A peer-reviewed journal that offers evidence-based clinical information and continuing education for dentists.

Effects of Tooth Whitening on The Appearance of Crowding And Buccal Corridors

This study analyzed whether tooth whitening can help mitigate the appearance of crowding, and the effects of bleaching on the perception of a narrow smile with large buccal corridors.


The demand for a beautiful smile continues to grow at a rapid pace. In 2019, global expenditures for cosmetic dentistry reached US$15.3 billion and are predicted to increase to US$17.3 billion by 2024.1 With the current emphasis on remaining youthful, the anti-aging benefits of cosmetic dentistry are often touted as a way to look and feel younger. Cosmetic options include procedures involving esthetic and restorative dentistry, as tooth whitening, bonding, veneers and orthodontic treatment are frequently employed to improve the esthetics of an aging face.2

The literature reveals that esthetic research in orthodontics has focused largely on tooth position, proportions of the face, and the effects of orthodontic treatment on the fullness of the smile and lips. The broader dental literature touts the esthetic benefits of veneers and vital tooth whitening. However, research regarding a relationship between teeth whitening, orthodontic crowding, and buccal corridor display is lacking. Therefore, the aims of this study were to determine: (1) whether tooth whitening could help mitigate the appearance of crowding, and (2) the effects of tooth whitening on the perception of a narrow smile with large buccal corridors.

Cosmetic Bonding or Veneers — Bonding and veneers are often used to address the esthetics of anterior teeth. Tooth size, shape, color, alignment and lip fullness can be improved with composite or ceramic veneers.3 Minor dental crowding can be addressed by the removal of select tooth structure and creative veneer reshaping (Figure 1). However, in cases where excessive crowding is addressed by this type of correction, teeth are often damaged and food traps can ensue. Periodontal inflammation and tissue breakdown are likely when food traps are created or plaque removal is inhibited. In addition, tooth reshaping may create occlusal changes that may necessitate orthodontic treatment and bite correction. Consequently, using veneers to reduce the appearance of crowding is limited to cases with minor misalignment of the anterior teeth, and may be contraindicated in cases with moderate to severe crowding.4

Occlusal view of veneers
FIGURE 1. Occlusal view of veneers placed on facial of upper incisors to mask irregular alignment.

Vital Tooth Whitening — Bleaching of vital teeth can improve smile esthetics. Modalities commonly used for whitening include in office “power whitening,” at-home, dentist-supervised bleaching tray whitening, and unsupervised whitening with over-the-counter products. High concentrations of hydrogen peroxide (25% to 40%) are used for in-office whitening procedures, whereas at-home, dentist-supervised bleaching tray techniques use 10% to 20% carbamide peroxide, which converts intraorally into 3.5% to 6.4% hydrogen peroxide. Over-the-counter whitening kits (with lower concentrations of carbamide peroxide) in the form of gels, paint-on products, strips and dentifrices are a third option.5

Each modality has its advantages and disadvantages. In-office treatments with higher concentrations of hydrogen peroxide are often costlier, but achieve more rapid results. One study found that an hour of in-office power whitening was equivalent to 40 hours of at-home, dentist-supervised whitening.6 However, when used as directed, at-home whitening can cause less sensitivity than in-office whitening, and may be less likely to cause enamel softening and surface roughness.7 Overall, tooth whitening is considered safe and effective when conducted properly and with appropriate supervision.7

Adult Orthodontic Treatment — Orthodontic treatment is increasingly popular with adults. The American Association of Orthodontists reports that as of 2016, 1.7 million Americans and Canadians adults were in active orthodontic treatment.8 This is up from the previous record high of 1.4 million adults undergoing treatment in 2014. Up to 75% of surveyed adults report they believe their personal relationships and careers had benefited from undergoing orthodontic treatment to improve their smile.9

Orthodontic treatment offers some major advantages compared to using crowns and veneers to treat dental misalignment: Instead of hiding the irregularities from view, it eliminates crowding and straightens misaligned teeth. In addition, orthodontic treatment can correct malocclusion, change the shape of the dental arches, and position teeth so restorations and implants can be more ideally placed. It can also improve oral function and speech.

