Advertisement
Browse Subject Areas
?

Click through the PLOS taxonomy to find articles in your field.

For more information about PLOS Subject Areas, click here.

  • Loading metrics

Three-dimensional morphology of first molars in relation to ethnicity and the occurrence of cleft lip and palate

  • Sandra Echtermeyer ,

    Contributed equally to this work with: Sandra Echtermeyer, Philine H. Metelmann

    Roles Data curation, Formal analysis, Investigation, Validation

    Affiliation Echtermeyer Dental Practice, Leipzig, Germany

  • Philine H. Metelmann ,

    Contributed equally to this work with: Sandra Echtermeyer, Philine H. Metelmann

    Roles Visualization, Writing – original draft, Writing – review & editing

    Affiliation Department of Orthodontics, Center for Dental, Oral and Craniomandibular Sciences, University Medicine Greifswald, Greifswald, Germany

  • Alexander Hemprich,

    Roles Conceptualization, Resources, Supervision

    Affiliation Department of Maxillofacial Surgery and Plastic Surgery, University Medicine Leipzig, Leipzig, Germany

  • Karl-Heinz Dannhauer,

    Roles Conceptualization, Methodology, Supervision

    Affiliation Department of Orthodontics, Center for Dental, Oral and Craniomandibular Sciences, University Medicine Greifswald, Greifswald, Germany

  • Karl-Friedrich Krey

    Roles Formal analysis, Project administration, Software, Supervision, Visualization, Writing – original draft

    kreyk@uni-greifswald.de

    Affiliation Department of Orthodontics, Center for Dental, Oral and Craniomandibular Sciences, University Medicine Greifswald, Greifswald, Germany

Three-dimensional morphology of first molars in relation to ethnicity and the occurrence of cleft lip and palate

  • Sandra Echtermeyer, 
  • Philine H. Metelmann, 
  • Alexander Hemprich, 
  • Karl-Heinz Dannhauer, 
  • Karl-Friedrich Krey
PLOS
x

Abstract

Objectives

This study aims to describe morphological peculiarities of maxillary and mandibular first molars in Europeans, Asians and Europeans with cleft lip and palate.

Material and methods

Reflex microscopy was used to obtain three-dimensional morphometric landmarks from 40 models (11 Europeans and 13 Asians without cleft lip and palate, 16 Europeans with unilateral cleft lip and palate). The cases were examined using traditional morphometry and geometric morphometry, and visualized using thin-plate splines.

Results

Classic morphometry showed no right/left differences in the study groups and no significant differences with regard to the cleft side in patients with cleft lip and palate. In Asians, a significantly greater mesiodistal width was found. Geometric morphometry showed an enlarged centroid size in Asians (maxilla and mandible). In cleft patients, the cleft site did not appear to impact the morphology of first molars.

Conclusion

Unilateral clefting did not affect the size and shape of molars; however, characteristic ethnicity-based differences were in fact identified. The results are relevant for orthodontic treatment with preadjusted appliances, and prosthetic CAD/CAM restorations.

Introduction

The development of molar crowns is a complex process that begins in the 28th week of gestation. The result are teeth shaped in a universal pattern with highly personalized details. In the case of a eugnathic dentition, this morphology enables a finely balanced function of static and dynamic occlusion. In a dysgnathic dentition, however, this functional balance can only be achieved through orthodontic treatment or by restoring lost tooth structure. It is therefore essential to have knowledge of the individual’s tooth shape and the average morphology of the individual’s ethnic group. The first molar (M1) plays a key role in this context [1]: Discrepancies in the mesiodistal diameter between permanent first molars on the left and right sides were described in connection with the development of asymmetric malocclusions [2].

To a large extent, tooth morphology is determined genetically [3] but can be modulated by external factors. High conformity in the tooth morphology of monozygotic and dizygotic twins [4] supports this hypothesis.

This study focuses on two hypothetical aspects that influence molar morphology:

  1. Ethnicity: The influence of ethnicity on tooth form has long been described [5] but is rarely factored into current orthodontic concepts. Forensic dentistry, however, frequently uses tooth morphology to assign ethnicity [6,7].
  2. Cleft lip and palate: Akcam et al. found evidence that there is a correlation between cleft lip and palate and smaller premolars [8]. However, this statement was based only on linear measurements on plaster models.

If these influences significantly change the morphology of teeth, the automatic CAD/CAM generation of restorations and orthodontic attachments should not be based on generalized algorithms.

