The literature provides no single criterion for malocclusion of primary dentition; therefore most occlusal characteristics were assessed in determining the relationship between malocclusion and feeding patterns. Our findings indicated that failure to breast-feed or breast-feeding for only a short period was associated with a higher prevalence of posterior crossbite and no maxillary space in the primary dentition. Sucking the breast places great demands on the perioral musculature. The constant repetitive effort promotes the correct development of these muscles, increasing their tone and ensuring that correct oral function is established. As a result, the duration of natural breast-feeding has a positive effect on the mobility of orofacial structures [17]. Early weaning may lead to an insufficient perioral muscular activity, which may cause negative consequences to swallowing, breathing and speaking, as well as malocclusion [18]. Warren et al. found that breast-feeding facilitates normal palate development and attenuates the formation of a deep, high-arched palate [10]. Viggiano et al. and Karjalainen et al. have reported that breast-feeding was a protective factor against development of posterior crossbite in the deciduous dentition [5,19]. Similarly, Kobayashi et al. reported that prolonged exclusive breast-feeding can strongly reduce the prevalence of posterior crossbite, and children who were breast-fed for more than 12 months had a 20-fold lower risk of posterior crossbite compared with children who were never breast-fed. Additionally, their risk was 5-fold lower than those who were breast-fed for between 6 and 12 months [20]. This evidence corroborates the findings of the present study. Although the absolute incidence of posterior crossbite was extremely low, its harmful effects on masticatory chewing cycle pattern and normal growth and development of the orofacial system were noticeable [21]. We suggest that early weaning may interfere with the normal development of alveolar ridges and the hard palate, and hence result in posterior crossbite, a lack of space or crowding in the upper arch.

The parents were very highly educated in our study and the rate of exclusive breast-feeding of children at 6 months of age was 22.9%. The corresponding rate in rural western China was 11.6 % [22], which was lower than our study. Scott et al. reviewed literature identifying factors associated with the initiation and duration of breastfeeding among Western women and found there was a strong and consistent association with demographic factors such as maternal age and level of education, there was a less consistent association with factors such as marital and socioeconomic status. [23]. However, Agboado et al. found there were no significant associations between breastfeeding cessation and marital status, mode of delivery, timing of breastfeeding initiation and socio-economic deprivation in Lancashire, UK [24]. These different finding highlights the fact that breastfeeding is multifactorial in nature and future programs aimed at promoting breastfeeding must take this into consideration.

Our study found a significant association between non-mesial step and bottle-feeding for over 18 months, compared to children who were bottle-fed for less than 18 months. A similar association was also found between the class II canine relationships and bottle-feeding. It is known that the terminal relationship of primary second molars seems to be the most important factor that could determine or influence the future relationship between the permanent molars and the subsequent development of occlusion. As Moyers and Waiuright reported, non-mesial step combined with a class II canine relationship increases the probability of developing Angle class II malocclusion in mixed dentition and permanent dentition [25]. Therefore, in many cases, early orthodontic intervention will be needed. Similarly, Nahás-Scocate et al. found that the older the child when bottle-feeding ceased (3–4 years old) and the shorter the breastfeeding duration (<3 months), the greater the chances of the child presenting distal step [26]. There are several theoretical mechanisms by which bottle-feeding might contribute to the development of malocclusion: (1) less muscle activity is necessary to extract milk from a bottle, resulting in decreased development of muscles involved in sucking, which may act as a functional matrix for inadequate mandibular growth; (2) The tongue acts only to control the milk outlet during bottle-feeding and bottle-fed children have an increased prevalence of abnormal swallowing patterns or tongue thrusting habits [27]; (3) over 60% of the children who were predominantly bottle-fed presented mouth breathing or mixed breathing, which may compromise occlusion [28]. Although Narbutyte et al. [29] found that the literature contains insufficient evidence to connect bottle-feeding with the development of skeletal malocclusions, the results of our study clearly showed that bottle-feeding may be related to abnormal maxillomandibular relationship via its inadequate provision of muscular stimulation. Nevertheless, the absolute OR value was low. The reason may be that the effects of bottle-feeding on occlusion were difficult to assess when bottle-feeding was concurrent with breast-feeding. In this study, 14.2% of infants were exclusively bottle-fed in the first 6 months while 62.9% were mixed-fed. This would weaken the effect of bottle-feeding on any specific type of malocclusion. Nevertheless, there was a weak trend associating bottle-feeding for over 18 months with inadequate mandibular growth.