Changing the dental arch shape with orthodontic treatment can create a broader smile that fills the buccal corridors (dark space between the posterior teeth and cheeks). Studies have found that a broad, tooth-filled smile with small buccal corridors is considered more esthetic than a narrow smile with large, dark buccal corridors.10–12


One reason for the increase in adults seeking orthodontic therapy is that treatment options can be more esthetic than in the past. Tooth-colored braces, lingual braces and clear aligners provide esthetic alternatives to traditional metal braces.13

Clear aligners are increasingly popular, and the popularity of lingual braces, a more esthetic (and sometimes more effective) option than clear aligners, has also continued to increase — albeit at a much slower pace. Temporary difficulties with speech, tongue irritation, the cost of the appliances, more complicated biomechanics, and a limited number of orthodontists offering lingual treatment are barriers to mainstream use of this approach.14


Barriers to Patient Acceptance of Complex Cosmetic Procedures — In spite of the advances in modern esthetic orthodontics, patient concerns include cost, lengthy treatment time, numerous office visits, maintaining the commitment to finishing treatment once started, discomfort, and incompatibilities that may occur with previous dental work. Disadvantages to crowns, veneers and bonding to improve smile esthetics include cost, loss of tooth structure, possible future loss/breakage of the restorations, multiple dental appointments, and potential pain during the procedure.

Less Invasive Options Are Typically Preferred — Patients obviously prefer an effective and cost-efficient way to improve their smile, and often inquire if they could improve esthetics without full orthodontic treatment or crowns and veneers.

These types of questions led the authors to speculate, what if these patients just bleached their teeth? Would a whiter smile hide the fact their teeth were misaligned? How much crowding could be masked by whitening alone? Would a lighter tooth shade make their smile look broader and hide any dark buccal corridors? This is a logical line of questioning because visual perceptions of an object can be altered by changing the object’s surroundings. Altering the color of clothing, changing the shade of makeup, or adding lipstick can create an illusion of tooth color change without actually altering tooth color.15,16 With this concept in mind, we set out to analyze (1) whether the perception of a crowded dentition could be improved with bleaching, and (2) what impact whitening might have on the appearance of a narrow smile with large buccal corridors.

Moderate crowding pretreatment
FIGURES 2A through 2C. Pretreatment images: Mild crowding pretreatment (A), moderate crowding pretreatment (B), and severe crowding pretreatment (C).


To determine if tooth whitening mitigated the unesthetic effects of crowding, we chose three patient models who consented for use of cropped photos of their teeth prior to their orthodontic treatment. The models were selected to represent examples of mild (1 to 3 mm), moderate (3 to 6 mm), and severe (6 to 9 mm) crowding (Figures 2A through 2C). Images of the teeth were digitally whitened using imaging software to visually compare to 1-M-1, 3-M-1, and 5-M-1 on the Vita Tooth Guide 3D Master. These particular shades and shade guide were chosen because they are evenly spaced according to hue, value and chroma. New photos were taken after orthodontic crowding had been corrected, and the teeth were shade matched with the original photos (Figures 3A through 3C).

Posttreatment images without crowding
FIGURES 3A through 3C. Posttreatment images without crowding: Mild crowding post-orthodontic treatment (A),
moderate crowding post-orthodontic treatment (B), and severe crowding post-orthodontic treatment (C).

The resulting images were randomly displayed to individual judges (30 laypeople, 30 dentists and 30 orthodontists) in a printed photo book. For each photo, the judge would rate how attractive the smile appeared on a 100-mm visual analog scale (VAS) by placing a mark on the provided line. A score of 0 represented the esthetically worst smile possible, whereas 100 represented the best smile possible (Figure 4). Four duplicate photos were randomly placed into the photo book to test for judge reliability.

Visual analogue scale for patient use
FIGURE 4. Using this visual analogue scale, participants were asked to place a mark on the line to indicate the attractiveness of the smile.


To determine the effect of tooth shade on perception of the buccal corridor, a consenting patient model with an esthetically pleasing smile — but somewhat darkly shaded teeth — was selected. The model whitened her teeth with a dentist-supervised, two-week at-home procedure. Photographs were taken prior to whitening, part way through the whitening procedure, and after whitening. This photographic series produced smile images with dark-, medium- and light-shaded teeth. Using imaging software, the buccal corridor was altered with a digital layering process to preserve the size and shape of the natural teeth, generating the appearance of large (22%), medium (15%) and narrow (2%) buccal corridors (Figure 5). The photographs were randomized and placed in a photo book for viewing by layperson, dentist and orthodontist judges. The judges again marked a VAS scale to rate how esthetic/attractive they found each smile.

