Understanding age-related changes in normal dental occlusion is crucial for orthodontic treatment, planning, and retention, especially considering the increasing demand from adult patients. This review aimed to systematically perform a scoping literature review regarding age-related dental changes in untreated normal occlusion subjects.
An electronic search was performed until May 2020 using the following terms: “Dental Occlusion,” “Dentition,” “Ageing,” and “Age groups.” Articles studying changes in dental occlusal traits in orthodontically untreated subjects with neutroclusion and comparing them between various age groups were included. Studies focusing on a single age group or written in languages other than English were excluded. The risk of bias of the studies was assessed with both the methodological index for nonrandomized studies and the risk of bias in nonrandomized studies of intervention tools.
From the initial 7681 articles, 40 were included. High methodologic heterogeneity was found among studies. Intercanine width was found to increase until 8.0 years and 15.0 years in the mandible and maxilla, respectively, whereas intermolar width increases in both arches until 26.0 years. After that, both parameters decrease into late adulthood. Crowding increases from mixed dentition until 60.0 years of age, especially in the mandible and females. Maxillary incisors procline with age while mandibular incisors retroclined.
The age-related evolution of occlusal traits, together with skeletal and soft tissue changes, should be considered during orthodontic planning. Well-designed prospective cohort studies are necessary for confirmatory evidence. Expectations for treatment stability should consider muscular balance and inform patients that crowding is a sign of aging that should not always be seen as a failure of orthodontic therapy. Protocol registration number: PROSPERO CRD42020138846.
Age-related occlusal changes influence orthodontic diagnosis, therapy, and stability.
This scoping review examined age-related dental changes in humans with normal occlusion.
Overjet, overbite, crowding, spacing, and tooth inclination showed age-related changes.
Arch width was most frequently analyzed (22 articles) and showed changes with age.
Dental occlusal changes are described from 3.0 years to 60.9 years of age.
Growth-related occlusal changes are well documented in the orthodontic literature and provide essential clinical guidance. However, age-related changes in normal dental occlusion in adults are less addressed in scientific literature, despite the fact that an increasing number of adult patients seek treatment. The biologic environment comprised skeletal, dental, and soft tissue and does constantly change throughout its lifetime, even when growth is no longer taking place.
In orthodontics, prior understanding of these changes is crucial to design an appropriate treatment plan and to inform the patient regarding the need for posttreatment retention. More importantly, this knowledge can help us understand whether the occurred posttreatment changes are due to the orthodontic intervention or part of the normal developmental or aging process. Some of these changes are age, sex, and sometimes ethnicity-related and should be considered during orthodontic diagnosis.
Postorthodontic relapse has been attributed to multiple causes, including the reorganization of periodontal and gingival tissues, lack of proper occlusal interdigitation, , soft tissue adaptation, oral habits, and growth changes. , Because orthodontic treatment alters the normal muscular pattern, it has been suggested that treatment should be limited within the equilibrium zone between intraoral and extraoral muscular forces to prevent relapse because of soft tissue pressure. Anthropological studies suggest that the lack of dental crowding in ancient humans was due to attrition as a consequence of their hard diet. , Animal studies have also shown that differences in the hardness of diet are related to significant changes in maxillary width and the presence of rotated or displaced teeth.
Many strategies have been proposed to avoid orthodontic relapse. Lifelong use of bonded retainers is one of the most popular options in an effort to avoid it, and it is currently seen as an inherent part of orthodontic treatment. However, there is no consensus in the literature regarding a predictable retention protocol. Long-term wear of fixed retainers presents a number of disadvantages, such as the need for maintenance, which increases the practitioner’s workload and the costs for the patients, the difficulty to maintain good oral hygiene, the possibility of retainer breakage, bond failure, or accidental activation, which can lead to active tooth movement. ,
Although various observational studies can be found regarding occlusal changes throughout a human lifetime, they normally address 1 or 2 specific parameters in a particular age group, sex, or ethnicity. However, to the best of our knowledge, a systematic analysis of these studies has not been undertaken so far. The objective of this article is to systematically perform a scoping review of the existing literature regarding age-related dental changes in untreated normal human occlusion. The comparative analysis of the included articles aims to serve as a baseline for orthodontic diagnosis and treatment planning.