The incidences of pacifier sucking and digit sucking in the present study were 4.1% and 19.1%, respectively. Previous studies on non-nutritive sucking habits have shown a variation in prevalence between 20% and 87% at age 36 months [20,25,30]. Pacifier and digit sucking incidences have been reported to be 18.2% and 61.5% in Australia [31], 70.3% and 10% in Sweden, and 50% and 19% in Norway [32]. Most previous studies have reported much higher incidences of NNS compared with our study. This difference suggests that the incidence of NNS varies from one population to another and may be related to local culture and customs. The present study showed that 40.9% children were weaned early and this increased the prevalence of pacifier sucking by up to 4.1 fold, but it did not increase the prevalence of digit sucking. Theories that endeavor to explain this effect suggest that children who are naturally breast-fed satisfy their sucking needs along with their psychological and affective requirements through close, intimate contact with the mother during breast-feeding and thus have less need to suck a pacifier, digit or other object [33]. Conversely, non-nutritive sucking habits are developed when children satisfy their instinctive sucking urge by sucking their thumbs or by prolonged use of pacifiers. Nobuya et al. also reported that pacifier sucking was more prevalent in children with a short breast-feeding duration than in those with a normal feeding duration [34]. Luz et al. examined 249 children with mixed dentition and found statistically significant associations between short breast-feeding duration (<6 months) and non-nutritive sucking habits, and between non-nutritive sucking habits and class II malocclusions [35]. Montaldo et al. reported that children who underwent bottle- or complementary feeding showed a higher risk of acquiring non-nutritive sucking habits after the first year of life, and that this was associated with a greater risk of crossbite, open bite and class II molar relationships [36]. Surprisingly no correlation was found between bottle-feeding duration and pacifier sucking or between bottle-feeding duration and digit sucking. This can be explained by the fact that Chinese parents will usually select a milk bottle or other way to calm their infants rather than a pacifier.

Different sucking habits are known to affect occlusion in different ways, and the disturbances found in the present study bear similarities to other findings elsewhere. In this study, a significant correlation was seen between pacifier sucking and excessive overjet as well as absence of lower arch developmental space. Pacifier sucking appeared to have an effect on the lower labial segment. Similar trends were reported by Warren et al., who found an association between prolonged pacifier sucking and a shorter intercanine width in the mandibular arch [10]. Aznar et al. found that a pacifier-sucking habit led to a significant reduction in the intercanine and intermolar mean width in the maxillary arch [37]. Melink et al. compared 30 children with a unilateral posterior crossbite and 30 children without a crossbite and found that the duration of pacifier habits were associated with posterior crossbites at the age of 4 or 5 years because of low tongue posture in the mouth [38]. These inconsistent findings can be explained by the force and frequency of muscle action involved in different pacifier sucking pattern or duration, which result in different effect on the development of the dental arches. About half of the children in the present study were 3–4-years old and sucking a pacifier was rare in those beyond 3 years of age, so they may be too young to present an anterior open bite. Moreover, as was mentioned above, Chinese parents seldom use a pacifier to placate their infants, so the number with the habit was not large enough to show the most serious and representative effects.

The finding that pacifier- and digit-sucking habits have different effects on occlusal traits has also been reported previously [10,39,40]. The present study found that digit sucking resulted in an increased prevalence of anterior crossbite and that it had a significant effect on the upper labial segment. Franco also found that for the first 3–4 years of life, the detrimental effect on occlusion is largely confined to the anterior segment [41]. The effect on development of dentofacial structures depends on the duration and frequency of the habit, the intensity of the sucking, the relationship of the dental arches and the direction and nature of the force exerted by the digit [42]. Generally, it is agreed that prolonged digit sucking is associated with increased overjet, greater maxillary arch depth, and greater prevalence of anterior open bite [37,40,41]. The present study pointed to the association between digit sucking and anterior open bite. This might be because a digit-sucking habit is difficult to give up and 62% of the children with the digit-sucking habit maintained it up to 3 years of age; this makes it more likely to impact occlusion and lead to vertical disturbances. Our study also showed that non-nutritive sucking habits were not associated with the prevalence of posterior crossbite and no maxillary space, nor did we find any effects on mesial step and class II canine relationship. We suggest that feeding practice may play a primary role in the development of posterior crossbite and no maxillary space. The evidence presented in this study suggests that children should be predominant breast-fed or exclusive breast-fed for no less than 6 months where possible and that parent should be more aware of the potentially deleterious effects of non-nutritive sucking habits on oral development. From another direction, our finding fitted in with the WHO recommendation of breastfeeding exclusively for 6 months.

However, there are some intrinsic limitations of cross section study and it was hard to take into consideration of the effect of heredity and other oral detrimental habits on malocclusion. The recall bias of the people who filled the questionnaire still existed, which might affect the accuracy of the duration of feeding practice. A future study with a larger sample size and isolating single variables is needed for a thorough understanding of the relationship between sucking and malocclusions.