Abstract
A nonlinear elastic microstructural model is used to investigate the relationship between structure and function in energy-storing and positional tendons. The model is used to fit mechanical tension test data from the equine common digital extensor tendon (CDET) and superficial digital flexor tendon (SDFT), which are used as archetypes of positional and energy-storing tendons, respectively. The fibril crimp and fascicle helix angles of the two tendon types are used as fitting parameters in the mathematical model to predict their values. The outer fibril crimp angles were predicted to be 15.1° ± 2.3° in the CDET and 15.8° ± 4.1° in the SDFT, and the average crimp angles were predicted to be 10.0° ± 1.5° in the CDET and 10.5° ± 2.7° in the SDFT. The crimp angles were not found to be statistically significantly different between the two tendon types (p = 0.572). By contrast, the fascicle helix angles were predicted to be 7.9° ± 9.3° in the CDET and 29.1° ± 10.3° in the SDFT and were found to be statistically highly significantly different between the two tendon types (p < 0.001). This supports previous qualitative observations that helical substructures are more likely to be found in energy-storing tendons than in positional tendons and suggests that the relative compliance of energy-storing tendons may be directly caused by these helical substructures.
Highlights
Tendons have varying mechanical requirements depending on their function
The predicted fibril crimp and fascicle helix angles according to the model fit are listed in table 1 and example fits to the experimental data are plotted in figure 2
The model used above provides a link between the microstructures and mechanical functions of the common digital extensor tendon (CDET) and superficial digital flexor tendon (SDFT), explaining that the relative compliance of energystoring tendons may be caused directly by the helical fibril arrangement of their fascicles, and not by differences in their fibril Young’s modulus or crimp angles
Summary
Positional tendons need to be stiff in order to keep joints in place, whereas energy-storing tendons play a role in locomotion [1] and are necessarily more compliant [2]. This specialization of mechanical properties between tendon types occurs despite them being composed of the same elementary materials—primarily collagen type I, which is organized into a hierarchical structure consisting of fibrous subunits of varying diameters, each of which is interspersed with a small amount of predominantly non-collagenous matrix [3]. Mathematical modelling can be used to determine how the geometrical arrangement of tendon subunits affects gross mechanical properties. Two models were developed [5,6] with the aim of having a microstructural
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