Mechanical differences between medial collateral ligament and lateral collateral ligament and influence of elastin degradation

Abstract

BACKGROUND: As crucial stabilizers of the knee joint, the medial collateral ligament and lateral collateral ligament play essential roles in restricting valgus and varus movements, respectively. However, the mechanical differences between the medial collateral ligament and lateral collateral ligament, the microstructure characteristics, and the effect of elastin degradation on their mechanical properties remain poorly understood. OBJECTIVE: To compare the mechanical differences between the medial collateral ligament and lateral collateral ligament, quantify the structural characteristics of the collagen fiber alignment, and investigate the effect of elastin degradation on the mechanical properties of both ligaments. METHODS: Left medial collateral ligaments and lateral collateral ligaments were harvested from adult pigs, frozen, and thawed. Quasi-static uniaxial tensile tests were performed to measure the mechanical properties of the medial collateral ligament and lateral collateral ligament, and the effects of repeated stretching on their mechanical properties were compared. Second harmonic generation imaging using a two-photon microscope was used to quantify the collagen fiber structure of the medial collateral ligament and lateral collateral ligament. After repeated stretching, the medial collateral ligament and lateral collateral ligament were incubated in elastase solution for 12 hours, followed by uniaxial tensile tests to determine the effect of elastin treatment on ligament mechanical properties. RESULTS AND CONCLUSION: (1) Quasi-static uniaxial tensile tests showed that the high-tension elastic modulus of the medial collateral ligament was higher than that of the lateral collateral ligament (P < 0.05), while there was no significant difference in the low-tension elastic modulus between the two groups (P > 0.05). Repeated stretching significantly reduced the low-tension elastic modulus of both the medial collateral ligament and lateral collateral ligament. (2) Elastase treatment significantly reduced the low-tension and high-tension elastic moduli of both the medial collateral ligament and lateral collateral ligament, and the decrease in the high-tension elastic modulus of the lateral collateral ligament was greater than that of the medial collateral ligament. After elastase treatment, both the low-tension and high-tension elastic moduli of the medial collateral ligament were higher than those of the lateral collateral ligament (P < 0.05). (3) Two-photon imaging showed that the collagen fibers of the medial collateral ligament maintained a crimped structure, and its fiber waviness was significantly higher than that of the lateral collateral ligament. (4) These results indicate that the medial collateral ligament has stronger elastic properties than the lateral collateral ligament, and elastase treatment has a greater effect on the mechanical properties of the lateral collateral ligament. These mechanical results may be related to the more crimped collagen fiber arrangement in the medial collateral ligament.

1. Introduction

The knee joint bears complex mechanical loads during movement and daily activities, and its stability mainly depends on the surrounding ligament structures [1-3]. Among them, the medial collateral ligament and lateral collateral ligament, as important stabilizers of the knee joint, respectively play key roles in restricting valgus and varus movements [4-7]. Medial collateral ligament injury is the most common ligament injury in knee trauma, while the lateral collateral ligament is often damaged together with other structures; isolated lateral collateral ligament injury is the second rarest knee ligament injury [8-11]. This significant difference in injury incidence may be related to the anatomical location, tissue structure, and physiological function of these two ligaments. However, as biological materials that resist elastic deformation, the biomechanical properties of the medial collateral ligament and lateral collateral ligament are significantly different, contributing differently to knee stability and movement patterns [12-13]. Therefore, understanding the mechanical differences between the medial collateral ligament and lateral collateral ligament and the regulatory role of microstructure on their mechanical properties is not only important for the diagnosis and treatment of knee injuries but also provides a theoretical basis for biomaterial design and tissue engineering.

Elastin is a highly extensible structural protein network that provides nearly elastic resistance to deformation in biological tissues [14-15]. Elastin accounts for about 5% of the dry weight of ligaments, but under uniaxial strain, it can provide up to 30% of the tensile stress, making it one of the important components determining the mechanical properties of ligaments [16-18]. Elastin and collagen together form the matrix network of ligaments, giving ligaments sufficient strength to withstand external forces and certain elasticity to adapt to dynamic movements [19]. However, the distribution and content of elastin may differ between the medial collateral ligament and lateral collateral ligament, which may be an important reason for the different mechanical properties of the two. In addition, elastin degradation is considered a non-negligible factor in many degenerative diseases and tissue aging processes, because elastin degradation significantly reduces the mechanical properties of ligaments, increasing the risk of tissue rupture and loss of function [20-23]. Current research shows that under external forces, the mechanical behavior of ligaments is closely related to the content and structure of elastin and collagen.