Although the morphology of molars has long been an issue in anthropology [9], few systematic investigations have been conducted in the field of orthodontics.

Questions

  1. Are there characteristic differences in the size and shape of first molars in the maxilla (upper jaw) and mandible (lower jaw) between Europeans, Asians and Europeans with unilateral cleft lip and palate?
  2. Are there differences in first molar morphology between the right and left side of the dental arch?
  3. Does unilateral clefting influence the morphology of first molars?

Materials and methods

For this retrospective study, 1000 plaster models were examined from the archives of a Department of Orthodontics.

After consulting the ethics committee of the institution, an ethics approval for this type of study was deemed unnecessary since no information was used that would allow connections to be made to private patient information. All models were analyzed anonymously and no patients were directly involved in the study. All plaster models were taken from an archive that was set up for research purposes and no plaster models were made for the sole purpose of this study.

All models were manufactured in the same dental laboratory (Alginate impressions: Tetra chromium, Kanidenta GmbH & Co. KG, Herford, Germany; Siladent Ortho Plaster, Siladent Dr. Böhme & Schöps GmbH, Goslar, Germany). Models were interrelated in the maximum intercuspal position.

The plaster models used in the investigation were obtained from patients showing mixed dentitions, representing the main orthodontic treatment age. Since abrasion of tooth structure can be considered a regular process that strongly influences morphology [10], only teeth of young people without signs of malfunction (grinding, pressing or abrasive habits) were used in the investigation.

The inclusion criteria were defined as follows:

  • For the group of Europeans and the group of Asians: no patients with syndromes or other craniofacial or dental malformations or systemic diseases
  • For the group of European cleft patients: unilateral complete cleft lip and palate
  • Normal development of dentition and vertical adjustment of the teeth
  • No hypoplastic teeth, no anomalies in tooth shape, no anomalies in number of teeth, no prominent Cusp of Carabelli; four-cusp molars in the maxilla and five-cusp molars in the mandible
  • Identifiability of all measurement points
  • No abrasions
  • No fillings, fissure sealings or prosthetic restoration
  • No previous orthodontic treatment with multibracket appliances

The use of such strict inclusion criteria meant that most plaster models were ill-suited for the study.

Based on these criteria, 40 models—11 European, 13 Asian, 16 European with unilateral cleft lip and palate, as described in Table 1 - were evaluated. The study has an 80% statistical power of detecting a difference of 0.512 (effect size) at alpha = 0.05. In general, conclusions based on the results of this study should be made with caution.

Owing to the small number of models that met the inclusion criteria, the unequal gender distribution was deemed acceptable in order to avoid reducing the sample size any further.

The overjet, overbite and dental arch width (Mühlberg analysis [11]) were measured to further characterize the groups (Table 2).

The three-dimensional measurement of the models was performed using a reflex microscope (Reflex Measurement Ltd, London, UK) with a 15x magnification, as introduced by Scott [12].

The measurement points (Fig 1, Table 3) in this investigation were selected based on earlier studies by Gómez-Robles et al. [13] and Hartmann [14].

thumbnail
Fig 1. Measuring points in the maxilla (“OK”) and mandible (“UK”).

Orientation: buccal (b), lingual (l), distal (d), mesial (m).

https://doi.org/10.1371/journal.pone.0185472.g001

The software C3D (Reflex Measurement Ltd, London, UK) was used to trace the xyz-coordinates for all measurements and to calculate the distances and angles.

Statistical analysis was performed using SPSS Statistics 19 (IBM—Armonk, NY, USA) and R 2.12.2 (http://cran.r-project.org). Palaeontological Statistics (PAST) [15,16] were employed to conduct analysis using geometric morphometry.

For the test results, p ≤ 0.05 was determined to be statistically significant (*).

Traditional morphometry

The measured distances and angles were calculated in three dimensions for each tooth. The specific distances and angles are shown in Fig 2 and described in Table 4. The measurement points were selected based on the work of Hartmann [14], Polychronis et al. [17] and Singleton et al. [18]. Distances were defined as connecting lines between cusp tips (occlusal polygon) as suggested by Peretz et al. [19] and Bailey [20].

thumbnail
Fig 2. Distances and angles.

Measured distances and angles were calculated from 3D coordinates using the software C3D. Orientation: buccal (b), lingual (l), distal (d), mesial (m).

https://doi.org/10.1371/journal.pone.0185472.g002

The calculated distances represent the mesio-distal and bucco-lingual distance of cusps and their angular relations.

