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Dento-alveolar characteristics in adolescents born extremely preterm

Marianne Rythén, Birgit Thilander, Agneta Robertson
DOI: http://dx.doi.org/10.1093/ejo/cjs034 475-482 First published online: 10 May 2012


It has been shown that children born extremely preterm (EPT) often suffer from medical complications and growth restrictions in early childhood. Catchup growth diminishes these effects but the children are known to have lower weight, height, and head circumference as school children. Effects on enamel development have been shown. How this affects the dento-alveolar outcome during adolescence is not known. Forty EPT children with a gestational age (GA) of less than 29 weeks, at 12–16 years of age, and matched healthy controls born at term, with a GA of 37–43 weeks, were examined. Data from the clinical examination, dental casts, and bitewing radiographs were collected and compared. Malocclusion was noted, and dento-alveolar length, width, palatal height, and mesio-distal tooth width were measured. Medical diagnoses, neurological, and neuropsychiatric disturbances were noted at the time of the survey. The two groups were compared with an epidemiological normal reference material. The results showed no differences between the controls and reference material. Angle Class II was the most frequent malocclusion associated with morbidity, neurological, and neuropsychiatric disturbances, followed by deep bite and overjet. Three or more malocclusions were almost twice as common among the EPT children compared with the controls. Significantly smaller incisors, canines, and first molars were found. In summary, the EPT children, during adolescence, had medical aberrations as well as dento-alveolar effects opposed to the healthy children born at term. Dentists should be aware of this and treatment plans should be made in due time.


Children born before gestational week 37 are considered preterm according to the World Health Organization. A total of 0.4 per cent of the infants born in Sweden 2007 were born before 29 weeks of gestation [extremely preterm (EPT)] and 85 per cent survived the first year (Swedish Medical Birth Registry, 2007).

Specific diagnoses or disabilities as well as medical complications are more common in infants with a lower gestational age (GA) and birth weight (BW) than in children born at term (Farooqi et al., 2006a; Hack, 2006; Hallin et al., 2010), resulting in more chronic conditions and functional limitations.

Due to postnatal morbidity, prolonged oral or nasal intubation is needed. A mature sucking ability is not developed until 34 weeks of gestation. The EPT children are fed through oral or nasogastric tubes until normal feeding can start. These aberrations may cause defects in the palate and dento-alveolar ridge (Kopra and Davis, 1991; Fadavi et al., 1992; Seow, 1997). The scientific evidence for some of these findings varies and some of the disturbances seem to decrease with growth (Seow, 1997).

Growth failure is common among the EPT children but catchup growth periods during infancy seem to diminish this aberration (Niklasson et al., 2003). However, the head circumference is smaller at birth and remains smaller without catchup growth up to the age of 11 years (Farooqi et al., 2006b). It is not fully understood if the skeletal facial growth in the EPT children follows this neurocranial growth pattern (Paulsson et al., 2004). Facial growth is a complex interaction of interstitial growth and surface apposition in a continuous remodeling process that continues into adulthood, where the facial pattern changes with growth acceleration during puberty (Thilander et al., 2005). It has been shown that the craniofacial morphology in EPT children at 8–10 years of age differs from children born at term. They have a shorter anterior cranial base (n-s), less convex profile (n-ss-pg), shorter maxillary length (sp-pm; Paulsson et al., 2008) and more malocclusion traits (Paulsson and Bondemark, 2009). Open bite and the effects on sagittal relations in children born preterm has been discussed (Harila-Kaera et al., 2002; Harila et al., 2007). If these findings are explained by preterm birth or medical aberrations and if they persist during adolescence, remains to be explained.

The dento-alveolar development is an interactive process. Time-related and significant changes in the dental arch (width, length, depth, and palatal height) occur from the primary to the permanent dentition (Thilander, 2009). The dento-alveolar development starts intrauterine (Thesleff and Hurmerinta, 1981; Deutsch and Pe’er, 1982; Nanci, 2008) and the neonatal and postnatal period may affect normal tooth development. The effect of preterm birth, morbidity, and postnatal medical treatments during the period of tooth development (mineralization defects, altered crown dimensions and morphology, tooth eruption, alveolar ridges, palatal asymmetry, and crossbites) have been discussed (Garn et al., 1979; Norén, 1983; Kopra and Davis, 1991; Viscardi et al., 1994; Seow, 1997; Seow and Wan, 2000).