Material and methods
Protocol and registration
This systematic scoping review was performed according to a protocol developed beforehand, and it follows the Preferred Reporting Items for Systematic Reviews and Meta-Analyses recommendations. In addition, our study adhered to the meta-analysis of observational studies in epidemiology guidelines. The protocol was registered on June 18, 2019 in the international prospective register of systematic reviews (PROSPERO), with protocol registration number: CRD42020138846.
Articles included in this review fulfilled the following inclusion criteria according to the Population, Exposure, Comparator, and Outcomes format: (1) population: human subjects with untreated normal occlusion ; (2) exposure: collection of characteristics of dental occlusion; (3) condition: various age groups in a lifetime (studies that compared ≥2 different age groups and did not focus on a particular single age group were included); (4) outcome: description of changes in dental and occlusal traits; and (5) study design: observational studies (cohort studies either prospective or retrospective and cross-sectional studies).
Articles including subjects who had undergone any type of orthodontic treatment, articles in languages other than English, nonhuman studies, case reports, and review articles were excluded.
Information sources and search strategy
To identify relevant articles, the electronic databases of PubMed, Embase, Cochrane, and Web of Science were searched up to May 4, 2020. Gray literature was checked by using the Open Gray database. A broad search strategy was formulated by the main reviewer (A.A.R.) with the assistance of a specialized librarian from the Biomedical Sciences Library of KU Leuven University, Belgium, and after discussion with one of the reviewers (M.C.L.P.). The index terms used for the electronic search were “Dental Occlusion” and “Dentition” combined with “Ageing,” “Age factors,” or “Age groups” to identify the age-related changes. The search was limited to human studies by using a PubMed search string instead of a PubMed filter. The search strategy was adapted for each database. The detailed search strategy for all the databases has been reported in Supplementary Table I . The reference lists of key articles were handsearched for relevant studies.
Articles were selected in 3 phases. In the first phase, 1 author (A.A.R.) excluded the duplicate articles using the Reference Manager EndNote X9 (Clarivate Analytics, Philadelphia, Pa) and the systematic review web application Rayyan ( rayyan.qcri.org ). Articles were then independently selected by 2 reviewers (A.A.R. and P.P.) on the basis of their title and abstract. In the second phase, the same 2 reviewers independently screened the full texts to identify those papers that satisfied the inclusion criteria. To ensure consistency between the 2 reviewers, 20 articles were screened independently, and the concordance between both was measured using the Kappa index. Finally, phase 3 resolved the conflicts of the selection process by a discussion with the third reviewer (M.C.L.P.).
Data collection and analysis
Data extraction for the included studies was performed by the first reviewer (A.A.R.) and was later rechecked by the second reviewer (P.P.). Finally, a data extraction sheet was formulated in Microsoft Excel (Redmond, Wash) after a discussion with the third reviewer (M.C.L.P.) to determine the variables to be extracted and to resolve disagreements in data extraction, which was performed for the following items: (1) participant characteristics: sample size, sex distribution, age of the included subjects with normal occlusion (mean and standard deviation or age range); and (2) study characteristics: author and year of publication, study design, number of observations recorded, period of observation, changes in dental occlusion (arch width, buccolingual tooth angulation, labiolingual tooth inclination, overjet, overbite, arch length, arch depth, crowding, spacing, and arch length discrepancy) and the mode of investigation.
Assessment of risk of bias
The methodological index for nonrandomized studies (MINORS) tool from Slim et al was used to assess the risk of bias of the included articles. This tool analyzes 12 variables and scores each item as 0 when not reported, 1 when reported but inadequate, 2 when reported and adequate. Comparative and noncomparative articles are scored differently. The overall score for comparative and noncomparative studies is 24 and 16, respectively.
Because this review only includes comparative studies, some items were not applicable because of their study design. These items were scored as 2 to prevent them from being misinterpreted as high risk of bias. For example, the item, adequate control group, was not applicable for cohort or cross-sectional studies. The item, contemporary groups, was applicable only to cross-sectional studies, whereas the items, follow-up period appropriate to the aim and loss of follow-up, were not applicable for cross-sectional studies.
