Median diastema: Prevalence, novel classification, and evidence-based investigation

Abstract

BACKGROUND:

Maxillary midline diastema (MMD) is a gap between the maxillary central incisors, posing cosmetic and psychosocial concerns for most patients. MMD results from multifactorial causes, including labial frenum, microdontia, genetics, and various oral habits. The purpose of this article was to review MMD, analyze its prevalence, describe a new shape-based classification, and identify potential causes with a focus on evidence-based practice.

MATERIALS AND METHODS:

This cross-sectional descriptive study utilized orthodontic clinical records of Saudi adults aged 18–30 years who had not sought orthodontic treatment. MMD presence was confirmed by examining dental models. Diastema size was measured at gingival, middle, and incisal levels using a Mitutoyo calliper with 0.1 mm accuracy. Pairs of Angle Class I models were used to ensure equivalent tooth size for specimens. Patient records were reviewed to determine etiologies, and Bolton’s tooth size ratio was calculated for analysis. Statistical analyses included Z-test, Shapiro–Wilk test, paired/independent t-tests, and post-hoc Bonferroni tests, conducted using SPSS version 26.

RESULTS:

The overall prevalence of MMD was 25% (n = 96), with 67% occurring in males. Among malocclusions, MMD prevalence was 42% in Class I, 32% in Class II, and 26% in Class III. MMD size distribution was 0.5–1 mm (18%), 1–2 mm (38%), and >2 mm (44%). A new shape-based classification identified trapezoidal (44%), triangular (40%), rectangular (12%), inverted triangular (2%), and biconcave (2%) forms. Major contributors included labial frenum elongation (25%), microdontia (19%), and midline shifting (72%). No significant differences in tooth sizes were found between MMD cases and Angle Class I controls (P > 0.05). Differences at the three measurement levels were negligible (P > 0.05) for both gender and malocclusion.

CONCLUSION:

This study highlights the polygenic nature of MMD and introduces a new shape-based classification. Key predisposing factors, such as midline shift and microdontia, were identified. These findings will guide clinicians in treating patients diagnosed with MMD.

Introduction

Maxillary midline diastema (MMD) is the space between the upper central incisors, a common dental aesthetic concern.[1,2] It is associated with psychological discomfort due to its impact on smile esthetics, prompting many patients to seek orthodontic treatment. Keene defined MMD as spacing greater than 0.5 mm between the proximal surfaces of adjacent teeth, classifying it as a malocclusion.[3] This study confirms a higher MMD prevalence in the maxilla compared to the mandible, with contributions from interrelated and independent factors.[4,5,6,7] Potential MMD causes include high labial frenum attachment, microdontia, mesiodens, peg-shaped lateral incisors, agenesis, cysts, dental malformations, genetic predisposition, and ethnic characteristics.[8] Defects in skeletal and dental hard tissue growth and imperfect interdental septa are documented as potential etiopathogenic factors.[9,10] Therefore, the reduction in size and the specific shape or central incisors are vital in measuring the level of MMD. Correct identification is critical when it comes to the steps to take, which include orthodontics and prosthodontics among other restorative treatments, including composite buildup, veneers, and crowns.[11,12] Thus, the presence of conservative and surgical treatment does not exclude the necessity of further investigations of the factors that predicate MMD, as well as accurate classification systems.

Accurate identification is critical for determining treatment steps, including orthodontics, prosthodontics, and restorative treatments, such as composite buildup, veneers, and crowns. Variations in MMD incidence and causes across cultures suggest multifactorial influences, including genetics, environmental interactions, and cultural practices. Research indicates a higher MMD prevalence in individuals of African descent compared to Caucasian or Asian populations, attributed to differences in dentition, jaw form, and phylogenetic factors.[13,14] These variations highlight the need to make diagnostic and treatment plans that will suit the different parts of the body and the genetic differences associated with many patient populations.