Images of buccal corridor and tooth shade
FIGURE 5. Buccal corridor and tooth shade.


Paired T-tests were run on the four duplicate photos and the judges were found to be reliable (p ≤ 0.05). Five different three-way analysis of variance assessments were conducted to evaluate the relationships between tooth shade, amount of crowding, size of the buccal corridor, and the educational background of the evaluating judge. A Student-Newman-Keuls test was performed when a statistically significant interaction was detected.


Bleaching and Crowded Dentition — Regarding the results of whitening on the perception of crowding, we found that digitally whitening the teeth improved the esthetic ratings only for the non-crowded and mildly crowded dentitions (p ≤ 0.05). Perception of the moderate and severely crowded dentition was not affected by whitening, and consistently scored much lower, regardless of the shade of the teeth (p ≤ 0.05). There was also a difference in the rating depending on the judge’s educational background. The orthodontists were much less influenced by whitening and more swayed by the degree of crowding than laypersons or dentists (p ≤ 0.05). Unlike orthodontists, laypersons and dentists rated smiles with light shaded teeth and mild crowding to be just as esthetic as the non-crowded smiles (p ≤ 0.05). From this it can be concluded that whitening non-crowded and mildly crowded dentitions can have a significant positive esthetic effect — at least for non-orthodontists.

Whitening and the Buccal Corridor — We hypothesized that lighter tooth shades might draw the viewer’s eye to the teeth and away from the buccal corridors, mitigating their negative esthetic effect. However, this was not true. Laypersons, dentists and orthodontists all preferred broad smiles with small buccal corridors, and changing the shade did not affect their preference (p ≤ 0.05). Dentists, especially, found narrow smiles with broad buccal corridors to be least attractive (p ≤ 0.05).


For patients who wish to improve their smiles, it is important to consider the specific factors that will be most effective for the patient’s individual situation. Cost, treatment time, potential discomfort, maintenance, and overall effectiveness must be evaluated in order to determine the best esthetic solution.

We found that for patients with minimal crowding, whitening appears to be an effective method for increasing the attractiveness of a smile. However, it is important to note the teeth of the models in our study were normally shaped and sized, and lacked significant wear or restorations. This conclusion may not be true for individuals with worn, or abnormally shaped or sized teeth. For moderate to severely crowded dentitions, bonding, veneers or orthodontic treatment will likely achieve a more favorable result than whitening alone.

Broad smiles with small buccal corridors were greatly favored by laypersons, dentists and orthodontists alike. We found that for individuals with narrow dental arches, whitening was not an effective way to mitigate the negative effects of the large, dark buccal corridors. Orthodontic arch development should be considered as a more ideal treatment option. Analysis showed a mild increase in the attractiveness score with whitening, even in narrow arches — but not nearly as significant as the improvement seen from arch broadening. It is important to consider that significant arch expansion in adults may not be possible with routine orthodontic treatment, and might require additional therapy, such as orthognathic surgery. Such drastic measures may not be desirable unless the improvement will be dramatic.

One final observation is that background, training and education affect the ratings of smile esthetics. While a broad smile with small buccal corridors is preferred by all, orthodontists tend to be more sensitive to crowding, dentists prefer wide dental arches, and laypeople show the strongest preference for extremely white teeth. As dental professionals, we should keep these preferences in mind when making recommendations to patients.


  • Tooth whitening, bonding, veneers and orthodontic treatment are frequently employed to improve a patient’s smile.2
  • Minor dental crowding can be addressed by the removal of select tooth structure and creative veneer reshaping, but this should be limited to cases with minor misalignment of the anterior teeth.
  • Orthodontic treatment offers some major advantages compared to using crowns and veneers to treat dental misalignment; instead of hiding the irregularities from view, it eliminates crowding and straightens misaligned teeth.
  • This study of the results of whitening on the perception of crowding demonstrates that it improved esthetic ratings, but only for the not-crowded and mildly crowded dentition groups. Perception of moderate and severely crowded dentition was not affected by whitening.
  • Study results also show that for individuals with narrow dental arches, whitening was not an effective way to mitigate negative esthetic perceptions of large, dark buccal corridors.

Acknowledgement: The authors wish to thank Grant Snider, DDS, for his help creating the figures used in the study.


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From Decisions in Dentistry. March 2020;6(3):22-25.

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