Importance: The maxillary labial frenulum and its potential contribution to breastfeeding difficulty may substantially affect public health. However, objective studies of the frenulum are limited. Objective: To measure the variations in length, thickness, and attachments of the maxillary labial frenulum in healthy newborns and to identify which anatomic measurements could be used in further research investigating the maxillary labial frenulum. Design, Setting, and Participants: This prospective cross-sectional study conducted measurements on images of maxillary labial frenula captured by digital photography from 150 healthy newborns admitted to the newborn nursery at a tertiary care children's hospital in Maryland between September 1, 2017, and April 1, 2018. Main Outcomes and Measures: The primary outcome was the measurement of numerous frenulum morphologic components. Results: Of 150 newborns enrolled, 77 were female, the mean (SD) gestational age was 38.60 (1.72) weeks, and the mean (SD) birth weight was 318 (57) g. The means and SDs of the morphologic components of the frenulum with the broadest distributions, which were most helpful in differentiating degrees of lip tethering, included the following: alveolar edge to frenulum gingival attachment, 1.53 (0.85) mm; frenulum length on stretch, 5.19 (1.68) mm; frenulum gingival attachment thickness, 0.84 (0.36) mm; frenulum labial attachment thickness, 2.83 (1.33) mm; and the percentage of free lip to total lip length, 87.38% (7.67%). Gingival attachment mean (SD) thickness differed between late-preterm (0.69 [0.24] mm) and term (0.88 [0.37] mm) infants (Cohen d, -0.52; 95% CI, -0.94 to -0.10). Conclusions and Relevance: To our knowledge, this cross-sectional study was the first to objectively measure the numerous morphologic components of the upper lip anatomy in healthy newborns. Variations in maxillary labial frenulum morphology were identified, and some combination of the stated measurements may be used to create a more robust classification system to advance quality research in the association of lip-tie with breastfeeding difficulty.
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