The risk of bias in nonrandomized studies of intervention (ROBINS-I) tool was used to assess the risk of bias of prospective and retrospective cohort studies. The tool evaluates confounding bias, selection bias, bias in classification of intervention, deviations from intended interventions, missing data, measurement of outcomes, and selection of reported results. The risk of bias within each study is then graded as low, moderate, serious, critical, or no information.
Kappa statistics were used to estimate the interrater reliability between 2 independent reviewers (A.A.R and P.P) for assessing the risk of bias of the articles.
A total of 11,679 articles were identified through the initial electronic database search, including 10 handsearched articles. In the first phase, 3299 duplicates were identified by using Endnote X9, and a total of 8380 articles were transferred to Rayyan, which in addition identified 699 duplicates that were excluded after screening by the first author (A.A.R). In the second phase, 7681 articles were screened on the basis of title and abstract, of which 7471 articles were excluded as they did not match the selection criteria. Two authors (A.A.R and P.P) independently assessed the eligibility of the remaining 210 papers by screening their full texts, and a good agreement between them was reported (Kappa, 0.8). A total of 40 articles were finally included in this review. The Preferred Reporting Items for Systematic Reviews and Meta-Analyses flowchart of the study selection process has been described in Figure 1 , and the reasons for the exclusion of articles after full-text reading have been provided in Supplementary Table II .
Table I shows the characteristics of the included studies. Eight cross-sectional and 32 cohort studies were included. Of the 32 cohort studies, 24 were retrospective, 4 prospective, and 4 studies used both methods. In these studies, the most reported populations were white (28 out of 40) , and Asian origin (12 of 40). The shortest and longest observation periods were 2.0 years and 47.6 years, respectively. , Sample sizes oscillated between 10 and 436 subjects, , and sex distribution was fairly even, except for 1 article that only reported on females. Dental occlusal changes were analyzed in subjects aged from 3.0 years , , to 60.9 years. The most common occlusal trait analyzed was arch width (22 out of 40 articles). , , , , , , , , , , , , , , ,
|Author (y)||Study design||Sample, n||Population||First observation||Last observation||No./period of observation||Mode of investigation||Arch width||AD, AL, and AP||Overjet/overbite||Crowding, spacing, ALD||Tooth angulation/inclination|
|Age, y||Sex, M/F||Age, y||Sex, M/F||Intercanine||Intermolar|
|Dekock (1972)||RCS and PCS||26||Northern European descent (Iowa facial Growth study)||12.0||16/10||26.3||16/10||4/14.3||Sliding calipers accurate to 0.1 mm, dental casts||NM||12.0-15.0 y for M AW ↑∗∗ of 1.16 mm in Mx and 0.58 mm in Mn||AD: 12.0-26.0 y ↓ in both M and F with no sex differences||NM||NM||NM|
