Introduction
This systematic review aimed to identify, evaluate, and provide a synthesis of the available literature on the proximal enamel thickness (PET) of permanent teeth.
Methods
The eligibility criteria were studies that assessed the PET of the permanent teeth. A search of studies in Medline (via PubMed), the Cochrane Library, Scopus, Web of Science, Embase, and Lilacs databases that measured PET was conducted until August 31, 2020. Two reviewers independently selected the studies, extracted the data, and assessed the risk of bias for systematic reviews involving cross-sectional studies. The quality of evidence was assessed using the Grading of Recommendations, Assessment, Development, and Evaluations approach. PET data generated from the systematic review were summarized by random-effects inverse-generic meta-analysis.
Results
From 1388 potentially eligible studies, 11 were considered for systematic review and meta-analysis. The measurement of PET was done with radiographs, microscopes, microtomographs, or profilometers. In total, 4019 mesial and distal surfaces involving 2118 teeth were assessed. All included studies showed low to moderate risk of bias, whereas GRADE revealed that the level of evidence was low. Greater mesial and distal enamel thickness was observed for the premolars and molars, whereas it was least for the mandibular central incisors. The least difference of 0.02 mm (95% confidence interval [CI], −0.07 to 0.11 and −0.06 to 0.09, respectively) between mesial and distal sides was observed for the maxillary and mandibular second molars, whereas the maximum difference of 0.12 mm (95% CI, 0.07-0.17 and 0.07-0.16, respectively) was observed for the maxillary central incisors and maxillary first premolars. The meta-analysis indicated a moderate level of heterogeneity (I 2 of 45%). The funnel plot revealed minimal publication bias.
Conclusions
The summary effect of the meta-analysis revealed that the thickness of the enamel on the distal aspect was greater than on the mesial aspect by an average of 0.10 mm (95% CI, 0.09-0.12). This finding would be of relevance to all disciplines of dentistry and especially for the clinician planning interproximal reduction, a procedure that is routinely done for clear aligner therapy.
Highlights
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Of 1388 eligible studies, 11 were considered for systematic review and meta-analysis.
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A total of 4019 mesial and distal surfaces involving 2118 teeth were assessed.
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All included studies showed low to moderate risk of bias.
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The meta-analysis reported a moderate level of heterogeneity (I 2 of 45%).
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Enamel on the distal aspect was greater than on the mesial aspect by an average of 0.10 mm.
Enamel stripping, also known as interproximal reduction (IPR) or reproximation, is an attractive alternative to tooth extraction in patients with mild-moderate crowding because it allows the transverse arch dimensions and anterior tooth inclinations to be maintained. , It is based on the natural phenomenon of physiological interproximal wear and allows for fitting the dental units to the arches. In this procedure, partial reduction of the enamel layer is carried out, resulting in the reduction of the mesiodistal tooth size so that a precise amount of space is created to allow the crowding to unravel. Therefore, this procedure is preferred to tooth extraction, which often results in more space than required to resolve dental crowding. The principle of IPR should follow 3 guidelines: leave a thin layer of enamel, remove more enamel from teeth with greater proximal enamel thickness (PET), and perform selective reduction at the 2 contact areas between paired adjacent teeth.
Several studies have reported on the PET of posterior and anterior teeth, , with various conclusions. Gillings and Buonocore reported that enamel on the distal surface was slightly thicker than on the mesial surface. However, Sarig et al reported that for the maxillary second premolars, mandibular incisors, and mandibular first premolars, the mesial enamel was thicker than the distal enamel, with Stroud et al suggesting that PET increases toward the distal aspect in the mandibular posterior dentition, with the exception of the second premolar.
PET would be a significant parameter in caries susceptibility, progress, and tooth sensitivity. Moreover, PET is an important aspect of tooth preparation in the field of pediatric dentistry and prosthetics for crowns and bridges. Although PET has not been extensively studied from these aspects, variations in PET would have clinically significant ramifications in treatment planning in these areas. Given the irreversible effects of enamel removal and a wide variation in the published data on PET, pooled data from the available literature could provide more solid conclusions.
Objectives
This study aimed to summarize the proximal enamel thickness of the permanent dentition from the incisors to the second molars through a systematic review of the literature. The review question in population, intervention, control, and outcomes format was “are there any differences in enamel thickness between the mesial and distal surfaces in permanent teeth?”
