INTRODUCTION

A large part of the population is concerned about posterior crossbite in the primary dentition. Estimates of its prevalence range from 13% to 25%.^1,2

Untreated persistent unilateral posterior crossbite may lead to mandibular rotation³ and to functional asymmetry of the temporomandibular joint.^4,5 Bilateral posterior crossbite occurs more rarely and can result in mandibular or condylar asymmetry.⁶ French and American guidelines for orthodontics recommend orthodontic treatment for posterior crossbite to prevent these complications.^7,8 It therefore seems relevant to look at early events and habits that might affect this malocclusion.

Posterior crossbite is often associated with anterior open bite,^1,9 another malocclusion that develops early in childhood. Most anterior open bites correct spontaneously.⁹ These two malocclusions seem to share common risk factors.

The major common risk factor is nonnutritive sucking habits, such as pacifier or thumb sucking.^1,10–13 The tongue, retained in a low position by pacifier or thumb, may be prevented from applying the pressure needed against the palate for transverse maxillary arch growth. It has been shown that tongue posture on the floor of the mouth is more frequent in children with posterior crossbite.¹⁴

A few studies have explored the relations between posterior crossbite^11,15,16 or anterior open bite^11,16,17 and breast-feeding habits, but these studies vary in the duration and type of breast- feeding considered (exclusive or mixed) and do not always control for nonnutritive sucking habits.

Mouth breathing is another commonly identified risk factor for both posterior crossbite and anterior open bite.^18,19 Mouth breathers seem to have a narrower maxilla^20,21 and a higher frequency of both posterior crossbite^22–24 and anterior open bite^25,26 than do nose breathers.

Because etiological mechanisms leading to posterior crossbite and anterior open bite may involve inadequate tongue capacity, factors that could affect lingual functions should be studied. Neurological immaturity in the case of preterm birth or neurological impairment occurring in some small-for-gestational-age (SGA) children may induce neuromotor dysfunction,²⁷ which could affect lingual functions. Preterm birth and SGA might therefore be associated with both malocclusions.

As noted above, these malocclusions share some of the same risk factors. However, except for nonnutritive sucking factors, risk factors for posterior crossbite and those for anterior open bite are poorly known. Our aim was therefore to identify early risk factors for posterior crossbite and for anterior open bite in the primary dentition.

MATERIALS AND METHODS

Population

The EDEN mother-child cohort, which aimed to investigate the pre- and early postnatal determinants of child health and development, included pregnant women in the university maternity hospitals of two French cities, Nancy and Poitiers.²⁸ Since oral examination of children was conducted in Nancy only, this study examines only the subjects in Nancy; 1034 women were recruited before the 24th week of gestation between September 2003 and January 2006 (Figure 1). Cohort noninclusion criteria were pregestational diabetes, multiple pregnancy, inability to read French, and plans to move away from the region in the following 3 years. Data at birth were available for 963 children, who were invited to a medical and oral examination at age 3. Oral examinations were performed by three dentists specially trained to follow the specific study protocol. Due to the unexpected unavailability of one of the three dentists, 186 children had no oral examination.

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The study was approved by the ethics committee of the Kremlin-Bicêtre Hospital and by the Data Protection Authority. Written consent was obtained from the mother for herself at inclusion and for her newborn child after delivery.

RESULTS

Mothers of the children who were examined were older and had a higher social status than those of the children not examined (Table 1). Frequencies of preterm birth and SGA were similar in both groups.

Table 1.  Description of the Population

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Posterior crossbite was found in 20% of the children (Table 2) and anterior open bite in 28% (Table 3). Posterior crossbite was closely associated with anterior open bite (P < .001); 57% of children with posterior crossbite also had an anterior open bite.

Posterior crossbite was not associated with sex (P  =  0.63) or parents’ occupational group (P  =  .64), but was associated with maternal age (P  =  .05). Because we had no hypothesis underlying this association, maternal age was not included in the multivariate models. Preterm birth, SGA, duration of breast-feeding, sucking habits at 3 years, and open lips were included in the multivariate models. Because lip posture was not recorded for three children, the multivariate models included 419 children.