Chung and Myers identified genetic and racial characteristics, along with tooth eruption sequences, as factors influencing MMD development. Transitional diastemas, common during mixed dentition, are most apparent before permanent canine eruption, often termed the “ugly duckling stage.”[15] However, many times diastemas are persistent or pathological, and they cannot be left untreated; the patient’s conditions have to be evaluated and treated. Clinical assessment is coupled with radiographic findings and even diagnostic tools to help determine between physiological and pathophysiological conditions.

Intervention strategies for MMD range from nonpharmacological approaches to comprehensive multimodal management. Smaller gaps can be closed using orthodontic appliances or direct restorative measures, such as composite resins, for immediate aesthetic results. Larger or complex cases may require orthodontic treatment, frenectomy, and prosthodontic restoration.[16] Advanced technologies, such as digital smile design and minimally invasive aesthetic procedures, enhance MMD management and enable personalized, effective therapy. This study proposes a new shape-based classification system, assesses etiological factors in detail, and evaluates microdontia’s role in MMD cases. The study explored MMD etiology in orthodontic cases and introduced a novel shape-based identification method. It also examined microdontia by comparing mesiodistal crown dimensions in MMD patients and Angle Class I controls.[17] Previous studies have shown variable distribution of MMD among malocclusion classes, which indicated differing prevalence among malocclusion classes: Class I (45%), Class II (34%), and Class III (21%). Our results present a similar trend but highlight regional variations.[18]

It also assessed for an exploration of numerous causes of etiology of the exact MMD in orthodontic cases and presented a fresh identification method based on its shape. It also aimed at describing an association of microdontia by determining the mesiodistal crown dimensions in the MMD patients and Angle Class I control individuals.

Materials and Methods

This cross-sectional study was conducted at the Orthodontic Clinic, College of Dentistry, Jouf University, to evaluate MMD prevalence, patterns, and causative factors. The study was approved by the Local Committee of Bioethics at Jouf University (Approval No. LCBE#JoufUniversity#4-07-44), adhering to the Declaration of Helsinki ethical guidelines. Written informed consent was obtained from all participants prior to the study. The study included 282 orthodontic clinical records of Saudi adults aged 18–30 years, based on specific inclusion criteria. Participants were required to have all natural anterior and posterior teeth, including maxillary central, lateral, and canine incisors, premolars, and first permanent molars. Exclusion criteria included patients with worn, restored, or edentulous teeth, misaligned teeth, dentofacial deformities, or a history of orthodontic treatment or orthognathic surgery. The sample size was calculated using G*Power software version 3.1.9.2, with 80% power, 0.05 alpha level, and 0.05 effect size, determining 282 records as sufficient. Dental models from selected clinical records were inspected to confirm MMD presence. Cases with diastema were evaluated to measure shape and size at incisal, middle, and cervical levels. Measurements were performed using a digital Mitutoyo slide caliper (Japan) with a resolution of 0.01 mm. A control group of Angle Class I dental models was assembled from the archive for tooth-size comparison. Mesiodistal dimensions of maxillary anterior teeth were recorded parallel to the occlusal or labial plane. Bolton’s tooth size ratio was calculated to compare maxillary and mandibular dentition, including anterior ratio and maxillary excess. Measurement reliability was assessed through error testing, with intra-observer reproducibility evaluated by remeasuring 10% of models after two weeks. Consistency was measured using inter- and intraclass correlation coefficients and Dahlberg’s formula: ME = [Σ(x₁ − x₂) ²/2n], where x₁ and x₂ are the first and second measurements, and n is the number of measurements.

Malocclusion was classified according to Angle’s standard classification (Class I, Class II, and Class III). A priori power analysis using GPower software version 3.1.9.2 was conducted, assuming an effect size of 0.05, alpha level of 0.05, and power of 80%, resulting in a required sample size of 282 orthodontic clinical records.