Material and methods
Protocol and registration
This systematic review was conducted and reported in accordance with the PRISMA guidelines (Preferred Reporting Items for Systematic Reviews and Meta-Analyses). The proposal was registered on the International Prospective Register of Systematic Reviews (PROSPERO, 180647).
Eligibility criteria
Inclusion and exclusion criteria were specified before the study ( Table I ). The main outcome was the measurement of the mesial and distal enamel thicknesses of the permanent teeth.
| Inclusion criteria | Exclusion criteria |
|---|---|
| Studies on human participants in whom the PET of permanent teeth was measured | Studies that assessed the PETs of primary teeth |
| English language studies | Studies that assessed the PETs of stone age dentition |
| Studies that evaluated PET by microscopy, profilometry, or radiography were included. Other assessment methods, such as ultrasonic imaging, flat plane radiography, x-ray computed tomography, and cone-beam computed tomography was also considered | Review articles, systematic reviews, and letters to the editor or author |
Information sources, search strategy
Electronic searches in Medline (via PubMed), the Cochrane Library, Web of Science, Scopus, Embase, and Lilacs were conducted until August 31, 2020. Search terms were based on both Medical Subject Headings (MeSH) and free text with combinations and were prepared for Medline via PubMed and adapted for Lilacs, Web of Science, Scopus, Embase, and Cochrane electronic databases ( Table II ).
| Search term | MeSH terms |
|---|---|
| Enamel | “dental enamel”[MeSH Terms] OR (“dental”[All Fields] AND “enamel”[All Fields]) OR “dental enamel”[All Fields] OR “enamel”[All Fields] OR “enamels”[All Fields] OR “enamel’s”[All Fields] OR “enameled”[All Fields] OR “enameling”[All Fields] OR “enamelling”[All Fields] |
| Thickness | “thick”[All Fields] OR “thickness”[All Fields] OR “thicknesses”[All Fields] |
| Width | “width”[All Fields] OR “widths”[All Fields] |
| Mesial | “mesial”[All Fields] OR “mesialization”[All Fields] OR “mesialized”[All Fields] OR “mesially”[All Fields] |
| Distal | “distal”[All Fields] OR “distalization”[All Fields] OR “distalize”[All Fields] OR “distalized”[All Fields] OR “distalizer”[All Fields] OR “distalizers”[All Fields] OR “distalizes”[All Fields] OR “distalizing”[All Fields] OR “distally”[All Fields] OR “distals”[All Fields] |
| Proximal | “proximal”[All Fields] OR “proximalization”[All Fields] OR “proximalize”[All Fields] OR “proximalized”[All Fields] OR “proximalizes”[All Fields] OR “proximalizing”[All Fields] OR “proximally”[All Fields] OR “proximals”[All Fields] |
| Permanent dentition | “dentition, permanent”[MeSH Terms] OR (“dentition”[All Fields] AND “permanent”[All Fields]) OR “permanent dentition”[All Fields] OR (“permanent”[All Fields] AND “dentition”[All Fields]) |
Study records
The selection of the studies consisted of 2 phases. Initial screening of articles identified in the databases searched involved independent screening of titles and abstracts by 2 reviewers (V.K., H.R.) on the basis of the research question and against the inclusion and exclusion criteria. In articles in which the title and abstract failed to provide sufficient information, the full text was reviewed to assess for relevance. In the second phase, full-text articles were then retrieved from these potentially eligible studies. To ensure that no relevant studies were missed, the reference lists of the remaining articles were hand-searched. Articles identified by this process were added to the pool of full-text articles to be evaluated. Any discrepancies with regard to the eligibility of an article were resolved by discussion with the third reviewer (M.S.M.) when necessary. This pool was then assessed for eligibility for both quantitative and qualitative reviews.
Data items and collection
The data extraction of the included articles was performed independently and in duplicate by 2 authors (V.K., H.N.E.). A predetermined and standardized table was used for data extraction. The predefined data to be extracted were title, authors, authors’ institutional affiliation, study type, age, gender, population, sample size, assessment technique, teeth assessed, mesial enamel thickness, distal enamel thickness, section assessed, and P value.
Outcome
The outcome for which the data will be sought would be the distal and mesial enamel thickness from which the difference would be obtained.
Risk of bias and quality assessment
The risk of bias was assessed using the risk of bias in nonrandomized studies of interventions for systematic reviews involving cross-sectional studies. The following domains were evaluated: (1) confounding bias, (2) selection bias, (3) bias due to deviation of intended interventions, (4) missing data, (5) measurement of outcomes, (6) bias in the selection of reported result. The bias in the classification of interventions was referred to as not applicable.
