Maximal bite force, facial morphology and sucking habits in young children with functional posterior crossbite

Objective The maintenance of normal conditions of the masticatory function is determinant for the correct growth and development of its structures. Thus, the aims of this study were to evaluate the influence of sucking habits on the presence of crossbite and its relationship with maximal bite force, facial morphology and body variables in 67 children of both genders (3.5-7 years) with primary or early mixed dentition. Material and methods The children were divided in four groups: primary-normocclusion (PN, n=19), primary-crossbite (PC, n=19), mixed-normocclusion (MN, n=13), and mixed-crossbite (MC, n=16). Bite force was measured with a pressurized tube, and facial morphology was determined by standardized frontal photographs: AFH (anterior face height) and BFW (bizygomatic facial width). Results It was observed that MC group showed lower bite force than MN, and AFH/ BFW was significantly smaller in PN than PC (t-test). Weight and height were only significantly correlated with bite force in PC group (Pearson’s correlation test). In the primary dentition, AFH/BFW and breast-feeding (at least six months) were positive and negatively associated with crossbite, respectively (multiple logistic regression). In the mixed dentition, breastfeeding and bite force showed negative associations with crossbite (univariate regression), while nonnutritive sucking (up to 3 years) associated significantly with crossbite in all groups (multiple logistic regression). Conclusions In the studied sample, sucking habits played an important role in the etiology of crossbite, which was associated with lower bite force and long-face tendency.


INTRODUCTION
Breast-feeding encourages normal growth and development of the alveolar processes and stomatognathic structures, correct intermaxillary relationship and nose breathing 20 . If "suck need" is not satisfied during regular feeding, it may be fulfilled by a sucking habit. Some studies have reported the effects of persistent nonnutritive sucking on sagittal and vertical dimensions of the maxilla and the mandible, dependent on the intensity and the duration of the habit 12,18 . Posterior crossbite occurs frequently in children, as a result of genetic or environmental influences (for example, nonnutritive sucking habits and mouth breathing), or a combination of both, and has been associated with asymmetrical growth and function of the hard structures and muscles 1,6,26,29 . Betts, et al. 2 (1995) stated that a posterior crossbite does not confine itself to dental displasias but is more often related to an underlying skeletal problem.
Bite force is one of the components of the chewing system, which may be influenced by dental occlusion, craniofacial morphology and masticatory muscle thickness. Its magnitude increases with age, with teeth in occlusal contact, and with increasing number of erupted teeth 26 . Craniofacial morphology evaluation is also an important tool in clinical practice and research, and can be achieved by different approaches, including photographic analyses, which is an inexpensive method, does not expose the patient to unnecessary irradiation, and can provide the evaluation of external craniofacial structures 7,27 .
In this way, the purposes of this study were to evaluate the association of sucking habits with the presence of posterior crossbite among children in the primary and early mixed dentition, and its relationship with maximal bite force and facial dimensions.

MATERIALS AND METHODS
This cross-sectional study comprised a convenience sample formed by healthy children of both genders aged from 3.5 to 7 years, who were to start treatment in the Department of Pediatric Dentistry and from day care centers. All children and their parents consented to participate in the study, which was approved by the Ethics Committee  (Table 1). The exclusion criteria for normocclusion groups were the presence of signs and/or symptoms of temporomandibular dysfunction 4 , and previous orthodontic treatment.
Data regarding the history, presence and duration of sucking habits were obtained from the parents/guardians, considering the following parameters: -breast-feeding over a period of at least six months (exclusive or not exclusive);bottle-feeding for 1 year or more; -nonnutritive sucking habit (pacifier or thumb sucking) that persisted up to the age of 3 years.
All analyses were done by the first author (PMC).     According to the multiple logistic regressions, AFH/BFW ratio, nonnutritive sucking habits and breast-feeding were the major independent predictors of crossbite in primary dentition (p<0.05). In the mixed dentition, univariate analyses showed that children with lower bite force and the absence of breast-feeding were significantly more likely to have a posterior crossbite; but they can not be considered predictors of this malocclusion, due to the no significant levels reached in the multiple logistic models. Multivariable analyses showed that nonnutritive sucking habits were significantly associated with the presence of crossbite in the mixed groups, that is, a nonnutrive sucking habit can predict the development of this malocclusion in both evaluated dentitions. Fisher's exact test also showed significant association between sucking habits and crossbite in both stages of the dentition. Bottle-feeding for 1 year or more was highly prevalent in both groups of the mixed dentition; for this reason, this variable was removed from the models.