Geometric morphometry

Geometric morphometry is a collection of methods used to describe and analyze spatial morphological variations in biological structures [21]. Separate individuals and groups can be described and compared based on corresponding measuring points.

First the centroid size was calculated for each tooth with reference to the xyz-coordinates. This is defined as the square root of the sum of the squared distances of the measuring points from the centroid [22]. This enabled the information on size to be preserved. Afterwards, the coordinates were scaled and rotated to a unified coordinate system using Procrustes transformation. The measuring points were then depicted in a scatterplot and compared statistically [23].

Geometric morphometry has already been used in studies within general dentistry and orthodontics. Detailed explanations for this method can be found in ibid [2426].

To make computation more efficient, a principal component analysis (PCA) was performed. PCA is a procedure for detecting hypothetical variables which explain as much as possible the variance in a multidimensional dataset. The new variables are linear combinations of the original variables.

Results

Traditional morphometry—Distances and angles

No differences between the right and left side were found for the measured distances and angles in any of the groups (Table 5). The standard deviations are within a range of 0.2 to 0.8mm and 5° to 9°. Patients with unilateral cleft lip and palate showed no difference between the cleft and non-cleft side (Table 6). For these reasons, the right and left sides were pooled for the subsequent analysis of group differences.

thumbnail
Table 5. Right/Left comparison of all groups.

T-Test for dependent samples.

https://doi.org/10.1371/journal.pone.0185472.t005

thumbnail
Table 6. Comparison of cleft and non-cleft side in the group of cleft patients.

https://doi.org/10.1371/journal.pone.0185472.t006

The different groups were compared by performing a t-test for unpaired samples since the measured values presented a normal distribution.

When comparing the study groups (Table 7), no differences in the maxilla were found between the group of Europeans and the group of cleft individuals. However, the mandible exhibited a significantly smaller F_J_O angle in cleft patients.

thumbnail
Table 7. Comparison of groups (pooled right and left sides).

ANOVA (Mann-Whitney pairwise), asterisks mark statistically significant results.

https://doi.org/10.1371/journal.pone.0185472.t007

Differences in the maxilla of Asians and Europeans was found, with Asians having a significantly increased R_F distance (bucco-lingual width).

Geometric morphometry

Centroid size (CS).

The paired t-test was used to study right/left differences in the centroid size of pooled groups at a given normal distribution (left: p = 0.8538, right: p = 0.1684, Shapiro-Wilk test). No significant differences in the sides were found for the maxilla (CSleft = 23.92, CSright = 23.97, p = 0.86726). Likewise, the mandible showed no significant differences between the sides (CSleft = 18.9, CSright = 18.96, p = 0.82382).

For patients with cleft lip and palate, no significant difference between the cleft side and non-cleft side was found in the maxilla (CScleft = 23.2; CSnoncleft = 23.4; p = 0.50737) and mandible (CScleft = 18.4; CSnoncleft = 18.6; p = 0.51 529)

Comparing the three groups (Fig 3), Europeans differed from Asians (p = 0.0022, ANOVA, Mann-Whitney pairwise), and cleft patients also differed from Asians (p = 0.0001). As expected, there was no difference (p = 0.7325) between Europeans and (also European) cleft patients.

thumbnail
Fig 3. Centroid sizes.

Comparison of centroid sizes of maxillary molars (left boxplot) and mandibular molars (right boxplot) in different study groups.

https://doi.org/10.1371/journal.pone.0185472.g003

The maxillary first molar was significantly larger in the group of Asians. Similarly, there was a significant difference in the mandible between Asians and Europeans (p = 0.03203) and Asians and cleft patients (p = 0.002532). Cleft patients did not differ from non-cleft Europeans (p = 0.3845).

Shape.

The 3D-coordinates were converted using Procrustes Transformation. Further analysis was performed using PCA. Groups were compared by means of NPMANOVA, a non-parametric test that compares groups on the basis of distances (here: Euclidean distances).

The shape of the maxillary molar differs significantly between Asians and cleft patients (p = 0.0011, Bonferroni-corrected, 9999 permutations), but not between Asians and Europeans (p = 0.1169) or Europeans and cleft patients (0.0769). The difference between the group of Asians and cleft patients is mainly due to a mid-distal compression and a slight elevation of the cusp in relation to the gingival portion (Fig 4).

thumbnail
Fig 4. Superimpositions and consensus forms.