|Herness et al (1973)||RCS||20||Ohio University growth study Middle and Northern European||5.0||9/11||11.0||9/11||4/6||Vernier calipers; dental casts, lateral cephalograms||NM||NM||NM||OB: 5.0-9.0 y ↑ by 1.9 mm∗∗ and∗ ↑ 0.9 mm until 11 y∗||NM||NM|
|Sinclair et al (1983)||RCS||65||Burlington Growth Center, Canada||9.06||33/32||20.06||33/32||3/11.01||Dial calipers accurate to 0.01 mm, Little irregularity index, dental casts||9.0-20.0 y in Mn a ↓∗∗ in both M and F of 0.75 mm, 13.0-20.0 y ↓∗∗ of 0.44 mm and F showed∗↓ of 0.73 mm||9.0-20.0 y for Mn ∗↓ by 0.88 mm in F, 13.0-20.0 y ∗↓ by 0.47 mm in F||AL: 9.0-13.0 y∗↓ of 2.88 mm, 13.0-20.0 y∗↓ of 1.95 mm, 9.0-20.0 y a∗∗ ↓ 4.83 mm in both M and F||OJ: 9.0-13.0 y ↑∗∗ of 0.44 mm, 13.0-20.0 y ↓∗∗ of 0.48 mm;
OB: 9.0-13.0 y ↑∗∗ of 0.4 mm, 13.0-20.0 y ↓∗∗ of 0.59 mm
|Crowding: 13.0-20.0 y ↑∗∗ by 0.7 mm in Mn for both M and F; ∗↑ in F > M||NM|
|Sinclair et al (1985)||RCS||65||Burlington Growth Center, Canada||9.06||33/32||20.06||33/32||3/11||Lateral cephalograms||NM||NM||NM||NM||NM||Angulation: 9.0-20.0 y
∗↑ in mesial tip of LM;
Inclination: 9.0-13.0 y L1 in M∗↑ 4.84°; 13.0-20.0 y in F L1 ↑ by 2.39°, 13.0-20.0 y U1L1∗↑ by 4.05° in M
|Bishara et al (1989)||RCS||32||Northern European Descent (Iowa Growth study)||13.3||18/14||26.0||18/14||2/12.75||Dental casts, lateral cephalograms||13.3-26.0 y ↓ in M and F in Mx by 0.47 mm and 0.43 mm, respectively; for Mn ↓ in M and slight ↑ in F||13.3-26.0 y ↓ in M and F for Mn by 0.57 mm and 0.36 mm, respectively; for Mx ↑ in M and slight ↓ in F||AL: 13.3-26.0 y total AL ↓ in M and F for Mx by 2.54 mm and 2.33 mm and for Mn by 2.61 mm and 1.25 mm, respectively||OJ: 13.3-26.0 y ↓ in M by 0.44 mm and slight ↑ in F by 0.11 mm;
OB: 13.3-26.0 y ↓ in M and F by 0.54 mm and 0.15 mm, respectively
|NM||Inclination: U1 ↓ in M by 0.69° and L1 by 1.79° with a slight ↑ in F by 1.50° in U1 and 0.51° in L1|
|Ursi et al (1993)||RCS||51||Northern European Descent (Bolton Brush study)||6.0||16/16||18.0||16/16||6/12||Lateral cephalograms||NM||NM||NM||NM||NM||Inclination: No sexual dimorphism at any age for M and F for U1 and L1|
|Chang et al (1993)||CSS||160||Chinese||11.95||40/40||NM||40/40||2/NA||Digitized lateral cephalograms tracings||NM||NM||NM||NM||NM||Inclination: 11.95 y to adults (NS) changes seen between either of sexes and between the age groups for U1-PP and L1-MP|
|Bishara et al (1994)||RCS||30||Northern European Descent (Iowa facial Growth study)||25.55||15/15||45.65||15/15||2/20.1||Digitized dental casts and lateral cephalograms landmarks, dial calipers accurate to 0.05 mm for tooth size||25.0-46.0 y a ↓∗∗ in F by 0.39 mm for Mx and ↓∗∗ 0.56 mm in Mn and a ↓∗∗ of 0.40 mm in Mn for M||25.0-46.0 y a (NS) ↓ in M and a (NS) ↑ in F was seen in both Mx and Mn||AL: 25.0-46.0 y in M ↓∗∗ of 0.47 mm in Mx left anterior segment while in F the total Mx AL∗↓ with a ↓ (NS) in left anterior segment||OB: 25.0-46.0 y in F ↑∗∗ by 1.04mm;