Two reviewers (H.R., H.N.E.) assessed all included studies independently. Disagreements were resolved through discussion and consensus, or the decision of the third reviewer (V.K.) was taken whenever necessary.
Data synthesis
The studies were grouped on the basis of the data assessed. For each article that met the validity criteria, data were extracted and compiled into a table of evidence. Analysis was prepared according to the Cochrane Handbook for Systematic Reviews. Data were analyzed in Revman 5.3. The continuous data were presented as mean difference and 95% confidence interval [CI]. An inverse variance method of pooling the data with a random-effects model was used for the meta-analysis. Heterogeneity of the included studies was also assessed on the basis of the individual tooth and by the assessment technique used. Heterogeneity was assessed with the Cochrane Q test along with I 2 statistics, which ranged from 0% to 100%. An I 2 index <25% is indicative of low heterogeneity, between 25% and 75% represents average heterogeneity, and more than 75% would indicate considerable heterogeneity.
Meta biases and confidence in cumulative evidence
Publication bias and selective reporting, if any, would be assessed. Publication bias would be assessed through a funnel plot. The quality of evidence was assessed using the Grading of Recommendations, Assessment, Development, and Evaluations approach.
Results
Study selection and characteristics
Searches of databases retrieved 2535 records, from which 1147 duplicates were removed, leaving 1388 unique records. The titles and abstracts of these records were screened by 2 independent reviewers (V.K., H.R.). One thousand three hundred and seventy-four were rejected on the basis of the exclusion criteria, resulting in 14 articles whose full texts were assessed for eligibility. Of these 14 articles, 11 studies provided sufficient data for quantitative assessment ( Fig 1 ).
All included studies were cross-sectional and published between 1998 and 2020. The studies had a sample size ranging from 32 to 402. Five studies mentioned the age range (from 12 to 60 years) of the patients from whom the samples were assessed. , , , , Most studies assessed the PET tooth wise and did not differentiate on the basis of gender. However, 1 study assessed the PET on the basis of tooth and gender but proceeded to combine gender data because no differences in PET between genders were observed. Of the 11 studies, 3 assessed premolars only , , ; 1 assessed canines only ; 2 assessed incisors only , ; 1 assessed premolars and molars ; 1 assessed incisors, canines, and premolars ; and 3 assessed incisors, canines, premolars, and molars. , , The measurement of PET was done with either bite-wing or intraoral periapical radiographs, , microscopes, , , profilometer , , or microcomputed tomography. , For the purpose of meta-analysis, the weighted mean and standard deviation for each study and the individual tooth were calculated using a standard mathematical formula. The maximum difference in PET for the maxillary teeth was observed in the canines. Shillingburg and Grace reported the mesial and distal enamel thicknesses to be 1.42 ± 0.26 mm and 1.67 ± 0.30 mm, respectively, resulting in a difference of 0.25 mm. The second premolars exhibited the maximum difference in the mandibular arch. The mesial and distal enamel thicknesses were 1.38 ± 0.23 mm and 1.8 ± 0.46 mm, respectively, resulting in a difference of 0.42 mm. Most of the studies reported that the enamel was thicker in the distal than in the mesial aspect ( Table III ).