As medical distinctions between EPT children and children born at term exist, it might be desirable to find out if these differences also involve the dento-alveolar variables during adolescence. Thus, the aim of the present study was to analyze the prevalence of malocclusions, size of the dental arches as well as tooth dimension during the adolescence periods.

Materials and methods


Eighty subjects, 40 ‘EPT children’ born before 29 weeks (25 boys, 15 girls), and 40 ‘controls’ (CTR) full-term ‘healthy’ children, individually matched regarding age; (+-5 months), gender, and the same residential area, and Public Dental Service Clinic as the EPT children, were included in the study (Table 1).

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Table 1

Demographics and dental age based on clinical examination in extremely preterm children (EPT) and controls (CTR).

The neonatal and postnatal medical history, BW, and GA were retrieved for from hospital medical records for the EPT children and from the Swedish Medical Birth Registration for the controls. A standard deviation (SD) score for birth weight (BWsds) was calculated according to Marsal et al. (1996). Small for GA is considered when BWsds is less than −2.0. Contrary to the controls, the EPT children had lower BW, lower GA, and lower BWsds. Four sets of twins were found among the EPT children. At the time of the clinical examination, 24 of the examined EPT children suffered from one or more medical diagnoses including chronic respiratory disease, heart disease (ventricular septum defect), allergies, body height/weight growth deficiencies, cerebral palsy, hearing, and visual defects. Neuropsychiatric disturbances were found in 11 children (ADHD, autism and dyslexia) (Figure 1). One girl was diagnosed with Turner mosaicism and one girl with Pierre Robin syndrome.

Figure 1

Medical diagnoses, neurological, and neuropsychiatric disturbances in extremely preterm children (n = 40). [HD = Heart Disease; CRD = Chronic Respiratory Disease; SCP = Syndromes and Cerebral Palsy; A = Allergy; GD = Growth Deficiency; D = Disturbances (visual, hearing, and neuropsychiatric)].

The children were divided into developmental periods (Table 2) as well as chronological age groups (13 and 16 years of age) and gender (Table 3). From the original 80 adolescents, 75 casts were analyzed. Four EPT children and one control refused to cooperate with dental casts. The malocclusion diagnoses in these cases were set at the clinical examination by one of the authors (M.R.) but were not included in the dento-alveolar measurements.

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Table 2

‘Developmental periods’, based on their dental stages (DS) in extremely preterm children (EPT) and controls (CTR).

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Table 3

‘Chronological age groups’ (13 and 16 years old) and ‘gender’ (female= F and male = M). Extremely preterm children (EPT, n = 36), controls (CTR, n = 39), and reference material (REF) according to Thilander (2009). Mean age and standard deviation (SD).

The controls were compared with a reference material (Thilander and Myrberg, 1973; Thilander, 2009).

Clinical examination

All children had a standardized clinical dental health examination performed by one of the authors (M.R.) described elsewhere (Rythén et al., 2012) and the dental stage (DS) was registered according to the method described by Björk et al. (1964).

Analogue X-ray bite-wings were taken in order to diagnose hypodontia, and study casts in a central occlusion (72 by MR and 3 by the children’s orthodontist) were made, if not provided by the children’s dentist or orthodontist.

Study cast analyses


The malocclusions according to Björk et al. (1964) were registered by the three authors together and compared with a Swedish reference material (Thilander and Myrberg, 1973). Sagittal diagnoses were registered with regard to mesial drift of the first permanent molars. Crowding was registered when more than or equal to 4 mm in each quadrant.

Dental arches.

The following measurements were performed on each study cast with a digital caliper (Cocraft®): Tooth size, arch length, width, and palatal height (Figure 2). Registration points where those used by Thilander (2009) in a normal ideal (non-orthodontic treated) sample, which also served as reference material.

Figure 2

(a–b) ‘Arch length’, according to Thilander (2009). The total circumference of the dental arch, divided into right/left posterior (P2–C) and anterior segments (I2–I2), was obtained by measuring the arch perimeter to the mesial surface at the first permanent molars, thus, representing the distance (P2–P2; P2 = second premolar, I2 = lateral incisor, and C = Canine.). (c–d) ‘Arch width’, according to Thilander (2009). The maxillary and mandibular intermolar width were obtained by measuring the distances between the central fossa of the corresponding first molars on the left and right side between (M1–M1). (P2 = second premolar and M1 = first molar). (e) ‘Palatal height’, according to Thilander (2009). The palatal height was achieved in the mid–palatal plane, determined by measuring the perpendicular distance from the occlusal plane constructed from the permanent first molars. Through a hole in a plastic sheet, the end of the caliper was pressed to the palatal contour. The distance minus the thickness of the sheet represented palatal height.