Left: Superimposition of the transformed coordinates of all maxillary molars (upper row) and mandibular molars (lower row). Color coding: black = Europeans, red = Asians, blue = cleft patients. Middle: Consensus forms of the groups. The comparison illustrates the small differences found. Right: Thin-plate spline deformation in 2D projection (color coding: red = expansion, blue = contraction) compared to the significant differences (maxilla: Asians-cleft, mandible: European-Asian).

https://doi.org/10.1371/journal.pone.0185472.g004

In the mandible, no differences between Europeans with and without cleft formation (p = 0.444) were found; however, the group of Asians differed significantly from both Europeans (p = 0.0009) and Europeans with cleft formation (p = 0.0006). The characteristic morphological difference in Asians is a bucco-lingual extension (Fig 4).

When analyzing the separate groups, no significant right/left differences were found in the maxilla (Europeans p = 0.7105; Asians p = 0.4708; cleft patients p = 0.1688) or in the mandible (Europeans p = 0.162; Asians p = 0.7551; cleft patients p = 0.9842).

Furthermore, the group of cleft patients showed no significant differences in molar shape between the cleft and non-cleft sides (p = 0.2193).

In the principal component analysis (PCA), the first six components are considered relevant (broken stick in scree plot at 6, Fig 5). The variance in shape in relation to the axis of the main components (principal component, PC) is shown in Table 8.

thumbnail
Fig 5. Scatterplot.

Scatterplot representing the first two components of the PCA (max = maxilla; mand = mandible; color coding: gray/black = Europeans, red = Asians, blue = cleft patients). However, the first three components only explain 30% of the variance in the maxilla and 34% in the mandible. Additionally, the deforming action of PC1 and PC2 for the maxillary first molar (+1.5 to -1.5) and mandibular first molar (+2 to -2) is shown in the 2D projection as a thin-plate spline (color coding: red = expansion, blue = contraction).

https://doi.org/10.1371/journal.pone.0185472.g005

thumbnail
Table 8. Shape variances corresponding to each PC axis for maxillary and mandibular first molars.

https://doi.org/10.1371/journal.pone.0185472.t008

Discussion

Critique of methodology

The low number of models in this study can be attributed to the high occurrence of fissure sealants and occlusal fillings, especially in the group of cleft patients. Moreover, cleft patients exhibit a high prevalence of caries and insufficient vertical development with abrasions of M1 [27]. Therefore, owing to the strict inclusion criteria, only 16 models were selected from a sample of 450 cleft patient models.

The prevalence of tooth wear is also high in children and adolescents: 30% of 14-year-old school children show abrasions with exposure of dentin [28].

The gender distribution in each study group is unbalanced, but consistent between the groups. Nevertheless, any identified gender differences should be interpreted with caution. Statements on sexual dimorphism of teeth are inconsistent. The teeth of male individuals are about 2–6% larger on average [29]. However, according to previous studies, only the canines consistently show features of sexual dimorphism [12]. No differences in molar shape were found between genders [30]. Evaluating female and male teeth as one group rather than separately based on gender can consequently be justified.

Characterization of groups based on orthodontic metrics

The measurement values for the group of Europeans were consistent with the expected average values of a population sample. Overbite and overjet were slightly increased, as was the prevalence of Angle Class II relationships, since some models were taken from patients prior to orthodontic treatment [31]. An enlarged middle transverse width was striking in the group of Asians. In contrast, the group of cleft patients showed significant sagittal (overjet) and transverse deficiencies. The high standard deviations in the group of cleft patients, particularly in the maxillary anterior dental arch width, are attributable to the highly individual characteristics of cleft formation [32]. It should be noted that orthodontic pre-treatment in the group of cleft patients had been done with removable appliances only.

Traditional morphometry

Reflex microscopy is a contactless, three-dimensional coordinate measurement that uses a projected laser spot. It has a measurement error of less than 0.15 mm [33]. Previous studies have proven the suitability of using reflex microscopy on models with cleft lip and palate [3436] and for measuring tooth morphology [37].

The measurement of distances and angles showed no significant left-right differences. In cleft patients, the asymmetry of the anterior tooth widths was associated with the general instability of development [38]. A differentiation of the cleft/non-cleft sides (Table 6) showed no abnormalities. When comparing the ethnic groups in this study, significant differences were found, particularly in the mandible. Asians were found to have larger teeth in all dimensions, especially in the mandible. Our results therefore suggest that ethnicity may affect tooth width. Its influence on the dental arch form and size has already been reported in the literature [39,40]. As a result, analyses that are based on the comparison of different tooth widths are not transferable to other populations [41]. This corresponds to the results of Hasegawa et al. [42].

