Examining differences in local collagen fiber crimp frequency throughout mechanical testing in a developmental mouse supraspinatus tendon model

Abstract

Crimp morphology is believed to be related to tendon mechanical behavior. While crimp has been extensively studied at slack or nondescript load conditions in tendon, few studies have examined crimp at specific, quantifiable loading conditions. Additionally, the effect of the number of cycles of preconditioning on collagen fiber crimp behavior has not been examined. Further, the dependence of collagen fiber crimp behavior on location and developmental age has not been examined in the supraspinatus tendon. Local collagen fiber crimp frequency is quantified throughout tensile mechanical testing using a flash freezing method immediately following the designated loading protocol. Samples are analyzed quantitatively using custom software and semi-quantitatively using a previously established method to validate the quantitative software. Local collagen fiber crimp frequency values are compared throughout the mechanical test to determine where collagen fiber frequency changed. Additionally, the effect of the number of preconditioning cycles is examined compared to the preload and toe-region frequencies to determine if increasing the number of preconditioning cycles affects crimp behavior. Changes in crimp frequency with age and location are also examined. Decreases in collagen fiber crimp frequency were found at the toe-region at all ages. Significant differences in collagen fiber crimp frequency were found between the preload and after preconditioning points at 28 days. No changes in collagen fiber crimp frequency were found between locations or between 10 and 28 days old. Local collagen fiber crimp frequency throughout mechanical testing in a postnatal developmental mouse SST model was measured. Results confirmed that the uncrimping of collagen fibers occurs primarily in the toe-region and may contribute to the tendon's nonlinear behavior. Additionally, results identified changes in collagen fiber crimp frequency with an increasing number of preconditioning cycles at 28 days, which may have implications on the measurement of mechanical properties and identifying a proper reference configuration.

Fig. 1

A schematic of the testing protocol with six points at which crimp was assessed

Fig. 2

Representative samples of type I (left), type II (middle), and type III (right) crimp in the supraspinatus tendon

Fig. 3

Crimp frequency at 10 days old demonstrates that crimp frequency decreases during the toe-region regardless of the mechanical testing protocol

Fig. 4

Crimp frequency demonstrates that crimp frequency decreases during the toe-region and increases from the preload to ten cycles of preconditioning at both locations

Fig. 5

Representative images from the 28 day insertion site throughout the mechanical test demonstrate an increase in crimp frequency between the preload and after ten cycles of preconditioning and a decrease in crimp frequency between after five and ten cycles of preconditioning and the toe-region

Fig. 6

Representative images for 4, 10, and 28 days at the preload, toe- and linear-region. Histology demonstrates the smaller collagen fibers and scale of crimp present at 4 days compared to 28 days.