Biomechanics of milk extraction during breast-feeding

Abstract

How do infants extract milk during breast-feeding? We have resolved a century-long scientific controversy, whether it is sucking of the milk by subatmospheric pressure or mouthing of the nipple-areola complex to induce a peristaltic-like extraction mechanism. Breast-feeding is a dynamic process, which requires coupling between periodic motions of the infant's jaws, undulation of the tongue, and the breast milk ejection reflex. The physical mechanisms executed by the infant have been intriguing topics. We used an objective and dynamic analysis of ultrasound (US) movie clips acquired during breast-feeding to explore the tongue dynamic characteristics. Then, we developed a new 3D biophysical model of the breast and lactiferous tubes that enables the mimicking of dynamic characteristics observed in US imaging during breast-feeding, and thereby, exploration of the biomechanical aspects of breast-feeding. We have shown, for the first time to our knowledge, that latch-on to draw the nipple-areola complex into the infant mouth, as well as milk extraction during breast-feeding, require development of time-varying subatmospheric pressures within the infant's oral cavity. Analysis of the US movies clearly demonstrated that tongue motility during breast-feeding was fairly periodic. The anterior tongue, which is wedged between the nipple-areola complex and the lower lips, moves as a rigid body with the cycling motion of the mandible, while the posterior section of the tongue undulates in a pattern similar to a propagating peristaltic wave, which is essential for swallowing.

Keywords: computational model; fluid-structure interaction; submental ultrasound; sucking pressure.

Fig. 1.

Tongue motility of a healthy infant during breast-feeding. (A) Submental US images with traced contours of the palate and the tongue’s upper surface in different frames of the movie clip (subject 24). (B) Contours of all of the palate (red) and the tongue (green) from 150 frames of subject 24. (C) The contours of B after rigid registration around the rigid palate and the imposed polar coordinates. (D) Motility of the anterior tongue around anterior coordinates 1–8 after scaling (data from subject 41). The pattern fits the motility of a rigid body. (E) Motility of the posterior tongue around polar coordinates 8, 13, 17, and 22 after scaling (data from subject 41). The pattern fits the motility of a peristaltic wave. (F) Frequency distribution of the tongue contours around all of the polar coordinates. All regions of the tongue have the same dominant frequency of 1.56 Hz (i.e., 0.64 s per suckling cycle) (data from subject 41). The original and processed movie is provided in Movie S1. HSPJ, hard–soft palate junction.

Fig. 2.

Nipple movements in the anterior–posterior direction during breast-feeding (data from subject 41). (A) Contours of the tongue (green) and nipple distal surface (purple) after rigid registration around the rigid palate. The black curve represents the average of all contours of the rigid palate. (B) Contours of the palate (red), tongue (green), and nipple (purple) of several frames during a single suckling cycle. The black arrow represents the distance between the nipple tip and the HSPJ (blue circle). (C) The scaled motility of the nipple tip (purple, length of black arrows in B) in comparison with the rigid motility of the anterior tongue (green, average of the motility over lines 1–8 as in Fig. 1D). Nipple movement is provided in Movie S2. HSPJ, hard–soft palate junction.

Fig. 3.

Physical model of milk extraction during breast-feeding. (A) The breast model. (B) The infant mouth model at maximal opening. (C) Sagittal cross-section of the infant mouth attached to the breast just at the beginning of latch-on with arrows to represent the closing–opening displacements and the subatmospheric sucking pressure. (D) Sagittal cross-section of the infant mouth and the breast just at the end of latch-on. (E) Vertical maximal width of the infant mouth during latch-on and suckling. (F) Sucking subatmospheric pressure during latch-on and suckling. (G) The teat length (as shown in D) during latch-on and suckling. (H) Volume flux of milk extracted by the infant during latch-on and suckling. (I) Accumulated milk during the simulation shown here (i.e., latch-on and two suckling cycles), obtained by integration of the data in H.