Mesiodistal widths of anterior teeth (canine-to-canine) were measured using digital calipers and compared between MMD patients and Angle Class I controls to detect tooth size discrepancies. Variables recorded included demographic data (age, gender), malocclusion classification (Angle’s Classes I, II, and III), diastema presence and size (gingival, middle, and incisal), diastema shape classification, and Bolton’s tooth-size ratios.

Data analysis was performed using SPSS version 26. Data were analyzed using descriptive statistics, frequencies, and Z-scores for comparison with normative data. Inferential analyses included independent samples t-tests, one-way ANOVA, and post-hoc Bonferroni correction. Statistical significance was set at P < 0.05 to ensure the validity of findings. Each statistical test was selected based on data type and distribution characteristics: independent t-tests compared tooth dimensions between groups; ANOVA examined differences across multiple malocclusion categories; Bonferroni corrections adjusted for multiple comparisons to control for type I error.

Results

Frequency of median diastema (MD)

In the sample, the prevalence rate of MD was 25% (96), and with 67% of the cases in the male subjects [Figure 1]. The distribution of the MD with respect to the different malocclusion classes was Class I 42%, Class II 32%, and Class III 26% [Figure 1]. However, 28% of these cases had coincident midline and 72% had shifting of midline [Figure 1].

Figure 1.

(a) Prevalence of MD 25% (n = 96); (b) 67% in male; (c) 32%, 26%, and 42%, respectively, in Class I, II, and Class III malocclusion; and (d) 28% in midline coincide cases

Classification of MD by size and shape

The size classification of MD revealed three categories: The distribution of MMD by size was 0.5–1 mm in 18% of cases, 1–2 mm in 38%, and >2 mm in 44%. Shape classification introduced five novel forms: This was followed by trapezoidal (44%), triangular (40%), rectangular (12%), inverted triangular (2%), and biconcave (2%) [Figures 2 and 3].

Figure 2.

(a) MD classification based on size. 0.5–1 mm (18%), >1–2 mm (38%), and > 2 (44%). (b) MD classification based on shape. Trapezoidal (44%), triangular (40%), rectangular (12%), inverted triangle (2%), and biconcave (2%)

Figure 3.

(a) MD size in three levels, (b) shape, trapezoidal, (c) triangular, (d) rectangular, (e) inverted triangle, and (f) biconcave

Risk factors involved in MD

According to the etiological analysis, the most frequent causes included enlarged labial frenum in 25% of cases, microdontia in 19%, heredity in 17%, and deep bite in 15%. The other minor anomalies observed were absent laterals, 10%; tongue thrusting, 4%; macrognathia, 4%; thumb sucking, 2%; peg laterals, 3%; and mesiodens, 1% [Figure 4].

Figure 4.

MD distribution based on etiology. 19%—microdontia, 4%—macrognathia, 10%—missing lateral, 3%—peg lateral, 1%—mesiodens, 25%—enlarged labial frenum, 15%—deep-bite, 2%—thumb sucking, 4%—tongue thrusting, and 17%—heredity

MMD size at different levels

Descriptive analysis of MD at incisal, middle, and cervical levels failed to reveal any effect of gender, midline coincidence, or the type of malocclusion (Class I, II, or III). The mean size at each level was as follows: incisal, 2.794; middle, 2.203; and cervical, 1.623. The statistical results are presented in Table 1.

Table 1.

Three different level MD size measurements. Comparison among gender, midline coincide, and malocclusion groups

Variables	Descriptive statistics			Mean			P
	Mean		SD	Male	Female
Incisal	2.794		1.233	2.698	2.984		NS
Middle	2.203		1.088	2.133	2.344		NS
Cervical	1.623		1.051	1.604	1.661		NS
Variables	Mean		P	Mean			P
	Shifting	Coincide		Class I	Class II	Class III
Incisal	2.733	2.948	NS	2.545	3.240	2.708	NS
Middle	2.122	2.411	NS	1.987	2.452	2.215	NS
Cervical	1.541	1.833	NS	1.477	1.612	1.743	NS

Tooth-size analysis

Analysis of mesiodistal tooth size mean width/height ratios between the MD cases and Angle Class I controls also did not show any significant differences (P > 0.05). The mean values of individual anterior teeth and the sum of the values of canine-to-canine dimensions were similar, which excluded microdontia in MD cases [Table 2].