OJ: 25.0-45.0 y a ↑ (NS) in both M and F is seen
|TSALD: 25.0-46.0 y in M total TSALD ↑∗∗ for Mx and Mn by −0.65 mm and −0.94 mm, respectively; in F in Mn anterior segment ↑∗∗ by −0.57||Inclination: 25.0-46.0 y in F U1-SN ↓∗∗ by 2.71°; in M and F U1L1∗↑ by 2.55° and 2.45°, respectively|
|Bishara et al (1996)||RCS||30||Northern European Descent (Iowa facial Growth study)||25.55||15/15||45.65||15/15||2/20.1||Digitized dental cast, dial caliper accurate to 0.05 mm for tooth size||25.0-46.0 y in F ↓∗∗ by 0.4 mm and 0.6 mm in Mx and Mn, respectively; for M a ↓∗∗ of 0.4 mm in mn is seen||25.0-46.0 y a (NS) ↓ in M and a (NS) ↑ in F was seen in both Mx and Mn||AL: 25.0-46.0 y in M ↓∗∗ of 1 mm and 1.2 mm for Mx and Mn, respectively; F ↓∗∗ of 1 mm and 0.8 mm for Mx and Mn, respectively||OJ: 25.0-46.0 y ↑ (NS) in M and F;
OB: 25.0-46.0 y in F ↑∗∗ by 1 mm
|TSALD: 25.0-46.0 y in M a total TSALD ↑∗∗ by −0.6 mm for Mx, In Mn anterior TSALD ∗↑ −0.9 mm and −0.6 mm for F∗∗||NM|
|Bishara et al (1997)||RCS||30||Northern European Descent (Iowa Growth study)||3.0||15/15||45.0||15/15||6/42||Dental casts, dial caliper accurate to 0.05 mm||3.0-13.0 y ↑∗∗ for Mx in both M and F is seen; for Mn ↑∗∗ in M until 8.0 y and in F until 13.0 y; 26.0-46.0 y a ↓∗∗ in both sexes was seen in Mn||3.0-13.0 y a ↑∗∗ in M and F for both Mx and Mn, 13.0-26.0 y in F a ↓∗∗ only in Mx is seen||NM||NM||NM||NM|
|Bishara et al (1998)||RCS||35||Northern European Descent (Iowa facial Growth study)||5.0||20/15||45.0||15/15||5/40||Lateral cephalograms||NM||NM||NM||OB: 5.0-10.0 y∗↑ in both M and F by 2.1 mm and 1.4 mm; 25.0-45.0 y in F a ↑∗∗ of 1mm||NM||NM|
|Bishara et al (1998)||RCS||30||Northern European Descent (Iowa facial Growth study)||3.0||15/15||45.0||15/15||6/42||Dental casts, dial caliper accurate to 0.05 mm||NM||NM||AL: 3.0-13.0 y∗↑ in Mx for both M and F, 13.0-45.0 y∗↓ in both sexes in Mx; 8.0-45.0 y∗↓ in both M and F in Mn||NM||NM||NM|
|Ferrario et al (1999)||CSS||70||Italy||12.8||10/10||20.0||25/25||2/7.2||Dental casts digitized||13.0-20.0 y (NS) ↓ of 0.06 mm in Mn||13.0-20.0 y (NS) ↓ of −0.35 mm in Mn||NM||NM||NM||NM|
|Ross-Powell et al (2000)||RCS||52||Black American||3.0||25/27||18.0||25/27||16/15||Dental casts photocopied and digitized||3.0-5.0 y ↑∗∗ of 2.5 mm in Mx 3.0-5.0 y↑∗∗ in mn of 0.9 mm and 0.7 mm in M and F||NM||AD: 12.0-18.0 y ↓∗∗ of 1.5 mm in Mx, 6.0-10.0 y in Mn ∗↑ of 2 mm followed by ↓ from 10 y onwards||NM||NM||NM|
|Ferrario et al (2001)||CSS||100||Northern Italians||14.6||21/22||19.8||26/31||2/5.2||Dental casts digitized||NM||NM||NM||NM||NM||Inclination: C, PM, and M were more inclined in adolescents than in adults in all the 4 quadrants|
|Henrikson et al (2001)||RCS and PCS||30||Swedish||13.6||11/19||31.1||11/19||2/17.5||Dental casts photocopied and digitized; Little irregularity index||13.0-31.0 y a total ↓∗∗ of 0.4 mm and 0.7 mm in Mx and Mn, respectively||13.0-31.0 y for M in Mn ↑∗∗ by 1.6 mm||AD: 13.0-31.0 y ↓∗∗ in both Mx and Mn by 1.8 mm and 1.6 mm. M show more ↓∗∗ than F in Mn||NM||Crowding: 13.0-31.0 y in Mn a combined ↑∗∗ of 1.4 mm in both M and F||NM|
