Opposite Effect of Cyclic Loading on the Material Properties of Medial Collateral Ligament at Different Temperatures: An Animal Study

Abstract

In traffic accidents, the medial collateral ligament (MCL) injury of the knee joint of pedestrians is common. Biofidelic material is important to realize MCL's native biomechanics in simulations to clarify the injury mechanisms of pedestrians. Pedestrians' MCLs usually experience cyclic loading at the intra-articular temperature of the knee joint before accidents. Temperature influences the material behaviors of ligaments. However, the mechanical properties of ligaments under cyclic loading have been widely evaluated only at room temperature rather than physiological temperature. Therefore, this study aimed to determine whether the difference between room and intra-articular temperatures influences the effect of cyclic loading on the mechanical properties of MCL. We measured the tensile properties of 34 porcine MCLs at room temperature (21-23°C) and intra-articular temperature (35-37°C), with either 10 cycles or 240 cycles of cyclic loading, a total of four different conditions. The structural responses and geometric data were recorded. After 240 cycles of cyclic loading, stiffness increased by 29.0% (p < 0.01) at room temperature and decreased by 11.5% (p = 0.106) at intra-articular temperature. Material properties were further compared because the geometric differences between samples were inevitable. At room temperature, after 240 cycles of cyclic loading, elastic modulus increased by 29.6% (p < 0.001), and failure strain decreased by 20.4% (p < 0.05). By contrast, at intra-articular temperature, after 240 cycles of cyclic loading, modulus decreased by 27.4% (p < 0.001), and failure strain increased by 17.5% (p = 0.193), insignificant though. In addition, there were no significant differences between the four groups in other structural or material properties. The results showed that temperature reversed the effect of cyclic loading on the mechanical properties of MCL, which may be caused by the high strength and thermally stable crosslinks of MCL. Therefore, for improving the fidelity of knee joint simulations and elucidating the injury mechanism of pedestrians, it is better to measure the mechanical properties of MCL at intra-articular temperature rather than room temperature.

Keywords: cyclic loading; knee joint; medial collateral ligament; pedestrian injury; temperature; tensile properties.

FIGURE 1

The femur-MCL-tibia complex mounted on a tensile apparatus. (A) Intact femur-MCL-tibia complex, and its condyle placed in a custom-made fixture (before sawing). The red parallel marker lines indicate the contact surfaces between the condyles and support plates. The blue marker lines mark the maximum allowable bone length that the fixtures can entirely wrap. (B) Osteotomized femur-MCL-tibia complex, and its condyle fixed in the fixture by pointed screws (after sawing). The pointed screws were screwed into the condyle through thread holes on the sidewalls of the fixture. (C) The fixtures were clamped by the upper and lower clamps of the testing machine after fixing the femur-MCL-tibia complex in the fixtures. An environmental chamber was used to control the experimental temperature. MCL = medial collateral ligament.

FIGURE 2

Test protocols of the four groups. Once the ambient temperature reached their respective specified values, 240 cycles of cyclic loading were applied to the samples of the RTCY and ATCY groups. The samples of the RTBL and ATBL groups were stood in the same temperature environment for the same time as the samples of the RTCY and ATCY groups, respectively, to keep the same temperature balance time; then, 10 cycles of cyclic loading were applied. After cyclic loading, all samples were stretched to failure at 10 mm/min.

FIGURE 3

A typical tensile curve of MCL. The maximum force/stress point was defined as the failure point, and the corresponding abscissa and ordinate were the failure displacement/failure strain and failure force/failure stress, respectively. R ² is the coefficient of determination of linear regression. Gradually reduce the number of data points for linear regression. Once R ² was greater than a threshold (e.g., 0.99), the data segment used for this linear regression was defined as the linear region. The slope of the linear region was the stiffness/modulus.

FIGURE 4

Tensile curves as a function of cyclic loading at two different temperatures. Force-displacement curves: (A) At room temperature (n = 9); (B) At intra-articular temperature (n = 8). Stress-strain curves: (C) At room temperature (n = 9); (D) At intra-articular temperature (n = 8). The curves end at their respective failure points. At room temperature, the average stiffness and modulus of the samples after 240 cycles of cyclic loading (i.e., RTCY) were higher than those of the samples only went through 10 cycles of cyclic loading (i.e., RTBL). At intra-articular temperature, the samples after 240 cycles of cyclic loading (i.e., ATCY) generally had lower stiffness and modulus than those after 10 cycles of cyclic loading (i.e., ATBL).

FIGURE 5

Geometric data of each group: (A) Ligament length; (B) Cross-sectional area. The RTBL and ATBL groups had a significant difference in ligament length (***p < 0.001). The box shows the mean ± interquartile range, and the whiskers show the minimum/maximum values.

FIGURE 6

Material properties of each group: (A) Modulus; (B) Failure strain; (C) Failure stress; (D) Failure strain energy. The box shows the mean ± interquartile range, and the whiskers show the minimum/maximum values. Significant differences are indicated as: *p < 0.05, **p < 0.01, ***p < 0.001.

FIGURE 7

The stress-strain response of a crosslinked collagen fibril under unidirectional stretching. The purple bars represent collagen molecules, and the pink lines represent crosslinks. Collagen molecules first uncoil their triple helix structures in the uncoiling stage. Then, a relatively gentle sliding stage is formed due to the sliding of molecules. Crosslinks also start to work in the sliding stage, restraining sliding and enhancing mechanical properties. If the strength of the crosslinks is weaker than that of the molecular backbones, the crosslinks will fail in the sliding stage, resulting in a two-stage response (the blue dot line). Otherwise, the molecular backbones will be stretched and generate a sharply rising curve segment, resulting in a three-stage response (the red line) (Svensson et al., 2013; Depalle et al., 2015).