| No. | Author/y/reference | Journal name | Study type | Population | Age, y | Sex | Sample size | Investigation used | Section assessed | Teeth assessed | Mesial enamel thickness | Distal enamel thickness | P |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 1 | Stroud et al/1998/1 | Angle Orthodontist | Cross-sectional | Caucasian | 20 to 39 | 59 male and 39 female | 98 | Radiograph (bitewing) | Maximum mesiodistal diameter of the tooth | Md first premolars | 0.99 ± 0.21 | 1.07 ± 0.23 | 0.001 |
| Md second premolar | 1.19 ± 0.21 | 1.22 ± 0.22 | 0.11 | ||||||||||
| Md first molars | 1.28 ± 0.23 | 1.40 ± 0.25 | <0.001 | ||||||||||
| Md second molars | 1.29 ± 0.20 | 1.48 ± 0.26 | <0.001 | ||||||||||
| 2 | Harris and Hicks/1998/17 | Arch Oral Biol | Cross-sectional | American Caucasoid | 13-17 y | Pooled, no difference noted | 115 | Radiograph (IOPA) | Maximum thickness | Mx central incisor right | 0.90 ± 0.12 | 1.06 ± 0.15 | <0.0 01 |
| Mx central incisor left | 0.91 ± 0.11 | 1.05 ± 0.14 | <0.001 | ||||||||||
| Mx lateral incisor right | 0.91 ± 0.12 | 1.01 ± 0.12 | <0.001 | ||||||||||
| Mx lateral incisor left | 0.91 ± 0.11 | 1.02 ± 0.14 | <0.001 | ||||||||||
| 3 | Hall et al/2007/9 | J Am Dent Assoc (JADA) | Cross-sectional | African American and White | NR | 40 male and 40 female | 80 | Radiograph (IOPA) | Contact point | Md right central | 0.72 ± 0.10 | 0.77 ± 0.11 | <0.0001 |
| Md left central incisor | 0.71 ± 0.10 | 0.77 ± 0.11 | <0.0001 | ||||||||||
| Md right lateral | 0.79 ± 0.11 | 0.95 ± 0.13 | <0.0001 | ||||||||||
| Md left lateral incisor | 0.81 ± 0.11 | 0.96 ± 0.14 | <0.0001 | ||||||||||
| 4 | Macha et al/1998/4 | Braz Oral Res | Cross-sectional | White adult | 19-31 | NR | 40 | Microscope | The central area that encompassed the greater crown width and the thicker enamel portion | Mx premolar right | 1.22 ± 0.17 | 1.28 ± 0.19 | 0.0759 |
| Mx premolar left | 1.22 ± 0.18 | 1.39 ± 0.17 | 0.09851 | ||||||||||
| 5 | Fernandes et al/2011/5 | Braz Oral Res | Cross-sectional | NR | NR | NR | 40 | Microscope | The central area that encompassed the greater crown width and the thicker enamel portion | Md second premolar right | 1.16 ± 0.17 | 1.24 ± 0.12 | 0.24 |
| Md second premolar left | 1.03 ± 0.22 | 1.10 ± 0.21 | 0.33 | ||||||||||
| 6 | Munhoz et al/2012/18 | Braz J Oral Sci | Cross-sectional | NR | NR | NR | 40 | Profilometer | Contact area | Mx premolar right | 1.22 ± 0.17 | 1.28 ± 0.19 | 0.001 |
| Mx premolar left | 1.22 ± 0.18 | 1.39 ± 0.17 | 0.001 | ||||||||||
| 7 | Vellini-Ferreira et al/2012/20 | Braz J Oral Sci | Cross-sectional | NR | NR | NR | 302 | Profilometer | Tooth long axis and contact surfaces coincided with the cutting plane of the disc | Mx central incisors | 0.854 ± 0.174 | 1.015 ± 0.173 | <0.001 |
| Mx lateral incisors | 0.860 ± 0.129 | 1.002 ± 0.176 | <0.001 | ||||||||||
| Mx canines | 1.027 ± 0.126 | 1.220 ± 0.145 | <0.001 | ||||||||||
| Mx first premolars | 1.220 ± 0.173 | 1.266 ± 0.187 | 0.001 | ||||||||||
| Mx second premolars | 1.101 ± 0.176 | 1.155 ± 0.149 | <0.001 | ||||||||||
| Md central incisors | 0.675 ± 0.144 | 0.872 ± 0.276 | <0.001 | ||||||||||
| Md lateral incisors | 0.734 ± 0.139 | 0.879 ± 0.158 | <0.001 | ||||||||||
| Md canines | 0.781 ± 0.106 | 1.014 ± 0.118 | <0.001 | ||||||||||
| Md first premolars | 1.051 ± 0.166 | 1.266 ± 0.187 | <0.001 | ||||||||||