|Slaj et al (2003)||PCS||30||Croatia||9.71||17/13||11.72||17/13||2/2.01||Dental casts digitized, Sliding caliper with 0.1 mm precision||9.7-11.72 y ↑∗∗ of 0.82 mm for left side from median palatal raphe||9.7-11.72 y∗↑ of 0.27 mm in right side of Mn||AL: 9.7-11.72 y ↓∗∗ by 1.07 mm and 0.94 mm in Mx and Mn;
AD: 9.7-11.72 y a ↓∗∗ of 0.69 mm in Mx and∗↓ of 0.67 mm in Mn
|Tsai (2003)||CSS||297||Taiwanese||4.0||11/10||30.0||25/29||6/26||Digitized lateral cephalograms tracing||NM||NM||NM||NM||NM||Inclination: Deciduous to adult∗↓ L1-MP with a mean value in F > M|
|Marshall et al (2003)||RCS||36||Northern European Descent (Iowa facial Growth study)||7.5||18/18||26.4||18/18||5/18.7||Dental casts, custom measuring gauge, digital calipers||NM||7.5-26.4 y a mean ∗∗↑ of 2.8mm in Mx and 2.2mm in Mn was reported||NM||NM||NM||Inclination: 7-26 y a ↓∗∗ or upright lingually of UM1, UM2 by 3.3° and 5.9° and LM1, LM2 ↓∗∗ upright buccally by 5° and 7.5°|
|Lux et al (2004)||RCS||18||Ireland, Belfast Growth study||7.6||10/8||15.66||10/8||5/8.06||Posteroanterior cephalograms digitized landmarks, dental casts, dial caliper nearest 0.02 mm||NM||7.0-15.0 y∗↑ in Mx for M and F by 2.44 mm and 1.39 mm, respectively||NM||NM||NM||NM|
|Tibana et al (2004)||PCS||27||Brazilian Caucasian||21.2||13/14||28.4||13/14||2/7.2||Dental casts, digital caliper with 0.01 mm precision and steel ruler, little irregularity index||21.0-28.0 y in Mn a ↓∗∗ of 0.39 mm.||21.0-28.0 y in Mn a∗↓ by 0.25 mm||AP: 21.0-28.0 y a ↓∗∗ of 0.67 mm and 0.71 mm in both Mx and Mn, respectively||OJ: (NS) ↓ of 0.02 mm;
OB: 21.0-28.0 y ↑∗∗ by 0.39 mm
|Crowding: 21.0-28.0 y a ↑∗∗ by 0.38 mm in Mx and 0.54 mm in Mn||NM|
|Yavuz et al (2004)||RCS||45||Turkey||10.0||23/22||14||23/22||4/3.63||Posteroanterior cephalograms||NM||10.0-14.0 y ↑∗∗ for both M and F and in both Mx and Mn||NM||NM||NM||NM|
|Thilander et al (2005)||RCS||169||Swedish||5.0||20/27||31||11/19||7/26||Digitized lateral cephalograms landmarks||NM||NM||NM||NM||NM||Inclination: 5.0-16.0 y Mx and Mn incisors proclined for M and F; Mean U1L1 is 130° in M and F at 10.0 y followed by a ↑ until adult|
|Hesby et al (2006)||RCS||36||Northern European Descent (Iowa facial Growth study)||7.6||18/18||26.4||18/18||5/18.8||Dental casts, Digital calipers, Posteroanterior cephalograms||NM||7.6-26.4 y a ↑∗∗ in Mx by 3.08mm and ↑∗∗ in Mn by 2.05mm||NM||NM||NM||NM|
|Moldez et al (2006)||CSS||157||Filipinos||7.0||21/29||22.1||21/21||4/15.1||lateral cephalograms digitized||NM||NM||NM||NM||NM||Inclination: Mx and Mn incisors proclined overall while from 7.0-10.0 y incisor proclination in F > M (P<0.05)|
|Arslan et al (2007)||RCS||65||Turkey||9.54||29/36||14.54||29/36||2/5||Dental casts, digital calipers with accuracy of ± 0.01 mm, Little irregularity index||9.5-14.5 y a total ∗↑ by 0.92 mm in Mx for M and F; M ∗↑ by 1.59 mm in Mx||9.5-14.5 y a total ↑∗∗ in Mx and Mn by 1.46 mm and 1.31 mm, respectively||AP and AD showed a (NS) ↓||OJ: 9.5-14.5 y a total ↑∗∗ by 0.23 mm;