Table 2.

Teeth size comparison of MD cases [n=96] with Angle Class I cases [n=96]. To elucidate the evidence of microdontia, nonsignificant differences (P>0.05) in relation to individual maxillary anterior teeth, sum of maxillary canine-to-canine, and maxillary excess amount level

Variables	Tooth/Teeth	Mean	SD	95% CI		P
				Lower	Upper
MD	11	8.040	0.929	–0.348	0.167	0.489
Class I	11	8.130	0.878
MD	12	6.397	0.882	–0.314	0.179	0.590
Class I	12	6.464	0.848
MD	13	7.191	0.879	–0.320	0.171	0.551
Class I	13	7.265	0.847
MD	21	7.995	0.996	–0.368	0.185	0.514
Class I	21	8.087	0.946
MD	22	6.334	0.912	–0.313	0.197	0.654
Class I	22	6.392	0.881
MD	23	7.239	0.950	–0.334	0.200	0.622
Class I	23	7.306	0.928
MD	13–23	43.194	5.144	–1.880	0.983	0.537
Class I	13–23	43.642	4.909

Z-score conversion

To compare the assessments using mesio-distal tooth size in the subjects with MMD, all measured values for a subject were converted into Z scores in relation to the means and standard deviations of the two parameters in the Angle Class I subjects. The values of mesio-distal tooth size measurements depended on the age and sex of the subjects in the 96 Angle Class I subjects.

The Z score was calculated with the following formula:

Z score (X) = (X–X)/SD

where X is the measured value of MMD subject, and X and SD are the mean and standard deviation of the Angle Class I subjects. Based on the Z scores, the distribution in the maxillary excess was compared for the MMD subjects [Figure 5a-c].

Figure 5.

(a, b) Maxillary excess based on Bolton’s calculations. In MMD, level of access showed 42% normal, 40% small and 18% large. (c) Graphical Z-score value of maxillary access of 96 MMD cases

There was no significant difference in maxillary anterior tooth dimensions comparing 96 MD cases with 96 Angle Class I controls concerning the individual or total width of maxillary canine-to-canine teeth. The mean mesiodistal dimensions for the single teeth in MD cases compared to the control group were slightly smaller but not statistically significantly; for instance, for tooth 11, 8.040 mm and 8.130 mm for control, respectively, and tooth 21, 7.995 mm and 8.087 mm for control, respectively (P > 0.05).

Likewise, the total of the MD measurements for 13–23 was slightly less in cases (43.194 mm) than in the controls (43.642 mm) but did not differ significantly (P = 0.537). Based on the analysis done, it may be concluded that microdontia as defined by the size of the teeth does not play a major role in the expression of MD in this study community. Additional results are provided in Figure 5.

Discussion

MMD is a common orthodontic treatment demand, which results in patients’ aesthetic and psychological issues. The purpose of this research was to assess the prevalence of MMD, propose a new classification by shape and size and examine causative factors for this condition. The proposed shape-based classification system (trapezoidal, triangular, rectangular, inverted triangular, and biconcave) aids clinicians by offering practical criteria for tailored treatment approaches. Compared with previous classifications, this shape-specific approach can enhance diagnostic precision and improve clinical outcomes.

