# Biomechanical Analysis of Different Pacifiers and Their Effects on the Upper Jaw and Tongue
## 1 Introduction  
The pacifier is a widely used tool for non-nutritive sucking (NNS) among infants and children, playing a crucial role in neonatal care. Non-nutritive sucking is not only physiologically present but also has protective effects, particularly against sudden infant death syndrome (SIDS). In neonatal intensive care units (NICUs), pacifiers are often introduced as a means of stimulating oral motor skills and preparing premature infants for feeding. The benefits of pacifier use are well-documented, with evidence suggesting that it contributes to brain maturation, promotes the development of gastrointestinal mucosa, activates the production of endorphins, improves sleep patterns, facilitates weight gain by aiding gastrointestinal absorption, and helps regulate the overall development of the newborn [1,2]. Sucking in infants follows a unique and dynamic pattern of tongue movement that has been closely studied using quantitative imaging techniques. These techniques have demonstrated that both infants and adults share similar kinetic patterns of tongue movements during deglutition, where the tongue moves in peristaltic-like waves to facilitate the swallowing process. In breastfeeding, the tongue's wave motion plays a critical role in generating intra-oral pressure to create suction for milk transfer [1]. This movement pattern is significantly more pronounced in breastfeeding, where the rigid anterior tongue movement stabilizes the breast in the mouth, while the posterior tongue performs peristaltic movements. In contrast, bottle-feeding and pacifier use introduce mechanical differences as the artificial nipple lacks the biomechanical compatibility present in breastfeeding, leading to variations in tongue movement and overall oral motor function [3]. In breast sucking, the less fatty tissue is present, the more difficult it is to activate sucking because the space in the oral cavity is large; the more the space narrows, the easier it is to create negative intra-oral pressure and thus a vacuum. The nipple sits between the hard and soft palate so the mouth opening must allow the entrance not only of the nipple but also of most of the mammary areola. The jaw movement combined with the infant's intra-oral pressure, lingual movement, and peristaltic movement allows the transfer of milk. The rigid anterior tongue movement pattern during breastfeeding reflects the need to stabilize the breast in the mouth. The current analysis of ultrasound video clips shows the dynamic pattern of the infant's tongue during breastfeeding. Results illustrate that the overall frequency spectrum and undulation pattern of the tongue during breastfeeding and bottle-feeding are similar [1]. However, the natural infant-mother biomechanical compatibility that exists in breastfeeding is missing in bottle-feeding due to differences in mechanical characteristics (e.g., size, stiffness, conformability, viscoelasticity) between the artificial nipple and the maternal nipple-areola complex. The pacifier, while beneficial for soothing and regulating infants, can influence craniofacial and oral development, particularly during the early stages of life when the soft and hard tissues are most vulnerable. Pacifiers can be categorized into conventional and orthodontic types. Conventional pacifiers usually feature a round or symmetrical nipple design, which does not support the natural development of the oral cavity. These pacifiers exert direct pressure on the palate, which may lead to palatal deformation and malocclusions, particularly when used excessively [4]. On the other hand, orthodontic pacifiers are specifically designed to promote healthy craniofacial development. They feature a flattened or angled nipple that mimics the natural positioning of the tongue, encouraging proper tongue posture and reducing the risk of deformities such as open bite or crossbite [5]. These pacifiers are intended to minimize harmful intra-oral pressure and support the correct development of the hard and soft tissues in the oral cavity, particularly the palate and jaw.