| Md second premolars | 1.376 ± 0.198 | 1.450 ± 0.172 | <0.001 | ||||||||||
| 8 | Sarig et al/2015/3 | Am J Orthod Dentofacial Orthop | Cross-sectional | NR | 12 to 59 | 48 male, 61 female | 109 | Microscope | Contact area | Mx central incisors | 0.81 ± 0.06 | 0.81 ± 0.27 | 0.998 |
| Mx lateral incisors | 0.76 ± 0.14 | 0.86 ± 0.28 | 0.206 | ||||||||||
| Mx canines | 1.10 ± 0.21 | 1.11 ± 0.26 | 0.747 | ||||||||||
| Mx first premolars | 1.13 + 0.15 | 1.25 + 0.37 | 0.593 | ||||||||||
| Mx second premolars | 1.41 ± 0.27 | 1.36 ± 0.30 | 0.652 | ||||||||||
| Mx molars | 1.35 ± 0.29 | 1.48 ± 0.17 | 0.237 | ||||||||||
| Md central incisors and lateral incisors | 0.64 ± 0.19 | 0.60 ± 0.18 | 0.581 | ||||||||||
| Md canines | 0.97 ± 0.21 | 1.30 ± 0.61 | 0.133 | ||||||||||
| Md first premolars | 1.16 ± 0.13 | 1.05 ± 0.24 | 0.227 | ||||||||||
| Md second premolar | 1.23 ± 0.10 | 1.30 ± 0.20 | 0.305 | ||||||||||
| Md molars | 1.37 ± 0.12 | 1.40 ± 0.20 | 0.72 | ||||||||||
| 9 | Akli et al/2020/19 | Am J Orthod Dentofacial Orthop | Cross-sectional | NR | NR | NR | 32 | Microcomputed tomography | 1.5 mm incisally and 1.5 mm gingivally from each of the 2 reference slices | Mx canines | 0.856 | 0.861 | 0.430 |
| 10 | Shillinburg and Grace/1973/21 | Journal-Southern California Dental Association | Cross-sectional | Assumed to be Caucasian, small percentage of African American and American Indian | NR | NR | 159 | Microscope | 9 sections | Mx central incisors | 0.98 ± 0.11 | 1.1 ± 0.31 | NR |
| Mx lateral incisors | 0.93 ± 0.35 | 0.97 ± 0.39 | NR | ||||||||||
| Mx canines | 1.42 ± 0.26 | 1.67 ± 0.3 | NR | ||||||||||
| Mx first premolars | 1.48 ± 0.2 | 1.54 ± 0.11 | NR | ||||||||||
| Mx second premolars | 1.27 ± 0.22 | 1.21 ± 0.28 | NR | ||||||||||
| Mx first molars | 1.34 ± 0.09 | 1.41 ± 0.1 | NR | ||||||||||
| Mx second molars | 1.27 ± 0.15 | 1.3 ± 0.17 | NR | ||||||||||
| Md incisors | 0.76 ± 0.24 | 0.87 ± 0.12 | NR | ||||||||||
| Md canines | 1.28 ± 0.1 | 1.24 ± 0.24 | NR | ||||||||||
| Md first premolars | 1.41 ± 0.15 | 1.51 ± 0.19 | NR | ||||||||||
| Md second premolars | 1.38 ± 0.23 | 1.8 ± 0.46 | NR | ||||||||||
| Md first molars | 1.46 ± 0.12 | 1.47 ± 0.09 | NR | ||||||||||
| Md second molars | 1.44 0.13 | 1.46 ± 0.14 | NR | ||||||||||
| 11 | Bian et al/2014/16 | Chinese Journal of Stomatology | Cross-sectional | Beijing residents, Han nationality | 12 to 60 y; mean (39.5 ± 10.6) | 85 women and 97 men | 182 | Microcomputed tomography | Contact area | Mx central incisor | 0.80 ± 0.21 | 0.81 ± 0.17 | NR |
| Mx lateral incisors | 0.72 ± 0.12 | 0.73 ± 0.24 | NR | ||||||||||
| Mx canines | 1.06 ± 0.18 | 1.12 ± 0.16 | NR | ||||||||||
| Mx first premolars | 1.19 ± 0.13 | 1.24 ± 0.26 | NR | ||||||||||
| Mx second premolars | 1.2 ± 0.21 | 1.25 ± 0.15 | NR | ||||||||||
| Mx first molars | 1.37 ± 0.23 | 1.46 ± 0.25 | NR | ||||||||||
| Mx second molars | 1.42 ± 0.18 | 1.43 ± 0.13 | NR | ||||||||||
| Md central incisors | 0.65 ± 0.19 | 0.63 ± 0.16 | NR | ||||||||||
| Md lateral incisors | 0.64 ± 0.22 | 0.66 ± 0.14 | NR | ||||||||||
| Md canines | 1.07 ± 0.25 | 1.16 ± 0.21 | NR | ||||||||||
| Md first premolars | NR | ||||||||||||
| Md second premolars | 1.15 ± 0.2 | 1.21 ± 0.18 | NR | ||||||||||
| Md first molars | 1.40 ± 0.14 | 1.42 ± 0.20 | NR | ||||||||||
| Md second molars | 1.41 ± 0.23 | 1.41 ± 0.16 | NR |
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