental Association (teeth.org.au): Teeth whitening
About the author
Dr James Tran is a general dentist at Lumi Dental in Melrose Park. He holds a BDent from the University of Sydney and a Graduate Diploma in Oral Implants, and is a Member of the International Congress of Oral Implantologists (MICOI).
This article is general information only and is not a substitute for an examination and individual advice from a registered dental practitioner. Suitability for any treatment varies from person to person.
