Aortic allografts: final destination?—a summary of clinical tracheal substitutes

The correlation of airway structure and function was first elucidated by Leonardo da Vinci. Early drawings demonstrate the anatomy of the uvula, larynx, vocal cords and the airway tree. In fact, much ahead of his time, Da Vinci came up with “the Da Vinci rule” where he elaborated upon his understanding of the cross-sectional area of the airway and tree branching. He proposed that the sum of the cross-sectional area of all tree branches above a branching point equals the cross-sectional area of the trunk or the branch immediately below the branching point (1,2). These studies have been revisited at Princeton University recently. In his early documents da Vinci mentions “the voice becomes weak in elderly because all the passages of the trachea are narrowed, in the same manner as the other entrails”. Although some assumptions of this Renaissance man were speculative, the concept of “structure and function” and the direct correlation of tissue and an accompanying purpose was emphasized repetitively in his work. The modern understanding of tracheal structure and function states an anatomical tubular structure containing cartilage to provide rigidity, intercartilagenous muscular area allowing longitudinal flexibility, dorsally muscular wall permitting esophageal motility and epithelialized inner lining necessary for clearance of the inner airway surface. The field of airway substitute generation attempts to recapitulate these attributes. However, this endeavor has appeared far from simple with various fields facing dilemma’s in one or more of these facets