Tooth size.

The mesio-distal crown diameter of each tooth was obtained by measuring the greatest distance between the contact points of each tooth. The tooth width of each tooth is given as a mean and SD from the total number of measured teeth. The buccal-lingual measurements were not included due to difficulties with identifiable registration points of measurements.

Error of the method

All measurements were performed twice at different occasions by one of the authors (M.R.). The error of the method was calculated according to the Dahlberg (1940) formula: SE=±d2/2n where d is the difference between the two measurements and n is the number of measurements. The accidental error varied from 0.15 (tooth size) and 0.22 mm (palatal height and arch width) to 0.49 mm (arch length), indicating a high degree of precision and accuracy.

Statistical methods

SPSS version 19 (SPSS, IBM, Chicago, Illinois) was used for the statistical analysis. Differences between the groups were analyzed with the Student’s T-test, Chi-Square Test, and Fisher’s Exact Test. Pearson’s correlation analyses were performed between tooth size and postnatal hospitalization. Adjustments according to Bonferroni (Bland and Altman, 1995) have not been performed.

Ethical aspects

Informed written consent was obtained from the EPT children, the control children, and their parents. Ethical consent was given by the Ethical Research Committee at the University of Gothenburg, Dnr: S 675-02.


Overall findings

Important medical, neurological, and neuropsychiatric distinctions between the adolescents born preterm and the healthy controls exist. Differences were also found between the EPT children and the controls especially for the frequency of malocclusion, described more in details below.

Dental stage

No differences in the DS between the EPT children and the controls were found (Table 2). A normal variation in the late mixed and early permanent dentitions was seen due to individual dental development.


Three or more malocclusions were almost twice as common among the EPT children compared with the controls (Table 4).

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Table 4

‘Number of diagnoses’ registered in extremely preterm children (EPT, n = 40) and controls (CTR, n = 40).

Occlusial anomalies.

Angle Class II malocclusion was twice as common in the EPT children compared with the controls as well as the reference material (Table 5). Overjet and overbite were also more frequent among the EPT children (Overjet: P = 0.04); in three of them, Angle Class II was associated with an overbite of more than or equal to 6 mm and in six of them, an overjet of more than or equal to 5 mm. No other differences regarding occlusion anomalies were registered. Medical diagnoses were registered in all Angle Classes (Figure 3).

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Table 5

‘Prevalence of occlusal, space, and dental anomalies’ in 40 extremely preterm children (EPT), 40 controls (CTR), and a Swedish reference material of 5459 schoolchildren (REF; Thilander and Myrberg, 1973). Given in number (n) and per cent (%).

Figure 3

Frequency of Angle Class I, II, and III, respectively, related to morbidity, neurological, and neuropsychiatric disturbances in extremely preterm children.

Regarding ‘space anomalies’, the frequency of crowding did not differ between the EPT children and the controls. Compared with the reference material, crowding was twice as frequently found in both groups. Crowding more more than or equal to 4 mm in each quadrant was registered; 12 EPT children and 11 controls were noted. This was in concordance with the reference material.

No differences in ‘dental anomalies’ between the EPT children and the controls or the reference material were found.

Dental arches

As shown in Tables 6–8, no differences in dento-alveolar length width and palatal height were found except for the dento-alveolar length and width in the EPT girls at 16 years of age.

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Table 6

‘Dento-alveolar length’ (mm) in extremely preterm children (EPT), controls (CTR), and normal material (REF) (Thilander, 2009); at 13 and 16 years (F = female and M = male) given as the mean and standard deviation (SD; *P < 0.05, **P < 0.01; EPT/CTR).

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Table 7

‘Dento-alveolar width’ (M1–M1, mm), in extremely preterm children (EPT), controls (CTR), and normal material (REF) (Thilander, 2009) at 13 and 16 years (F = female and M = male) given as the mean and standard deviation (SD; *P < 0.05; **P < 0.01 EPT/CTR).

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Table 8

‘Palatal height’ (mm), in extremely preterm children (EPT), controls (CTR), and normal material (REF) (Thilander, 2009) at 13 and 16 years (F = female and M = male) given as the mean and standard deviation (SD; *P < 0.05; EPT/CTR).