Geometric morphometry—Centroid size

The group of Asians presented a significantly greater centroid size in both the maxilla and the mandible. This could be due to an actual enlargement (which also corresponds with clinical experience) or it could be influenced by the unequal gender distribution. The identified differences support the studies of Endo et al. [41] and Hasegawa et al. [42].

Geometric morphometry—Shape

Methods of geometrical morphometry provide considerably more information than linear measurements [43]. In this study, the entire morphology of the crown was mapped by measuring points. This method, based on molar morphology, offers significant advantages for detecting similarities and differences between population groups.

No signs of directional asymmetry of the maxillary or mandibular molars were found in this study. This corresponds to the findings of Noss et al. [44]. Furthermore, no differences in molars were detected with regard to the cleft side. Thus, nonlocal mechanisms seem to be responsible for dental anomalies in cleft patients [45].

Cleft lip and palate and first molars

Kraus et al. [46] described numerous anomalies, including shape abnormalities, in maxillary and even mandibular teeth associated with cleft formation. Animal experiments also confirm an association between MSX1 mutation, orofacial clefts and aplasia of teeth [45]. Despite these references to additional complex effects, the present study suggests that the process of cleft formation (7th - 11th week) has no local or temporal effect on the development of first molar crowns which occurs later (initial calcification of M1: 28th - 32nd week). Size reduction and asymmetry, as described by Sofaer [38] and Werner & Harris [47], could not be confirmed. However, these studies were only based on the mesiodistal crown diameter and the sample sizes were larger.

Questions & answers

  1. Are there any characteristic differences in the size and shape of first molars in the maxilla and mandible between Europeans, Asians and Europeans with unilateral cleft lip and palate? Differences were found involving larger crown dimensions in Asians and altered sagittal-transverse relationships.
  2. Are there differences between the right and the left side of the dental arch?
  3. There was no evidence of side-to-side differences based on the available data.
  4. Does unilateral clefting influence the morphology of molars?

A unilateral cleft does not seem to affect the morphology of molars.

Conclusion

Unilateral clefting did not affect the size and shape of molars. By contrast, ethnic differences in the size and shape of teeth were confirmed.

The results are relevant for orthodontic treatment that uses preadjusted appliances and prosthetic CAD/CAM restorations.

Most modern orthodontic multibracket-appliances are preadjusted to a set “average” tooth morphology. In patients with a tooth morphology that does not correspond exactly to the one the appliance was programmed for, individual adjustments are imperative in order to align the teeth in an ideal position. From this perspective, the results support the use of fully customized CAD/CAM manufactured appliances in orthodontics. In terms of the automatic generation of restorations, ethnic differences should be considered and the morphologies proposed by algorithms that are based on a particular ethnic group should not be applied uncritically to another ethnic group.

However, the lack of evidence of side-to-side differences does allow the possibility of mirroring crowns or brackets/attachments in a CAD/CAM workflow.

Supporting information

S1 Table. Patient characterization based on orthodontic metrics.

https://doi.org/10.1371/journal.pone.0185472.s001

(CSV)