OB: 9.5-14.5 y total ↑∗∗ of 0.41 mm
|Crowding: 9.5-14.5 y ↓(NS) of −0.12 mm and −0.19 mm in Mx and Mn||NM|
|Thilander (2009)||RCS||436||Swedish||5.0||20/27||31.0||11/19||6/26||Dental casts, digital calipers||5.0-16.0 y in Mx ↑ of 4 mm; in Mn a same degree of ↑ until 10 y followed by ↓ in M from 16-31 y||13.0-16.0 y ↓ in Mn||AL: 7.0-13.0 y ↓ by 1 mm in Mx and 3 mm in Mn a 5.0-10.0 y ↑ of anterior segment in Mx and Mn
AD: 7.0-13.0 y ↑ of 5 mm and 3 mm in Mx and Mn, respectively
|Chen et al (2010)||PCS||28||Chinese||13.0||13/15||18.0||13/15||6/5||Lateral cephalograms digitized||NM||NM||NM||NM||Spacing: 13.0-18.0 y in posterior Mn with a total ↑∗∗ for M is 5.79 mm and in F is 5.12 mm||NM|
|Louly et al (2011)||CSS||66||Brazilian||9.0||3/5||12.0||10/8||4/3||Dental casts, Korkhaus compass and digital pachymeter||9.0-12.0 y (NS) ↑ of 1.2 mm in Mx and a same amount of (NS) ↓ in Mn||9.0-12.0 y (NS) ↑ of 1.7 mm in Mx and a (NS) ↑ of 1.2 mm in Mn||AL: 10.0-12.0 y a∗↑ in anterior segment in Mx of 1.4 mm
AD: 9.0-10.0 y M∗ > F
|Heikinheimo et al (2012)||RCS||33||Finnish||7.0||15/18||32.0||15/18||5/25||Dental casts, vernier calipers||7.0-10.0 y for M and F ∗↑ in Mx of 1.05 mm and 1.59 mm while in Mn by 0.29 mm and 1.12 mm is reported; 15.0-32.0 y∗↓ for M and F in both Mx and Mn is present||7.0-15.0 y in F ∗↑ of 1.07 mm and 0.16 mm in Mx and Mn, For M ∗↑ of 1.56 mm and 0.25 mm in Mx and Mn; 15.0-32.0 y ∗↓ in both M and F||NM||OJ: 7.0-10.0 y ↑(NS) followed by ↓(NS) until 32.0 y during 15.0-32.0 y most reduction occurs;
OB:↑(NS) in both M and F from 7.0-12.0 y followed by ↓(NS) from 12.0-32.0 y
|Inada et al (2012)||CSS||34||Kagoshima, Japan||5.2||0/11||20.3||0/23||2/15.1||Digitized dental casts, lateral and frontal Cephalograms||NM||NM||NM||NM||NM||Inclination: 5.0-20.0 y ↑∗∗ by 10.9° of U1 and 15.4° of U2|
|Ahn et al (2012)||RCS||66||Korean Dental Growth Study||6.0||16/50||14.0||16/50||10/8||Dental casts, 3-dimensional laser scanner and reconstruction||6.0-8.0 y in F in deciduous canines ↑ of 1.86 mm and 1.62 mm in both Mx and Mn was reported while in M 2 mm and 1.26 mm ↑ in Mx and Mn was reported||NM||NM||NM||NM||NM|
|Tanoi et al (2012)||RCS and PCS||10||Tokyo, Japan||22.0||8/2||42.0||8/2||2/20||Dental casts digitized, Little irregularity index||22.0-42.0 y ∗↓ in Mx and Mn of 0.54 mm and 0.83 mm, respectively||22.0-42.0 y ∗↓ in Mx and Mn of 0.65 mm and 0.49 mm, respectively||AL: 22.0-42.0 y ∗↑ in Mx anterior segment by 0.76 mm; in Mn anterior and posterior segment ↓∗∗ by 1.03 mm and 1.49 mm, respectively||OJ: 22.0-42.0 y a ↑∗∗ of 0.84 mm;
OB: 22.0-42.0 y a ↑∗∗ of 0.65 mm
|Crowding: 22.0-42.0 y a ↑∗∗ of 1.8 mm in Mn anterior segment||Angulation: 22.0-42.0 y U6-U3 ∗↑ in mesially
Inclination: 22.0-42.0 y a∗↑ of U1 by 0.83 and ∗↓ in L1 by −1.17
