Collagen fiber re-alignment and uncrimping in response to loading: Determining structure-function relationships using a developmental tendon mouse model

Collagen fiber re-alignment and uncrimping are postulated mechanisms of structural response to load. It has been suggested that fibers re-orient in the direction of load and then uncrimp before collagen is tensioned and that in general, the structure is a result of the function tendons perform. However, little is known about how fiber re-alignment and uncrimping change in response to load, how this change relates to tendon mechanical properties, and if these changes are dependent on the underlying structure. Throughout postnatal development, dramatic structural and compositional changes occur in tendon. Postnatal tendons, with immature collagen networks, may respond to load in a different manner and timescale than mature collagen networks. Therefore, the overall objective of this study was to quantify the mechanical properties and structural response to load in a developmental mouse tendon model at 4, 10, 28 and 90 days old. Local collagen fiber re-alignment and crimp frequency were quantified throughout mechanical testing and local mechanical properties were measured. Throughout development, fiber re-alignment occurred at different points in the mechanical testing protocol. At early development, re-alignment was not identified until the linear (4 days) or toe-regions (10 days) of the mechanical test suggesting that fibers required a prolonged exposure to mechanical load before responding and that the immature collagen network present may delay re-alignment. The uncrimping of collagen fibers was identified during the toe-region of the mechanical test at all ages suggesting that crimp contributes to tendon nonlinear behavior. Additionally, results at 28 and 90 days suggested that collagen fiber crimp frequency decreased with increasing number of preconditioning cycles, which may affect toe-region properties. Mechanical properties and cross-sectional area increased throughout development. The insertion site demonstrated lower moduli values and a more disorganized fiber distribution compared to the midsubstance at all ages suggesting it experiences multi-axial loads. Further, the tendon locations demonstrated different re-alignment and crimp behaviors suggesting that locations may respond to load differently and develop at different rates. Results from this study suggest that structure affects the tendon\u27s ability to respond to load and that the loading protocol applied may affect the measurement of mechanical properties