References

  1. 1. Andrews LF. The six keys to normal occlusion. Am J Orthod 1972;62(3):296–309. pmid:4505873
  2. 2. Ballard ML. Asymmetry in tooth size: a factor in the etiology, diagnosis and treatment of malocclusion. Angle Orthod 1944;14(3):67–70.
  3. 3. Lundström A. Tooth morphology as a basis for distinguishing monozygotic and dizygotic twins. Am J Hum Genet 1963;15:34–43. pmid:13931711
  4. 4. Biggerstaff RH. Morphological variations for the permanent mandibular first molars in human monozygotic and dizygotic twins. Arch Oral Biol 1970;15(8):721–730. pmid:5272548
  5. 5. Hellman M. Racial characters in human dentition Part I. A racial distribution of the Dryopithecus pattern and its modifications in the lower molar teeth of man. Proc Am Philos Soc 1928;67(2):157–174.
  6. 6. Pretty I, Sweet D. forensic dentistry: A look at forensic dentistry–Part 1: The role of teeth in the determination of human identity. Br Dent J 2001;190(7):359–366.
  7. 7. Edgar HJ. Prediction of race using characteristics of dental morphology. J Forensic Sci 2005;50(2):269–273. pmid:15813536
  8. 8. Akcam MO, Toygar TU, Özer L, Özdemir B. Evaluation of 3-dimensional tooth crown size in cleft lip and palate patients. Am J Orthod Dentofacial Orthop 2008;134(1):85–92. pmid:18617107
  9. 9. Erdbrink DP. A quantification of lower molar patterns in deutero-Malayans. Z Morph Anthropol 1967(1):40–56.
  10. 10. Ungar PS, Williamson M. Exploring the effects of tooth wear on functional morphology: a preliminary study using dental topographic analysis. Palaeontol Electron 2000;3(1):1–18.
  11. 11. Mühlberg G, Bräuninger H, Weiskopf J. Zur kritischen Bewertung des Pontschen Indexes unter Berücksichtigung des geschlechtsbedingten Einflusses. Dtsch Stomat 1969;19:689–701.
  12. 12. Scott GR. Classification, sex dimorphism, association, and population variation of the canine distal accessory ridge. Hum Biol 1977:453–469. pmid:892765
  13. 13. Gómez‐Robles A, Olejniczak AJ, Martinón‐Torres M, Prado‐Simón L, Bermúdez de Castro J. Evolutionary novelties and losses in geometric morphometrics: a practical approach through hominin molar morphology. Evolution 2011;65(6):1772–1790. pmid:21644962
  14. 14. Hartman SE. A cladistic analysis of hominoid molars. J Hum Evol 1988;17(5):489–502.
  15. 15. Hammer Ø, Harper DAT, Ryan PD. PAST 2.12: Paleontological Statistics Software Package for Education and Data Analysis. Palaeontologia Electronica 2001 http://palaeo-electronica.org/2001_1/past/issue1_01.htm.
  16. 16. Hammer Ø, Harper DAT. Paleontological data analysis. Malden, USA: Blackwell Publishing Ltd.; 2006.
  17. 17. Polychronis G, Christou P, Mavragani M, Halazonetis DJ. Geometric morphometric 3D shape analysis and covariation of human mandibular and maxillary first molars. Am J Phys Anthropol 2013;152(2):186–196. pmid:24009105
  18. 18. Singleton M, Rosenberger AL, Robinson C, O'neill R. Allometric and metameric shape variation in Pan mandibular molars: a digital morphometric analysis. Anat Rec 2011;294(2):322–334.
  19. 19. Peretz B, Nevis N, Smith P. Morphometric analysis of developing crowns of maxillary primary second molars and permanent first molars in humans. Arch Oral Biol 1998;43(7):525–533. pmid:9730270
  20. 20. Bailey SE. A morphometric analysis of maxillary molar crowns of Middle-Late Pleistocene hominins. J Hum Evol 2004;47(3):183–198. pmid:15337415
  21. 21. Bookstein FL. Morphometric tools for landmark data: geometry and biology. 2nd ed. Cambridge, UK: Cambridge University Press; 1997.
  22. 22. Zelditch ML, Swiderski D, Sheets DH. Geometric Morphometrics for Biologists: A primer. 2nd ed. London, UK: Elsevier Academic Press; 2004.
  23. 23. Dryden IL, Mardia KV. Statistical shape analysis. 2nd ed. Chichester, UK: J. Wiley; 1998.
  24. 24. Krey K, Dannhauer K. Morphometric analysis of facial profile in adults. Journal of Orofacial Orthopedics/Fortschritte der Kieferorthopädie 2008;69(6):424–436. pmid:19169639
  25. 25. Stancheva N, Dannhauer K, Hemprich A, Krey K. Three-dimensional analysis of maxillary development in patients with unilateral cleft lip and palate during the first six years of life. J Orofac Orthop 2015;76(5):391–404. pmid:26123734
