Ligaments and tendons are both involved with movement of the bones of the musculoskeletal system. Tendons connect muscle to bone, and ligaments connect bone to bone. Between these ligament connections articulating the bones of joints, are a number of anatomical parts that provide cushion, protection, and shock absorption. Within the space occupied by the ligaments and articulating bones of the joint is the joint capsule. Within this joint capsule is the synovial joint. The synovial joint is made out of a synovial membrane enclosing a capsule filled with synovial fluid. The synovial fluid has several functions, including the exchange of oxygen and other maintain that maintain the health of the joint, shock absorption, and reduction of friction. Despite this synovial capsule having a volume that would fill barely half a teaspoon in an adult knee, it is able to reduce the friction between articulating bones nearly to zero.
Ligaments are tightly wrapped strands of dense, connective tissue. Ligaments are composed of collagen fibers. These bundles of collagen fibers are very tough and able to withstand all but the most extreme pathological movements and stresses against them, yet they are elastic structures that allow for movement of the articulating bones or joints. The ligaments are the reason that we are not able to turn, twist, extend, or flex our limbs or torso beyond a certain points. Despite their composition of elastic, collagen fibers, many ligaments are actually stronger than the bones they support. To further state this point, there are examples of ligaments that were so strong that after shortening or losing their elasticity beyond a point that they could no longer support healthy movements, the bones that they were attached to broke before the ligaments themselves were stressed to the point of rupture.
Despite the elastic properties of ligaments, they are designed to be strong enough to prevent the joint and joint capsule from moving beyond a healthy range of motion. If the joint was pulled, pushed, bent, turned, flexed, or extended beyond a healthy range of motion the joint could become damaged or the ligaments of that joint could become sprained or ruptured.
There are multiple ligaments in each joint. For example, the human spine has several ligaments, including the anterior longitudinal ligament, posterior longitudinal ligament, supraspinous ligament, interspinous ligament, and ligamentum flavum. The knee also has several important ligaments that stabilize the knee joint, while allowing for movement. The knee joint has some ligaments that are completely outside the joint capsule, and some within the joint capsule. The ligaments most commonly involved with knee injuries include the posterior cruciate ligament, anterior cruciate ligament, lateral collateral ligament, and medial collateral ligament.
Ligaments may be involved in causing back pain if due to age related changes to them that cause them to lose their elasticity, or due to trauma or injury that causes them to lengthen. Ligaments that have gone anatomical changes due to disease, age, or trauma may no longer be able to protect the bones and joints they support the same, and the joints may become vulnerable to osteoarthritis and other orthopedic problems.