Ligaments appear as dense, white, hypovascular bands of tissue and vary in size, shape, and orientation depending on their anatomical location and function. For example, medial collateral ligaments (MCLs) of the knee are broad or ribbon-shaped structures while anterior cruciate ligaments (ACLs) of the knee are cylindrical-shaped structures in the midsubstance that expand into the femur and tibia. The ACL consists of two major bundles (anterior medial [AM] and posterior lateral [PL] bundles) that merge together to provide functional heterogeneity under knee flexion and extension. ACL AM and PL bundles have unique tensioning patterns with knee motion (Harner et al. 1999). Ligaments are inserted into the bone in a complex way with region-dependent mechanical inhomogeneity and mineral distribution (Moffat et al. 2008). It has long been postulated that the controlled matrix heterogeneity inherent at the ligament-to-bone interface serves to minimize the formation of stress concentrations, and to enable the transfer of complex loads between soft tissue and bone. Insertion sites also provide blood supply to the ligament.
Ligaments are composed of fibroblasts surrounded by ECM proteins. The ECM of ligaments typically contains 60-80% water. The approximate two thirds of water are crucial for cellular function and viscoelastic behavior of ligaments. The ECM typically accounts for 20-40% of the solid portion of the ligament. The ECM of ligaments are composed mainly of collagen which accounts for approximately 70-80% of its dry weight. Type I collagen accounts for 85-90% of the collagen and the rest is composed of collagen types III, VI, V, XI, and XIV. The other 20-30% of the ECM are ground substances, including proteoglycans (<1%), elastin, and other proteins and glycoproteins such as actin, laminin, and integrins (Bray et al. 2005; Frank 2004). Not all of these components are fully understood and are still an active area of research. At the molecular level, collagen is synthesized as procollagen molecules that are secreted into the extracellular space. Once outside the cell, a post-transitional modification catalyzed by a specialized enzyme called lysyl oxidase takes place. Lysyl oxidase promotes stable crosslink formation within and between procollagen molecules. The triple helical collagen molecules then line up and begin to form fibrils and subsequently fibers. Crosslinking is critical for the high strength of collagen fibers. During growth and development, crosslinks are relatively immature and soluble but with age they mature and become insoluble and increase in strength (Frank 2004). This process is important for understanding the healing process of ligaments.
Fibroblasts are relatively few in number compared to the extracellular matrix in the total volume of ligaments. Although these cells may appear physically and functionally isolated in the ligament tissue, recent evidence has indicated that normal ligament cells may communicate by means of prominent cytoplasmic extensions (Benjamin and Ralphs 2000). Gap junctions have also been detected contributing to cell connections and in linking complex networks of cellular processes (Chi et al. 2005; Lo et al. 2002a). Cell-to-cell communication raises the possibility and the potential to coordinate cellular and metabolic responses throughout the tissue. Ligaments have three dimensional (3D) anisotropic structures. Ligament fibroblasts and collagen fibers are blended within a fascicle and are organized into parallel rows that run along the long axis of the ligament (Lo et al. 2002a, b). The cells are spindle-shaped and have long cytoplasmic projections connected by gap junctions. A complex interconnected array of cells (termed cellular matrix) extends from one end of the ligament to the other and is believed to assist communication throughout the ligament and coordinate responses to both biochemical and biomechanical signals (Bray et al. 2005; Chi et al. 2005).
One of the main functions of ligaments is to stabilize joints and help guide them through the normal range of motion but limiting excessive bone displacements under high loads. Ligaments are subjected to tensile loads as they transfer load from bone to bone along the longitudinal direction of the ligament. Thus, their biomechanical properties are usually studied via a uniaxial (along the longitudinal direction) tensile test of a bone-ligament-bone complex. The effects of age on mechanical properties of a ligament vary from ligament to ligament. For example, in the rabbit model, the stiffness and ultimate load of the femur-MCL-tibia complex (FMTC) dramatically increase during skeletal maturation from 6 to 12 months of age (Woo et al. 1986,
1990). This corresponds with a change in failure mode from the tibial insertion to the midsubstance reflecting closure of the tibial epiphysis during maturation (Woo et al. 1986). However, there was no significant change from 1 to 4 years. In contrast, the human femur-ACL-tibia complex (FATC) demonstrated a rapid decrease in the stiffness and ultimate load with increasing age (Woo et al. 1991). A rapid reduction in ligament properties and mass could occur following immediate injury (Woo et al. 1986, 1987, 1990, 1991; Newton et al. 1990; Noyes 1977).
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