Tooth size

The mesio-distal tooth width differed between the controls and the reference material (Table 9). In both the mandible and the maxilla, EPT children had significantly smaller incisors, canines, and molars compared with the controls. This was found especially in adolescents with prolonged postnatal hospitalization (Figure 4).

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Table 9

‘Mesio-distal crown diameters’ (mm) of permanent teeth in extremely preterm children, controls (CTR), and normal material (Thilander, 2009; REF; F = female and M = male) given as the mean and standard deviation (SD; *P < 0.05, **P < 0.01; EPT/CTR).

Figure 4

Upper central incisor—tooth size (mm) and postnatal morbidity shown as number of days hospitalized (days).

Special findings

All but two of the EPT children with Angle Class II were associated with some kind of morbidity, neurological, or neuropsychiatric disturbances. The preterm child born with Pierre Robin syndrome had bimaxillary crowding, Angle Class II, deep bite, and incisor malformations. The child with Turner syndrome mocaicism had a prenormal occlusion, a lateral open bite and unilateral crossbite. Both children had smaller mesio-distal tooth dimensions compared with the mean value of teeth from preterm children.

Similarities between twins in the present study were Angle Class I in all siblings. In one set of twins, both siblings had crowding and dental anomalies.


The present study has shown that some dento-alveolar anomalies occurred frequently in the EPT children in comparison with the healthy children born at term (Figure 1). It is worth noting that the data of the controls were constantly close to that of the reference material (Thilander and Myrberg, 1973; Thilander, 2009), thus strengthening the present results.

Different information regarding the frequency of dento-alveolar anomalies exists (Harila et al., 2007; Paulsson et al., 2008), depending on small study sizes and missing reference material. In the present study, Angle Class II was the most frequent malocclusion found in the EPT children, contrary to the findings by Paulsson et al. (2008). An explanation of this difference might be that their study had excluded syndromes and neuromuscular disorders. In the present study, large overjet was found in many of the Angle Class II EPT children, thus indicating Angle Class II division 1 type. To verify such a differential diagnosis, chephalometric analyses is desired, as retrognatic mandible might be a growth inhibition caused by medical morbidity, early nutritional disturbances or feeding difficulties (Bucher et al., 2002). The preterm children in the present study, being born before additional supplements were added to feeding formulas, may have suffered from malnutrition with effects on growth in general (Kushel and Harding, 2000, 2004; McCormick et al., 2010) as well as the dento-alvelor development.

The modern use of nasal intubation instead of oral intubation may reduce the effect of oral defects on the alveolar ridge and palatal arch seen in earlier studies in younger children (Kopra and Davis, 1991; Fadavi et al., 1992; Seow, 1997). No major dento-alveolar differences were found between the groups, except for smaller dento-alveolar length and width in EPT girls at 16 years of age.

Differences in tooth size, especially for incisors and first molars, were noted. Contradicting effects on tooth size has been shown in earlier studies (Garn et al., 1979; Fearne and Brook, 1993; Seow and Wan, 2000; Harila-Kaera et al., 2001; Harila et al., 2003) in children with low BW or short GA. An interesting finding in the present study was the association with prolonged hospitalization postnatally (indicating an effect of the morbidity postnatally) and tooth dimension. Enamel developmental defects seen as enamel mineralization disturbances have been associated with prematurity (Seow, 1997; Brogårdh-Roth et al., 2011; Rythén et al., 2012). If the difference in tooth size is an effect of less enamel laid-down or a smaller tooth in general, may be discussed.

In conclusion, the EPT children, examined during adolescence, had medical aberrations as well as dento-alveolar effects in contrast to the healthy children born at term, which may explain the differences in results compared with earlier studies (Harila-Kaera et al., 2002; Harila et al., 2007; Paulsson et al., 2008). The main finding was a high frequency of Angle Class II in the EPT children with physical and mental disturbances. Preterm birth as such may not be the cause of this anomaly but rather the association with mentioned morbidity. Dentists should be aware of this and treatment plans should be made in due time. For further knowledge in EPT children and their facial and occlusal development, cephalometric studies, in combination with clinical studies in a larger population of young adults, are needed.


The Research and Development Council in the Region of V ä stra G ö taland, Sweden; The Research and Development Council in S ö dra Ä lvsborg County, Sweden; The Dental Society of Gothenburg; T he Foundation of Sigge Persson and Alice Nyberg for Research in Odontology.


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