The overall rate of MMD in this study was 25%, again similar to previous surveys which suggest that MMD is a frequent complaint among orthodontic patients irrespectively of ethnicity.[1,2] The occurrence is relatively higher in male patients (67%); this has been supported by research showing that skeletal growth and arch shape variations between male and female may be related to the formation of diastema.[3] The distribution across malocclusion classes (Class I: The result indicates that though MD is higher in Class I malocclusion (90–42%) compared to Class II and Class III malocclusion (Class II 30–32%, Class III 26%) studied but this does not mean that it is absent in Class II and Class III malocclusion as suggested by previous studies on the subject. Although our analysis did not show statistically significant tooth-size differences suggestive of classic microdontia, future studies should consider alternative criteria, including tooth proportions and morphological anomalies, to better understand their relationship with MMD.

The new classification system proposed in this study divides MMD into categories according to size and form. They also categorized the diastemas using size whereby the diastemas bigger than 2 mm were most frequent (44%) a factor that proved that treatment methodologies for bigger gaps required further study. Shape classification introduced five subcategories with trapezoidal/triangular being the most frequent. They were able to identify that dentists use this classification to help decide what orthodontic or restorative treatment is needed, as the shape of the tooth plays an important role in this decision.[5,6]

In this study, the most prevalent cause of the manifestation of MMD as determined was enlarged labial frenum (25%), advancing microdontia (19%), hereditary factors (17%), and deep bite (15%). Labial frenum is today acknowledged as the leading factor causing considerable diastema because its excessive growth prevents central incisors from undergoing mesial migration.[7] Consequently, microdontia and tooth size differences readily explain diastema formation. The hereditary influence represents a direct agreement with the genetic link to MMD that has been demonstrated in several others.[9,10] Lack of laterals and peg-shaped teeth shows other nonsurgical factors which support MD as a multifactored disease, as the prior research.[11,12] presented. Clinically, smaller diastemas (<2 mm) may be effectively managed through orthodontic treatments or direct restorations like composite resins. Conversely, diastemas > 2 mm or those with significant labial frenum elongation typically require combined orthodontic, surgical (frenectomy), and prosthetic interventions. The proposed shape-based classification further aids clinical decision-making, especially when selecting restorative materials and techniques.

The analysis of mesiodistal tooth measurements and the comparison of the findings between the MD cases and Angle Class I controls suggest that microdontia may not always be the source of the problem. The conclusions drawn from this study are in agreement with earlier studies that proposed that factors other than tooth size determine diastema formation, including arch length and differences in skeletal structures.[13]

The findings vindicate implemented and proposed that MMD is a complex disorder arising from multiple causes in order to choose an individualized treatment plan. The size of the diastema naturally determines whether it can be treated with orthodontic appliances only or whether surgical and prosthetic treatments are needed as well.[14] That is why in cases when labial frenum becomes enlarged, the only efficient treatment remains surgery followed by the orthodontic treatment.[15]

That is why the new classification proposed in this study can be useful to clinicians as a more suitable approach for the assessment of MMD. As the data showed, MMD can be presented in different sizes and shapes, so this classification helps to reveal the differences in diastema and, therefore, improve treatment planning. For example, trapezoidal diastemas which were the most frequent type noted in the present analysis (44%) may demand different orthodontic control than linear diastemas as triangular or rectangular forms. These findings highlight the importance of acknowledging the differences in shapes when arriving at diagnosis and decisions on treatment as the shape of a diastema is determinant of the alignment and closure strategies to be used.[16,17,18,19,20,21] As aforementioned, this novel classification can also be used as a reference for additional research in terms of standardizing the diagnostic criteria for MMD.

In this present work, etiological analysis underscores the fact of MMD that it is a multidimensional disorder and there are considerable implications regarding the course of clinical management. However, the labial frenum was deemed the most recurrent patient complaint by the respondents at 25%; however, it was revealed to present with other complications, such as a deep bite and probable hereditary factors, which call for a team approach to treatment. Knowledge of these relations is fundamental to the psychological analysis of interventions for an individual whose academic performance needs to be improved and where the gap is covered and where possibly structural or hereditary problem is.[22,23,24,25,26] Potential contributions of arch perimeter discrepancies, soft-tissue factors, and skeletal relationships are recognized; however, these require explicit investigation in future research. Current evidence highlights the necessity of comprehensive etiological assessments incorporating cephalometric analysis, cone-beam computed tomography, and detailed soft-tissue examinations.

