Introduction
Tendons and ligaments are soft connective structures that carry stress from muscle to bone and from one bone to another. These particular tissues play crucial roles in the biomechanical function of the musculoskeletal system. Additionally, they stabilize and control diarthrodial joint movement. However, repeated and excessive use of the tissues frequently results in tissue damage. These injuries affect joints' mobility and stability, causing aberrant loading that may harm other soft tissues in and around the joint and cause pain and other morbidities like osteoarthritis (a form of arthritis that develops due to wearing of the elastic tissue or cartilage at the ends of bones, which serves as a bone's protective covering).
What Are the Basic Biomechanics of Ligaments and Tendons?
The primary function of ligaments and tendons is to transmit tensile forces from muscle to bone and from one bone to another. Therefore, the mechanical properties of tendons are improved by appropriate mechanical stresses applied at physiological levels. However, sufficient mechanical stress could reverse the decreasing mechanical characteristics of aged tendons and ligaments and reduce Young's modulus (a property of the material, measured as the ratio of tensile stress to tensile strain).
The collagen production and marker degradation (PICP procollagen type I C-terminal propeptide) increase in response to physical training. While collagen synthesis and breakdown increase during the early training phases, leading to a net increase in the collagen in tendons.
What Are the Mechanical Properties of Tissues?
The mechanical properties of ligaments and tendons vary based on their design and function in joints.
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The Young’s modulus and the tensile strength in the medial collateral ligament (ligaments aid in the stability of the knee joint) are 332.2 ± 58.3 MPa and 38.6 ± 4.8 MPa.
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The inferior glenohumeral ligament has a wide range of motion and flexibility, ranging from 5 to 42 MPa of the Young’s modulus and 1 to 6 MPa of tensile strength.
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The mechanical characteristics of the tendon and ligament highly depend on the direction of application (anisotropic), where the MCL (medial collateral ligament) is more significant in the longitudinal direction than in the transverse.
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The viscoelastic properties of ligaments and tendons are characterized by stress relaxation and creep, or the gradual rise in deformation under a constant load. The viscoelasticity prevents tissue damage during running or walking, where the maximal stress in the tissue substance declines.
What Are the Factors Affecting the Tensile Properties of Tendons and Ligaments?
The various factors can affect the biomechanical characteristics of ligaments and tendons, especially the tensile strength. Aging and the degree of exercise can affect ligaments and tendons' attributes. The tissue stiffness of younger people is greater than that of older people. Eventually, tendons and ligaments change in response to applied motion and tension. Any injury to the limb often results in immobilization, which increases joint stiffness and decreases the structural and mechanical properties of the MCL tissue; long-term training and exercise result in remobilization.
What Is the Most Common Tissue Injury?
Muscles, tendons, and ligaments are the most frequently damaged soft tissues. Common soft-tissue injuries include tendinitis (inflamed fibrous materials that connect muscle to bone) and bursitis (a painful condition that affects the bursae, which are small fluid-filled sacs that cushion the bones, tendons, and muscles while moving the joint), as well as sprains, strains, and contusions of the shoulder and knee resulting from ACL (anterior cruciate ligament) and MCL (medial collateral ligament) injuries. These accidents often happen when participating in sports or exercising but can also occur while doing routine daily chores.
What Are the Biomechanics of Tendon Injuries?
According to biomechanics, microtrauma is assumed to initially happen in the tendon, rupturing a limited number of collagen fibers while increasing the strain on the remaining ones. The tendon is more subjected to repeated stress, resulting in tendinosis or tendonitis. Usually, tendon damage spreads to the remaining intact tendon before tendinopathy symptoms manifest in response to high strains applied by stretching bioreactors, which would break down the collagen and result in degeneration. Apoptosis (biologically driven cell death) and altered MMP (matrix metalloproteinases) expression caused by tenocytes can also result in tendon degeneration.
Heat and internal shear stresses have also been proposed to contribute to intratendinous degeneration. The treatment of tendon injuries differs from one person to another. The exercises are used in the vast majority of cases. Activity may increase tolerance for severe exercise, resulting in training-induced adaptations in mechanical characteristics that help athletes avoid injury.
What Are the Biomechanics of the Anterior Cruciate Ligament (ACL) Injury?
The ACL is a tissue that stabilizes the knee joint and connects the thigh bone (femur) to the shin bone (tibia). The ACL has a high prevalence of injury during sports (basketball, soccer, tennis, and volleyball) and physical activity with a sudden change in direction. When the ACL is injured, the sensitive synovium around it is disturbed and does not begin to regenerate for one to two months. This thin synovium has been proven crucial in protecting the ACL from the abrasive synovial fluid and aiding in vascular supply. As a result, ACL is less capable of healing, cellular proliferation, and extracellular matrix (ECM) synthesis than MCL.
What Are the Biomechanics of the Medial Collateral Ligament (MCL) Injury?
The MCL injury in the knee can recover naturally. Therefore, conservative treatment is applied to restore knee stability and biomechanical features, with or without immobilization. It has been demonstrated that immobilization following ligament damage resulting in increased collagen fibrils decreased structural and mechanical properties of the ligament. The healing phase after MCL injury is distinguished into three stages.
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Stage 1: The inflammatory phase is characterized by hematomas or blood clots that last for a few weeks.
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Stage 2: The reparative phase, during which fibroblasts multiply and create a matrix of proteoglycan and collagen, particularly type III collagen, to connect the ripped edges, which lasts over six weeks.
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Stage 3: The remodeling phase is characterized by collagen fiber alignment, and matrix maturation can continue for a year.
The mechanical characteristics of the healing MCL remain constant and involve increased quantitative and decreased qualitative properties of ligamentous tissue.
Conclusion
Functional tissue engineering is a brand-new discipline that opens up many fresh opportunities. There will undoubtedly be better results when growth factors, gene therapy, cell therapy, and biological scaffolds are used to promote recovery. The ECM-derived bioscaffolds will be crucial since they have the potential to speed up the healing process, provide a link between the torn ends, and are effectively used to enhance MCL healing.