  26. 26. Krey K, Dannhauer K, Hierl T. Morphology of open bite. J Orofac Orthop 2015;76(3):213–224. pmid:25929714
  27. 27. Kirchberg A, Makuch A, Hemprich A, Hirsch C. Dental caries in the primary dentition of german children with cleft lip, alveolus, and palate. Cleft Palate Craniofac J 2014;51(3):308–313. pmid:23237430
  28. 28. Milosevic A, Young PJ, Lennon MA. The prevalence of tooth wear in 14-year-old school children in Liverpool. Community Dent Health 1994 Jun;11(2):83–86. pmid:8044716
  29. 29. Garn SM, Lewis AB, Swindler DR, Kerewsky RS. Genetic control of sexual dimorphism in tooth size. J Dent Res 1967;46(5):963–972. pmid:5234039
  30. 30. Ferrario VF, Sforza C, Tartaglia GM, Colombo A, Serrao G. Size and shape of the human first permanent molar: a Fourier analysis of the occlusal and equatorial outlines. Am J Phys Anthropol 1999;108(3):281–294. pmid:10096680
  31. 31. Reich U, Dannhauer KH. Craniofacial morphology of orthodontically untreated patients living in Saxony, Germany. J Orofac Orthop 1996;57(4):246–258. pmid:8765800
  32. 32. Krey K, Dannhauer K, Hemprich A. Dental arch morphology in adult cleft patients. VJO 2012;9(4).
  33. 33. Speculand B, Butcher G, Stephens C. Three-dimensional measurement: the accuracy and precision of the reflex microscope. Br J Oral Maxillofac Surg 1988;26(4):276–283. pmid:3166961
  34. 34. Dürwald J, Dannhauer K. Vertical Development of the Cleft Segments in Infants with Bilateral Cleft Lip and Palate. J Orofac Orthop 2007;68(3):183–197. pmid:17522803
  35. 35. Börnert H, Dannhauer K, Schmalzried D. Vertical Changes in the Positions of the Cleft Segments of Patients with Unilateral Cleft Lip and Palate Changes from Birth to Palatoplasty at the Age of 10–14 Months. J Orofac Orthop 2002;63(1):51–61. pmid:11974453
  36. 36. Krey K, Börngen J, Dannhauer K. Three-dimensional analysis of the deciduous dentition of patients with bilateral cleft lip and palate and delayed cleft closure. J Orofac Orthop 2009;70(3):237–246. pmid:19484416
  37. 37. Teaford MF. Measurements of teeth using the reflex microscope. Biostereometric Technology and Applications 1991;Proc. SPIE 1380(Biostereometric Technology and Applications):33–44.
  38. 38. Sofaer J. Human tooth-size asymmetry in cleft lip with or without cleft palate. Arch Oral Biol 1979;24(2):141–146. pmid:299139
  39. 39. Ferrario VF, Sforza C, Colombo A, Carvajal R, Duncan V, Palomino H. Dental arch size in healthy human permanent dentitions: ethnic differences as assessed by discriminant analysis. Int J Adult Orthodon Orthognath Surg 1999;14(2):153–162. pmid:10686839
  40. 40. Kook Y, Nojima K, Moon H, McLaughlin RP, Sinclair PM. Comparison of arch forms between Korean and North American white populations. Am J Orthod Dentofacial Orthop 2004;126(6):680–686. pmid:15592215
  41. 41. Endo T, Shundo I, Abe R, Ishida K, Yoshino S, Shimooka S. Applicability of Bolton's tooth size ratios to a Japanese orthodontic population. Odontology 2007;95(1):57–60. pmid:17660982
  42. 42. Hasegawa Y, Rogers J, Kageyama I, Nakahara S, Townsend G. Comparison of permanent mandibular molar crown dimensions between Mongolians and Caucasians. Dent Anthrop 2007;20:1–6.
  43. 43. Bernal V. Size and shape analysis of human molars: comparing traditional and geometric morphometric techniques. Homo 2007;58(4):279–296. pmid:17662983
  44. 44. Noss JF, Scott GR, Potter RHY, Dahlberg AA. Fluctuating asymmetry in molar dimensions and discrete morphological traits in Pima Indians. Am J Phys Anthropol 1983;61(4):437–445. pmid:6624887
  45. 45. van den Boogaard M, Dorland M, Beemer FA, van Amstel HK. MSX1 mutation is associated with orofacial clefting and tooth agenesis in humans. Nat Genet 2000;24:343.
  46. 46. Kraus BS, Jordan RE, Pruzansky S. Dental abnormalities in the deciduous and permanent dentitions of individuals with cleft lip and palate. J Dent Res 1966 Nov-Dec;45(6):1736–1746. pmid:5226539
  47. 47. Werner SP, Harris EF. Odontometrics of the permanent teeth in cleft lip and palate: systemic size reduction and amplified asymmetry. Cleft Palate J 1989;26(1):36–41. pmid:2917415