The frequency of posterior crossbite was higher in children born preterm and lower in SGA children (Table 2). Posterior crossbite was also more frequent in children who sucked their thumbs or pacifiers at 3 years and in children with open lips. Duration of breast-feeding was not associated with posterior crossbite. Anterior open bite was more frequent in children breast-fed less than 6 months, in thumb suckers and especially in pacifier suckers at 3 years, and in children with open lips than in the other children. Anterior open bite was not associated with preterm birth or with SGA (Table 3).

Table 2. Posterior Crossbite According to Neonatal Characteristics, Duration of Breast-feeding, Sucking Habits, and Open Lips

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Table 3.  Anterior Open Bite According to Neonatal Characteristics, Duration of Breast-feeding, Sucking Habits, and Open Lips

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Table 4.  Links Between Posterior Crossbite and Anterior Open Bite

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DISCUSSION

This study suggests that preterm birth is a risk factor specific to posterior crossbite and SGA a possible protective factor for posterior crossbite. Our study confirmed that open lips increased the risk for posterior crossbite, with or without anterior open bite.

One strength of our study is that neonatal characteristics and duration of breast-feeding were collected prospectively, which reduced possible errors. The fact that the oral examinations were carried out by only three dentists, specially trained for the study, reduced possible misclassification. Furthermore, the criteria set to identify posterior crossbite were objective and accurate enough for us to believe that the collected information was reliable. Moreover, if misclassification had occurred, it would have been independent of the risk factors and thus have led to an underestimation of the relations.

Only one study has explored the relations between preterm birth and posterior crossbite,³¹ and the small size of its sample prevented it from reaching a conclusion. An association between gestational age and alteration of palatal morphology has been found in very preterm children.³² In case of preterm birth, even moderately as in the present study, immaturity of lingual functions might have an impact on palatal growth. Preterm babies are more often SGA than others and SGA babies are less often found to have a posterior crossbite. That explains why the association of preterm birth with posterior crossbite became significant when SGA was taken into account. Unexpectedly, SGA appeared as a protective factor for posterior crossbite. A previous study reported a nonsignificant similar trend.¹⁷ Tongue overuse could explain this inverse relation between SGA and posterior crossbite. The fast weight catch-up in most SGA babies suggests that they eat more in their first months of life³³ and thus probably suck and swallow more often than others. These links between neonatal characteristics and early posterior crossbite need to be confirmed, and these possible explanations should be considered with caution. No association was observed between preterm birth or SGA and anterior open bite. These results suggest that the mechanisms leading to posterior crossbite are different from those leading to anterior open bite.

A nonnutritive sucking habit is the best-known risk factor for posterior crossbite and for anterior open bite.^1,10 The association found here seems to concern only children with an ongoing habit, for both malocclusions. Dimberg explored the evolutions of posterior crossbites and anterior open bites from 3 to 11.5 years of age,³⁴ but more longitudinal studies are needed to explore the links between age at which the nonnutritive sucking habit ceased and posterior crossbite and anterior open bite, and their spontaneous correction.

To identify mouth breathers, various authors have used different proxies. We used open lips as a proxy for mouth breathing, but some normal breathers might have been misclassified as mouth breathers, for example, because of open lips due to a cold on the day they were examined. Open lips might therefore slightly overestimate mouth breathing. Nonetheless, the frequency of 21% of open lips in our population is close to that of 19% of mouth breathers in Swedish children of the same age.³⁵ Besides, this potential misclassification should be independent of the studied risk factors and the link between mouth breathing and posterior crossbite using open lips as a proxy is therefore probably only slightly underestimated.

Removal of premature contacts in the primary dentition can prevent a posterior crossbite from persisting into the permanent dentition.³⁶ Later, a quad-helix appliance or an expansion plate can treat posterior crossbite in the mixed dentition.³⁷ It follows that, in some cases of posterior crossbite, early orthodontic treatment is less complex than later comprehensive treatment.^38,39 Early diagnosis, therefore, seems to be of meaningful help. However, 3 years is a very early age, and longitudinal studies are needed to understand the evolution of these malocclusions and their relationships with risk factors.

CONCLUSIONS

• Preterm birth seems to be a risk factor for early posterior crossbite.

• Ongoing sucking habits seem to be risk factors for early posterior crossbite.

• SGA appears to be associated with a lower risk of early posterior crossbite.

• This study helps to identify new risk factors for posterior crossbite.

• Posterior crossbite and anterior open bite may result from different mechanisms.