|Stahl de Castrillon et al (2013)||RCS||32||German University of Rostock growth study||6.0||16/16||17.0||16/16||11/11||Dental casts, lateral cephalograms digitized||NM||NM||NM||OJ:(NS)
|NM||Inclination: 6.0-11.0 y U1 and L1 remained stable. Differences between sexes for U1 and L1 Inclination at age 9.0, 10.0, and 15.0 were seen∗|
|Bae et al (2014)||RCS||68||South Korean||9.0||36/32||19.0||18/13||6/10||Lateral cephalograms digitized||NM||NM||NM||OJ and OB: M showed less amount of OB and OJ than F at 9.0 y with a NS ↑||NM||Inclination: (NS) ↑ in U1L1 of 4.5°, U1 and L1 with age|
|Sayania et al (2017)||RCS||95||Burlington Growth Center, Canada.||6.0||13/8||16.0||42/50||8/10||Dental casts digitized||NM||NM||NM||NM||NM||Inclination: From 6.0-16.0 y U6 upright palatally and L6 buccally. M> F|
|Eslami Amirabadi et al (2018)||CSS||237||Iranian||5.53||24/27||14.9||61/73||3/9.37||Dental casts, Korkhaus’ compass, digital caliper||9.0-14.0 y a ↑∗∗ in Mxby 4.6 mm; M∗ > ↑ F||5.0-9.0 y a ↑∗∗ in Mx by 6.2 mm, 9.0-14.0 y ↑∗∗ in Mx 2.42 mm||AD: 9.0-14.0 y a ↑∗∗ in Mx by 5.68 mm||NM||NM||NM|
|Massaro et al (2018)||RCS and PCS||22||White Brazilians||13.3||12/10||60.9||12/10||3/47.6||Dental casts digitized||17.0-60.0 y a ↓∗∗ in Mn by −0.69 mm||17.0-60.0 y a (NS) ↑ of 0.44 mm and 0.7 mm in Mx and Mn, respectively||AL: 13.0-17.0 y ↓∗∗ in Mx by −1.11 mm; 17.0-60.0 y ↓∗∗ in mx 1.1 mm and 1.06 mm in Mn;
AD: 13.0-17.0 y in Mx a ↑∗∗ of 1.74 mm
|OJ: (NS) ↓ of 0.06 mm,
OB: 13.0-17.0 y and 17.0-60.0 y a ↓∗∗ by 0.79 mm and 0.61 mm
|Crowding: 13.0-17.0 y a ↑∗∗ by 0.87 mm in Mn; 17.0-60.0 y a ↑∗∗ in Mx and Mn by 0.82 mm and 1.54 mm, respectively||NM|
|Yang et al (2019)||PCS||44||Beijing, China||10.0||22/22||12.0||22/22||3/2||Dental casts digitized||10.0-12.0 y a ↑∗∗ of 0.95 mm in Mx||10.0-12.0 y a ↑∗∗ of 1.46 mm in Mx||AL: 10.0-12.0 y a ↑∗∗ in Mx by total of 0.58 mm and in the anterior segment by 0.96 mm||NM||NM||Inclination: 10.0-12.0 y in right Mx U6 a ↓∗∗ by −3.36° and left. U6 ↓∗∗ by −5.13°|
|Stern et al (2020)||RCS||31||German Caucasian growth study||3.55||16/14||15.05||11/11||4/11.5||Dental cast and extraoral photographs digitized||3.0-15.0 y ∗↑ in Mx for M and F by 5.8 mm and 5.3 mm and in Mn by 4.1 mm and 4 mm respectively||3.0-15.0 y a ↑ in Mx for M and F by 1.8 mm and 0.5 mm, and in Mn by 2 mm and 0.6 mm; M ∗ > F||NM||OJ: (NS)
Risk of bias
Table II shows the risk of bias of the included articles according to the MINORS tool. The score of each article was graphically plotted in Figure 2 . Articles were sorted from older to newer publication dates, and a discrete tendency to better quality articles with time can be observed. The lowest score was 16, and the highest was 22. In addition, 31 of the 40 articles scored >75% of the maximum score for comparative studies (24). Two articles achieved 91.6% of the maximum score, studying 2 different populations from 5.5 years to 60.9 years. , Seven articles , , , adequately reported the establishment of a protocol before the start of the study, and the sample size was estimated only in 2 articles. ,