Since there were no large discrepancies of patients with MMD and control on mesiodistal tooth dimensions, it endangers the conventional fundamental understanding of microdontia as a major causal factor of diastema. Thus, this supports the idea that perhaps MMD is not a result of one principle etiology but rather an interplay between dental and skeletal parameters. The authors acknowledge that some of these parameters, like arch perimeter, soft tissue flexibility, and lower limb length inequalities may be contributing more than has been previously estimated. These ideas, therefore, stress the importance of comprehensive etiologic investigations, including cephalometric “tissue” analysis and evaluation of soft tissues, in order to identify specific etiologic factors in individual cases with MMD.[27]

This research adds to the recent calls identifying the beauty and utility of responding to MD. Apart from psychological and social problems associated with an appealing smile, untreated gap can result in the subsequent anomalies inclusive of communication impediments, periodontal diseases, and faulty occlusal loading.[28,29,30] These risks can, however, be reduced if an early diagnosis is made and intervention taken early enough. Diagnostic bench-top techniques using digital imaging and three-dimensional modeling in conjunction with artificial intelligence could finally help clinicians unravel diagnostic accuracy as well as predict outcomes of diastema interventions. As such, subsequent studies should use these technologies to optimize treatment and improve the quality of patients’ lives.

The present study acknowledges several methodological limitations. First, its retrospective design relied on existing clinical records, which may limit the control over data quality and completeness, potentially introducing selection and information biases. Future research adopting a prospective approach is recommended to mitigate such limitations. Additionally, the study population was restricted to Saudi adults aged 18–30 years, reducing the generalizability of findings to other ethnic groups and age categories. Subsequent studies should incorporate broader, multi-ethnic samples across different age ranges. Although sample size justification was provided using G*Power analysis, the selected small effect size (0.05) might not sufficiently power the study to detect subtle clinical differences; thus, future research should consider using larger effect sizes informed by prior studies or pilot data. Measurement reliability was assessed only through intra-observer reproducibility, and the absence of reported interobserver reliability could impact the consistency of measurements. Future studies should include interobserver reliability assessments. Furthermore, the exclusion criteria employed, particularly excluding patients with restored or worn teeth, may have omitted clinically relevant etiologies of maxillary midline diastema (MMD), potentially affecting the comprehensiveness of the etiological evaluation. Future research should expand inclusion criteria to encompass these patients, ensuring a thorough etiological assessment. The current study also primarily evaluated dental and skeletal factors, but exploration of soft tissue dynamics and functional habits (e.g., tongue thrusting) was limited. Future research should incorporate detailed soft tissue and functional habit assessments through clinical examination and advanced imaging techniques. Lastly, while Bonferroni correction was employed, the rationale for selecting multiple statistical tests could have been clearer, raising concerns about type I error risk. Future research should explicitly justify each statistical test and consider additional adjustments such as false-discovery rate to control for multiple comparison errors effectively.

Conclusion

In this work, the frequency of MMD and its polygenic etiology is discussed, as well as a new size and shape classification is proposed. The results also pinpoint on the etiologic factor that includes labial frenum, microdontia, and genetic predisposition. The suggested classification and the detailed analysis of the cases will prove to be helpful to clinicians when examining MMD-related aesthetic and functional deficits, which should help to better differentiate the diagnosis and individualized approaches to the treatment.

Author contributions

All authors have contributed to conceptualizing of the work, writing, and reviewing the manuscript.

Consent for publication

All authors reviewed the final manuscript and consented for publication.

Conflict of interest

The authors declare no conflict of interests.

Data availability statement

The authors confirm that the data supporting the findings of this study are available within the article.

Funding Statement

None declared.